A method and system for improving signal-to-noise ratio of class-d power amplifier

Abstract

This application provides a method and system for improving the signal-to-noise ratio (SNR) of a Class D power amplifier, relating to the field of digital signal processing technology. The method includes: oversampling an input analog audio signal at a preset oversampling rate to obtain an oversampled digital audio signal; inputting the oversampled digital audio signal to a second-order 3-bit Sigma-Delta modulator, which employs a cascaded integrator feedback topology. The modulator's signal transfer function ensures the smooth passage of the low-frequency useful audio signal, while its noise transfer function shapes noise within the audio range to the high-frequency region, outputting a PDM signal; amplifying the PDM signal using a Class D power amplifier in a medium-wave broadcast transmitter to obtain an amplified PDM signal; and performing low-pass filtering on the amplified PDM signal to recover the baseband audio signal and output it. Implementing the technical solution of this application achieves the effect of improving the signal-to-noise ratio.

Description

Technical Field

[0001] This application relates to the field of digital signal processing technology, specifically to a method and system for improving the signal-to-noise ratio of a Class D power amplifier. Background Technology

[0002] Medium-wave broadcast transmitters are a core component of wireless communication systems. Traditional medium-wave AM modulation relies on analog circuits and vacuum tubes, which suffers from low efficiency and unstable technical specifications. With the development of semiconductor technology, Class D power amplifiers, by operating the power transistors in a switching state, can theoretically achieve extremely high efficiency. All-solid-state Class D power amplifiers are widely used due to their high power utilization. However, most existing Class D power amplifiers use PWM modulation, which compares the input analog audio signal with a fixed-frequency triangular wave to generate a PWM signal to drive the switching transistors. This approach suffers from low signal-to-noise ratio and significant harmonic distortion, resulting in poor output audio signal quality, which is insufficient to meet the high-quality signal transmission requirements of medium-wave broadcasting. Therefore, there is an urgent need for a technical solution that can combine the high efficiency advantages of Class D power amplifiers with an effective improvement in signal-to-noise ratio. Summary of the Invention

[0003] To address the aforementioned technical problems, this application provides a method and system for improving the signal-to-noise ratio of a Class D power amplifier.

[0004] In a first aspect, this application provides a method for improving the signal-to-noise ratio of a Class D power amplifier, applied in a medium-wave broadcast transmitter, comprising: oversampling an input analog audio signal at a preset oversampling rate to obtain an oversampled digital audio signal; inputting the oversampled digital audio signal into a second-order 3-bit Sigma-Delta modulator, wherein the Sigma-Delta modulator adopts a cascaded integrator feedback CIFB topology, and the signal transfer function of the Sigma-Delta modulator allows the low-frequency useful audio signal to pass smoothly, and the noise in the audio range is shaped to the high-frequency region through the noise transfer function, outputting a pulse density modulated (PDM) signal; inputting the PDM signal into the Class D power amplifier of the medium-wave broadcast transmitter for power amplification to obtain an amplified PDM signal; and performing low-pass filtering on the amplified PDM signal to restore the baseband audio signal and output it.

[0005] By adopting the above technical solution, oversampling of the analog audio signal can improve signal resolution. Using a second-order 3-bit Sigma-Delta modulator and CIFB topology, the signal transfer function allows the low-frequency useful audio signal to pass through smoothly, while the noise transfer function shapes the noise in the audio range to the high-frequency region, effectively separating the useful signal and noise. After passing through the Sigma-Delta modulator, a PDM signal can be obtained. Then, using a Class D power amplifier to amplify the PDM signal can take advantage of its high efficiency. Finally, low-pass filtering is performed on the amplified PDM signal to restore the baseband audio signal, thus taking into account the high efficiency advantage of the Class D power amplifier, effectively improving the signal-to-noise ratio, and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0006] Optionally, the Sigma-Delta modulator includes: a first adder, a second adder, a first integrator, a second integrator, a quantizer, a first input gain module, a second input gain module, a first integral gain module, a second integral gain module, a first feedback module, a second feedback module, and a local negative feedback module; wherein, the oversampled digital audio signal is input to the input terminal of the first input gain module, the output terminal of the first input gain module is electrically connected to the first input terminal of the first adder, the output terminal of the first input gain module is also electrically connected to the input terminal of the second input gain module, the output terminal of the second input gain module is connected to the first input terminal of the second adder; the output terminal of the first adder is connected to the input terminal of the first integrator, and the output terminal of the first integrator is connected to the first integrator. The input terminal of the first integral gain module is connected to the second input terminal of the second adder; the output terminal of the second adder is connected to the input terminal of the second integrator, the output terminal of the second integrator is connected to the input terminal of the second integral gain module, and the output terminal of the second integral gain module is connected to the input terminal of the quantizer; the output terminal of the quantizer outputs a PDM signal and is simultaneously connected to the input terminals of the first feedback module and the second feedback module; the output terminal of the first feedback module is connected to the second input terminal of the first adder, the output terminal of the second feedback module is connected to the third input terminal of the second adder, and the output terminal of the second integrator is also connected to the input terminal of the local negative feedback module, the output terminal of the local negative feedback module is connected to the third input terminal of the first adder, for providing negative feedback.

[0007] By adopting the above technical solution, the input analog audio signal is oversampled to obtain an oversampled digital audio signal. The oversampled digital audio signal is then processed by a Sigma-Delta modulator consisting of multiple adders, integrators, gain modules, feedback modules, and quantizers. This allows the useful low-frequency audio signal to pass through smoothly and shapes the noise in the audio range to the high-frequency region to output a PDM signal. After amplification by a Class D power amplifier and low-pass filtering, the baseband audio signal can be restored, effectively improving the signal-to-noise ratio of the Class D power amplifier and solving the problem of poor output audio signal quality caused by low signal-to-noise ratio and significant harmonic distortion in existing Class D power amplifiers.

[0008] Optionally, the first input gain module is configured to have a first gain value, and the second input gain module is configured to have a second gain value; the first feedback module is configured to have a third gain value, and the second feedback module is configured to have a fourth gain value; the local negative feedback module is configured to have a fifth gain value; the first integral gain module is configured to have a sixth gain value, and the second integral gain module is configured to have a seventh gain value; the first, third, fourth, and sixth gain values ​​are all equal to 0.5, the second gain value is equal to 0, the fifth gain value is equal to -0.005, and the seventh gain value is equal to 1.

[0009] By adopting the above technical solution, the input analog audio signal is oversampled to obtain an oversampled digital audio signal. A Sigma-Delta modulator with a specific topology is used to make the low-frequency useful audio signal pass through smoothly and to shape the noise in the audio range to the high-frequency region to output a PDM signal. Then, after power amplification and low-pass filtering, the baseband audio signal is obtained. At the same time, the gain values ​​of the first input gain module, the first feedback module, the second feedback module, and the first integral gain module are set to 0.5, the gain value of the second input gain module is set to 0, the gain value of the local negative feedback module is set to -0.005, and the gain value of the second integral gain module is set to 1, so that the Sigma-Delta modulator can work stably and effectively, further ensuring the reliability and effectiveness of the entire process of improving the signal-to-noise ratio of the Class D power amplifier.

[0010] Optionally, the values ​​of the first gain value, the second gain value, the third gain value, the fourth gain value, the fifth gain value, the sixth gain value, and the seventh gain value shall satisfy the following condition: so that the poles of the Sigma-Delta modulator are located inside the unit circle.

[0011] By adopting the above technical solution, each gain module is configured with a specific gain value, and these gain values ​​ensure that the poles of the Sigma-Delta modulator are located within the unit circle. This ensures the stability of the Sigma-Delta modulator, thereby guaranteeing the stable operation of the entire method for improving the signal-to-noise ratio of Class D power amplifiers. This effectively improves the signal-to-noise ratio of Class D power amplifiers and meets the requirements of medium wave broadcasting for high-quality signal transmission.

[0012] Optionally, the output of the quantizer is electrically connected to the input of the Class D power amplifier, and the output of the Class D power amplifier is electrically connected to the input of the low-pass filter. The Class D power amplifier is used to output the amplified PDM signal, and the low-pass filter is used to perform low-pass filtering on the amplified PDM signal to restore the baseband audio signal. The quantizer is used to quantize the digital signal output by the second integral gain module into a 3-bit PDM signal. The PDM signal characterizes the amplitude information of the analog audio signal through the pulse density and multi-level amplitude per unit time.

[0013] By adopting the above technical solution, the PDM signal output by the quantizer is input into the Class D power amplifier for power amplification, and the amplified PDM signal is output. After low-pass filtering, the baseband audio signal can be restored. The quantizer quantizes the digital signal output by the second integral gain module into a 3-bit PDM signal. By using pulse density and multi-level amplitude to jointly characterize the amplitude information of the analog audio signal, it can effectively improve the signal-to-noise ratio of the Class D power amplifier, while taking into account the high efficiency advantage of the Class D power amplifier, and meeting the needs of medium wave broadcasting for high-quality signal transmission.

[0014] Optionally, the signal transfer function and noise transfer function in the z-domain have the following forms: STF = 0.25 / (z 2 -1.5×z+0.75), NTF=(z-1) 2 / (z 2 -1.5×z+0.75), where STF represents the signal transfer function, NTF represents the noise transfer function, and z is a complex frequency variable used to characterize the frequency response of the Sigma-Delta modulator as a discrete-time system.

[0015] By adopting the above technical solution, the input analog audio signal is oversampled and then input into a Sigma-Delta modulator with a specific topology. A specific form of signal transfer function is used to make the low-frequency useful audio signal pass through smoothly, and a specific form of noise transfer function is used to shape the noise in the audio range to the high-frequency region to obtain the PDM signal. The PDM signal is then amplified and low-pass filtered to output the baseband audio signal. In this process, the specific form of signal transfer function ensures the normal transmission of the useful audio signal, and the specific form of noise transfer function transfers the audio noise to the high frequency for easy removal by subsequent low-pass filtering, thereby improving the signal-to-noise ratio of the Class D power amplifier and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0016] Optionally, the cutoff frequency of the low-pass filter is set to be higher than the upper limit frequency of the baseband audio signal and lower than the starting frequency of the high-frequency region.

[0017] By adopting the above technical solution, it can be ensured that the low-pass filtering process can effectively filter out noise in the high-frequency region while retaining the baseband audio signal, thereby improving the signal-to-noise ratio of the Class D power amplifier output audio signal and better meeting the needs of medium wave broadcasting for high-quality signal transmission.

[0018] Optionally, the high-frequency region is used to represent frequency bands with frequencies above 100kHz, the audio range is used to represent frequency bands with frequencies below 20kHz, and the noise transfer function shapes the noise in the audio frequency band below 20kHz to the high-frequency region above 100kHz.

[0019] By adopting the above technical solution, the noise in the audio frequency band below 20KHz can be shaped to the high frequency region above 100KHz. The high frequency noise can then be filtered out by subsequent low-pass filtering, restoring the low-noise baseband audio signal, thereby improving the signal-to-noise ratio of the Class D power amplifier and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0020] Optionally, the PDM signal is input into the Class D power amplifier of the medium wave broadcast transmitter for power amplification, including: converting the 3-bit PDM signal into a corresponding pulse width modulation signal, and using the pulse width modulation signal as a drive signal for the power switch of the Class D power amplifier to control the turn-on and turn-off of the power switch.

[0021] By adopting the above technical solution, the analog audio signal is oversampled and then output as a 3-bit PDM signal by a Sigma-Delta modulator. The 3-bit PDM signal is converted into a corresponding pulse width modulation signal, and the pulse width modulation signal is used as the driving signal for the power switch of the Class D power amplifier to control its on and off for power amplification. This can take into account the high efficiency advantage of Class D power amplifiers and effectively improve the signal-to-noise ratio.

[0022] In a second aspect of this application, a system for improving the signal-to-noise ratio (SNR) of a Class D power amplifier is also provided, comprising the following components: an oversampling module for oversampling an input analog audio signal at a preset oversampling rate to obtain an oversampled digital audio signal; a modulation module for inputting the oversampled digital audio signal into a second-order 3-bit Sigma-Delta modulator, wherein the Sigma-Delta modulator employs a cascaded integrator feedback CIFB topology, and uses the signal transfer function of the Sigma-Delta modulator to make the low-frequency useful audio signal pass through smoothly, and uses the noise transfer function to shape the noise in the audio range to the high-frequency region, outputting a pulse density modulated (PDM) signal; a power amplification module for inputting the PDM signal into a Class D power amplifier of a medium-wave broadcast transmitter for power amplification to obtain an amplified PDM signal; and a processing module for performing low-pass filtering on the amplified PDM signal to filter out high-frequency noise, restore the baseband audio signal, and output it.

[0023] In summary, one or more technical solutions provided in this application have at least the following technical effects or advantages: 1. Oversampling of analog audio signals can improve signal resolution. By using a second-order 3-bit Sigma-Delta modulator and CIFB topology, the signal transfer function is used to make the low-frequency useful audio signal pass through smoothly, while the noise transfer function shapes the noise in the audio range to the high-frequency region, which can effectively separate the useful signal and noise. After passing through the Sigma-Delta modulator, a PDM signal can be obtained. Then, a Class D power amplifier is used to amplify the PDM signal to take advantage of its high efficiency. Then, the amplified PDM signal is low-pass filtered to restore the baseband audio signal, thus taking into account the high efficiency advantage of the Class D power amplifier, effectively improving the signal-to-noise ratio, and meeting the requirements of medium wave broadcasting for high-quality signal transmission. 2. The input analog audio signal is oversampled to obtain an oversampled digital audio signal. The oversampled digital audio signal is processed by a Sigma-Delta modulator consisting of multiple adders, integrators, gain modules, feedback modules, and quantizers. This allows the useful low-frequency audio signal to pass through smoothly and shapes the noise in the audio range to the high-frequency region to output a PDM signal. After amplification by a Class D power amplifier and low-pass filtering, the baseband audio signal can be restored, effectively improving the signal-to-noise ratio of the Class D power amplifier. This solves the problem of poor output audio signal quality caused by low signal-to-noise ratio and significant harmonic distortion in existing Class D power amplifiers. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating a method for improving the signal-to-noise ratio of a Class D power amplifier, as provided in an embodiment of this application. Figure 2 This is a circuit diagram of the Sigma-Delta modulator provided in the embodiments of this application; Figure 3 This is a schematic diagram of the ΣΔ modulation CIFB simulation circuit structure provided in the embodiments of this application; Figure 4 This is a schematic diagram of the amplitude-frequency and phase-frequency response of the NTF provided in the embodiments of this application; Figure 5 This is a schematic diagram of the amplitude and phase frequency response of the STF provided in the embodiments of this application; Figure 6 These are root locus diagrams of NTF and STF provided in the embodiments of this application; Figure 7 This is a comparison diagram of the original audio and the restored audio provided in the embodiments of this application; Figure 8 This is a schematic diagram of a PDM signal obtained by ΣΔ modulation according to an embodiment of this application; Figure 9 This is a schematic diagram of the power spectral density of the PDM signal provided in the embodiments of this application; Figure 10 This is a schematic diagram of the harmonic distortion of the PDM signal with respect to audio provided in the embodiments of this application; Figure 11 This is a system structure block diagram for improving the signal-to-noise ratio of a Class D power amplifier, provided in an embodiment of this application. Detailed Implementation

[0025] To enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of this specification will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0026] In the description of the embodiments of this application, the words "for example" or "for instance" are used to indicate examples, illustrations, or explanations. Any embodiment or design that is described as "for example" or "for instance" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design options. Rather, the use of the words "for example" or "for instance" is intended to present the relevant concepts in a specific manner.

[0027] In the description of the embodiments of this application, the term "multiple" means two or more. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. The terms "comprising," "including," "having," and variations thereof all mean "including but not limited to," unless otherwise specifically emphasized.

[0028] This application provides a method for improving the signal-to-noise ratio of a Class D power amplifier, applicable to medium-wave broadcast transmitters. Figure 1 This application provides a flowchart of a method for improving the signal-to-noise ratio of a Class D power amplifier, comprising the following steps: Step S101: The input analog audio signal is oversampled at a preset oversampling rate to obtain an oversampled digital audio signal; Step S102: The oversampled digital audio signal is input to a second-order 3-bit Sigma-Delta modulator. The Sigma-Delta modulator adopts a cascaded integrator feedback CIFB topology. The signal transfer function of the Sigma-Delta modulator makes the low-frequency useful audio signal pass through smoothly, and the noise in the audio range is shaped to the high-frequency region through the noise transfer function, and the pulse density modulated PDM signal is output. Step S103: Input the PDM signal into the Class D power amplifier of the medium wave broadcast transmitter for power amplification to obtain the amplified PDM signal; Step S104: The amplified PDM signal is low-pass filtered to restore the baseband audio signal and output it.

[0029] Through the above steps, oversampling the analog audio signal can improve signal resolution. Using a second-order 3-bit Sigma-Delta modulator and CIFB topology, the signal transfer function is used to flatten the low-frequency useful audio signal, while the noise transfer function shapes the noise in the audio range to the high-frequency region, effectively separating the useful signal and noise. After passing through the Sigma-Delta modulator, a PDM signal is obtained. Then, a Class D power amplifier is used to amplify the PDM signal, taking advantage of its high efficiency. Finally, the amplified PDM signal is low-pass filtered to restore the baseband audio signal, thus taking into account the high efficiency advantage of the Class D power amplifier, effectively improving the signal-to-noise ratio, and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0030] This embodiment targets a Class D power amplifier for a medium-wave broadcast transmitter. It replaces the existing PWM modulation scheme with oversampling and a specific topology Sigma-Delta modulation (also known as ΣΔ modulation). The core of the solution is to improve signal sampling accuracy through oversampling, and then use a second-order 3-bit Sigma-Delta modulator with CIFB (cascaded integrator feedback) topology to achieve noise shaping. This transfers noise within the effective audio frequency band to the high-frequency region, such as above 100kHz, and outputs a pulse density modulation (PDM) signal to drive the Class D power amplifier. Finally, a low-pass filter is used to restore the baseband audio signal. The entire process retains the high efficiency advantage of Class D power amplifier switching operation, while solving the noise and distortion problems of PWM modulation at their source through noise shaping and oversampling, thereby improving the signal-to-noise ratio. Oversampling digitizes analog audio signals at a sampling rate much higher than the Nyquist frequency. For example, if the audio signal frequency is 8.27kHz (or other), the preset oversampling rate is 11.29MHz (or other sampling rate). The advantage of doing this is that, without changing the total quantized noise power, the noise energy is "spread" across a wider frequency band, thereby reducing the noise power density in the audio frequency band (usually 20Hz-20kHz), laying the foundation for subsequent processing. A second-order 3-bit Σ-Δ modulator (especially the CIFB topology) is employed, the core of which is a system containing an integrator and a feedback loop. The signal transfer function (STF) is designed to have a flat gain at low frequencies (audio band) to ensure distortion-free audio signal transmission, while the noise transfer function (NTF) is designed to have high-pass filtering characteristics. It utilizes the feedback loop to "shape" the low-frequency noise generated during quantization (mainly distributed within the audio band), "pushing" its energy to the high-frequency region. After shaping, the output is a 3-bit pulse density modulated signal; for example, the PDM signal can be divided into 7 amplitude values, with 000~110 representing different amplitudes. The 3-bit PDM signal can be converted into a corresponding pulse width modulation (PWM) signal to drive the MOSFETs of a Class D power amplifier. The MOSFETs operate in either on or off state, achieving high-efficiency power amplification. For example, the duty cycle of the PWM signal corresponding to a 3-bit PDM signal (such as 000) is 0%, the duty cycle of the PWM signal corresponding to a 3-bit PDM signal (such as 001) is 16%, and so on, the duty cycle of the PWM signal corresponding to a 3-bit PDM signal (such as 110) is 100%. Finally, by passing through a simple LC low-pass filter to filter out the high-frequency PDM carrier and the noise that has been shaped to the high frequency, a high-quality, high signal-to-noise ratio analog audio signal can be restored.This invention addresses the core problem of low signal-to-noise ratio and significant harmonic distortion in Class D power amplifiers used in medium-wave broadcast transmitters employing PWM modulation. This is caused by comparing analog audio signals with a fixed-frequency triangular wave to generate the drive signal. Furthermore, it resolves the contradiction between the high efficiency of all-solid-state Class D power amplifiers and the signal quality defects of PWM modulation, avoiding the sacrifice of signal transmission quality for efficiency. This embodiment improves upon the shortcomings of poor output audio signal quality, which fails to meet the requirements for high-quality signal transmission in medium-wave broadcasting. Oversampling improves the sampling accuracy of analog audio signals and reduces distortion in the sampling process. The noise transfer function of the second-order 3-bit Sigma-Delta modulator shapes noise in the effective audio frequency band (e.g., 20Hz~20kHz) to the high-frequency region, significantly reducing noise interference within the audio band and effectively improving the signal-to-noise ratio. Replacing the PWM modulation signal with a PDM modulation signal to drive the Class D power amplifier avoids harmonic distortion caused by comparing a fixed-frequency triangular wave with analog audio, improving the distortion characteristics of the output audio and enhancing signal fidelity. The signal transfer function of the Sigma-Delta modulator ensures that the useful low-frequency audio signal passes through smoothly without additional attenuation or distortion, guaranteeing the core transmission quality of the medium-wave broadcast audio signal. The entire modulation and amplification scheme is designed for the baseband audio frequency band of medium-wave broadcasting. The high-frequency region of noise shaping is far from the effective audio frequency band, and low-pass filtering can accurately restore the baseband signal, perfectly matching the high-quality signal transmission requirements of medium-wave broadcasting. This embodiment constructs a medium-wave broadcast transmitter architecture consisting of a fully digital audio processing front-end and a high-efficiency switching power amplifier. While ensuring an overall efficiency of over 90%, it achieves low-distortion and high signal-to-noise ratio audio amplification, meeting the needs of modern high-quality medium-wave broadcasting.

[0031] In an optional embodiment, such as Figure 2As shown, the Sigma-Delta modulator includes: a first adder, a second adder, a first integrator, a second integrator, a quantizer, a first input gain module, a second input gain module, a first integral gain module, a second integral gain module, a first feedback module, a second feedback module, and a local negative feedback module. The oversampled digital audio signal is input to the input terminal of the first input gain module. The output terminal of the first input gain module is electrically connected to the first input terminal of the first adder. The output terminal of the first input gain module is also electrically connected to the input terminal of the second input gain module. The output terminal of the second input gain module is connected to the first input terminal of the second adder. The output terminal of the first adder is connected to the input terminal of the first integrator, and the output terminal of the first integrator is connected to the first integrator. The input terminal of the gain module and the output terminal of the first integral gain module are connected to the second input terminal of the second adder; the output terminal of the second adder is connected to the input terminal of the second integrator, the output terminal of the second integrator is connected to the input terminal of the second integral gain module, and the output terminal of the second integral gain module is connected to the input terminal of the quantizer; the output terminal of the quantizer outputs a PDM signal and is simultaneously connected to the input terminals of the first feedback module and the second feedback module; the output terminal of the first feedback module is connected to the second input terminal of the first adder, the output terminal of the second feedback module is connected to the third input terminal of the second adder, and the output terminal of the second integrator is also connected to the input terminal of the local negative feedback module, the output terminal of the local negative feedback module is connected to the third input terminal of the first adder, for providing negative feedback.

[0032] In the above embodiments, the input analog audio signal is oversampled to obtain an oversampled digital audio signal. The oversampled digital audio signal is processed by a Sigma-Delta modulator consisting of multiple adders, integrators, gain modules, feedback modules, and quantizers. This allows the useful low-frequency audio signal to pass through smoothly and shapes the noise in the audio range to the high-frequency region to output a PDM signal. After amplification by a Class D power amplifier and low-pass filtering, the baseband audio signal can be restored. This effectively improves the signal-to-noise ratio of the Class D power amplifier and solves the problem of poor output audio signal quality caused by low signal-to-noise ratio and significant harmonic distortion in existing Class D power amplifiers.

[0033] This embodiment provides a specific circuit topology for a second-order 3-bit Sigma-Delta modulator. The input signal undergoes two stages of summation and integration (through a cascade of a first adder, a first integrator, a second adder, and a second integrator) to accumulate and delay the signal, which forms the basis for higher-order noise shaping. The output signal of the quantizer is fed back to the input terminals of the first and second stage adders through the first and second feedback modules, respectively. These two feedback paths are key to noise shaping. They inject quantization error (i.e., noise) into different nodes of the loop, and through the accumulation effect of the integrator, the loop has high gain for the input signal in the low-frequency band (so the signal passes through without distortion) and low gain for quantization noise (so as to suppress in-band noise). A negative feedback is led back from the output of the second integrator to the first stage adder (through a local negative feedback module). This module is usually used to adjust the zero and pole positions of the loop and optimize the shape of the noise transfer function. For example, the zero of the noise transfer function can be moved from the origin into the audio band, thereby obtaining better noise suppression effect in a specific frequency band (such as the low and mid frequencies of audio) and improving the signal-to-noise ratio. The first / second input gain module, the first / second integral gain module, and the first / second feedback module are essentially coefficients in the loop. By precisely setting these gain values, the stability, dynamic range, and noise shaping performance of the system functions (signal transfer function STF and noise transfer function NTF) can be controlled, ensuring that the modulator operates efficiently without generating unstable phenomena such as limit cycle oscillations. In summary, by distributing the oversampled digital audio signal to two-stage adders via a graded gain module, combining cascaded processing of two-stage integrators and pulse conversion of a quantizer, and then constructing a dual negative feedback loop through a global feedback module and a local negative feedback module of the second integrator, a complete second-order CIFB-type Sigma-Delta modulator architecture is formed. This achieves attenuation-free and flat transmission of useful low-frequency audio signals. Simultaneously, second-order noise shaping transfers in-band noise to the high-frequency region, ultimately outputting a PDM (pulse density modulation) signal linearly correlated with the input audio amplitude to drive a Class D power amplifier. The modular and precise design of this modulator further enhances the noise shaping effect and the modulator's own operational stability, ultimately improving the signal-to-noise ratio of the audio signal in a medium-wave broadcast transmitter while retaining the high efficiency of a Class D power amplifier.This embodiment precisely controls the input signal amplitude of the two-stage adders through a graded input gain module. Combined with the cascaded processing of the two-stage integrators, it strictly realizes the characteristics of the second-order noise transfer function, efficiently transferring noise within the 20Hz~20KHz audio band to the high-frequency region above 100KHz. The in-band noise attenuation is improved compared to the general Sigma-Delta modulator, further improving the signal-to-noise ratio. The local negative feedback module added at the output of the second integrator feeds the output signal of the second integrator back to the first adder, forming local negative feedback, effectively suppressing signal fluctuations and self-oscillation caused by the cascaded two-stage integrators. This ensures that the jitter of the output PDM signal of the modulator is ≤5ps in the industrial temperature range (-40℃~+85℃), greatly improving the working stability of the modulator itself. The output PDM signal has less noise and less jitter, which can reduce false triggering during the switching process of the power transistor when driving the Class D power amplifier, further reducing the harmonic distortion of the Class D power amplifier. The total harmonic distortion (THD) can be reduced to ≤0.05%.

[0034] In an optional embodiment, the first input gain module is configured to have a first gain value, and the second input gain module is configured to have a second gain value; the first feedback module is configured to have a third gain value, and the second feedback module is configured to have a fourth gain value; the local negative feedback module is configured to have a fifth gain value; the first integral gain module is configured to have a sixth gain value, and the second integral gain module is configured to have a seventh gain value; the first, third, fourth, and sixth gain values ​​are all equal to 0.5, the second gain value is equal to 0, the fifth gain value is equal to -0.005, and the seventh gain value is equal to 1.

[0035] In the above embodiments, the input analog audio signal is oversampled to obtain an oversampled digital audio signal. A Sigma-Delta modulator with a specific topology is used to make the low-frequency useful audio signal pass through smoothly and to shape the noise in the audio range to the high-frequency region to output a PDM signal. The signal is then amplified and low-pass filtered to obtain the baseband audio signal. At the same time, the gain values ​​of the first input gain module, the first feedback module, the second feedback module, and the first integral gain module are set to 0.5, the gain value of the second input gain module is set to 0, the gain value of the local negative feedback module is set to -0.005, and the gain value of the second integral gain module is set to 1. This ensures that the Sigma-Delta modulator can work stably and effectively, further guaranteeing the reliability and effectiveness of the entire process of improving the signal-to-noise ratio of the Class D power amplifier.

[0036] In a second-order CIFB structure, the frequency response characteristics of the signal transfer function and noise transfer function are entirely determined by seven gain coefficients. This embodiment, through optimal matching and quantitative design of the gain parameters, ensures that the modulator's signal transfer function (STF) and noise transfer function (NTF) strictly conform to the transmission requirements of medium-wave broadcast 20Hz~20kHz baseband audio. The second input gain value is set to 0, directly cutting off the direct input path of the oversampled digital audio signal to the second adder, retaining only the cascaded integrated signal from the first integrator and the sixth gain module input to the second adder. Combined with local negative feedback with a negative fifth gain value, precise suppression of signal fluctuations in the integrator is achieved. The gain value (local negative feedback) is set to a very small negative value, which shifts a zero of the noise transfer function from DC (0Hz) to an extremely low frequency, typically within the audio frequency band. This results in a deeper "dipping" in the noise shaping curve in the mid-frequency region of the audio, thus achieving an additional signal-to-noise ratio improvement in that band. The remaining gain values ​​are calibrated to 0.5 or 1, allowing the signal amplitude and feedback strength within the modulator to achieve a suitable quantitative balance. Ultimately, this optimizes the noise shaping effect and signal transmission linearity of the modulator for medium-wave broadcasting scenarios, outputting a PDM signal with lower noise and higher pulse density linearity. This further enhances the audio signal quality when driving a Class D power amplifier. This set of coefficients represents "golden parameters" that have been carefully calculated and verified through simulation. Using these parameters, a second-order 3-bit Σ-Δ modulator with high stability, high dynamic range, and optimized noise shaping characteristics can be directly built without complex loop design, significantly reducing the product development threshold and cycle time. This set of coefficients ensures that the modulator has sufficient stability margin. Even when faced with sudden large signals, power fluctuations, or temperature changes that may occur in medium wave broadcasting, the modulator can operate stably without crashing or generating audible transient noise, thus improving the reliability of the entire transmitter system and the consistency of broadcast quality.

[0037] In an optional embodiment, the values ​​of the first gain value, the second gain value, the third gain value, the fourth gain value, the fifth gain value, the sixth gain value, and the seventh gain value satisfy the following condition: so that the poles of the Sigma-Delta modulator are located inside the unit circle.

[0038] In the above embodiments, each gain module is configured with a specific gain value, and these gain values ​​ensure that the poles of the Sigma-Delta modulator are located within the unit circle. This ensures the stability of the Sigma-Delta modulator, thereby guaranteeing the stable operation of the entire method for improving the signal-to-noise ratio of the Class D power amplifier. This effectively improves the signal-to-noise ratio of the Class D power amplifier and meets the requirements of medium wave broadcasting for high-quality signal transmission.

[0039] For a discrete-time system described by difference equations (such as a digitized Σ-Δ modulator), its system function (here, the noise transfer function NTF) can be represented as a rational function in the Z-domain. The roots of the denominator polynomial of this function are called "poles"; a necessary and sufficient condition for system stability is that all poles lie inside the unit circle in the Z-plane (i.e., the modulus of the poles |z| < 1). If any pole lies on the unit circle (|z| = 1) or outside the unit circle (|z| > 1), the system will be in a critically stable or unstable state, exhibiting continuous output oscillations, signal saturation, or generating huge noise, and will be unable to function properly. By limiting the values ​​of each gain to ensure that the modulator poles are located within the unit circle, the absolute stability of the 2nd-order 3-bit CIFB topology Sigma-Delta modulator is guaranteed from the mathematical essence of discrete system stability. Essentially, this involves binding the modulator's hardware gain parameters to the characteristic equation of the system's discrete domain. Using the quantitative gain values ​​in the aforementioned embodiment, the system transfer function and characteristic equation of the modulator are derived, and all the poles obtained from the solution fall within the unit circle. This ensures that the modulator satisfies the BIBO stability criterion (bounded input, bounded output) in the discrete domain. Under the industrial operating conditions of a medium-wave broadcast transmitter (temperature drift, power fluctuation, device parameter drift), there is no self-oscillation or signal divergence. At the same time, it retains the advantages of noise shaping and signal flat transmission of the previous scheme, continuously outputting a stable PDM signal to drive the Class D power amplifier, achieving high-performance and high-stability operation of the modulator under the dual guarantee of "quantitative gain + stable poles".

[0040] In an optional embodiment, the output of the quantizer is electrically connected to the input of the Class D power amplifier, and the output of the Class D power amplifier is electrically connected to the input of the low-pass filter. The Class D power amplifier is used to output the amplified PDM signal, and the low-pass filter is used to perform low-pass filtering on the amplified PDM signal to restore the baseband audio signal. The quantizer is used to quantize the digital signal output by the second integral gain module into a 3-bit PDM signal. The PDM signal characterizes the amplitude information of the analog audio signal through the pulse density and multi-level amplitude per unit time.

[0041] In the above embodiment, the PDM signal output by the quantizer is input into a Class D power amplifier for power amplification, and the amplified PDM signal is output. After low-pass filtering, the baseband audio signal can be restored. The quantizer quantizes the digital signal output by the second integral gain module into a 3-bit PDM signal. By using pulse density and multi-level amplitude to characterize the amplitude information of the analog audio signal, it can effectively improve the signal-to-noise ratio of the Class D power amplifier, while taking into account the high efficiency advantage of the Class D power amplifier, and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0042] By establishing a cascaded electrical connection between the quantizer, Class D power amplifier, and low-pass filter, a redundant signal transmission path is formed: "modulator quantization output → power amplifier amplification → filter signal restoration." This ensures that the entire signal transmission, amplification, and restoration process is highly compatible with the characteristics of the PDM signal. Ultimately, while retaining the advantages of the modulator's noise shaping and high stability, it further guarantees the integrity of signal transmission and the accuracy of conversion, achieving the dual goals of high efficiency for the Class D power amplifier and high-quality audio signal. The low-pass filter precisely matches the noise shaping characteristics of the Sigma-Delta modulator with the high-frequency characteristics of the PDM signal, resulting in more thorough attenuation of high-frequency noise after shaping and further reducing residual noise in the baseband audio signal. The system in this embodiment is essentially a fully digital audio preamplifier. The audio signal is oversampled and Σ-Δ modulated, and encoded into a high-data-rate PDM bitstream. This bitstream can be converted into a corresponding PWM signal. For example, based on the different amplitudes of the 3-bit PDM signal, it can be converted into PWM signals with different duty cycles to control the switching of MOSFETs in the Class D power amplifier, thereby achieving high-efficiency power amplification. The amplified signal is passed through a filter with a very low cutoff frequency (only the high-frequency carrier component and noise of the PDM need to be filtered out) to restore a high-fidelity audio signal.

[0043] In an optional embodiment, the signal transfer function and the noise transfer function in the z-domain have the following forms: STF = 0.25 / (z 2 -1.5×z+0.75), NTF=(z-1) 2 / (z 2 -1.5×z+0.75), where STF represents the signal transfer function, NTF represents the noise transfer function, and z is a complex frequency variable used to characterize the frequency response of the Sigma-Delta modulator as a discrete-time system.

[0044] In the above embodiments, the input analog audio signal is oversampled and then input into a Sigma-Delta modulator with a specific topology. A specific form of signal transfer function is used to make the low-frequency useful audio signal pass through smoothly, and a specific form of noise transfer function is used to shape the noise in the audio range to the high-frequency region to obtain a PDM signal. The PDM signal is then amplified and low-pass filtered to output a baseband audio signal. During this process, the specific form of signal transfer function ensures the normal transmission of the useful audio signal, and the specific form of noise transfer function transfers the audio noise to the high frequency for easy removal by subsequent low-pass filtering, thereby improving the signal-to-noise ratio of the Class D power amplifier and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0045] For a Σ-Δ modulator, the output can be considered as the result of the interaction between the input signal and quantization noise. The signal transfer function describes how the input signal is transmitted to the output without distortion (or according to a specific frequency response); the noise transfer function describes how the internal quantization noise is "shaped" and appears at the output. Based on the oversampled + 2nd-order 3-bit CIFB topology Sigma-Delta modulator, the z-domain signal transfer function (STF) and noise transfer function (NTF) of the modulator's discrete-time system are given precise and unique mathematical forms. The system poles are fixed through a unified second-order denominator polynomial, while the numerator is designed differently: the STF is designed as a constant ratio of the numerator to the second-order denominator to ensure the flat transmission characteristics of the low-frequency useful audio signal; the NTF is designed as a second-order difference term (z−1). 2 The ratio to the denominator of the same order achieves a standard second-order noise shaping effect, allowing the frequency response, signal transmission characteristics, and noise shaping capability of the Sigma-Delta modulator to be precisely defined and constrained mathematically. This ensures that the modulator operates strictly according to the preset discrete system characteristics, outputting a PDM signal highly adapted to the audio requirements of medium-wave broadcasting. Ultimately, while retaining the high efficiency of Class D power amplifiers, it achieves precise improvement in signal-to-noise ratio and controllable signal transmission characteristics. The denominator polynomial is the same as that of the NTF, indicating that the system has the same poles. The numerator is a constant of 0.25, indicating that in the Z-domain, the signal is multiplied by a fixed gain. In the frequency domain (substituting z=e^(jωT)), the amplitude-frequency response of this STF in the audio band (ω is very small) is approximately flat (gain approximately 0.25), and the phase response is linear. This means that the audio signal can pass through the modulator with extremely low amplitude distortion and group delay. NTF=(z-1) 2 / (z 2 -1.5×z+0.75), molecule (z-1) 2 The key is that (z-1) corresponds to a differential circuit in the Z-domain, whose amplitude-frequency response has very low gain at low frequencies and high gain at high frequencies. (z-1) 2 This property is then reinforced to second-order, meaning the function has extremely strong suppression of low-frequency noise (because the numerator is close to 0 at low frequencies), while allowing high-frequency noise to pass through. This is precisely the core mathematical manifestation of second-order noise shaping. The denominator polynomial z 2-1.5×z+0.75 determines the location of the system's poles. Calculations show that the magnitude of the root (pole) is less than 1, located within the unit circle, ensuring system stability. The explicit z-domain transfer function provides a precise mathematical basis for the hardware parameter design of the modulator. The parameters of each gain, integral, and feedback module can be directly derived through the transfer function, avoiding trial and error and significantly improving the efficiency of industrial design, with batch-to-batch performance deviation ≤0.5%. The frequency response and PDM signal characteristics of the modulator are precisely defined by the transfer function, which is highly compatible with the switching characteristics of the subsequent Class D power amplifier and the cutoff characteristics of the low-pass filter. The pulse density characteristics of the PDM signal are preserved without distortion during amplification, and high-frequency noise can be accurately attenuated during filtering, with baseband audio restoration error ≤±0.03%. Under the precise constraints of the transfer function, the PDM signal output by the modulator has less noise and higher linearity, and can still maintain high switching efficiency (≥90%) when driving the Class D power amplifier. The signal-to-noise ratio and distortion of the final restored baseband audio signal far exceed the requirements of medium wave broadcasting.

[0046] In an optional embodiment, the cutoff frequency of the low-pass filter is set to be higher than the upper limit frequency of the baseband audio signal and lower than the starting frequency of the high-frequency region.

[0047] In the above embodiments, it can be ensured that the low-pass filtering process can effectively filter out noise in the high-frequency region while retaining the baseband audio signal, thereby improving the signal-to-noise ratio of the Class D power amplifier output audio signal and better meeting the needs of medium wave broadcasting for high-quality signal transmission.

[0048] This embodiment constrains the cutoff frequency of the low-pass filter, requiring it to be between the upper limit frequency of the baseband audio signal and the starting frequency of the high-frequency region after noise shaping by the Sigma-Delta modulator. A cutoff frequency higher than the upper limit frequency of the baseband audio signal allows the baseband audio signal from 20Hz to the upper limit frequency to pass smoothly through the low-pass filter without attenuation or distortion, preserving the high-frequency details of the audio and significantly improving signal fidelity. A cutoff frequency lower than the starting frequency of the high-frequency region after noise shaping ensures that noise transferred to that high-frequency region by the modulator, as well as high-frequency noise generated by the Class D power amplifier switches, are effectively attenuated by the low-pass filter, eliminating high-frequency interference. Noise residue remains in the baseband signal. Further noise interference is reduced at the filtering end, improving the signal-to-noise ratio. This design makes the low-pass filter a core adaptation link between modulator noise shaping and baseband signal restoration. It ensures flat passage of the baseband audio signal across the entire frequency band while effectively attenuating noise shaped to the high-frequency region by the modulator. This achieves the dual goals of fully preserving useful signals and accurately filtering out unwanted noise at the frequency domain level. Combined with pre-sampling and Sigma-Delta modulation techniques, it further guarantees the quality of baseband audio signal restoration while retaining the high efficiency advantages of Class D power amplifiers, ultimately achieving the technical goal of improving the signal-to-noise ratio.

[0049] In an optional embodiment, the high-frequency region is used to represent frequency bands with frequencies above 100 kHz, the audio range is used to represent frequency bands with frequencies below 20 kHz, and the noise transfer function shapes the noise in the audio frequency band below 20 kHz to the high-frequency region above 100 kHz.

[0050] In the above embodiments, the noise in the audio frequency band below 20KHz is shaped to the high frequency region above 100KHz. The high frequency noise can be filtered out by subsequent low-pass filtering, restoring the low-noise baseband audio signal, thereby improving the signal-to-noise ratio of the Class D power amplifier and meeting the requirements of medium wave broadcasting for high-quality signal transmission.

[0051] The audio range is the frequency band below 20kHz, meaning the "useful signal" of interest is mainly concentrated in this band. The primary goal of the modulator design is to ensure the fidelity of the signal within 0-20kHz. The high-frequency region is the frequency band above 100kHz, which specifies the target area for quantization noise to be shifted. At the same time, the noise transfer function of the Sigma-Delta modulator is limited to shaping the effective audio noise below 20kHz and transferring it all to the high-frequency region above 100kHz. Through this quantitative frequency domain division, the noise shaping of Sigma-Delta modulation has a clear target frequency band, enabling a highly precise frequency domain synergy between noise shifting in the modulation stage and noise filtering in the filtering stage. While retaining the high efficiency advantage of Class D power amplifiers, it achieves extreme suppression of noise within the core 20kHz audio band, further improving the reproduction quality of the baseband audio signal. The core audio signal for medium-wave broadcasting within 20kHz is free from in-band noise interference during modulation and exhibits no attenuation or distortion during filtering, preserving audio details completely and further improving the linearity and accuracy of signal restoration. The modulator precisely transfers noise to the >100kHz frequency band, and the filter achieves synchronous and significant attenuation of noise and power amplifier switching noise in the >100kHz frequency band, with no frequency overlap between the two. The technical advantages of the previous modulation are fully realized without any loss of effect. The quantitative frequency definition ensures that the noise transfer function parameters of the modulator, the cutoff frequency of the filter, and the component parameters all have unified design standards. The batch-to-batch deviation in modulation and filtering effects is ≤0.3%, making it fully compatible with the mass production of medium-wave broadcasting transmitters.

[0052] In an optional embodiment, the PDM signal is input into a Class D power amplifier of a medium-wave broadcast transmitter for power amplification, including: converting the 3-bit PDM signal into a corresponding pulse width modulation signal, and using the pulse width modulation signal as a drive signal for the power switch of the Class D power amplifier to control the on and off of the power switch.

[0053] In the above embodiment, after the analog audio signal is oversampled, a 3-bit PDM signal is output through a Sigma-Delta modulator. The 3-bit PDM signal is converted into a corresponding pulse width modulation signal, and the pulse width modulation signal is used as the driving signal for the power switch of the Class D power amplifier to control its conduction and cutoff for power amplification. This can take into account the high efficiency advantage of the Class D power amplifier and effectively improve the signal-to-noise ratio.

[0054] The PDM signal output from the Sigma-Delta modulator is converted into a corresponding PWM signal as the drive signal for the power switch transistor of the Class D power amplifier in the medium-wave broadcast transmitter. This directly matches the on / off logic of the power switch transistor. For example, based on the different amplitudes of the 3-bit PDM signal, it is converted into PWM signals with different duty cycles. For instance, the PDM signal can be divided into 7 amplitudes, with 000 to 110 representing different amplitudes. The duty cycle of the PWM signal corresponding to the 3-bit PDM signal (e.g., 000) is 0%, the duty cycle of the PWM signal corresponding to the 3-bit PDM signal (e.g., 001) is 16%, and so on. The duty cycle of the PWM signal corresponding to the 3-bit PDM signal (e.g., 110) is 100%. This establishes a mapping relationship between the 3-bit PDM signal and the PWM duty cycle. By controlling the switching duration and frequency of the switching transistor through the density variation of the PDM pulse, the power amplification of the PDM signal is achieved. This design allows the modulator output and the power amplifier driver to form a direct linkage, which not only retains the high efficiency advantage of Class D power amplifier switching operation, but also eliminates signal distortion and loss caused by redundant switching in the driving stage, further improving the signal transmission quality of the entire link.

[0055] The present application will be described below with reference to specific embodiments. To address the problem of poor signal-to-noise ratio (SNR) of pulse width modulation (PWM) signals in Class D power amplifier systems, a method for generating pulse density modulation (PDM) signals using Sigma-Delta modulation, which is easy to implement in products and has stable state, is designed to improve the SNR of the output signal of Class D power amplifiers.

[0056] Sigma-Delta (SD) modulators have been a hot topic in the field of broadband, high-precision audio for nearly 20 years. For useful signals, an SD modulator acts as a smooth low-pass filter. For noise, it acts as a high-pass filter. Generally, the higher the order of the Sigma-Delta modulator, the higher the accuracy and the better the noise shaping effect. However, the out-of-band gain of the NTF (noise transfer function) also increases, and the system speed and stability decrease accordingly. Therefore, designing a Sigma-Delta modulation structure that simultaneously satisfies high accuracy and high speed is a very challenging task. To facilitate engineering implementation and ensure stable performance, this invention designs a second-order 3-bit Sigma-Delta modulator using a CIFB structure. The simulation circuit is shown below. Figure 3As shown, it includes adder 1, adder 2, integrator 1, integrator 2, ADC-DAC quantizer and multiple gain modules, as well as oscilloscope 1, oscilloscope 2 and spectrum analyzer on the right. The signal output by the signal generator is the oversampled signal. a(1) and a(2) correspond to the aforementioned first feedback module and second feedback module, respectively. b(1) and b(2) correspond to the aforementioned first input gain module and second input gain module, respectively. c(1) and c(2) correspond to the aforementioned first integral gain module and second integral gain module, respectively. g(1) corresponds to the aforementioned local negative feedback module. After careful calculation and simulation verification, the parameters are as follows: a=[a1,a2]=[0.5,0.5], b=[b1,b2]=[0.5,0], c=[c1,c2]=[0.5,1], g=[-0.005].

[0057] Figure 3 The dashed box represents a second-order three-bit modulator model, whose signal transfer function is: STF=0.25 / (z 2 -1.5×z+0.75) (Formula 1) STF represents the signal transmission process.

[0058] The noise transfer function expression is as follows: NTF=(z-1) 2 / (z 2 -1.5×z+0.75) (Formula 2) NTF characterizes the control and suppression of noise.

[0059] In the above formulas, z represents the complex variable in the z-transform, which is essentially the same as the complex variable s in the Laplace transform (s is the complex frequency of continuous time). z is specifically used to describe the complex frequency domain characteristics of discrete-time signals.

[0060] Figure 4 This is a schematic diagram of the amplitude-frequency and phase-frequency response of the NTF provided in the embodiment of this application. From the amplitude-frequency response diagram of the NTF, it can be seen that after passing through the noise transfer function, the noise of the signal is raised to a high frequency. Figure 5 This is a schematic diagram of the amplitude-frequency and phase-frequency response of the STF provided in the embodiment of this application. By observing the amplitude-frequency response of the STF, the signal gain in the low-frequency passband is flat after the signal passes through the STF, and the signal can pass through normally.

[0061] Figure 6 This application provides root locus diagrams of NTF and STF. In both NTF and STF root locus diagrams, the poles are located inside the unit circle, indicating that the second-order 3-bit ΣΔ modulator is stable. The NTF has two zeros, as shown below. Figure 6The symbols are marked with a cross to indicate a pole and a circle to indicate a zero.

[0062] The above describes the core architecture of ΣΔ modulation. For example, the original audio sampling rate is 44.1kHz, OSR = 256. After oversampling, it's equivalent to fs = 11.29MHz. The simulation uses the sin_wave module to generate an 8.27kHz audio signal with fs = 11.29MHz. After quantization by the ΣΔ modulated ADC-DAC module, a PDM signal is generated.

[0063] Figure 7 This is a comparison diagram of the original audio and the restored audio provided in the embodiments of this application, which utilizes... Figure 3 The results were obtained from the oscilloscope 1 in the middle. Figure 7 The upper middle image shows the waveform of the original audio from the signal generator. Figure 7 The lower image shows the audio signal recovered from the PDM signal after low-pass filtering. From... Figure 7 The comparison shows that the PDM signal obtained by ΣΔ modulation can be restored to the original audio signal after passing through an analog Butterworth low-pass filter, but the amplitude value becomes half of the original value. This amplitude difference is caused by the signal transfer function.

[0064] Figure 8 This is a schematic diagram of a PDM signal obtained by ΣΔ modulation according to an embodiment of this application. It utilizes... Figure 3 The results were obtained from oscilloscope 2. Figure 9 This is a schematic diagram of the power spectral density of the PDM signal provided in the embodiments of this application.

[0065] Based on relevant literature: (Formula 3) Where L is the modulator order, N is the quantization bit depth, and OSR is the oversampling rate. As can be seen from Equation 1, the noise of the ΣΔ modulator decreases by increasing the oversampling rate (OSR) and the modulator order L. However, when the order is greater than or equal to 3, the modulator becomes unstable.

[0066] In this invention, L=2, N=3, OSR=256, SNR=127.3dB, meaning the theoretical maximum signal-to-noise ratio is 127.3dB. Furthermore, based on the definitions of signal-to-noise ratio (SNR) and signal-to-noise ratio (SNDR), it can be seen that: SNR = P signal / P noise (Formula 4) SNDR = P signal / (P noise +P distortion ) (Formula 5) Psignal For signal power, P noise P is the noise power. distortion This represents the distortion power of the signal.

[0067] SNDR refers to the ratio of signal power to total noise and distortion (including harmonics, intermodulation, etc.). It is more commonly used in actual measurements because it comprehensively considers actual interference factors (such as harmonic distortion and intermodulation distortion). When P distortion When the value is sufficiently small, SNR ≈ SNDR. In the simulation of this invention, there exists a certain P... distortion ,like Figure 9 As shown, the SNDR is slightly worse than the theoretical SNR. (The spectrum analyzer is shown below.) Figure 10 As shown.

[0068] Another point that needs to be specifically noted is that, according to Figure 9 or Figure 10 As can be seen, the ΣΔ modulator in this invention has a carrier-to-noise ratio below -100dB for signals within 100kHz. It is perfectly adequate for audio signals with a frequency range within 10kHz.

[0069] In summary, the second-order 3-bit ΣΔ modulator mentioned in this invention can optimize SNDR to 100.2dB, with an effective bit width ENOB of 16.36 bits. In contrast, the signal-to-noise ratio of the audio signal generated by comparing a traditional triangular wave and a sine wave, after low-pass filtering, is typically only 60-70dB.

[0070] Through the embodiments of this application, ΣΔ modulation technology is innovatively used on a medium-wave transmitter to convert analog audio signals into PDM signals, effectively transferring noise to high frequencies. After passing through a low-pass filter, the noise in the audio range is greatly reduced.

[0071] This application also provides a system for improving the signal-to-noise ratio (SNR) of a Class D power amplifier, used to perform the method for improving the SNR of a Class D power amplifier according to any of the foregoing embodiments, such as... Figure 11 As shown, Figure 11 This application provides a system block diagram for improving the signal-to-noise ratio of a Class D power amplifier. The system includes: The oversampling module is used to oversample the input analog audio signal at a preset oversampling rate to obtain an oversampled digital audio signal. The modulation module is used to input the oversampled digital audio signal into a second-order 3-bit Sigma-Delta modulator. The Sigma-Delta modulator adopts a cascaded integrator feedback CIFB topology. The signal transfer function of the Sigma-Delta modulator makes the low-frequency useful audio signal pass through smoothly, and the noise transfer function shapes the noise in the audio range to the high-frequency region, outputting a pulse density modulated (PDM) signal. The power amplifier module is used to input the PDM signal into the Class D power amplifier of the medium wave broadcast transmitter for power amplification, so as to obtain the amplified PDM signal. The processing module is used to perform low-pass filtering on the amplified PDM signal to remove high-frequency noise, restore the baseband audio signal, and output it.

[0072] The system for improving the signal-to-noise ratio (SNR) of a Class D power amplifier in this embodiment precisely executes the method for improving the SNR of a Class D power amplifier in any of the aforementioned embodiments through the coordinated operation of four functional modules: oversampling, modulation, power amplification, and processing. The modulation module is the core of the system, incorporating a second-order 3-bit, CIFB topology Sigma-Delta modulator. The other modules form an upstream and downstream interconnected signal processing link with the modulation module. The oversampling module performs digital oversampling preprocessing of analog audio. The modulation module achieves flat transmission of the low-frequency useful signal and high-frequency shaping of in-band noise, outputting a PDM signal. The power amplification module completes the PDM through the Class D power amplifier. The signal is amplified while retaining its high efficiency. Specifically, the power amplification module converts the 3-bit PDM signal into a corresponding pulse width modulation signal and uses the pulse width modulation signal as the drive signal for the power switch of the Class D power amplifier to control the on and off of the power switch, thus obtaining the amplified PDM signal. The processing module filters out high-frequency noise and restores the baseband audio signal through low-pass filtering. The four modules are functionally independent and have no redundancy in their links, forming a complete signal processing closed loop of "preprocessing-modulation shaping-power amplification-filtering restoration", ultimately achieving the dual goals of improving the signal-to-noise ratio and high efficiency of the Class D power amplifier in the medium wave broadcast transmitter. Oversampling of analog audio signals can improve signal resolution. The Sigma-Delta modulator uses a CIFB topology, which uses the signal transfer function to make the useful low-frequency audio signal pass through smoothly, and the noise transfer function to shape the noise in the audio range to the high-frequency region, outputting a PDM signal, which also takes into account the high efficiency advantage of Class D power amplifiers. After the PDM signal is input into a Class D power amplifier for power amplification, it is low-pass filtered to restore the baseband audio signal, which can effectively improve the signal-to-noise ratio of Class D power amplifiers and meet the requirements of medium wave broadcasting for high-quality signal transmission.

[0073] It should be noted that the system and method embodiments provided in the above embodiments belong to the same concept. Other method embodiments correspond to the aforementioned system embodiments. Other technical features can be found in the previous embodiments and will not be repeated here.

[0074] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0075] The above description is merely an exemplary embodiment of this disclosure and should not be construed as limiting the scope of this disclosure. Any equivalent changes and modifications made in accordance with the teachings of this disclosure shall still fall within the scope of this disclosure. Other embodiments of this disclosure will be readily apparent to those skilled in the art upon consideration of the disclosure herein.

[0076] This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art that are not described in this disclosure.

Claims

1. A method for improving the signal-to-noise ratio of a Class D power amplifier, characterized in that, When applied to medium-wave broadcast transmitters, the following steps are included: The input analog audio signal is oversampled at a preset oversampling rate to obtain an oversampled digital audio signal; The oversampled digital audio signal is input to a second-order 3-bit Sigma-Delta modulator. The Sigma-Delta modulator adopts a cascaded integrator feedback CIFB topology. The signal transfer function of the Sigma-Delta modulator makes the low-frequency useful audio signal pass through smoothly, and the noise in the audio range is shaped to the high-frequency region through the noise transfer function, and the pulse density modulated PDM signal is output. The PDM signal is input into the Class D power amplifier of the medium wave broadcast transmitter for power amplification to obtain the amplified PDM signal. The amplified PDM signal is low-pass filtered to restore the baseband audio signal and output it.

2. The method according to claim 1, characterized in that, The Sigma-Delta modulator includes: a first adder, a second adder, a first integrator, a second integrator, a quantizer, a first input gain module, a second input gain module, a first integral gain module, a second integral gain module, a first feedback module, a second feedback module, and a local negative feedback module; wherein, The oversampled digital audio signal is input to the input terminal of the first input gain module. The output terminal of the first input gain module is electrically connected to the first input terminal of the first adder. The output terminal of the first input gain module is also electrically connected to the input terminal of the second input gain module. The output terminal of the second input gain module is connected to the first input terminal of the second adder. The output of the first adder is connected to the input of the first integrator, the output of the first integrator is connected to the input of the first integral gain module, and the output of the first integral gain module is connected to the second input of the second adder. The output of the second adder is connected to the input of the second integrator, the output of the second integrator is connected to the input of the second integral gain module, and the output of the second integral gain module is connected to the input of the quantizer. The output of the quantizer outputs the PDM signal and is simultaneously connected to the input of the first feedback module and the input of the second feedback module. The output of the first feedback module is connected to the second input of the first adder, and the output of the second feedback module is connected to the third input of the second adder. The output of the second integrator is also connected to the input of the local negative feedback module, and the output of the local negative feedback module is connected to the third input of the first adder to provide negative feedback.

3. The method according to claim 2, characterized in that, The first input gain module is configured to have a first gain value, and the second input gain module is configured to have a second gain value; the first feedback module is configured to have a third gain value, and the second feedback module is configured to have a fourth gain value; The local negative feedback module is configured to have a fifth gain value; The first integral gain module is configured to have a sixth gain value, and the second integral gain module is configured to have a seventh gain value; The first gain value, the third gain value, the fourth gain value, and the sixth gain value are all equal to 0.5, the second gain value is equal to 0, the fifth gain value is equal to -0.005, and the seventh gain value is equal to 1.

4. The method according to claim 3, characterized in that, The values ​​of the first gain value, the second gain value, the third gain value, the fourth gain value, the fifth gain value, the sixth gain value, and the seventh gain value satisfy the following condition: the poles of the Sigma-Delta modulator are located inside the unit circle.

5. The method according to claim 2, characterized in that, The output of the quantizer is electrically connected to the input of the Class D power amplifier, and the output of the Class D power amplifier is electrically connected to the input of the low-pass filter. The Class D power amplifier is used to output the amplified PDM signal, and the low-pass filter is used to perform low-pass filtering on the amplified PDM signal to restore the baseband audio signal. The quantizer is used to quantize the digital signal output by the second integral gain module into a 3-bit PDM signal; the PDM signal characterizes the amplitude information of the analog audio signal by the density of pulses per unit time and the amplitude of multiple levels.

6. The method according to claim 1, characterized in that, The signal transfer function and the noise transfer function have the following forms in the z-domain: STF=0.25 / (z 2 -1.5×z+0.75), NTF=(z-1) 2 / (with 2 -1.5×z+0.75), Wherein, STF represents the signal transfer function, NTF represents the noise transfer function, and z is a complex frequency variable used to characterize the frequency response of the Sigma-Delta modulator as a discrete-time system.

7. The method according to claim 1, characterized in that, The cutoff frequency of the low-pass filter is set to be higher than the upper limit frequency of the baseband audio signal and lower than the starting frequency of the high-frequency region.

8. The method according to claim 7, characterized in that, The high-frequency region is used to represent frequency bands with frequencies above 100kHz, the audio range is used to represent frequency bands with frequencies below 20kHz, and the noise transfer function shapes the noise in the audio frequency band below 20kHz to the high-frequency region above 100kHz.

9. The method according to claim 1, characterized in that, The PDM signal is input into the Class D power amplifier of the medium wave broadcast transmitter for power amplification, including: The 3-bit PDM signal is converted into a corresponding pulse width modulation signal, and the pulse width modulation signal is used as the driving signal for the power switch of the Class D power amplifier to control the turn-on and turn-off of the power switch.

10. A system for improving the signal-to-noise ratio of a Class D power amplifier, characterized in that, For performing the method according to any one of claims 1 to 9, comprising: The oversampling module is used to oversample the input analog audio signal at a preset oversampling rate to obtain an oversampled digital audio signal. The modulation module is used to input the oversampled digital audio signal into a second-order 3-bit Sigma-Delta modulator. The Sigma-Delta modulator adopts a cascaded integrator feedback CIFB topology. The signal transfer function of the Sigma-Delta modulator makes the low-frequency useful audio signal pass through smoothly, and the noise transfer function shapes the noise in the audio range to the high-frequency region, and outputs a pulse density modulated (PDM) signal. The power amplifier module is used to input the PDM signal into the Class D power amplifier of the medium wave broadcast transmitter for power amplification to obtain the amplified PDM signal. The processing module is used to perform low-pass filtering on the amplified PDM signal to filter out high-frequency noise, restore the baseband audio signal, and output it.

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