Baseband digital modulation device and method based on FPGA and DDS

Abstract

The invention discloses a baseband digital modulation device and method based on an FPGA (Field Programmable Gate Array) and a DDS (Direct Digital Synthesizer), relates to the technical field of telemetering systems, and aims to solve the problem of high signal sidelobe energy caused by adopting 2FSK (Frequency Shift Keying) modulation in the prior art. The device comprises an FPGA (Field Programmable Gate Array) and a DDS (Direct Digital Synthesizer), and the FPGA comprises a pre-modulation filtering module which is configured to convert a PCM (Pulse Code Modulation) signal input into the FPGA into a bipolar signal and perform pre-modulation filtering on the bipolar signal; a phase calculation module; the DDS configuration module is configured to configure a register and an interface signal of the DDS, so that the DDS works in a quadrature modulation mode. According to the method, 2FSK modulation is replaced by quadrature modulation, so that the signal sidelobe energy is effectively reduced, and the telemetering baseband signal finally output by the DDS has better out-of-band rejection capability.

Description

Technical Field

[0001] This document relates to the technical field of telemetry systems, and in particular to a baseband digital modulation device and method based on FPGA and DDS. Background Art

[0002] Telemetry transmitter is an important part of telemetry system. Traditional telemetry transmitter is generally realized by analog solution. With the development of software radio technology, the demand for digital telemetry transmitter is increasing. Digital telemetry transmitter generally adopts direct digital frequency synthesis (DDS) technology to modulate the received PCM signal to DDS chip to obtain FSK intermediate frequency signal, and then mix the intermediate frequency signal to S band and perform power amplification for wireless transmission.

[0003] Traditional telemetry transmitter baseband digital modulation generally adopts FSK, that is, the value represented by the binary PCM signal (1 or 0) is modulated into two different frequencies (f1 and f2), and the PCM 0 and 1 code streams are translated into two different frequency control words inside the FPGA, and the frequency control words are sent to the DDS to achieve the purpose of digital intermediate frequency output. In addition to the highest main spectrum, the intermediate frequency signal output by this modulation method also has a high primary sidelobe energy, resulting in a high sidelobe energy of the RF signal after mixing. Therefore, when multiple products work simultaneously at different frequency ranges, it is possible that the main spectrum power of the telemetry signal is submerged in the sidelobe. Summary of the invention

[0004] This specification provides a baseband digital modulation device and method based on FPGA and DDS, which is used to solve the problem that the existing telemetry transmitter uses 2FSK modulation, resulting in high sidelobe energy, which may cause the main spectrum power of the telemetry signal to be submerged in the sidelobe.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] In a first aspect of the present specification, a baseband digital modulation device based on FPGA and DDS is provided, comprising:

[0007] The device includes an FPGA and a DDS, and the FPGA includes:

[0008] A pre-adjustment filtering module is configured to convert the PCM signal input to the FPGA into a bipolar signal, and pre-adjust the bipolar signal to convert the binary square wave PCM signal into a 16-bit digital slow-changing PCM signal;

[0009] A phase calculation module, configured to realize phase resolution of IQ data by performing phase accumulation on the 16-bit digital slowly varying PCM signal;

[0010] A quantization lookup module is configured to calculate I / Q data in a preset sine-cosine lookup table according to a result of phase solution, and output the I / Q data to a DDS for modulation;

[0011] A DDS configuration module, configured to configure the registers and interface signals of the DDS so that the DDS operates in an orthogonal modulation mode;

[0012] The DDS is configured to receive the I / Q data and perform orthogonal modulation to output a telemetry baseband signal.

[0013] In some preferred implementations, the configuration signal of the DDS is generated by the FPGA, and the FPGA generates the register configuration signal and the interface signal configuration signal of the DDS according to the timing of the DDS.

[0014] In some preferred implementations, the pre-tuning filtering module performs pre-tuning filtering through a preset pre-tuning filter, and the pre-tuning filter is a raised cosine roll-off filter that meets the first Nyquist criterion.

[0015] In some preferred implementations, the number of stages of the raised cosine roll-off filter is 50.

[0016] In some preferred embodiments, the 6 dB cutoff frequency of the raised cosine roll-off filter corresponds to the bit rate of the binary square wave PCM signal.

[0017] In some preferred embodiments, the sine-cosine lookup table is constructed by quantizing the angle into P layers.

[0018] In some preferred implementations, during configuration, the frequency control word of the DDS is configured according to the baseband output frequency.

[0019] In some preferred embodiments, the signal coefficient k of the bipolar signal after pre-adjustment filtering is f for:

[0020] k f = k*bitrate*fs / 4;

[0021] Where k is the telemetry signal modulation degree, bitrate is the bit rate of the input PCM signal, and fs is the preset sampling rate.

[0022] In some preferred implementations, the I / Q data includes I-channel data and Q-channel data; the I / Q data is acquired by:

[0023] The expression of the frequency modulated signal is:

[0024]

[0025] Where S(t) is the frequency modulated signal, A is the signal amplitude, ω c is the carrier angular frequency, k f is the signal coefficient of the bipolar signal after pre-modulation and filtering, and x(τ) is the preset modulation signal;

[0026] Let A = 1, and discretize S(t) to satisfy:

[0027]

[0028] Where x(n) is the signal after pre-filtering, cos(ω c t) is the cosine component of the carrier, sin(ω c t) is the sinusoidal component of the carrier, k f is the signal coefficient after pre-adjustment filtering;

[0029] The formula for calculating the I-channel data I(n) and the Q-channel data Q(n) by the FPGA is:

[0030]

[0031] In the formula, k f is the signal coefficient after pre-adjustment filtering, and x(n) is the signal after pre-adjustment filtering.

[0032] A second aspect of this specification provides a baseband digital modulation method based on FPGA and DDS, based on the baseband digital modulation device based on FPGA and DDS described in the first aspect, the method comprising:

[0033] Converting the PCM signal input to the FPGA into a bipolar signal, and pre-filtering the bipolar signal to convert the binary square wave PCM signal into a 16-bit digital slow-changing PCM signal;

[0034] Phase calculation of IQ data is achieved by performing phase accumulation on the 16-bit digital slowly varying PCM signal;

[0035] According to the result of phase solution, a preset sine-cosine lookup table is used to calculate I / Q data in the sine-cosine lookup table, and the I / Q data is output to DDS for modulation. The register and interface signal of the DDS are configured as an orthogonal modulation mode to output a telemetry baseband signal according to the I / Q data.

[0036] At least one of the above technical solutions adopted in the embodiments of this specification can achieve the following beneficial effects:

[0037] The device and method proposed by the present invention first filter the input PCM code stream before performing orthogonal modulation, then convert the shaped PCM data stream into mutually orthogonal I-channel data and Q-channel data in FPGA, and use FPGA to configure the register and interface signal of DDS so that it works in orthogonal modulation mode, thereby obtaining an intermediate frequency signal. Based on the device and method proposed by the present invention, orthogonal modulation is used instead of the commonly used 2FSK modulation mode, which effectively reduces the signal sidelobe energy, so that the output signal of the telemetry transmitter has better out-of-band suppression capability.

[0038] The device proposed in the present invention is based on the hardware foundation of FPGA+DDS. It utilizes the orthogonal modulation function of DDS, first shapes and filters the PCM inside the FPGA to generate the mutually orthogonal I / Q data required by the DDS, and then configures the DDS using the FPGA to realize the output of the telemetry baseband signal. This modulation method can effectively suppress the side lobes of the telemetry baseband signal spectrum.

[0039] In addition, the present invention can be extended to the scenario where multiple products with different frequencies work at the same time. By improving the out-of-band suppression capability of the product, the frequency interval between the operating frequencies of multiple products can be reduced. Within a limited frequency range, more products can be capable of working at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0041] Figure 1 A schematic diagram of the structure of a baseband digital modulation device based on FPGA and DDS provided in an embodiment of this specification;

[0042] Figure 2 The present invention provides a flowchart of a baseband digital modulation method based on FPGA and DDS according to an embodiment of the present invention. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in combination with the specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

[0044] The technical solutions provided by various embodiments of the present application are described in detail below in conjunction with the accompanying drawings.

[0045] Please refer to Figure 1 The first embodiment of the present application proposes a baseband digital modulation device based on FPGA and DDS, including FPGA and DDS, wherein the FPGA includes:

[0046] A pre-adjustment filtering module is configured to convert the PCM signal input to the FPGA into a bipolar signal, and pre-adjust the bipolar signal to convert the binary square wave PCM signal into a 16-bit digital slow-changing PCM signal;

[0047] A phase calculation module, configured to realize phase resolution of IQ data by performing phase accumulation on the 16-bit digital slowly varying PCM signal;

[0048] A quantization lookup module is configured to calculate I / Q data in a preset sine-cosine lookup table according to a result of phase solution, and output the I / Q data to a DDS for modulation;

[0049] A DDS configuration module, configured to configure the registers and interface signals of the DDS so that the DDS operates in an orthogonal modulation mode;

[0050] The DDS is configured to receive the I / Q data and perform orthogonal modulation to output a telemetry baseband signal.

[0051] In this embodiment, after the PCM enters the FPGA, it is first converted into a bipolar signal, and then the signal is pre-filtered to convert the binary square wave PCM into a 16-bit digital slow-changing PCM.

[0052] Among them, the preset sine-cosine lookup table refers to a data table obtained after quantizing and layering the angles in order to calculate the trigonometric function values. In the aforementioned steps, since the I / Q data has been phase resolved, the sine value and cosine value corresponding to the closest quantized angle can be found in the sine-cosine lookup table based on the phase value in the resolution result, and after certain processing (such as multiplication by a coefficient, etc., which can be determined according to the actual application scenario), the I / Q data with a fixed bit width is obtained, and then these data are output to the DDS for modulation.

[0053] Preferably, a sine-cosine lookup table can be created by quantizing the angle into 1024 hierarchical values, and the sine-cosine lookup table is stored in a storage unit (such as ROM or RAM) of the FPGA, which is initialized when the FPGA is configured so that it can be quickly accessed in this embodiment.

[0054] In this embodiment, when converting the PCM signal into a bipolar signal, a specific conversion rule may be determined according to the binary value of the PCM signal.

[0055] In this embodiment, the DDS is configured to the orthogonal modulation mode through the DDS configuration module to implement orthogonal modulation. The frequency control word of the DDS is configured according to the baseband output frequency, and finally the DDS outputs the telemetry baseband signal.

[0056] Furthermore, the configuration signal of the DDS is generated by the FPGA, and the FPGA generates the register configuration signal and the interface signal configuration signal of the DDS according to the timing of the DDS.

[0057] Furthermore, the pre-tuning filtering module performs pre-tuning filtering through a preset pre-tuning filter, and the pre-tuning filter is a raised cosine roll-off filter that meets the first Nyquist criterion.

[0058] Among them, the number of stages of the raised cosine roll-off filter is 50; the 6dB cutoff frequency of the raised cosine roll-off filter corresponds to the bit rate of the binary square wave PCM signal; the sine-cosine lookup table is constructed by quantizing the angle into P layers; when configuring, the frequency control word of the DDS is configured according to the baseband output frequency.

[0059] Furthermore, the signal coefficient k of the bipolar signal after pre-adjustment filtering is f for:

[0060] k f = k*bitrate*fs / 4;

[0061] Where k is the telemetry signal modulation degree, bitrate is the bit rate of the input PCM signal, and fs is the preset sampling rate.

[0062] Furthermore, the expression of the frequency modulated signal is:

[0063]

[0064] Where S(t) is the instantaneous value of the FM signal at time t, A is the signal amplitude, ω c is the carrier angular frequency, k f is the signal coefficient of the bipolar signal after pre-modulation and filtering, and x(τ) is the preset modulation signal;

[0065] Let A = 1, and discretize S(t) to satisfy:

[0066]

[0067] Where x(n) is the signal after pre-filtering, cos(ω c t) is the cosine component of the carrier, sin(ω c t) is the sinusoidal component of the carrier, k fis the signal coefficient after pre-adjustment filtering;

[0068] The formula for calculating the I-channel data I(n) and the Q-channel data Q(n) by the FPGA is:

[0069]

[0070] In the formula, k f is the signal coefficient after pre-adjustment filtering, and x(n) is the signal after pre-adjustment filtering.

[0071] Please refer to Figure 2 The second embodiment of the present application proposes a baseband digital modulation method based on FPGA and DDS, such as Figure 2 As shown, the method includes:

[0072] Step S1, converting the PCM signal input to the FPGA into a bipolar signal, and pre-filtering the bipolar signal to convert the binary square wave PCM signal into a 16-bit digital slow-changing PCM signal;

[0073] Step S2, implementing phase resolution of IQ data by performing phase accumulation on the 16-bit digital slowly varying PCM signal;

[0074] Step S3, according to the result of phase solution, using a preset sine-cosine lookup table, calculating I / Q data in the sine-cosine lookup table, and outputting the I / Q data to the DDS for modulation, the register and interface signal of the DDS are configured as an orthogonal modulation mode to output a telemetry baseband signal according to the I / Q data.

[0075] It should be noted that the above embodiment provides a baseband digital modulation device based on FPGA and DDS, and only uses the division of the above functional modules as an example. In practical applications, the above functions can be assigned to different functional modules as needed, that is, the modules or steps in the embodiments of the present invention can be decomposed or combined. For example, the modules in the above embodiments can be combined into one module, or further divided into multiple sub-modules to complete all or part of the functions described above. The names of the modules and steps involved in the embodiments of the present invention are only for distinguishing the modules or steps, and are not regarded as improper limitations of the present invention.

[0076] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A baseband digital modulation device based on FPGA and DDS, characterized in that: The device includes an FPGA and a DDS, and the FPGA includes: A pre-adjustment filtering module is configured to convert the PCM signal input to the FPGA into a bipolar signal, and pre-adjust the bipolar signal to convert the binary square wave PCM signal into a 16-bit digital slow-changing PCM signal; A phase calculation module, configured to realize phase resolution of IQ data by performing phase accumulation on the 16-bit digital slowly varying PCM signal; A quantization lookup module is configured to calculate I / Q data in a preset sine-cosine lookup table according to a result of phase solution, and output the I / Q data to a DDS for modulation; A DDS configuration module, configured to configure the registers and interface signals of the DDS so that the DDS operates in an orthogonal modulation mode; The DDS is configured to receive the I / Q data and perform orthogonal modulation to output a telemetry baseband signal.

2. The baseband digital modulation device based on FPGA and DDS according to claim 1, characterized in that: The configuration signal of the DDS is generated by the FPGA, and the FPGA generates the register configuration signal and the interface signal configuration signal of the DDS according to the timing of the DDS.

3. The baseband digital modulation device based on FPGA and DDS according to claim 1, characterized in that: The pre-tuning filtering module performs pre-tuning filtering through a preset pre-tuning filter, and the pre-tuning filter is a raised cosine roll-off filter that meets the first Nyquist criterion.

4. The baseband digital modulation device based on FPGA and DDS according to claim 3, characterized in that: The number of stages of the raised cosine roll-off filter is 50.

5. The baseband digital modulation device based on FPGA and DDS according to claim 3, characterized in that: The 6 dB cutoff frequency of the raised cosine roll-off filter corresponds to the bit rate of the binary square wave PCM signal.

6. The baseband digital modulation device based on FPGA and DDS according to claim 3, characterized in that: The sine-cosine lookup table is constructed by quantizing the angle into P layers.

7. The baseband digital modulation device based on FPGA and DDS according to claim 3, characterized in that: When configuring, the frequency control word of the DDS is configured according to the baseband output frequency.

8. The baseband digital modulation device based on FPGA and DDS according to claim 3, characterized in that: The signal coefficient k of the bipolar signal after pre-adjustment filtering f for: k f =k*bitbrate*fs / 4; Where k is the telemetry signal modulation, bitrate is the bit rate of the input PCM signal, and fs is the preset sampling rate.

9. The baseband digital modulation device based on FPGA and DDS according to claim 1, characterized in that: The I / Q data includes I-channel data and Q-channel data; the I / Q data is obtained by: The expression of the frequency modulated signal is: Where S(t) is the frequency modulated signal, A is the signal amplitude, ω c is the carrier angular frequency, k f is the signal coefficient of the bipolar signal after pre-modulation and filtering, x(τ) is the preset modulation signal, τ is the time constant, and -∝ is the lower limit of the time domain; Let A = 1, and discretize S(t) to satisfy: Where x(n) is the signal after pre-filtering, cos(ω c t) is the cosine component of the carrier, sin(ω c t) is the sinusoidal component of the carrier, k f is the signal coefficient after pre-adjustment filtering; The formula for calculating the I-channel data I(n) and the Q-channel data Q(n) by the FPGA is: In the formula, k f is the signal coefficient after pre-adjustment filtering, and x(n) is the signal after pre-adjustment filtering.

10. A baseband digital modulation method based on FPGA and DDS, based on the baseband digital modulation device based on FPGA and DDS according to any one of claims 1 to 9, characterized in that: The method comprises: Converting the PCM signal input to the FPGA into a bipolar signal, and pre-filtering the bipolar signal to convert the binary square wave PCM signal into a 16-bit digital slow-changing PCM signal; Phase calculation of IQ data is achieved by performing phase accumulation on the 16-bit digital slowly varying PCM signal; According to the result of phase solution, a preset sine-cosine lookup table is used to calculate I / Q data in the sine-cosine lookup table, and the I / Q data is output to DDS for modulation. The register and interface signal of the DDS are configured as an orthogonal modulation mode to output a telemetry baseband signal according to the I / Q data.

Images