Digital modulation circuit and digital interphone
By using FD6818 RF baseband chip and large-capacity capacitor in digital modulation circuit, the complex design and frequency band limitation problems in the existing technology are solved, and low-cost, wide-band digital modulation is realized, suitable for high- and low-band intercoms, and guaranteed Signal integrity and distortion.
Patent Information
- Application Number
- CN202323620165.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- Filing Date
- 2023-12-28
- Publication Date
- 2024-12-17
- Estimated Expiration
- 2033-12-28
AI Technical Summary
Existing digital modulation circuits require external loop filters and defined reference clocks, resulting in complex design, high cost and inability to meet the modulation requirements of low-frequency intercoms.
Using the FD6818 RF baseband chip, it uses its own audio full-through function and external reference crystal oscillator to directly connect the digital baseband unit to output it to the chip audio signal input, skip the voltage-controlled crystal oscillator and digital signal processing unit, and use large-capacity coupling and Decoupling capacitors to achieve wide-band modulation.
Reduces device requirements and costs, expands the applicable frequency band range, ensures signal integrity and distortion, and is suitable for high and low frequency band intercoms.
Smart Images
Figure CN222192365U_ABST
Abstract
Description
Technical Field
[0001] The utility model relates to the field of communication, in particular to a digital modulation circuit and a digital intercom. Background Art
[0002] Existing digital modulation circuits, such as Figure 1 As shown, it includes a digital baseband unit, a voltage-controlled crystal oscillator and an analog baseband unit connected in sequence. When transmitting, the voice is input to the AD port of the digital baseband unit through a microphone, and AD conversion is performed in the digital baseband unit. After conversion, the digital baseband unit performs coding and compression processing to become a digital baseband signal to be modulated, and then the DA of the digital baseband unit converts it into an analog signal and outputs the analog signal to be modulated; the analog signal to be modulated is directly sent to the voltage-controlled end of the voltage-controlled crystal oscillator for modulation, and the analog signal to be modulated changes the reference clock frequency, thereby changing the locking frequency of the phase-locked loop in the analog baseband unit, and completing the modulation of the carrier.
[0003] In the existing digital modulation circuit, the loop filter and reference clock required for the phase-locked loop inside the analog baseband unit need to be external, and the modulation method performed by the voltage-controlled end of the voltage-controlled crystal oscillator needs to be guaranteed by a loop filter with a frequency much higher than the analog signal to be modulated. This requires technicians to adjust external parameters to obtain the optimal loop bandwidth. This loop bandwidth ultimately determines the locking time of the phase-locked loop and the phase noise of the signal generated by the voltage-controlled oscillator. The design requirements are relatively strict and the R&D cost is relatively high.
[0004] Moreover, the modulation signal of the existing digital modulation circuit is modulated by the reference clock to control the linearity of the modulation frequency deviation, but the modulation range of the reference clock is limited, generally below 20PPM of the reference clock. When the frequency is low and the modulation frequency deviation needs to be larger than the voltage control range of the reference clock, the modulation requirements cannot be met. Therefore, low-frequency band walkie-talkies cannot use this reference clock modulation method. Utility Model Content
[0005] One of the purposes of the utility model is to provide a digital modulation circuit with low cost and wide applicable frequency band.
[0006] Another object of the utility model is to provide a digital intercom using the digital modulation circuit.
[0007] In order to achieve the above purpose, the utility model adopts the following technical solutions:
[0008] The digital modulation circuit comprises a digital baseband unit, a coupling capacitor, a decoupling capacitor and an analog baseband unit. The analog baseband unit adopts an FD6818 radio frequency baseband chip. The output end of the digital baseband unit is connected to the non-inverting input end of the automatic gain controller of the FD6818 radio frequency baseband chip through the coupling capacitor. The decoupling capacitor is connected between the inverting input end of the automatic gain controller of the FD6818 radio frequency baseband chip and ground. The external crystal oscillator pin of the FD6818 radio frequency baseband chip is connected to an external reference crystal oscillator.
[0009] The reference crystal oscillator adopts a temperature compensated crystal oscillator.
[0010] In the FD6818 RF baseband chip, the output of its analog / digital converter is directly connected to the input of its ∑∆ modulator through its direct channel.
[0011] The coupling capacitor and the decoupling capacitor both use capacitors with specifications not less than 0805 and a capacity not less than 10uF.
[0012] The digital intercom has a digital modulation circuit, and the digital modulation circuit adopts the digital modulation circuit as described above.
[0013] After adopting the above scheme, the digital modulation circuit and digital intercom of the utility model adopt the FD6818 radio frequency baseband chip in its analog baseband unit. Since the FD6818 radio frequency baseband chip has a full audio pass-through function, when working, the analog signal to be modulated no longer needs to be modulated using the voltage-controlled end of the voltage-controlled crystal oscillator. Since there is no need to use a crystal oscillator with voltage control, the requirements for the device are low, thereby reducing costs. Moreover, the digital modulation circuit of the utility model is no longer limited by the modulation range of the reference clock compared to the prior art because the output end of the digital baseband unit is connected to the audio signal input end of the FD6818 radio frequency baseband chip. It is not only suitable for high-frequency intercoms, but also for low-frequency intercoms (such as 30-45MHz, 66-88MHz), and the applicable frequency band is relatively wide.
[0014] Furthermore, in the utility model, in the FD6818 radio frequency baseband chip, the output end of its analog / digital converter skips its digital signal processing unit and is directly connected to the input end of its ∑∆ modulator through its direct channel. The digital signal to be modulated output by the analog / digital converter of the FD6818 radio frequency baseband chip no longer passes through its digital signal processing unit for digital signal processing, such as high-pass filtering, low-pass filtering, pre-emphasis, etc., thereby avoiding these processes from damaging or severely distorting the integrity, distortion, amplitude frequency, etc. of the digital signal to be modulated, thereby ensuring the integrity and distortion of the digital signal to be modulated.
[0015] Furthermore, in the utility model, the coupling capacitor and the decoupling capacitor both use capacitors with specifications not less than 0805 and capacitance not less than 10uF to avoid problems such as loss and distortion of low-frequency signals of coupling signals and distortion of high-frequency signals. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Figure 1 It is a circuit principle block diagram of an existing digital modulation circuit.
[0017] Figure 2 The utility model is a circuit principle block diagram of the digital modulation circuit.
[0018] Figure 3 for Figure 2 In the figure, the circuit schematic diagram of the FD6818 RF baseband chip and its peripheral circuits. DETAILED DESCRIPTION
[0019] The utility model is a digital modulation circuit, such as Figure 2 and Figure 3 As shown, it includes a digital baseband unit, a coupling capacitor C14, a decoupling capacitor C16 and an FD6818 RF baseband chip U1 as an analog baseband unit, the output end of the digital baseband unit is connected to the positive input end (pin 11) of the automatic gain controller of the FD6818 RF baseband chip U1 through the coupling capacitor C14, the decoupling capacitor C16 is connected between the negative input end (pin 12) of the automatic gain controller of the FD6818 RF baseband chip U1 and the ground, and the external crystal oscillator pin (pin 5) of the FD6818 RF baseband chip is connected to an external reference crystal oscillator X1. The reference crystal oscillator X1 can use a conventional temperature-compensated crystal oscillator.
[0020] In the present invention, the digital baseband unit adopts a conventional digital baseband processing chip, such as the SCT3288 digital baseband processing chip.
[0021] The utility model discloses a digital modulation circuit. When the microphone collects the voice signal of the user, converts it into an analog level and then sends it to the AD port input pin of the digital baseband unit. The AD conversion is performed through the AD module. After the conversion, the digital baseband unit performs coding and compression processing to become a digital baseband signal to be modulated. The digital baseband unit converts it into an analog signal through the DA module of the digital baseband unit and then outputs it. The analog signal to be modulated output from the digital baseband unit enters the FD6818 RF baseband chip U1 from the MIC INPUT port of the FD6818 RF baseband chip U1 through the coupling capacitor C14, performs signal modulation through the FD6818 RF baseband chip U1, and outputs the digital modulation signal from the RF output end (the 18th pin) of the FD6818 RF baseband chip U1.
[0022] After adopting the above scheme, the digital modulation circuit of the utility model adopts the FD6818 RF baseband chip in its analog baseband unit. Since the FD6818 RF baseband chip has its own audio full-pass function, when working, the analog signal to be modulated no longer needs to be modulated by the voltage-controlled end of the voltage-controlled crystal oscillator. Since there is no need to use a voltage-controlled crystal oscillator, only a conventional temperature-compensated crystal oscillator is needed, which has lower requirements on the device and can reduce costs.
[0023] Moreover, since the output end of the digital baseband unit of the digital modulation circuit of the present invention is connected to the audio signal input end of the FD6818 RF baseband chip U1, compared with the prior art, it is no longer limited by the modulation range of the reference clock and is not only suitable for high-frequency signal modulation, but also for low-frequency signal modulation (such as 30-45MHz, 66-88MHz), and the applicable frequency band is wider.
[0024] Furthermore, in the utility model, in the FD6818 RF baseband chip U1, the output end of its analog / digital converter skips its digital signal processing unit and is directly connected to the input end of its ∑∆ modulator through its direct channel. Connecting the output end of the analog / digital converter of the FD6818 RF baseband chip to the input end of its ∑∆ modulator through its direct channel, skipping its digital signal processing unit, can be achieved by pre-setting the FD6818 RF baseband chip, which is an existing conventional function of the FD6818 RF baseband chip U1. When working, the digital signal to be modulated output by the analog / digital converter of the FD6818 RF baseband chip U1 no longer passes through its digital signal processing unit for digital signal processing, such as high-pass filtering, low-pass filtering, pre-emphasis, etc., thereby avoiding these processes from damaging or severely distorting the integrity, distortion, amplitude frequency, etc. of the digital signal to be modulated, thereby ensuring the integrity and distortion of the digital signal to be modulated.
[0025] Furthermore, in the present invention, the MIC INPUT port of the FD6818 RF baseband chip U1 adopts a single-port input. Since the smaller the ceramic capacitor, the smaller the quality factor and withstand voltage, the larger the ESR series resistance / low withstand voltage, and the material used is a low dielectric material, the temperature coefficient and voltage coefficient are poor, and the capacity changes greatly in a large voltage signal and a large temperature range, which will cause problems such as loss and distortion of the low-frequency signal of the coupled signal, and distortion of the high-frequency signal. Therefore, in the present invention, the coupling capacitor C14 and the decoupling capacitor C16 both use capacitors with specifications not less than 0805 and a capacity not less than 10uF to avoid problems such as loss and distortion of the low-frequency signal of the coupled signal, and distortion of the high-frequency signal.
[0026] The utility model discloses a digital interphone, which is provided with a digital modulation circuit. The digital modulation circuit adopts the digital modulation circuit as described above.
[0027] The above is only a preferred embodiment of the present invention, and all equivalent changes and modifications made within the scope of the claims of the present utility model shall fall within the scope of the claims of the present utility model.
Claims
1. A digital modulation circuit, characterized in that: It includes a digital baseband unit, a coupling capacitor, a decoupling capacitor and an analog baseband unit. The analog baseband unit adopts an FD6818 radio frequency baseband chip. The output end of the digital baseband unit is connected to the non-phase input end of the automatic gain controller of the FD6818 radio frequency baseband chip through the coupling capacitor. The decoupling capacitor is connected between the inverting input end of the automatic gain controller of the FD6818 radio frequency baseband chip and the ground. The external crystal oscillator pin of the FD6818 radio frequency baseband chip is connected to an external reference crystal oscillator.
2. The digital modulation circuit according to claim 1, characterized in that: The reference crystal oscillator adopts a temperature compensated crystal oscillator.
3. The digital modulation circuit according to claim 1, characterized in that: In the FD6818 RF baseband chip, the output of its analog / digital converter is directly connected to the input of its ∑∆ modulator through its direct channel.
4. The digital modulation circuit according to claim 1, characterized in that: The coupling capacitor and the decoupling capacitor both use capacitors with specifications not less than 0805 and a capacity not less than 10uF.
5. A digital intercom having a digital modulation circuit, characterized in that: The digital modulation circuit thereof adopts the digital modulation circuit as described in any one of claims 1-4.