FM voice demodulation circuit and method low in cost and easy to implement

Abstract

The invention discloses a low-cost and easy-to-implement FM voice demodulation circuit and method, and the circuit comprises an FM voice extraction circuit and an FM voice restoration circuit, the FM voice extraction circuit carries out the frequency mixing of a local oscillator signal and a high-frequency RF signal received by an antenna, obtains an intermediate frequency IF signal, carries out the filtering processing, and outputs the intermediate frequency IF signal to the FM voice restoration circuit. Further mixing the filtered intermediate frequency IF signal with the other path of local oscillation signal to obtain a baseband voice signal and realize the extraction of the voice signal; and after the baseband voice signal is input into the FM voice restoration circuit for filtering, amplification and conversion processing, a restored voice signal is obtained, and demodulation of the voice signal is realized. The method is simple and easy to implement, system complexity and cost are reduced, and the method is suitable for occasions sensitive to cost and low in signal quality requirement.

Description

Technical Field

[0001] The invention belongs to the technical field of voice demodulation, and in particular relates to a low-cost and easy-to-implement FM voice demodulation circuit and method. Background Art

[0002] The traditional FPGA-based FM voice demodulation solution first obtains the baseband signal through down-conversion, then processes the baseband signal through FPGA to obtain the I / Q signal, and finally obtains the audio signal through noise reduction, filtering, phase correction and other processing. In the process, the orthogonal component and the in-phase component need to be operated on by arc tangent and differential, which is relatively complex and resource-intensive to implement. The use of FM receiving chips also has the problems of low frequency accuracy, low selectivity, and cumbersome use. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide a low-cost and easy-to-implement FM voice demodulation circuit.

[0004] Another object of the present invention is to provide a speech demodulation method using the circuit.

[0005] Technical solution: The low-cost and easy-to-implement FM voice demodulation circuit of the present invention comprises an FM voice extraction circuit and an FM voice restoration circuit. The FM voice extraction circuit comprises a dual-channel local oscillator module, a primary mixing module, a filtering module and a secondary mixing module. The FM voice restoration circuit comprises an LC low-pass filter, an LC high-pass filter, a first amplifier, a comparator, an F / V converter and a power amplifier circuit. The dual-channel local oscillator module generates dual-channel local oscillator signals. After one channel of the local oscillator signal is mixed with a high-frequency RF signal received by an antenna in a primary mixing module, an intermediate frequency IF signal is output to the filtering module. The filtering module performs filtering processing on the intermediate frequency IF signal. After the filtered intermediate frequency IF signal is mixed with another channel of the local oscillator signal generated by the dual-channel local oscillator module in a secondary mixing module, a baseband voice signal is output to realize voice signal extraction. The baseband voice signal is input into the FM voice restoration circuit, and after being processed by the LC low-pass filter, the LC high-pass filter, the first amplifier, the comparator, the F / V converter and the power amplifier circuit in sequence, a restored voice signal is obtained to realize voice signal demodulation.

[0006] Optionally, the dual-channel local oscillator module includes a single-chip microcomputer, a frequency multiplier and a crystal oscillator. The single-chip microcomputer is used to program the frequency multiplier, the crystal oscillator is used as a reference clock for the frequency multiplier, and the crystal oscillator is used to stabilize the frequency of the dual-channel local oscillator signal output by the frequency multiplier. One channel of the dual-channel local oscillator signal output by the frequency multiplier is input to the primary mixing module, and the other channel is input to the secondary mixing module.

[0007] Optionally, the single-chip microcomputer and the frequency multiplier both have SPI interfaces, the frequency accuracy of the frequency multiplier is 3HZ, and the crystal oscillator frequency does not exceed 350MHZ.

[0008] Optionally, the primary mixing module includes a first filter and a first mixer, wherein the high-frequency RF signal received by the antenna is input into the radio frequency input terminal RF of the first mixer after filtering by the first filter, and at the same time, a local oscillator signal output by the frequency multiplier of the dual-channel local oscillator module is also input into the analog input terminal LO of the first mixer, and the filtered high-frequency RF signal and the local oscillator signal are mixed once in the first mixer, and the intermediate frequency IF signal is output to the filtering module through the output terminal IF of the first mixer.

[0009] Optionally, the center frequency of the first filter is 2 GHZ, and the passband range is 80 MHZ; the frequency input range of the radio frequency input terminal RF and the analog input terminal LO of the first mixer is 1.5 GHZ-4.5 GHZ.

[0010] Optionally, the filtering module includes an attenuator, a second amplifier, a second filter, a third amplifier and a third filter. The intermediate frequency IF signal output by the primary mixing module is processed by the attenuator, the second amplifier, the second filter, the third amplifier and the third filter in sequence to obtain a filtered intermediate frequency signal.

[0011] Optionally, the attenuation value of the attenuator is 30dB; the amplification factor of the second amplifier at the intermediate frequency IF signal frequency is 30dB; the passband range of the second filter is 243MHZ±25KHZ, and the out-band attenuation is 30dB; the amplification factor of the third amplifier at the intermediate frequency IF signal frequency is 20dB; the out-band frequency signal of the third filter has an attenuation value of at least 3dB.

[0012] Optionally, the secondary mixing module includes a fourth amplifier and a second mixer. The fourth amplifier amplifies the intermediate frequency signal output by the filtering module and inputs the signal into the radio frequency input terminal RF of the second mixer. At the same time, another local oscillator signal generated by the dual-channel local oscillator module is input into the analog input terminal LO of the second mixer. After the amplified intermediate frequency signal and the other local oscillator signal are secondary mixed in the second mixer, the baseband voice signal is output through the output terminal.

[0013] Optionally, the frequency of one local oscillator signal is 1757.053 MHZ, and the frequency of another local oscillator signal is 243.113 MHZ; the frequency of the intermediate frequency IF signal after one mixing is 243.030 MHZ; and the frequency of the baseband voice signal is 83 KHZ.

[0014] The FM voice demodulation method using the circuit comprises the following steps:

[0015] (1) FM speech extraction;

[0016] First, the high-frequency RF signal received by the antenna is filtered and sent to the primary mixing module. After receiving one local oscillator signal from the dual-channel local oscillator module, the primary mixing module mixes out an intermediate frequency IF signal; then the intermediate frequency IF signal is input into the filtering module for filtering and amplification, and then sent to the secondary mixing module. After receiving the other local oscillator signal from the dual-channel local oscillator module, the secondary mixing module mixes out a baseband voice signal;

[0017] (2)FM voice restoration;

[0018] The baseband voice signal extracted from FM voice is input into the low-pass filter for low-pass filtering, and then into the high-pass filter for high-pass filtering to obtain a clean voice spectrum signal, and then enters the first amplifier to amplify the signal, and then enters the comparator to convert it into a square wave signal, and then enters the F / V converter to convert it into a voltage signal, and finally enters the power amplifier circuit to restore it to a voice signal and output it from the speaker.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant technical effects: (1) low hardware complexity, high precision, and good selectivity. The functions can be realized by using a common LC filter circuit, RF amplifier circuit, comparison circuit, F / V conversion, and audio power amplifier circuit; (2) low software complexity and high flexibility. It is only necessary to set up the SPI peripherals through single-chip microcomputer programming; (3) low cost. No FPGA (XILINX-XC6SLX75T, etc.) and multi-layer PCB are required. The FM voice extraction circuit and the FM voice restoration circuit can be realized on a 2-layer PCB. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a FM demodulation circuit diagram using a FM receiving chip TA2003 in the prior art;

[0021] Figure 2 This is a schematic diagram of the FM voice demodulation circuit of the present invention;

[0022] Figure 3 This is a schematic diagram of the FM voice extraction circuit in the present invention;

[0023] Figure 4 This is a schematic diagram of the FM voice restoration circuit in the present invention;

[0024] Figure 5 This is the frequency attenuation curve of TB0208A;

[0025] Figure 6 This is a schematic diagram of an active low-pass filter circuit with an octave attenuation of -18dB;

[0026] Figure 7 This is a schematic diagram of an 11th-order LC passive low-pass filter circuit;

[0027] Figure 8 This is the measured attenuation curve of the 11th-order low-pass filter circuit;

[0028] Fig. 9 This is the spectrum after 11th-order low-pass filtering;

[0029] Fig.10 This is the schematic diagram of the AD650 F / V conversion circuit;

[0030] Fig.11 This is a schematic diagram of the input square wave signal of AD650;

[0031] Fig.12 This is a schematic diagram of the output voltage signal of AD650;

[0032] Fig.13 This is the schematic diagram of the LM386 power amplifier circuit. DETAILED DESCRIPTION

[0033] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0034] The circuit diagram of the commonly used FM receiver chip TA2003 is as follows Figure 1 As shown, the principle is that the RF signal received by the antenna needs to be mixed with the signal generated by two external LC resonant networks, and the resonant networks are composed of dual variable capacitors (0-20pF), which have low frequency accuracy and selectivity. They are also cumbersome to use in practice, and sometimes lead to failure to receive broadcast signals normally.

[0035] The low-cost and easy-to-implement FM voice demodulation circuit of the present invention is as follows: Figure 2 As shown, it includes an FM voice extraction circuit and an FM voice restoration circuit, wherein the function of the FM voice extraction circuit is to completely and effectively extract the 83KHZ voice spectrum from the radio frequency signal, and the function of the FM voice restoration circuit is to restore the 83KHZ voice spectrum to a clear voice signal.

[0036] like Figure 3 As shown, the FM voice extraction circuit includes a dual-channel local oscillator module, a primary mixing module, a filtering module and a secondary mixing module. The dual-channel local oscillator module generates dual-channel local oscillator signals (1757.053MHZ and 243.113MHZ), which will be used for subsequent mixing operations. The primary mixing module downconverts the high-frequency RF signal (2GHZ) received by the antenna to an intermediate frequency IF signal (243.030MHZ). The filtering module filters out interference signals near 243MHZ and amplifies the effective signal. The secondary mixing module mixes an 83KHZ baseband voice signal from the filtered intermediate frequency IF signal.

[0037] The dual-channel local oscillator module includes an STM8 microcontroller, an HMC series frequency multiplier and a 50M crystal oscillator. After the STM8 microcontroller is powered on, it connects and controls the HMC series frequency multiplier through the SPI interface. The HMC series frequency multiplier will output the local oscillator frequency signal set by the program. The 50M crystal oscillator is connected to the HMC series frequency multiplier as a clock. The HMC series frequency multiplier will use the 50M crystal oscillator as a reference clock and multiply the local oscillator signal. The digital circuit STM8 microcontroller is used to program the HMC series frequency multiplier to ensure the accuracy of the local oscillator frequency and improve the selectivity of the mixing frequency. At the same time, a 50M crystal oscillator is used to stabilize the frequency of the local oscillator signal output by the HMC series frequency multiplier. The local oscillator signal output by the HMC series frequency multiplier is input to the primary mixing module. The microcontroller can also choose other models as long as it has an SPI interface. Of course, the size and price factors must also be considered. The frequency multiplier can also have other options, as long as it has an SPI communication interface, and the frequency accuracy (3HZ) and price factors must also be considered. The reference clock of the HMC series frequency multiplier has a frequency range limit, as long as it does not exceed 350MHZ. 50MHZ is selected here to facilitate the frequency multiplication calculation.

[0038] The primary mixer module includes a TA series filter and an HMC series mixer. The high-frequency RF signal received by the antenna is filtered by the TA series filter and then input to the RF input terminal RF of the HMC series mixer. At the same time, the local oscillator signal output by the HMC series frequency multiplier of the dual-channel local oscillator module is also input to the analog input terminal LO of the HMC series mixer. The RF signal and the local oscillator signal are mixed once in the HMC series mixer. The input RF signal (2.000083GHZ) and the local oscillator signal (1757.053MHZ) are subtracted to output the IF signal (243.030MHZ). Other types of filters can also be selected, as long as they are bandpass filters with a center frequency of 2GHZ and a passband range of 80MHZ. Other types of mixers can also be selected, as long as the frequency input range of the RF terminal and the LO terminal (1.5GHZ-4.5GHZ) is met. The intermediate frequency signal is output to the filter module through the output terminal IF.

[0039] The filter modules include RFSA series attenuators, INA series amplifiers, TB0208A filters, ERA series amplifiers and LFCN series filters. The RFSA series attenuators attenuate the intermediate frequency signals to prevent them from overexciting the subsequent devices. When selecting other types of attenuators, it is sufficient that the attenuation value can reach 30dB. The INA series amplifiers amplify the intermediate frequency signals. When selecting other types of amplifiers, it is sufficient that the amplification factor reaches 30dB at the frequency point of 243MHZ. The TB0208A filter performs the passband filtering of 243MHZ. When selecting other bandpass filters, it is sufficient that the passband range is 243MHZ±25KHZ and the out-of-band attenuation reaches 30dB. The ERA series amplifiers amplify the intermediate frequency signals. When selecting other types of amplifiers, it is sufficient that the amplification factor reaches 20dB at the frequency point of 243MHZ. The LFCN series filter completes the low-pass filtering of the 243MHZ signal. When selecting other types of filters, it is sufficient to meet the attenuation value of the out-of-band frequency signal of at least 3dB. The signal flow of the filter module is as follows: the intermediate frequency signal first enters the RFSA attenuator for 3dB attenuation, and then enters the INA series amplifier for 30dB amplification, then enters the TB0208A bandpass filter to filter out the out-of-band interference, then enters the ERA series amplifier for 20dB amplification, enters the LFCN series filter for low-pass filtering, and finally outputs a relatively clean intermediate frequency signal (243.030MHZ).

[0040] The secondary mixing module includes an AD series amplifier and an AD series mixer. The AD series amplifier completes the 42dB amplification of the intermediate frequency signal. When other types of amplifiers are selected, it is sufficient to meet the 42dB amplification value at the 243MHZ frequency point. The AD series mixer completes the mixing of the intermediate frequency signal, and obtains an 83KHZ baseband voice signal after performing a difference frequency operation with the local oscillator signal. When other types of mixers are selected, it is sufficient to meet the 500MHZ bandwidth range and -10dBm local oscillator drive. Specifically: the AD series amplifier amplifies the signal output by the filter module and inputs it to the RF input terminal RF of the AD series mixer. At the same time, the local oscillator signal output by the dual-channel local oscillator module is input to the analog input terminal LO of the AD series mixer. After the amplified signal and the local oscillator signal are secondary mixed in the AD series mixer, the baseband voice signal is output through the output terminal.

[0041] In this embodiment, the frequency of a local oscillator signal is set to 1757.053 MHZ (with an accuracy of up to 3 Hz, i.e., 1757.053 MHZ±3 Hz), and a 243.030 MHZ±3 Hz signal can be generated after one mixing, and the result does not need to be repeatedly adjusted, which is convenient to use.

[0042] The reason why the signal after the first mixing is selected at 243.030MHZ is due to other considerations. In order to retain the voice signal as much as possible and weaken the invalid interference signal, the SAW filter TB0208A is used in the filtering module. Its frequency attenuation curve is shown in the figure below. Figure 5 shown. Figure 5 The center frequency of the filter is 243MHZ and its passband range is 243MHZ±25KHZ.

[0043] It is known that the frequency range of the speech signal of the present invention is 83KHZ±20KHZ, so the frequency after the first mixing needs to be 243.030MHZ, because its attenuation is -3.7dB at 243.030MHZ±20KHZ, which is the smallest attenuation compared to the center frequency, so that the valid signal is retained. At the same time, considering that the interference signal is mainly concentrated at the harmonics of 83KHZ such as 166KHZ, 249KHZ, etc., its attenuation is greater than -13dB at 243.030MHZ±166KHZ, so that the invalid signal is weakened as much as possible.

[0044] like Figure 4 As shown, the FM voice restoration circuit includes an 11th-order LC low-pass filter, a 7th-order LC high-pass filter, an AD series amplifier, an LT series square wave comparator, an AD650F / V converter and an LM series power amplifier circuit. The 11th-order LC low-pass filter circuit completes the low-pass filtering function of 83KHZ. When other filters are selected, it is sufficient to meet the attenuation value of -2.7dB@98KHZ / -68dB@167KHZ. The 7th-order LC high-pass filter circuit completes the high-pass filtering function of 60KHZ. When other filters are selected, it is sufficient to meet the attenuation value of -2.2dB@60KHZ / -107dB@1.5KHZ. The AD series amplifier completes the 80dB amplification of the 83KHZ signal. When other amplifiers are selected, it is sufficient to meet the 80dB amplification value. The LT series square wave comparator completes the function of converting the 83KHZ sine wave signal into a square wave signal. When other comparators are selected, it is sufficient to meet the functions of the adjustable reference source and bipolar input signal. The AD650F / V converter converts the 83KHZ square wave signal into a voltage signal. When selecting other converters, it is sufficient to meet the 1MHZ frequency operating range and 0.005% nonlinearity. The LM series power amplifier circuit completes the power amplification function. When selecting other power amplifier circuits, it is sufficient to meet the voltage gain of 200.

[0045] The voice signal obtained by the above voice extraction circuit still has a lot of interference signals, especially its harmonic components are relatively large. If a common active filter is used, its attenuation is about -20dB / octave, which will not work when the interference at 166KHZ is almost as large as the effective signal, resulting in interference signals in the filtered signal. Figure 6 As shown, an active low-pass filter circuit is constructed using an RC4558DN operational amplifier in the prior art.

[0046] The present invention adopts an 11th-order LC passive low-pass filter, and the circuit structure is as follows: Figure 7 As shown in the figure, it includes five inductors L and six capacitors C. By adjusting the LC parameters, the attenuation is finally achieved to -60dB / octave. The measured attenuation curve of the 11th-order LC passive low-pass filter is shown in Figure 8 As shown, Figure 8 It is the factory parameter indicator of the LC low-pass filter. It can be seen that the signal attenuation value is -1.4dB at 83KHZ, and the attenuation value is -67.7dB at 167KHZ. The attenuation at the effective frequency band of 83KHZ is as small as possible, and the attenuation at the invalid interference frequency band of 167KHZ is as large as possible, which plays a good low-pass filtering role. Fig. 9 This is the measured effect after the LC low-pass filter is connected to this circuit. It can be seen that the interference signal amplitude at 167KHZ is about 45dB lower than the effective signal amplitude at 83KHZ, achieving the expected effect. The spectrum after filtering by the 11th-order LC passive low-pass filter is as follows: Fig. 9 shown.

[0047] To restore the speech signal, the commonly used method is orthogonal demodulation, which can be implemented in hardware (such as Figure 1 There are two methods: TA2003) and software. The software method is mostly implemented through FPGA programming, which involves orthogonal demodulation algorithms and many filtering algorithms, and its implementation process is relatively complicated. The present invention adopts the F / V conversion method, such as Fig.10 As shown in the figure, AD650 is used to realize F / V conversion, which has small linear error (0.005%), fast response speed (1cycle+1uS), large dynamic range (0-1MHZ), and is very suitable for the application of this system. Fig.10 shown.

[0048] Fig.10 This is the circuit connection diagram of AD650 in F / V conversion application. Pin 9 (COMP input port) is the frequency input pin. The external frequency signal is from F IN The input passes through the capacitor and resistor and then enters pin 9. Fig.11 As shown; Fig.11 The AD650 input square wave signal 83KHZ±20KHZ is output at pin 3 after AD650 F / V conversion. Pin 3 is the voltage output port. After passing through the capacitor, it outputs V OUT The corresponding voltage signal can be obtained. The actual waveform is as follows: Fig.12 shown. Fig.12The figure shows the output analog voltage signal after AD650 F / V conversion. It changes in direct proportion to the frequency of the input square wave signal 83KHZ±20KHZ, showing a good linear relationship.

[0049] The voltage signal analyzed by AD650 can then be restored through the LM386 power amplifier circuit to achieve voice restoration, and finally output through the speaker. The implementation circuit is as follows Fig.13 As shown, it shows the typical power amplifier circuit of LM386, and its voltage gain can reach 200.

[0050] The present invention avoids the problems of low precision, poor selectivity and cumbersome use when using the frequency modulation receiving chip TA2003 by adopting a filtering circuit and an F / V conversion circuit. By adopting a single-chip digital programming method to set the local oscillator frequency, the required frequency can be obtained by simply modifying the program for different applications, so the flexibility is high; at the same time, it is simpler and more feasible than the FPGA program that uses a variety of complex algorithms. The selected circuits are all independent general-purpose devices, basically 16-pin or 20-pin SMT packages, and there will be no large-sized BGA packages, so a 2-layer PCB can be used to place all devices, and the final overall implementation cost is relatively low. The present invention uses an RF analog circuit to directly filter, amplify, compare, and F / V convert the baseband signal, and obtain a voice signal after the audio power amplifier. After experimental verification, this solution is simple and easy to implement, reduces system complexity and cost, and is suitable for occasions that are cost-sensitive and have low signal quality requirements.

[0051] The present invention provides a low-cost and easy-to-implement FM voice demodulation method, comprising the following steps:

[0052] (1)FM voice extraction circuit

[0053] The function of this circuit is to completely and effectively extract the 83KHZ voice spectrum from the radio frequency signal. The signal flow of this circuit is as follows: first, the high-frequency RF signal received by the antenna is filtered and sent to the primary mixing module, which then receives the local oscillator signal (1757.053MHZ) from the dual-channel local oscillator module and mixes out the IF signal (243.030MHZ). Then the IF signal enters the filter module for filtering and amplification, and then is sent to the secondary mixing module, which mixes out the 83KHZ baseband voice signal after receiving the local oscillator signal (243.113MHZ) from the dual-channel local oscillator module. See the specific flow chart. Figure 3 .

[0054] (2)FM voice restoration circuit

[0055] The function of this circuit is to restore the 83KHZ voice spectrum to a clear voice signal. The signal flow of this circuit is as follows: first, the 83KHZ baseband voice signal enters the LC low-pass filter for low-pass filtering, and then enters the LC high-pass filter for high-pass filtering to obtain a clean voice spectrum signal, and then enters the AD series amplifier to amplify the signal by 80dB, and then enters the LT series square wave comparator to convert it into a square wave signal, and then enters the AD650F / V converter to convert it into a voltage signal, and finally enters the LM series power amplifier circuit to restore it to a voice signal and output it from the speaker. See the specific flow chart Figure 4 .

Claims

1. A low-cost and easy-to-implement FM voice demodulation circuit, characterized in that: The invention comprises an FM voice extraction circuit and an FM voice restoration circuit. The FM voice extraction circuit comprises a dual-channel local oscillator module, a primary mixing module, a filtering module and a secondary mixing module. The FM voice restoration circuit comprises an LC low-pass filter, an LC high-pass filter, a first amplifier, a comparator, an F / V converter and a power amplifier circuit. The dual-channel local oscillator module generates dual-channel local oscillator signals. After one channel of the local oscillator signal is mixed with a high-frequency RF signal received by an antenna in a primary mixing module, an intermediate frequency IF signal is output to the filtering module. The filtering module performs filtering processing on the intermediate frequency IF signal. After the filtered intermediate frequency IF signal is mixed with another channel of the local oscillator signal generated by the dual-channel local oscillator module in a secondary mixing module, a baseband voice signal is output to realize voice signal extraction. The baseband voice signal is input into the FM voice restoration circuit, and after being processed by the LC low-pass filter, the LC high-pass filter, the first amplifier, the comparator, the F / V converter and the power amplifier circuit in sequence, a restored voice signal is obtained to realize voice signal demodulation.

2. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 1 is characterized in that: The dual-channel local oscillator module includes a single-chip microcomputer, a frequency multiplier and a crystal oscillator. The single-chip microcomputer is used to program the frequency multiplier, and the crystal oscillator is used as the reference clock of the frequency multiplier. The crystal oscillator is used to stabilize the frequency of the dual-channel local oscillator signal output by the frequency multiplier. One channel of the dual-channel local oscillator signal output by the frequency multiplier is input to the primary mixing module, and the other channel is input to the secondary mixing module.

3. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 2 is characterized in that: The single chip microcomputer and the frequency multiplier both have SPI interfaces, the frequency accuracy of the frequency multiplier is 3HZ, and the crystal oscillator frequency does not exceed 350MHZ.

4. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 1 is characterized in that: The primary mixing module includes a first filter and a first mixer, wherein the high-frequency RF signal received by the antenna is input into the radio frequency input terminal RF of the first mixer after filtering by the first filter, and at the same time, a local oscillator signal output by the frequency multiplier of the dual-channel local oscillator module is also input into the analog input terminal LO of the first mixer, and the filtered high-frequency RF signal and the local oscillator signal are mixed once in the first mixer, and the intermediate frequency IF signal is output to the filtering module through the output terminal IF of the first mixer.

5. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 4 is characterized in that: The center frequency of the first filter is 2 GHZ, and the passband range is 80 MHZ; the frequency input range of the radio frequency input terminal RF and the analog input terminal LO of the first mixer is 1.5 GHZ-4.5 GHZ.

6. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 1 is characterized in that: The filtering module includes an attenuator, a second amplifier, a second filter, a third amplifier and a third filter. The intermediate frequency IF signal output by the primary mixing module is processed by the attenuator, the second amplifier, the second filter, the third amplifier and the third filter in sequence to obtain a filtered intermediate frequency signal.

7. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 6 is characterized in that: The attenuation value of the attenuator is 30dB; the amplification factor of the second amplifier at the intermediate frequency IF signal frequency point is 30dB; the passband range of the second filter is 243MHZ±25KHZ, and the out-band attenuation is 30dB; the amplification factor of the third amplifier at the intermediate frequency IF signal frequency point is 20dB; the out-band frequency signal of the third filter has an attenuation value of at least 3dB.

8. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 1 is characterized in that: The secondary mixing module includes a fourth amplifier and a second mixer. The fourth amplifier amplifies the intermediate frequency signal output by the filtering module and inputs it into the radio frequency input terminal RF of the second mixer. At the same time, another local oscillator signal generated by the dual-channel local oscillator module is input into the analog input terminal LO of the second mixer. After the amplified intermediate frequency signal and the other local oscillator signal are secondary mixed in the second mixer, the baseband voice signal is output through the output terminal.

9. The low-cost and easy-to-implement FM voice demodulation circuit according to claim 1, characterized in that: The frequency of one local oscillator signal is 1757.053MHZ, and the frequency of the other local oscillator signal is 243.113MHZ; the frequency of the intermediate frequency IF signal after one mixing is 243.030MHZ; the frequency of the baseband voice signal is 83KHZ.

10. A low-cost and easy-to-implement FM voice demodulation method, characterized in that: The following steps are involved: (1)FM speech extraction; First, the high-frequency RF signal received by the antenna is filtered and sent to the primary mixing module. After receiving one local oscillator signal from the dual-channel local oscillator module, the primary mixing module mixes out an intermediate frequency IF signal; then the intermediate frequency IF signal is input into the filtering module for filtering and amplification, and then sent to the secondary mixing module. After receiving the other local oscillator signal from the dual-channel local oscillator module, the secondary mixing module mixes out a baseband voice signal; (2)FM voice restoration; The baseband voice signal extracted from FM voice is input into the low-pass filter for low-pass filtering, and then into the high-pass filter for high-pass filtering to obtain a clean voice spectrum signal, and then enters the first amplifier to amplify the signal, and then enters the comparator to convert it into a square wave signal, and then enters the F / V converter to convert it into a voltage signal, and finally enters the power amplifier circuit to restore it to a voice signal and output it from the speaker.

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