Intercom amplitude modulation circuit

Abstract

An intercom amplitude modulation circuit comprises an audio processing circuit, a radio frequency power amplifier circuit, a PWM pulse modulation circuit, an automatic gain control circuit and a microprocessor; the audio processing circuit outputs an audio alternating current signal after processing an input audio signal; the microprocessor outputs a constant voltage and superimposes and combines the constant voltage with the audio alternating current signal and then sends the combined signal to the PWM pulse modulation circuit; the PWM pulse modulation circuit outputs an AM audio modulation voltage which varies synchronously with the audio signal to the radio frequency power amplifier circuit; the automatic gain control circuit rectifies the AM audio modulation voltage to obtain a negative half cycle signal and adjusts the amplitude of the input audio signal in combination with the output of the microprocessor so that the AM audio modulation voltage is within a set range; and the radio frequency power amplifier circuit modulates the obtained AM audio modulation voltage to an input transmission carrier signal and then amplifies and outputs the modulated signal. The circuit is small in size and high in efficiency.

Description

Technical Field

[0001] This invention relates to the field of walkie-talkies, and in particular to an amplitude modulation circuit for walkie-talkies. Background Technology

[0002] Currently, handheld amplitude modulation (AM) radios typically use large-signal modulation to achieve AM functionality. Large-signal modulation is more efficient than small-signal modulation, which is beneficial for the design of handheld device battery capacity. Common large-signal modulation methods include the following two:

[0003] One method is to use a transformer to achieve modulation, see [link to relevant documentation]. Figure 2 The transmitted audio signal is amplified by the audio power amplifier and then coupled to transformer L1 via C1. One end of transformer L1 is connected to the battery or power supply pin, and the other end outputs power to the RF power amplifier or pre-drive circuit. Using this scheme, the unmodulated power is 4W. To achieve a peak power output (PEP) of 12W, the transformer needs a minimum volume of 15*15*18mm (length*width*height). This means the transformer is large and has moderate efficiency. Furthermore, the audio power amplifier exhibits static power consumption when there is no audio modulation.

[0004] Another method is to use voltage regulation, see [link to relevant documentation] Figure 3 The transmitted audio signal is processed by amplifier B1 and then sent to transistor Q1 for linear amplification. The signal is then output from the collector of transistor Q1 to power the RF power amplifier or pre-drive circuit. While this approach is small in size, it has the lowest efficiency. Furthermore, the collector output voltage of transistor Q1 needs to be adjusted via VSET to achieve the required supply voltage for the carrier power. Additionally, to ensure distortion-free amplitude modulation (AM) half-cycles, the ideal collector output voltage of transistor Q1 is half the supply voltage. This means that when there is no audio signal, transistor Q1 loses 50% of its efficiency. Summary of the Invention

[0005] The main objective of this invention is to overcome the aforementioned defects in the existing technology of amplitude modulation using large signal modulation in walkie-talkies, and to propose an amplitude modulation circuit for walkie-talkies that is small in size and highly efficient.

[0006] The present invention adopts the following technical solution:

[0007] An amplitude modulation (AM) circuit for a walkie-talkie, characterized by comprising an audio processing circuit, an radio frequency (RF) power amplifier circuit, a PWM pulse modulation circuit, an automatic gain control (AGC) circuit, and a microprocessor; the audio processing circuit processes the input audio signal and outputs an AC audio signal; the microprocessor outputs a constant voltage at its first output terminal, which is then superimposed and merged with the AC audio signal before being sent to the PWM pulse modulation circuit; the PWM pulse modulation circuit outputs an AM audio modulation voltage that changes synchronously with the audio signal to the RF power amplifier circuit; the AGC circuit detects the AM audio modulation voltage to obtain a negative half-cycle signal and, in conjunction with the amplitude modulation control signal output from the second output terminal of the microprocessor, adjusts the amplitude of the input audio signal so that the AM audio modulation voltage is within a set range; the RF power amplifier circuit modulates the obtained AM audio modulation voltage onto the input transmit carrier signal, amplifies it, and then outputs it.

[0008] Preferably, the automatic gain control circuit includes transistors Q2 and Q3, resistors R2, R3, R6, R20, R23, R16, R21, diode D1, capacitors C8, C10, C11, and C9; the base of transistor Q2 is connected to one end of resistor R3, and its collector is connected to the audio processing circuit; resistor R6 is connected to the other end of resistor R3 and one end of resistors R20, R2, C8, and C10; the other end of resistor R20 is connected to the collector of transistor Q3, the base of transistor Q3 is connected to one end of resistor R23, the other end of resistor R23 is connected to the anode of diode D1, the cathode of diode D1 is connected to the RF power amplifier circuit to detect the negative half-cycle signal, and the emitter of transistor is connected to one end of resistors R16, R21, C11, and C9.

[0009] Preferably, it further includes an amplification circuit, which is connected between the automatic gain control circuit and the second output terminal of the microprocessor to amplify the amplitude modulation control signal to obtain the amplitude modulation index. When the amplitude modulation of the negative half-cycle signal is deepened, the automatic gain control circuit controls the amplitude of the audio signal to be linearly attenuated.

[0010] Preferably, it further includes a buffer amplifier circuit, which is connected to the output terminal of the audio processing circuit and the first locking terminal of the microprocessor to superimpose, merge and amplify the constant voltage output by the first output terminal of the microprocessor with the audio AC signal.

[0011] Preferably, the constant voltage output from the first output terminal of the microprocessor is amplified by the buffer amplifier circuit and then sent to the PWM pulse modulation circuit, so that the PWM pulse modulation circuit outputs a constant voltage and sends it to the radio frequency power amplifier circuit to power it.

[0012] Preferably, the buffer amplification circuit includes an amplifier U2A, resistors R5, R7, R8, R10, R19, R26, R28, R29, capacitors C3, C4, and C28; one end of resistor R7 is connected to the first output terminal of the microprocessor, the other end of resistor R7 is connected to one end of resistor R8 and capacitor C3, the other end of resistor R8 is connected to one end of capacitor C4, resistor R5, and resistor R10, the other end of resistor R10 is connected to the positive input terminal of amplifier U2A and the audio processing circuit, the negative input terminal of amplifier U2A is connected to one end of resistors R28 and R19, the other end of resistor R28 is connected to the output terminal of amplifier U2A and one end of resistor R29, the other end of resistor R29 is connected to one end of resistor R26 and capacitor C28, and the other end of resistor R26 is connected to the PWM pulse modulation circuit.

[0013] Preferably, the PWM pulse modulation circuit includes a modulation chip U1, which has an FB pin. The FB pin is connected to one end of resistor R33, and the other end of resistor R33 is connected to one end of resistors R30 and R31, as well as the other end of resistor R26 in the buffer amplifier circuit. The formula for calculating the DC voltage without audio modulation output by the PWM pulse modulation circuit is as follows:

[0014]

[0015] Where VA is the voltage output from the output terminal of amplifier U2A, and VFB is the internal reference voltage of modulation chip U1.

[0016] Preferably, the audio processing circuit includes a baseband chip, which performs gain adjustment, pre-emphasis, and filtering on the audio signal.

[0017] Preferably, the output terminal of the automatic gain control circuit is connected to the input or output terminal of the baseband chip.

[0018] As can be seen from the above description of the present invention, compared with the prior art, the present invention has the following beneficial effects:

[0019] 1. The circuit of this invention includes an audio processing circuit, an RF power amplifier circuit, a PWM pulse modulation circuit, an automatic gain control circuit, and a microprocessor. The microprocessor outputs a constant voltage, which is superimposed and merged with the audio AC signal before being sent to the PWM pulse modulation circuit. The PWM pulse modulation circuit outputs an AM audio modulation voltage that changes synchronously with the audio signal to the RF power amplifier circuit. The automatic gain control circuit detects the AM audio modulation voltage to obtain the negative half-cycle signal and, in conjunction with the amplitude modulation control signal output by the microprocessor, adjusts the amplitude of the input audio signal so that the AM audio modulation voltage is within a set range. The RF power amplifier circuit modulates the obtained AM audio modulation voltage onto the input transmit carrier signal, amplifies it, and then outputs it. The large signal modulation method of this invention is used to achieve amplitude modulation, resulting in a small overall size and high efficiency.

[0020] 2. In the circuit of the present invention, the first output terminal of the microprocessor can output a constant voltage, which is amplified by the buffer amplifier circuit and then sent to the PWM pulse modulation circuit U1. The output of the PWM pulse modulation circuit U1 is a constant voltage, which is sent to the radio frequency power amplifier circuit to power it, that is, to provide the power supply voltage required for the carrier power. By setting the magnitude of the DC voltage output by the first output terminal of the microprocessor, the buffer amplifier circuit can output different VA voltage values, so as to conveniently and accurately adjust the non-audio modulated carrier power to meet various different standards without the need to modify the hardware data.

[0021] 3. In the circuit of this invention, the automatic gain control circuit includes transistors Q2 and Q3. The negative half-cycle signal detected by diode D1 can be applied to transistor Q3. At the same time, the collector of transistor Q3 is connected to the base of transistor Q2 to achieve synchronous control of transistor Q2. The collector of transistor Q2 is connected to the MIC-IN input terminal. When the amplitude modulation of the negative half-cycle signal is deepened, transistor Q3 controls transistor Q2 to linearly attenuate the audio signal at the MIC-IN input terminal, thereby reducing the modulation depth and avoiding severe distortion caused by amplitude overshoot in the input signal range that is 20dB higher than the standard 60% modulation depth. Attached Figure Description

[0022] Figure 1 This is a circuit diagram of the present invention;

[0023] Figure 2 For existing technology circuits Figure 1 ;

[0024] Figure 3 For existing technology circuits Figure 2 .

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] in:

[0027] 10. Baseband chip; 20. RF power amplifier circuit; 30. PWM pulse modulation circuit; 40. Automatic gain control circuit; 50. Microprocessor; 60. Buffer amplifier circuit; 70. Amplifier circuit. Detailed Implementation

[0028] The present invention will be further described below through specific embodiments.

[0029] In this invention, the terms "first," "second," and "third," etc., are used only to distinguish similar objects and are not necessarily used to describe a specific order or sequence, nor should they be construed as indicating or implying relative importance. The use of terms such as "upper," "lower," "left," "right," "front," and "rear" to indicate orientation or positional relationships is based on the orientation or positional relationships shown in the accompanying drawings and is only for the convenience of describing the invention, not to indicate or imply that the device referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation on the scope of protection of this invention. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0030] Furthermore, in the description of this application, unless otherwise stated, "multiple" means two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0031] See Figure 1An amplitude modulation circuit for a walkie-talkie includes an audio processing circuit, an RF power amplifier circuit 20, a PWM pulse modulation circuit 30, an automatic gain control circuit 40, and a microprocessor 50. The microprocessor 50 has a first output terminal PWR-SET, a second output terminal ANL-SET, and a third output terminal TX-DC-EN. The first output terminal PWR-SET provides the DC voltage for the red light, the second output terminal ANL-SET outputs the amplitude modulation control signal, and the third output terminal TX-DC-EN outputs the enable control signal for the PWM pulse modulation circuit 30. The audio processing circuit processes the input audio signal and outputs an audio AC signal; the first output terminal of the microprocessor 50 outputs a constant voltage, which is superimposed and merged with the audio AC signal and then sent to the PWM pulse modulation circuit 30; the PWM pulse modulation circuit 30 outputs an AM audio modulation voltage that changes synchronously with the audio signal to the RF power amplifier circuit 20; the automatic gain control circuit 40 detects the AM audio modulation voltage to obtain the negative half-cycle signal and, in conjunction with the amplitude modulation control signal output from the second output terminal of the microprocessor 50, adjusts the amplitude of the input audio signal so that the AM audio modulation voltage is within the set range; the RF power amplifier circuit 20 modulates the obtained AM audio modulation voltage onto the input transmit carrier signal, amplifies it, and then outputs it.

[0032] The audio processing circuit includes a baseband chip 10, which performs gain adjustment, pre-emphasis, and filtering on the audio signal. Specifically, the audio processing circuit also includes capacitor C13, resistors R4 and R1, capacitor C2, resistor R14, capacitor C18, resistor R12, resistor R15, and capacitor C5. One end of capacitor C13 is connected to the voice input terminal, and the other end is connected in series with resistors R4, R1, and C2. The input terminal of baseband chip 10 is connected to capacitor C2, and the output terminal is connected in series with resistors R14, C18, R15, and C5. One end of resistor R12 is connected between capacitor C18 and resistor R15, and the other end is grounded.

[0033] Furthermore, it also includes a buffer amplifier circuit 60, which is connected to the output terminal of the audio processing circuit and the first output terminal of the microprocessor 50 to superimpose, merge and amplify the constant voltage output by the first output terminal of the microprocessor 50 with the audio AC signal, and then send it to the PWM pulse modulation circuit 30.

[0034] The buffer amplifier circuit 60 includes amplifier U2A, resistors R5, R7, R8, R10, R19, R26, R28, R29, and capacitors C3, C4, and C28. One end of resistor R7 is connected to the first output terminal of microprocessor 50. The other end of resistor R7 is connected to one end of resistor R8 and one end of capacitor C3. The other end of resistor R8 is connected to one end of capacitor C4, resistor R5, and resistor R10. The other end of resistor R10 is connected to the positive input terminal of amplifier U2A and capacitor C5 of the audio processing circuit. The negative input terminal of amplifier U2A is connected to one end of resistors R28 and R19. The other end of resistor R28 is connected to the output terminal of amplifier U2A and one end of resistor R29. The other end of resistor R29 is connected to one end of resistor R26 and capacitor C28. The other end of resistor R26 is connected to PWM pulse modulation circuit 30. The other ends of capacitors C3, C4, R5, R19, and C28 are grounded. The power supply terminal of amplifier U2A is also connected to VCC and capacitor C12.

[0035] Furthermore, it also includes an amplifier circuit 70, which is connected between the automatic gain control circuit 40 and the second output terminal of the microprocessor 50 to amplify the amplitude modulation control signal to obtain the amplitude modulation index. When the amplitude modulation of the negative half-cycle signal is deepened, the automatic gain control circuit 40 controls the amplitude of the audio signal to be linearly attenuated, thereby reducing the modulation depth and avoiding severe distortion caused by amplitude overshoot in the input signal range that is 20dB higher than the standard 60% modulation depth.

[0036] The amplifier circuit 70 includes amplifier U2B, resistors R11, R13, R17, R18, R22, capacitors C6 and C7, etc. Resistor R22 is connected to the output terminal and negative input terminal of amplifier U2B. One end of resistor R18 is connected to the negative input terminal of amplifier U2B, and the other end is grounded. The positive input terminal of amplifier U2B is connected to one end of capacitor C6, resistor R11, and resistor R17. The other end of resistor R17 is connected to one end of resistor R13 and capacitor C7. The other end of R13 is connected to the second output terminal of microprocessor 50. The other ends of capacitor C6, resistor R11, and capacitor C7 are grounded.

[0037] The automatic gain control circuit 40 includes transistors Q2 and Q3, resistors R2, R3, R6, R20, R23, R16, and R21, diode D1, capacitors C8, C10, C11, and C9. The base of transistor Q2 is connected to one end of resistor R3, and its collector is connected to resistor R4 of the audio processing circuit, i.e., connected to the input terminal of baseband chip 10. Resistor R6 is connected to the other end of resistor R3 and resistor R... 20. One end of resistor R2, capacitor C8, and capacitor C10; the other end of resistor R20 is connected to the collector of transistor Q3. The base of transistor Q3 is connected to one end of resistor R23, and the other end of resistor R23 is connected to the anode of diode D1. The cathode of diode D1 is connected to RF power amplifier circuit 20 to detect the negative half-cycle signal. The emitter of transistor Q3 is connected to one end of resistor R16, resistor R21, capacitor C11, and capacitor C9. Resistor R21 is connected to the output terminal of amplifier circuit 70. The other ends of capacitors C8, C10, R2, R16, C11, and C9 are grounded. Transistor Q3 is a PNP transistor, and transistor Q2 is an NPN transistor.

[0038] Furthermore, the output terminal of the automatic gain control circuit 40 of the present invention can also be connected to the output terminal of the baseband chip 10, which can also adjust the amplitude of the audio AC signal input to the audio processing circuit so that the AM audio modulation voltage is within a set range.

[0039] In this invention, the constant voltage output from the first output terminal of the microprocessor 50 is amplified by the buffer amplifier circuit 60 and then sent to the PWM pulse modulation circuit 30, causing the PWM pulse modulation circuit 30 to output a constant voltage, which is then sent to the radio frequency power amplifier circuit 20 to power it. The PWM pulse modulation circuit 30 includes a modulation chip U1, which has an FB pin. The FB pin is connected to one end of resistor R33, and the other end of resistor R33 is connected to one end of resistors R30 and R31, as well as the other end of resistor R26 in the buffer amplifier circuit 60. According to the KCL formula, the formula for calculating the DC voltage without audio modulation output by the PWM pulse modulation circuit 30 is as follows:

[0040]

[0041] Where VA is the voltage output from the output terminal of amplifier U2A, and VFB is the internal reference voltage of modulation chip U1.

[0042] Furthermore, the modulation chip U1 also has two connected SW pins. The SW pins are connected to a filter circuit for filtering the constant output voltage, including inductor L5, capacitors C31, C33, and C34. One end of inductor L5 is connected to the SW pin, and the other end of inductor L5 is connected to one end of capacitors C31, C33, and C34. The other ends of capacitors C31, C33, and C34 are grounded. Another filter circuit, including inductor L3, capacitors C21 and C23, can also be connected between the PWM pulse modulation circuit 30 and the RF power amplifier circuit 20 for filtering.

[0043] The DC voltage output from the first output terminal PWR-SET of the microprocessor 50 is amplified by amplifier U2A to output voltage VA. This voltage is then fed into the FB pin of the modulation chip U1 via resistors R29 and R26, capacitor C28, etc. After comparison and processing with the VFB pin within the modulation chip U1, the pulse width of the internal oscillation frequency is adjusted. The voltage is then output from the SW pin and filtered by L5 and L3 to form a constant DC voltage, which is supplied to the RF power amplifier circuit 20 to generate the non-audio modulated carrier power. By setting different DC voltages output from the first output terminal PWR-SET of the microprocessor 50 to generate different VA voltage values, the non-audio modulated carrier power can be easily adjusted.

[0044] The RF power amplifier circuit 20 includes amplifier Q4, inductors L1, L2, and L4; capacitors C17, C26, C15, C16, and C25; and resistors R24, R25, and R27. One end of capacitor C27 is connected to the input terminal RF-IN to input the transmit carrier signal, and the other end is connected to one end of inductor L4 and one end of capacitor C25. The other end of inductor L4 is connected to capacitor C26, resistor R24, and the gate of amplifier Q4. The other end of resistor R24 ​​is connected to one end of capacitor C15, resistor R25, and resistor R27, and the other end of resistor R27 is connected to capacitor C16. The drain of amplifier Q4 is connected to one end of inductor L1, capacitor C17, and inductor L2, and the other end of inductor L2 is connected to the output terminal RF-OUT. The source of amplifier Q4, capacitors C17, C26, C25, C15, C16, and the other end of resistor R25 are grounded. Amplifier Q4 uses either a transistor or a field-effect transistor.

[0045] The working principle of the walkie-talkie amplitude modulation circuit of the present invention is as follows:

[0046] Voice is converted into an audio electrical signal by the microphone and input to the audio processing circuit from the MIC-IN terminal. After passing through the AF-IN terminal, it enters the baseband chip 10U1 for gain adjustment, pre-emphasis, filtering, and other processing, and is output from the AF-OUT terminal of the baseband chip 10. The output audio AC signal is superimposed and merged with the DC voltage output from the first output terminal PWR-SET of the microprocessor 50. After being amplified by the buffer amplifier circuit 60, it is sent to the modulation chip U1 of the PWM pulse modulation circuit 30. The PWM pulse modulation circuit 30U1 outputs an AM audio modulation voltage that can change synchronously with the voice and modulates it onto the transmit carrier signal input from the RF-IN terminal. Then, after being amplified by the transistor Q4 of the RF power amplifier circuit 20, it is output from the RF-OUT terminal.

[0047] The louder the voice signal, the larger the AM audio modulation voltage and the greater the amplitude modulation (AM). To prevent AM overshoot and severe distortion, the second output terminal ANL-SET of the microprocessor 50 outputs a constant voltage during large signal conditions. This voltage is amplified by amplifier U2B in the amplifier circuit 70 to obtain the amplitude modulation index that limits AM overshoot, resulting in the maximum amplitude modulation depth. The automatic gain control circuit 40 detects the negative half-cycle signal from the AM audio modulation voltage, which changes synchronously with the voice signal, using diode D1. The amplitude modulation control signal output from the second output terminal of the microprocessor 50, combined with the detected negative half-cycle signal, adjusts the amplitude of the input audio signal to keep the AM audio modulation voltage within a set range. This ensures that the amplitude modulation does not suffer from severe distortion within a certain depth range.

[0048] Specifically, when the detected negative half-cycle signal acts on transistor Q3 of the automatic gain control circuit 40, the collector of transistor Q3 is connected to the base of transistor Q2 to achieve synchronous control of transistor Q2. The collector of transistor Q2 is connected to the MIC-IN input terminal. When the amplitude modulation of the negative half-cycle signal deepens, transistor Q3 controls transistor Q2 to linearly attenuate the audio signal at the MIC-IN input terminal to a certain extent, thereby reducing the modulation depth and avoiding severe distortion caused by amplitude overshoot in the input signal range that is 20dB higher than the standard 60% modulation depth.

[0049] for Figure 3In existing technical solutions, if the load current is 1A, the load voltage is 6V, and the supply voltage is 12V, then the load power is 1*6=6W, and the total power provided by the power supply is 1*12=12W, with an efficiency of 6 / 12=50%. However, with the circuit of this invention, if the load current is 1A, the load voltage is 6V, and the supply voltage is 12V, the load power is 1*6=6W. According to the PWM DC / DC step-down principle, the efficiency can reach approximately 80-90%, so the total power provided by the power supply is 6 / (80%~90%)=6.7~7.5W, significantly improving efficiency. The above are merely specific embodiments of this invention, but the design concept of this invention is not limited thereto. Any non-substantial modifications to this invention using this concept should be considered as infringing upon the protection scope of this invention.

Claims

1. An amplitude modulation circuit for a walkie-talkie, characterized in that: The system includes an audio processing circuit, an RF power amplifier circuit, a PWM pulse modulation circuit, an automatic gain control circuit, and a microprocessor. The audio processing circuit processes the input audio signal and outputs an AC audio signal. The microprocessor outputs a constant voltage at its first output terminal, which is then superimposed and merged with the AC audio signal before being sent to the PWM pulse modulation circuit. The PWM pulse modulation circuit outputs an AM audio modulation voltage that changes synchronously with the audio signal to the RF power amplifier circuit. The automatic gain control circuit detects the AM audio modulation voltage to obtain a negative half-cycle signal and, in conjunction with the amplitude modulation control signal output from the second output terminal of the microprocessor, adjusts the amplitude of the input audio signal so that the AM audio modulation voltage is within a set range. The RF power amplifier circuit modulates the obtained AM audio modulation voltage onto the input transmit carrier signal, amplifies it, and then outputs it.

2. The walkie-talkie amplitude modulation circuit as described in claim 1, characterized in that: The automatic gain control circuit includes transistors Q2 and Q3, resistors R2, R3, R6, R20, R23, R16, R21, diode D1, and capacitors C8, C10, C11, and C9. The base of transistor Q2 is connected to one end of resistor R3, and its collector is connected to the audio processing circuit. Resistor R6 is connected to the other end of resistor R3 and one end of resistors R20, R2, C8, and C10. The other end of resistor R20 is connected to the collector of transistor Q3. The base of transistor Q3 is connected to one end of resistor R23, and the other end of resistor R23 is connected to the anode of diode D1. The cathode of diode D1 is connected to the RF power amplifier circuit to detect the negative half-cycle signal. The emitter of transistor Q2 is connected to one end of resistors R16, R21, C11, and C9.

3. The walkie-talkie amplitude modulation circuit as described in claim 1, characterized in that: It also includes an amplifier circuit connected between the automatic gain control circuit and the second output terminal of the microprocessor to amplify the amplitude modulation control signal to obtain the amplitude modulation index. When the amplitude modulation of the negative half-cycle signal is deepened, the automatic gain control circuit controls the amplitude of the audio signal to be linearly attenuated.

4. The walkie-talkie amplitude modulation circuit as described in claim 1, characterized in that: It also includes a buffer amplifier circuit, which is connected to the output terminal of the audio processing circuit and the first locking terminal of the microprocessor to superimpose, merge and amplify the constant voltage output from the first output terminal of the microprocessor with the audio AC signal.

5. The walkie-talkie amplitude modulation circuit as described in claim 4, characterized in that: The constant voltage output from the first output terminal of the microprocessor is amplified by the buffer amplifier circuit and then sent to the PWM pulse modulation circuit, so that the PWM pulse modulation circuit outputs a constant voltage and sends it to the radio frequency power amplifier circuit to power it.

6. The walkie-talkie amplitude modulation circuit as described in claim 5, characterized in that: The buffer amplification circuit includes amplifier U2A, resistors R5, R7, R8, R10, R19, R26, R28, R29, capacitors C3, C4, and C28. One end of resistor R7 is connected to the first output terminal of the microprocessor, and the other end of resistor R7 is connected to one end of resistor R8 and capacitor C3. The other end of resistor R8 is connected to one end of capacitor C4, resistor R5, and resistor R10. The other end of resistor R10 is connected to the positive input terminal of amplifier U2A and the audio processing circuit. The negative input terminal of amplifier U2A is connected to one end of resistors R28 and R19. The other end of resistor R28 is connected to the output terminal of amplifier U2A and one end of resistor R29. The other end of resistor R29 is connected to one end of resistor R26 and capacitor C28. The other end of resistor R26 is connected to the PWM pulse modulation circuit.

7. The walkie-talkie amplitude modulation circuit as described in claim 6, characterized in that: The PWM pulse modulation circuit includes a modulation chip U1, which has an FB pin. The FB pin is connected to one end of resistor R33, and the other end of resistor R33 is connected to one end of resistors R30 and R31, as well as the other end of resistor R26 in the buffer amplifier circuit. The formula for calculating the DC voltage without audio modulation output by the PWM pulse modulation circuit is as follows: ； Where VA is the voltage output from the output terminal of amplifier U2A, and VFB is the internal reference voltage of modulation chip U1.

8. The walkie-talkie amplitude modulation circuit as described in claim 1, characterized in that: The audio processing circuit includes a baseband chip, which performs gain adjustment, pre-emphasis, and filtering on the audio signal.

9. The walkie-talkie amplitude modulation circuit as described in claim 8, characterized in that: The output of the automatic gain control circuit is connected to the input or output of the baseband chip.

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