FM walkie-talkie circuit

The FM intercom circuit introduced here has an intercom distance of more than 100m in open areas and can be used as a wireless microphone in conjunction with an FM radio. The circuit is shown in the figure. Transistor V, inductor coil L1, capacitors C1, C2, etc. form a capacitor three-point oscillation circuit, which generates a carrier frequency signal with a frequency of about 100MHz. Integrated power amplifier circuit LM386 and capacitors C8, C9, C10, Cll, etc. form a low-frequency amplifier circuit. Speaker BL doubles as a microphone. When the circuit is working in the receiving state, put the receiving/receiving switch in the "receiving" position, and the signal received from the antenna ANT passes through the ultrasonic transistor V, the inductor L1, the capacitors C1, C2 and the high-frequency choke coil L2. The regeneration detection circuit performs detection. The detected audio signal is coupled to the input end of the low-frequency amplifier via capacitor C8. After amplification, it is coupled by capacitor Cll to drive the speaker BL to produce sound. When the circuit is working in the sending state, S2 is placed in the "sending" position, and the speaker turns the voice into an electrical signal. After being amplified by the IC at low frequency, the signal is added to the oscillation tube by the output coupling capacitors Cll, S2, R3, C4, etc. The base of V causes the bc junction capacitance of the tube to change with the change of the voice signal, and the bc junction capacitance of the tube is connected in parallel to both ends of L1, so the frequency of the oscillation circuit also changes accordingly, realizing the frequency modulation function. , and transmit the modulated wave from the antenna through capacitor C3. V selects silicon high-frequency low-power tubes with fT>=600MHz and B>=60, such as 3DG80, 3DG56, etc. L1 is wound 6 times with 0.8mm enameled wire flatly, with an inner diameter of 6mm, and then stretched into an air-core coil with a spacing of 1mm. L2 is made of 0.lmm enameled wire wound around a 1/8W, 100K resistor for 100 turns. C1, C2, and C3 use mica or high-frequency ceramic capacitors. S2 uses a four-pole two-position toggle switch. BL uses an electric speaker with a diameter of 5cm. The antenna uses a 0.8-meter rod antenna (when used as a wireless microphone, it can be replaced by a multi-stranded cord of the same length). The power supply uses a 9V laminated battery. The component parameters of the two walkie-talkies should be as consistent as possible. When debugging, first put S2 in the "receiving" position. At this time, the speaker should have greater noise. If the noise disappears when you touch the transistor case with your hand, it means that the receiving circuit is working basically normally. Then put the S2 in the "sending" position, put an FM radio nearby, and adjust the receiving frequency to about 100MHz. At this time, there should be a loud howling sound in the radio, and the howling sound will disappear after about 10 meters away. , speak into the microphone, and you should be able to hear a clear and loud voice on the radio. If there is no sound or the sound is low, you can adjust the frequency of the radio. After the two walkie-talkies have completed the above-mentioned debugging, conduct an interoperability test and appropriately adjust the spacing of L1 so that both reception and transmission can be unified on the same frequency. When the frequency overlaps with the local radio station, the resonant capacitor C1 needs to be replaced to prevent mutual interference and affect normal use.