Homemade FM wireless microphone circuit for teaching

The wireless FM microphone produced by the author is based on Q5337 as the core, plus a first-level low-frequency amplification and RF power amplification circuit (which can improve the sensitivity of the microphone and RF transmission power). This FM microphone is used for classroom teaching in the large classroom of 250m2 in my unit. It has been used for 6 years and the effect is very good. This microphone has high speech clarity, and several main measures have been taken: The signal output by the MIC is first sent to the BG1 tube for amplification, of which R1 and C1 are additional treble pre-emphasis circuits. C2 and C3 are the input and output coupling capacitors of the BG1 tube. Their values ​​are smaller to attenuate the bass and enhance the mids and highs. The circuit of the diodes D3, D4, C4, and R7 connected in reverse parallel at the output end of the BG1 tube uses the characteristic that the internal resistance becomes smaller when the diodes are forward-conducted to limit the amplitude of the strong signal, while the normal intensity signal is not affected. It also has a good suppression effect on howling caused by positive feedback between the microphone and the speaker. After the microphone signal is amplified by BG1, it is added to the varactor inside the IC through L5, and the high-frequency signal is frequency modulated to obtain a larger frequency deviation. C7, C8, C9, and L1 form an FM signal tuning circuit, and its operating frequency is between 88MHz and 108MHz. The high-frequency signal output from the first pin of the IC is tuned and frequency-selected by L2 and C10, then sent to C11 and then coupled to the BG2 tube for radio frequency amplification (BG2 can use a general UHF tube), and then the FM microphone signal is radiated into the space. The whole machine is housed in the casing of a pocket semiconductor radio. The MIC is led out with an 80cm long single-core shielded flexible wire. This microphone lead also serves as a transmitting antenna. The high-frequency signal output by C13 is isolated from the ground by the inductor L4 and connected to the outer layer of the shielded wire. The MIC is mounted in a suitable latex tube and secured to the latex tube with a tie clip. When in use, clip the microphone to your chest near your collar, hang the machine on your trouser belt, and spread out the microphone wire to achieve the best transmission effect. For L1, L2 and L3, use an enameled wire of about ∮0.5mm to wrap 5 times around a round rod with a diameter of 5mm. There is a tap on L2. Ordinary small color coded inductors can be used for L4, L5 and L6. When debugging, first adjust the tightness of L1 so that the radio can receive the signal emitted by the FM microphone in the FM segment, and then adjust C16 to make the signal stronger. Finally, shorten the radio antenna and adjust it to L3 to maximize the transmitting distance. If a simple field strength meter ("Electronic News" has been introduced many times) is used for debugging, the best effect can be achieved. The frequency of this machine is stable and will not drift after being used for several months after being adjusted once. When I use a 4.5V power supply, there is no directionality within the transmitting-receiving distance of 25 meters. When listening to an FM radio, it feels like an FM radio station.