I made the wireless FM microphone is Q5337 core, plus a low frequency amplifier and RF power amplifier and other components (which can increase the sensitivity of the microphone and RF transmission power.) The FM microphone in my unit 250m2 for a large classroom teaching, has been in use for 6 years with good results.
The higher voice clarity microphone, mainly taken several measures: MIC output signal is then sent BG1 tube amplification, in which R1 and C1 is an additional treble pre-emphasis circuit. C2 and C3 are the input and output BG1 pipe coupling capacitor, the value used small attenuation of the bass in order to enhance the alt. BG1 tube output reverse parallel diode D3, D4 and C4, R7 of the circuit, is the use of the forward diode resistance becomes smaller when the characteristics of the strong signal from the limiter function, and normal strength of the signal is not affected, while between the microphone and speaker on the positive feedback caused by the whistle has a good inhibitory effect. Microphone signal is amplified by the BG1 through L5 added to the internal IC varactor, the frequency of high-frequency signal modulation, the larger the deviation can be obtained.
C7, C8 and C9, L1 tuning circuit composed of frequency modulated signal, the operating frequency between 88MHz ~ 108MHz. {6} IC feet first high frequency signal output by the selected frequency tuning L2 and C10 C11 and then coupled to the BG2 evacuation tube RF amplifier (BG2 available UHF general management), the frequency of the microphone to the space radiation signal. Machine installed in a pocket-sized transistor radios of the enclosure. MIC 80cm long with a single core shielded cord leads, this doubles as a microphone wire antennas. C13 output and high-frequency signal inductively isolate L4, received the outer shield. MIC mounted on a suitable latex tube, and then a tie clip with latex tubes fixed together. Use the microphone caught in the chest near the collar at the machine hanging on the belt, so that the microphone cable to start, the launch was the best.
L1, L2, L3 with ∮ 0.5mm magnet wire around the rod 5mm in diameter on a circle around the 5, L2 on a tap. L4, L5 and L6 ordinary small Inductors available. L1 debugging to emphasize the tightness, so that the radio section of the FM transmitter microphone can receive the FM signal, and then adjust the signal stronger C16. Finally, tune the radio antenna shortened L3, so that emission from the farthest. If simple field strength meter ("newsletter" has been introduced several times) with the commissioning, to transferred to the best results.
Frequency stability of the machine, a few months after moving, will not use drift. I use the 4.5V power supply, transmitter - receiver distance of 25 meters is no direction, with the FM radio listening, I feel like a FM radio station.