Let me tell you how to make the mini fm stereo transmitter

The use of this dedicated FM stereo transmitter IC chip BHl416F, compact machine, 5V single power supply, the whole current of 31mA; can be used for stereo wireless microphone, CD sound source emission or low-power FM stereo radio.

BHl416F is an FM stereo transmitter IC, with the following features: Built-in pre-emphasis, limiter, LPF circuit, excellent sound quality; integrated into the stereo pilot signal from the signal of the full circuit; the use of PLL frequency synthesizer, transmitter frequency is stable ; PLL oscillation frequency from the four I / O pin is set in the range of 70MHz ~ 120MHz step 100kHz to set, up to 14 frequency points; operating voltage 4V ~ 6V; audio bandwidth of 20Hz ~ 15kHz.

See the whole circuit drawings. Op amp IC2a and IC2b respectively, for R, L channel microphone amplifier, magnification 11, the gain 20.8dB.

Stereo audio signals and microphone signals are sent from the IC2c, d consisting of mixed-level, and then input to the ICl.

In IC1, the audio signal is first pre-emphasis, to emphasize the high frequency components; then limiting to prevent over-modulation; then filtered by the low-pass filter components than 15kHz (the actual cut-off frequency of the circuit 18.4kHz). 7.6MHz crystal oscillator reference frequency generator for the oscillator, the output frequency divided by 4 and get 50 points after the 38kHz sub-carrier frequency signal provided to the MPX circuit, for the DSB is modulated LR signal superimposed to the L + R signal. 38kHz signal is then divided by 2 to obtain 19kHz pilot signal, and the MPX output summing output from IC1 ⑤ feet. 7.6MHz signal frequency divided by 4 and 19 100kHzPLL dividing the reference frequency. In part by the PLL phase detector, with the VCO LC oscillator, RF amplifier, programmable counter, and the connection for an external phase detector output Trl, loop filter, VCO and frequency of the resonant circuit selector switch. Feet from the ICl ⑤ synthetic signal obtained by VRl adjust the level, then by R9, C20, R13, C23, R12, R10 filter consisting of filter, to improve the channel separation. High-frequency oscillator circuit OSC by the internal and external varactor diode D1 L1 and composition, L1 unlined tin copper wire with Φ0.5mm between around 5 turns, diameter 5mm.

From IC1's (11) pin output RF signal, the 50Ω load up on 101dBμPD (112μVPD), equivalent to 0.25mmW (about-6dBm), not enough to launch long-distance, so increase the level high-frequency amplifier circuit, with double pass filter (BPF, to filter out high-order harmonic components of high frequency amplification Tr2's emitter current in the absence of signal is 4mA; collector resonant inductor L3 with Φ0.6mmm keep out tin copper wire 6 around between turns, diameter 6mm, near the end of two turns at the +5 V output tap for the resonant frequency is adjusted by the semi-variable capacitor TCl; if the abnormal oscillation, you need to adjust the wiper position.

Output carrier frequency set by the switch SW1, set, see Table. But preferably in FM radio frequency band, the radio in order to avoid interference. In practice, the need to adjust the length or diameter L1 to change the inductance. First, the SW1 switch set to "0000", then the frequency should be 76.8MHz, varactor diode voltage VT at below 1V, if the output frequency high, you should compress the length of L1; then SWl switch set to "1000 "Then VT about 5V, the output frequency should be 88MHz, if the frequency is low, we must lengthen the L1. If the adjustment is invalid, you can change the diameter of L1, such as to diameter 5.2mm, the frequency range up to meet the requirements.