Next is a resonance stage and the final stage built with a minimum 1W transistor which must have a heatsink. The VCO (Voltage Controlled Oscillator) in this IC uses a Colpitts oscillator circuit. You must see it as a quick and dirty 5-hour effort, because that’s all the time the transmitter took to design, build, and test. The PLL circuitry of the IC is so precise that there is practically no frequency drift. The circuit is based on BH1417 PLL stereo transmitter IC from Rhom semiconductors. Sometimes they dont even tell you which inductance is required and you have to calculate an estimate, which is the main reason why many high frequency RF projects fail in the first place.
Then there is a final level adjustment, used to set the deviation of the transmitter, and then a buffer stage with low output impedance, that drives the output through a resistor to avoid instability from capacitive loads. Jumpers allow to disable each of these signals for testing and adjustment purposes. A rather old fashioned, but low-noise and low-distortion analog multiplier chip modulates the L-R signal, produced by an op amp differential amplifier, onto the 38kHz subcarrier. Resistor R3 serves as the collector load for the transistor Q1. The modulated RF output is available at pin 11 and it is fed to the antenna through a filter network consisting of components L2, L3, C20, and C21. The job of this filter network is to remove harmonics. The whole operation should not take more than 5 seconds. The Amp offers 15 dB in the area of 80Mhz to 110 Mhz. L4, L5, and L6 are 5mm diameter air coils, 8 turns, with wire 1mm wire diameter.An easy project, with great results. Eight available frequencies are controlled by grounding S1, S2 and S3 pins on header connector.
The circuit is designed around the BA1404 single chip FM stereo transmitter from ROHM. The IC requires only a small number of external parts so it is well suited for hobbyist projects. The output signal from the emitter of the transistor is coupled to the input of the power amplifier transistor using the coupling capacitor. As this signal is amplified, the variable capacitor in the power amplifier section tends to maintain an output matching with that of the oscillator. Transmission range is 100 meters but can be increased with better antenna or RF amplifier. Then connect the circuit to the power supply and connect the positive lead of your multimeter to pin 8 of the IC and the negative lead to ground. Resistors R8 to R11 are the pull-up resistors for the D0 to D3 (pins 15 to 18 of the IC) respectively.