It's been long time I haven't done any radio communication circuits. I spent lots of time in building AM transmitter and FM transmitter circuit. When I was engineering student, we didn't had enough time to do the practical implementation of the radio. During the college time, I spent lot of time reading communication related theories but not how to actually build one. I remember Simon Hykin books which was our textbook but really the book was too much theoretical. No practical example circuits, just block diagram representing modulators, mixers, VCO, PLL etc. For example in the analog modulation chapters, like amplitude modulation, frequency modulation, there was no actual schematic of modulation and mixing and the RF filters etc. The book explained with block diagram like a square law modulator was drawn in boxes. No inside of how actually the circuit works. After my engineering degree, I searched internet and studied the communication circuits like modulator, mixers and I have built few of them.
Here the input is the audio signal using the electret microphone as shown in the FM transmitter schematic diagram above. The resistor R1 is for turning on the electret mic, without voltage applied to it via the resistor R1, the mic won't function. The audio from the mic enters into the self biased pre-amplifier circuit made up of the BC547 Q1 transistor, R2 and R3. The resistors R2 and R3 are biasing resistors and the biasing method used for the pre-amplifier is called self biasing. The collector output is fed back into the base of the Q1 transistor in order to put the Q1 transistor in active region. The capacitors C1 and C2 are just coupling capacitors, which blocks dc signal and allows low frequency audio signal to pass through it. R3 resistor is the load of the pre-amplifier circuit and we can call it audio load. After amplification of the microphone audio signal, the amplified signal is passed into the actual oscillator with frequency modulator which is made up of the another BC547 BJT transistor Q2. The RF oscillator circuit comprises of Q1, R4, R5, C4,C5,C6 and the LC tank circuit made up of variable capacitor VC1(wrong symbol in the schematic, i forgot to put the variable capacitor symbol) and inductor L1. Their LC values depends upon the frequency of the carrier signal it will generate. The formula for LC circuit is,
\(f = \frac{1}{2 \pi \sqrt(L,C)}\)
The oscillator circuit is a Colpitt oscillator variant, where C5 and C6 forms the feedback network structure like in standard Colpitts oscillator which are required for sustained oscillation. As you can see, part of the signal at the collector of Q2 goes back to its emitter via the C5 and C6 feedback circuit. This feedback signal is required for continuous oscillation to occur. The capacitor C4 is important to provide stability to the RF colpitts oscillator circuit. C2 capacitor is just coupling capacitor that couples the amplified audio signal from the pre-amplifier circuit to the base of the oscillator circuit so that the DC operating point is not disturbed.
The RFC(Radio Frequency Choke) inductor is used to prevent loss of ac signal in the oscillator while providing the DC voltage to the RF oscillator.
This video below explains in much greater details how the two transistors FM transmitter works.
