Analysis the component role and FM frequency

[daisyyyyyy]

Hi, I get a schematic design above for a FM transmitter. However, I have no idea with the role of each component except some of them and how to determine its value. The most confusing is why there are 2 tank circuits? What are their purpose and which tank circuit should I use to calculate my the FM frequency?

 

[Audioguru]

The circuit is similar to the one I fixed. TR1 is biased wrongly because it is saturated when the battery is new and/or the transistor has high hFE. TR1 is cutoff when the battery is low and/or the transistor has low hFE.
The value of C1 is too low so it will cut all low audio frequencies and pass squeaky sounds.

TR1 is the mic preamp transistor. TR2 is the RF oscillator transistor and C5 provides the positive feedback. TR3 is an RF amplifier that isolates the tuned LC in the oscillator from the antenna. You need to know about transistor circuits to know the function of each component and how to calculate them.

It is difficult to calculate the frequency because stray capacitance from the layout and wiring and from Q2 change the frequency. C4 and C7 should be adjustable trimmer capacitors then C4 can tune the frequency and C7 can peak the output to max at that frequency.

The circuit will sound muffled on an FM radio because it is missing pre-emphasis (look it up) and all FM radios have matching de-emphasis.

 

[daisyyyyyy]

Thanks for answering my question. I'm facing what you mentioned.

Squeaky sounds pass well, but bass is somehow missing. After testing with FM radio, I can hear the output but really full of noise.

I receive the output around 97Mhz. Although it is hard to calculate, if I want to check whether it is the frequency from antenna, then which tank circuit should I use to calculate? Is C4L1?

 

[Audioguru]

Bass sounds are missing because the value of C1 is much too small and cuts frequencies below about 1600Hz. Use 1μF for frequencies down to about 35Hz.

The radio frequency is calculated with L1, C4, C5, the capacitance of TR2, some of the capacitance of TR3 and stray wiring capacitance.