Question

Given the schematic of an electret microphone fm transmitter,

http://eee3223.wdfiles.com/local--files/zk50group01/fm_trasm.gif

What will I need to change to make it adjustable on a wide transmission range. Which component will affect the "gain" of the microphone input?

Answer 1

There are 2 "gains" there: the 1st works on microphone sensitivity with Q1 transistor, while the 2nd affects the transmitter output power that Q2 transistor transfers to the antenna.

Answer 2

So if I wanted to make it more sensitive on the microphone's input, I'd need to change the transistor? I thought it would be one of the resistor values.

I heard to change the output frequency, you'll need to adjust the coil.

Answer 3

@Agentjamesbond007
You're right, indeed. In order to change the gain of the microphone amplifier (Q1) you can play on the ratio R4/R5: with a greater ratio you obtain an higher gain; but take care, do not change to much absolute values of resistances because they also affect transistor biasing.

Answer 4

So If I was to build a prototype, I'd need to use a potentiometer for R4/R5?

Would the tuning capacitor (C4) changed the output frequency (Hertz)?

Answer 5

1) yes, you can use a potentiometer for one of the two resistances
2) yes, the frequency is:
\[f=\frac{ 1 }{ 2 \pi \sqrt{LC} }\]

Answer 6

Oh, I see. Now, if we keep the inductor (coil L1) constant, we'd just need to change the tuning capacitor (c4) to change the output frequency.

If I were to replace the electet microphone with a stereo audio plug, How would I connect the pinout? Would the left/right connexion go upward (to R1) and the ground connexion to downward (to ground)? Or will it not work at all?

Answer 7

@Agentjamesbond007
The stereo signal has to become a mono signal, that is you have to mix-up both channel, joining them at the input of the Q1 amplifer (C1). A possible circuit is the following one, where we suppose a typical PC audio board output with a 2V pk-pk output, on a 100 Ohm resistance (one channel only is depicted).
Note that R2 now is equal to R3, because the input is supposed to be a balanced signal.
The circuit is an high-pass active filter, with a cut-off frequency around 220 Hz and a gain around 12 dB, with a negligible distortion for an input signal with 2Vpk-pk amplitude and 1KHz frequency.
You may substitute R4 with a potentiometer to have greater gain on lower input signals, taking care to not cause distortion.