Connect FSK modulator with fm transmitter circuit?

Thread Starter


Joined Apr 15, 2019

I am wondering how to superimpose a square wave input signal onto a fm carrier signal?

Using this circuit as an fm transmitter

and then this is the modulator

I've given a lot of thought and research for this project. It is for school. One thing I have noticed is, the fm transmitter uses a microphone as a varying resistor which modifies the base current to the transistor. There is some parasitic capacitance, so this varies the fm carrier signal.

My issue is two things.
1. You are varying the input base current of a transistor with a voltage divider. Replacing the input base current with an oscillatory signal and much larger peak to peak will change this a lot. Is it possible to replace the voltage divider with an oscillatory input for it to perform the same function?
2. The modulator outputs square waves from 0V to +5V. This is an issue because this causes the transistor to be effectively "ON" or "OFF". Will this affect the overall functionality of the circuit?


Joined Mar 10, 2018
An asumption on my part, that you want to do both ends of a FSK communications link ?

If you want to decode this generally speaking you need accurate f1 and f2. A 555
timer not the best approach for this, doable, but not the best.

Google "lm555 fsk" lots of hits. Use a PLL to decode, like a

Another approach is - this both generates and decodes
an FSK stream.

Project files

Regards, Dana.
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