Hi, I'm trying to build a FM voice audio circuit from discrete components with the LM555 as an oscillator . I get how to modulate the input wave with the 555, but how would you demodulate/ get a modulated sine wave from the square output? My first idea was a sharp low pass filter to remove all the odd harmonics, but the filter would depend on the frequency--which is changing, right?
Does anyone have any ideas?

Thank you very much!

 

the frequency--which is changing, right?

Usually the modulation changes the frequency only slightly.
Perhaps use a fixed frequency and modulate the pulse width? Fig 18 of the Texas Instrument datasheet for the 555 shows how it's done.

 

http://www.radio-electronics.com/in...eption/fm-demodulation-detection-overview.php

 

It would not take much to get the full deviation required for the FM broadcast band using an NE555. The problem would be one whether there is enough energy left in the 100th or so harmonic to be useful for the receiver.

You might have to resort to using a frequency multiplier ahead of your bandpass filter.


This has been done, but the example I saw was in the AM broadcast band and the AM receiver was used to slope-detect the signal.