Subject: Re: Zero Crossing Detectors, FM Demodulation
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Mon, 02 Dec 2002 04:31:44 GMT
NNTP-Posting-Date: Sun, 01 Dec 2002 23:31:44 EST
The easiest way to demodulate an FM signal based on the zero crossings is to
feed it into a monostable circuit input. I assume the 122KHz signal is a
rectangular pulse. If not, it is relatively easy to make it.
The period of the monostable should be slightly lower than the minimum
signal period. I assume that the monostable is triggered by say the rising
Let's see what the monostable period would be...
Firstly, the max frequency would be 122+5*5=172KHz
The period of the 172KHz signal is 1/172KHz=5.81us
I guess a 5us monostable would do the trick. Take a look at the 74121
Now, in order to get the demodulated signal, just filter the monostable
output with a simple low pass filter which cutoff frequency is significantly
lower than 122-5*5=97KHz, but higher than the highest freq in your
modulation spectrum. The filter could be a simple RC or something more
elaborated, depending on what you want.
The main disadvantage of the zero crossing FM detector is that the gain and
S/N ratio are very low. But they are realy easy to implement.
Sorry no schematic, but if you find the above difficult to follow, I can
give you more details. I need more details too :-)
"morphiend" wrote in message
> I am working on a project for a class I am currently enrolled in and
> need to construct a zero-crossing detector FM demodulation circuit.
> I know the specifications of the incoming FM'd signal:
> fc = 122kHz.
> fm = 5kHz.
> deviation/modulation of 5.
> I was wondering if anyone had any circuit diagrams that contained such
> a device and the constraints for tuning it to recieve the
> aforementioned signal.
> Mike Koss