FM demodulation problem

Hi,

I've designed a FM demodulator with (IdQ-QdI)/(I=B2+Q=B2).

My FM modulator is center on 70 MHz with 250 kHz deviation.

The demodultor works fine from 10 kHz to 250 kHz (message)... but under
10 kHz I lost a lot of amplitude in my IdQ-QdI signal...

Is it because i made the derivative by x[n]-x[n-1] ? Is it working
better with a FIR filter with 3 taps (-1,+1,0) ?

If yes what are the location of the coeffcients ? (-1,+1,0) or
(+1,-1,0) or( 0,-1,+1) ??

I think with lower rate (< 10kHz) my derivative is too small in
amplitude that at higer rate (> 10 kHz) ?

Thanks

I found my stupid problem !!!!!!!!!!

I forgot a capacitor on my DAC ouput, so the low frequency don't pass
!!!!!

Now it work fine from 1 Hz to 250 kHz with no problem

SORRY

patrick.melet@dmradiocom.fr wrote:
> I found my stupid problem !!!!!!!!!!
> 
> I forgot a capacitor on my DAC ouput, so the low frequency don't pass
> !!!!!
> 
> Now it work fine from 1 Hz to 250 kHz with no problem
> 
> SORRY
> 
Sometimes it seems that the only way to find one of these obvious 
answers is to expose yourself to public embarrassment by asking for help.

It certainly happens to me often enough.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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