I have a problem to understand how to design a low pass filter for demodulation, using in AM radio reciver.
what are we looking for to design a good low pass filter in demodulation? and how the cut off frequency effect quality of the signal. please help

 

for a low pass filter the signal passes through a resistor to the next stage, and through a capacitor to ground. As the capacitor value goes up,lower frequencies will pass through it, a smaller value cap will pass higher frequencies.

 

I have a problem to understand how to design a low pass filter for demodulation, using in AM radio reciver.
what are we looking for to design a good low pass filter in demodulation? and how the cut off frequency effect quality of the signal. please help

Hi Rark:

It's been a long time since I've heard a good question about this. There's more to it than meets the eye (and ear!)

It's important to have a good low-pass filter after the detector diode, as a little bit of R.F. leakage can cause tremendous distortion on the following amplifier stages.

HOWEVER.......and this is a BIG however.....if the RC time constant of the lowpass filter is too high, you encounter a most obnoxious form of distortion called "diagonal clipping". It's also called "failure to follow" distortion. This is caused when the discharge cycle of the filter is too long to follow the envelope of the detected R.F. carrier. When this happens, the diode "falls out" of conduction at the most inopportune times...particularly on high frequency modulation peaks.

The ideal time constant for the filter is a function of the R.F. bandwidth AND the modulation percentage. I don't have the formula right with me, but I'll look it up when I get home. (This is a real blast from the past!)

One solution to reduce diagonal clipping is to use a FULL WAVE detector. This allows you to use twice the R.f. bandwidth before the detector before diagonal clipping occurs (all other things being equal).


Hope this helps. Look up diagonal clipping references on the web, as well.

Eric

 

Here's a good demonstration of diagonal clipping. And the magic formula as well!

http://www.ece.mtu.edu/labs/EElabs/EE3305/AM_receiver.pdf


Eric

 

Hello Eric
Thank you so much, your answer is really helpful and it is what I'm looking for.