I started a thread re: AM demodulation using I and Q a few weeks ago
and I am still having a problem. According to Frerking, AM should be
A=sqrt(i^2+q^2). This works if I tune off the AM zero beat frequency
by 1/2 the low pass filter bandwidth. I don't understand this
A little more detail:
I am using a pair of mixers fed with a sin/cos local osc. to generate
I/Q at baseband. SSB demod works fine for upper and lower sidebands
with excellent opposite sideband rejection. I can zero beat the AM
carrier and receive upper, lower, or both sidebands to stereo output.
I am using a DDS LO and I am able to accurately zero beat the AM
carrier to less than 1hz. When I do this while listening to both
sidebands or I/Q in stereo, I can hear the sidebands come into phase
as I tune the radio.
Once I have the signal zero beat, I flip it into the AM algorithm and
I hear a great deal of distortion. Assuming I use a low pass filter
(actually a bandpass) from 30-4000hz, prior to the sqrt(i^2+Q^2) math,
if I move off zero beat by 2000hz, the AM sounds great. I can move
either higher or lower by 2000 with the same results. It almost seems
like the filter is acting like a mixer??
I'm missing something here. When I modeled this receiver using Matlab
Simulink, it works fine at zero beat. If I get this simple
demodulator working I am going to move onto synch AM. I actually like
listening to the independent sidebands in stereo since it takes care
of some of the fade.