AM digital demodulation methods
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From: brent on 25 Apr 2010 07:07 On Apr 25, 1:03 am, Jerry Avins <j...(a)ieee.org> wrote: >Synchronous demodulation does improve on peak detection, but not on sqrt(I^2+Q^2). I think you are wrong on this point. I am in this group primarily because I have responsibility to "maintain " a software radio product that we bought from another company. So I have had to try to learn DSP to maintain understand and maintain this radio. I am not yet capable of designing a radio like this, but I am getting closer as time goes by. Anyhow, In their design they use a digital PLL. They run a FFT to analyze the carrier and then build an PLL around the signal to get synchronous detection. If the carrier gets squirrely, the PLL will break lock, and if it does they revert to I/Q detection so as to not lose the signal. My point is, I know they did not go to great lengths to build thisFFT/ PLL detection scheme if I/Q demodulation would give the same result. I cannot tell you right now what the math is, but I know synchronous detection is better in some respects.
From: Jerry Avins on 25 Apr 2010 10:46 On 4/25/2010 7:07 AM, brent wrote: > On Apr 25, 1:03 am, Jerry Avins<j...(a)ieee.org> wrote: > >> Synchronous demodulation does improve on peak detection, but not on sqrt(I^2+Q^2). > > I think you are wrong on this point. That would hardly be novel! > I am in this group primarily because I have responsibility to > "maintain " a software radio product that we bought from another > company. So I have had to try to learn DSP to maintain understand and > maintain this radio. I am not yet capable of designing a radio like > this, but I am getting closer as time goes by. > > Anyhow, In their design they use a digital PLL. They run a FFT to > analyze the carrier and then build an PLL around the signal to get > synchronous detection. If the carrier gets squirrely, the PLL will > break lock, and if it does they revert to I/Q detection so as to not > lose the signal. > My point is, I know they did not go to great lengths to build thisFFT/ > PLL detection scheme if I/Q demodulation would give the same result. > I cannot tell you right now what the math is, but I know synchronous > detection is better in some respects. Without judging the merits of this particular scheme, I have to keep in mind other instances where designers -- Even Edwin Armstrong, the father of FM -- acted on superstition rather than fact. The scheme you describe leaves me with a question: If synchronous demodulation loses the signal even when I/Q demodulation tracks it, in what way is synchronous demodulation superior? Have you had a chance to compare the outputs of the two detectors when both are working? Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: gretzteam on 25 Apr 2010 11:31 >> Now, I'm not building an AM radio, and I'm mostly interested in very low >> frequency (up to 20Hz), but I need quite good performance. My sampling rate >> and carrier are fixed and won't change. This is not under my control and >> were decided because of other stuff that this system is doing. >> >> So another way to ask the original question: >> When taking the absolute value of a digital signal, what really happens? >> I'm trying to see this from a frequency domain perspective. > >I'd have to do the math -- a Laplace transform -- to give a definitive >answer even for a continuous signal, and the discrete-time case is more >involved. There seems to be a conflict among authorities. In the one >hand, the waveform is what one gets from a push-push doubler, and should >contain no odd harmonics, not even the fundamental. On the other hand, >the ITT Reference Data for Radio Engineers (4th edition; 1949) gives a >formula that I don't believe involving finite values of coefficients for >all harmonics, including the fundamental. > >> My original experiment - reinventing the wall-wart I guess - showed that >> the carrier is shifted to DC pretty well. > >I wouldn't say that. There's a strong second harmonic, so you could also >say that the carrier is shifted up. What we know is that the average >value is extracted and that harmonics are produced. Each of those >harmonics will exhibit sidebands. > >> I'm trying to get a feel for what >> happens to the sidebands, how much distortion is introduced when I move out >> of DC? I understand abs() is nonlinear, but there might be some analysis >> possible? >> >> Now, since multiplying by sin/cos is a perfect shift, I know the sidebands >> won't get affected, and my problem becomes designing a good lowpass filter. > >What do you mean by "not affected"? they get shifted along with the carrier. > >Jerry Yes but the sidebands don't get distorted at all during the sin/cos multiplication, not matter what other stuff is in the spectrum from 0 to fs/2, right? Everything just gets shifted, and performance will depend on the lowpass filter. However, can we say that for the abs() case? Can we say that the sidebands are shifted along with the carrier back to DC without distortion? I don't care if everything else becomes all distorted, cause it will get lowpass filtered. Thanks.
From: gretzteam on 25 Apr 2010 11:34 >My point is, I know they did not go to great lengths to build thisFFT/ >PLL detection scheme if I/Q demodulation would give the same result. >I cannot tell you right now what the math is, but I know synchronous >detection is better in some respects. Maybe they try to avoid I/Q demodulation because it involves two multipliers running 'fast', and twice the lowpass filtering? This is kind of what I'm trying to avoid, if I can get decent performance with another scheme. However, a bandpass filter is not free either, so maybe it all boils down to the same thing! Thanks,
From: Vladimir Vassilevsky on 25 Apr 2010 11:59
Jerry Avins wrote: > If synchronous demodulation loses the signal > even when I/Q demodulation tracks it, in what way is synchronous > demodulation superior? Have you had a chance to compare the outputs of > the two detectors when both are working? BTW, in the modern ICs, they demodulate AM in a quasi-synchronous way: the bandpass signal is digitized by comparator and the output of the comparator drives the analog switch which does synchronous rectification. The advantage is much better linearity compared to diode detector. Obviously this could be improved by adding a narrowband filter in front of the comparator; however it is not worth extra cost for consumer applications. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com |