AM digital demodulation methods
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From: Jerry Avins on 23 Apr 2010 16:34 On 4/23/2010 4:24 PM, Tim Wescott wrote: > Jerry Avins wrote: >> On 4/23/2010 1:52 PM, gretzteam wrote: >>>>> Use it, but understand it. Understand the implication of in-band >>>>> interference. Understand the need to exclude out-of-band signals from >>>>> the demodulation process. (The baseband low-pass filter can't remove >>>>> aliases.) >>>> >>>> I am assuming that he is properly prepping the signal prior to the >>>> multiplication by sin/cos and will pick appropriate filters at >>>> baseband. >>> >>> >>> Ok I must admit that I'm more confused than before! Why do you still >>> need a >>> bandpass filter for method 2? Isn't multiplying by sin/cos shifting the >>> carrier frequency to DC? >> >> What Brent said. Keep in mind that you not only shift the carrier to >> baseband, you also shift everything else down by a similar amount. >> Where do the aliases of the out-of-band signals go? >> >>> About method 1 having the problem of peak values not being close to full >>> scale, can we say that this is not a problem when fs>> carrier? >> >> When the carrier is adequately oversampled, method 1 works. I leave it >> to you to determine what "adequate" means. How many samples per >> carrier cycle are needed to ensure that one is at least 95% of either >> peak? Is that a reasonable expenditure of resources? > > Except that by his original description he's not peak-seeking -- he's > averaging the absolute value. That _ought_ to work better, but I don't > know by how much. How does the average value of samples of the carrier relate to the approximate value of the envelope? Would it help if the "carrier" were triangular? Jerry -- "It does me no injury for my neighbor to say there are 20 gods, or no God. It neither picks my pocket nor breaks my leg." Thomas Jefferson to the Virginia House of Delegates in 1776. ���������������������������������������������������������������������
From: Jerry Avins on 23 Apr 2010 16:43 On 4/23/2010 4:32 PM, Vladimir Vassilevsky wrote: > > > Clay wrote: > > >> This is a wonderful example of how a trivial method in analog (diode >> and RC filter) is not really how you want to do it in digital. Now if >> he has an analytic signal .... > > Except for the diode detector doesn't work like the simplified textbook > desccription. An accurate analysis requires involved math. Except for diagonal clipping with deep modulation and the limited charging current, what is missing from the classical analysis? Whenever it was in my control, I used a full-wave peak detector. That suppresses the IF in the output, leaving the weaker second harmonic to dominate. You then need only a much smaller capacitor, making the onset of diagonal clipping more remote. Besides, 456 KHz can get through many Hi-Fi power amps, but the response at 902 is much less. Jerry -- "It does me no injury for my neighbor to say there are 20 gods, or no God. It neither picks my pocket nor breaks my leg." Thomas Jefferson to the Virginia House of Delegates in 1776. ���������������������������������������������������������������������
From: glen herrmannsfeldt on 23 Apr 2010 16:16 Clay <clay(a)claysturner.com> wrote: (snip) > This is a wonderful example of how a trivial method in analog (diode > and RC filter) is not really how you want to do it in digital. Now if > he has an analytic signal .... I was almost going to mention that square wave demodulation is commonly used for the usual laboratory "lock-in amplifier", but again that is analog. The higher harmonics are normally just filtered out, but maybe not in the digital world. -- glen
From: gretzteam on 23 Apr 2010 17:03 > > >How does the average value of samples of the carrier relate to the >approximate value of the envelope? Would it help if the "carrier" were >triangular? > Yes you have a point here! All I've proven so far is that when the input signal contains only a carrier, full scale, then the output of the lowpass filter is pretty much exactly 0.63 (2/pi), which is the average value of a full scale sine wave. I was pretty happy to see this, but that's probably not AM demodulation just yet! But isn't this what the Analog version does when using bandpass, full wave rectifier and capacitor? Thanks
From: Vladimir Vassilevsky on 23 Apr 2010 17:05
Jerry Avins wrote: > On 4/23/2010 4:32 PM, Vladimir Vassilevsky wrote: > >> >> >> Clay wrote: >> >> >>> This is a wonderful example of how a trivial method in analog (diode >>> and RC filter) is not really how you want to do it in digital. Now if >>> he has an analytic signal .... >> >> >> Except for the diode detector doesn't work like the simplified textbook >> desccription. An accurate analysis requires involved math. > > Except for diagonal clipping with deep modulation and the limited > charging current, what is missing from the classical analysis? Even with ideal diode and ideal source, the angle of conduction is determined by transcendental equation. Now account for impedance, nonlinearity and add noise, and it gets really messy. > Whenever it was in my control, I used a full-wave peak detector. That > suppresses the IF in the output, leaving the weaker second harmonic to > dominate. You then need only a much smaller capacitor, making the onset > of diagonal clipping more remote. Besides, 456 KHz can get through many > Hi-Fi power amps, but the response at 902 is much less. Jerry, do you know what was the rationale for choosing 455kHz vs 465kHz standard IF ? VLV |