AM digital demodulation methods
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From: HardySpicer on 24 Apr 2010 20:04 On Apr 25, 12:39 am, Jerry Avins <j...(a)ieee.org> wrote: > On 4/23/2010 5:56 PM, Vladimir Vassilevsky wrote: > > > > > > > Jerry Avins wrote: > > >> On 4/23/2010 5:33 PM, HardySpicer wrote: > > >>> On Apr 24, 8:24 am, Vladimir Vassilevsky<nos...(a)nowhere.com> wrote: > > >>>> HardySpicer wrote: > > >>>>> Synchronous demodulation using a PLL will give you 3dB improvement > >>>>> over ordinary envelope detection. > > >>>> This is wrong. > > >>> It's in the textbooks...read it! > > >> What is ordinary envelope detection? Peak detection? > > > Doesn't matter; It is very simple. Think of |I| vs sqrt(I^2 + Q^2) > > Still, when someone claims "3dB improvement", I want to know what is > improved upon. > > Jerry > -- > "I view the progress of science as ... the slow erosion of the tendency > to dichotomize." --Barbara Smuts, U. Mich. > ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ If I can remember that far back I believe it is in received SNR (baseband). Look up synchronous demodulation vs envelope detection. Of course you digital guys have I and Q and make things even more complicated. Do you remember when we just used a cats whisker! The I and Q method vs a PLL - I have no idea. How do you get I and Q - do you need a PLL to get I and Q? If so then I expect it is the same result. It was Taub and Shilling or some such that had the details but most older coms books will have the info. People are so locked into digital nowadays that they forget the basics which all comes from analogue. You cannot understand digital without understanding analogue first. I don't mean you Jerry of course but many of the new breed of engineer that gets taught exclusively digital. (does happen!!) For example, what is an exclusive OR in a PLL? To me it is a multiplier!! Hardy
From: Jerry Avins on 25 Apr 2010 00:52 On 4/24/2010 3:30 PM, gretzteam wrote: >> The size of the capacitor (a.k.a. lowpass filter) matters. >> >> Put a 100pF capacitor across your signal and you filter out stuff you >> can't hear (and presumably, in this context, don't care about). It >> won't have a noticeable effect on audio frequencies. (But you should >> check to see what impedance 100pF represents at your carrier and >> modulating frequencies.) >> >> Put a 1000uF capacitor across your signal hand and you filter out >> everything down nearly to DC; in other words, you've reinvented the >> wall-wart (power supply, AC->DC converter, etc.). (Calculate the >> impedance of 1000uF at your carrier and modulating frequencies.) >> >> So... design your trailing LPF to eliminate signals above (say) >> 20kHz, feed it a carrier modulated by a 2000Hz signal, and see what >> you get out the far end. >> >> Hope this helps... > > Hi, > Thanks, this helps! > > 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 -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Jerry Avins on 25 Apr 2010 01:03 On 4/24/2010 8:04 PM, HardySpicer wrote: > On Apr 25, 12:39 am, Jerry Avins<j...(a)ieee.org> wrote: >> On 4/23/2010 5:56 PM, Vladimir Vassilevsky wrote: >> >> >> >> >> >>> Jerry Avins wrote: >> >>>> On 4/23/2010 5:33 PM, HardySpicer wrote: >> >>>>> On Apr 24, 8:24 am, Vladimir Vassilevsky<nos...(a)nowhere.com> wrote: >> >>>>>> HardySpicer wrote: >> >>>>>>> Synchronous demodulation using a PLL will give you 3dB improvement >>>>>>> over ordinary envelope detection. >> >>>>>> This is wrong. >> >>>>> It's in the textbooks...read it! >> >>>> What is ordinary envelope detection? Peak detection? >> >>> Doesn't matter; It is very simple. Think of |I| vs sqrt(I^2 + Q^2) >> >> Still, when someone claims "3dB improvement", I want to know what is >> improved upon. >> >> Jerry >> -- >> "I view the progress of science as ... the slow erosion of the tendency >> to dichotomize." --Barbara Smuts, U. Mich. >> ����������������������������������������������������������������������� > > If I can remember that far back I believe it is in received SNR > (baseband). Look up synchronous demodulation vs > envelope detection. Of course you digital guys have I and Q and make > things even more complicated. Do you remember when we just used a cats > whisker! > The I and Q method vs a PLL - I have no idea. How do you get I and Q - > do you need a PLL to get I and Q? If so then I expect it is the same > result. > > It was Taub and Shilling or some such that had the details but most > older coms books will have the info. People are so locked into digital > nowadays that they forget the basics > which all comes from analogue. You cannot understand digital without > understanding analogue first. I don't mean you Jerry of course but > many of the new breed of engineer that gets taught exclusively > digital. > (does happen!!) For example, what is an exclusive OR in a PLL? To me > it is a multiplier!! I/Q demodulation is pure envelope detection, even for carriers not much higher than the highest modulating frequency. What you seem to refer to as envelope detection is peak detection, which is merely a damn good approximation under typical conditions of operation. There are a number of ways to make an analytic (I-Q) signal. A Hilbert transformer is one, Clay Turner's Tips & Tricks article is better. Often the best produces both I and Q in the sampling process. Synchronous demodulation does improve on peak detection, but not on sqrt(I^2+Q^2). So does exalted carrier, for the same fundamental reason. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Jerry Avins on 25 Apr 2010 01:06 On 4/24/2010 1:25 PM, glen herrmannsfeldt wrote: ... > As I understand it, many college level digital design classes > are taught entirely with simulations. Students never see any > actual hardware! The nature of many of the questions we see here bear that out. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: steveu on 25 Apr 2010 05:00
>As I understand it, many college level digital design classes >are taught entirely with simulations. Students never see any >actual hardware! I am told that it is becoming quite normal for entire electronics degree courses to be free of any lab work. Therefore, analogue, power and other topics are in the same position as digital. Cost, health and safety issues, and the narrow inclinations of lecturers were cited to me recently by a lecturer. One still runs his power electronics courses to include things that can actually go bang. Regards, Steve |