AM digital demodulation methods
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From: HardySpicer on 23 Apr 2010 17:33 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!
From: Jerry Avins on 23 Apr 2010 17:36 On 4/23/2010 5:03 PM, gretzteam wrote: >> >> >> 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. How many samples per carrier cycle do you have? How many carrier cycles do you average over? How long does that take, and what does that imply about the highest envelope frequency you can demodulate without attenuation? > 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? The analog version is essentially the same as the digital version, the salient difference being an effectively infinite sample rate. Peak detection works just fine with that degree of oversampling! Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: Jerry Avins on 23 Apr 2010 17:44 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? Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
From: brent on 23 Apr 2010 17:45 On Apr 23, 4:33 pm, "gretzteam" <gretzteam(a)n_o_s_p_a_m.yahoo.com> wrote: > >Synchronous demodulation using a PLL will give you 3dB improvement > >over ordinary envelope detection. > > 3dB improvement of what? I don't quite understand how to qualify such a > system. I assume you mean SNR, but how does it get measured? > > >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? > > Ok this is the part I don't understand! Can you elaborate a bit more? > > Thanks a lot! I will explain, as long as you take your assigned role as "blind". I will take my assigned role as "dumb". I was going to try to explain a little further, but I suggest you actually try to mess around a bit with scilab or matlab if you have it and see what you can generate. If your signals do not have any noise or outside interference, then just do the mixing process and don't worry about it. If you are collecting data from a noisy environment and there is embedded energy in your signal that is outside the frequency of your carrier, then you need to think about a Band Pass Filter and think about aliasing.
From: Vladimir Vassilevsky on 23 Apr 2010 17:51
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! "Making errors is a way of a man. Insisting on errors is a way of an idiot." Who said that? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com |