On 4/23/2010 4:33 PM, gretzteam wrote: I wrote:>> 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?Rarely is a carrier to be demodulated sampled at more than twice the carrier frequency; that would be a waste. The sampling theorem tells us that we have to sample more than twice the frequency corresponding to the bandwidth of interest. There are some practical restrictions (the second edition of Understanding Digital Signal Processing by Rick Lyons has an excellent analysis of them) but in general, sampling 20 KHz wide signal on a 356 KHz carrier can be accomplished with a 50 KHz sample rate. A bandpass filter assures that the AM signal is not contaminated by adjacent channels. With bandpass sampling, we need to exclude signals both above and below the band being sampled. Even if the signal were sampled at 1 MHz, a low-pass filter would be needed to substantially eliminate all signals above 500 KHz. Incidentally, sampling at 1 MHz provides a little over 2 samples per carrier cycle, with little chance that either of them will be near a carrier peak and hence representative of the envelope. With bandpass sampling at 50 KHz, there will be only one sample for every 9 or so carrier cycles. There is then no hope of peak detection. 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. ���������������������������������������������������������������������
AM digital demodulation methods
Started by ●April 23, 2010
Reply by ●April 23, 20102010-04-23
Reply by ●April 23, 20102010-04-23
On 4/23/2010 5:05 PM, Vladimir Vassilevsky wrote: ...> Jerry, do you know what was the rationale for choosing 455kHz vs 465kHz > standard IF ?The AM band is roughly 560 to 1600 KHz, a nearly 3:1 range. A little thought reveals that the LO must be above the signal in order avoid a band switch in the receiver. Someone did an image analysis to pick the IF, and 455 came out. I don't know the calculation or if it made sense. I remember 455, 456, even 460. Standard IF transformers locked in 45x. It couldn't be very different. It needs to be below the AM band itself and not so low that the selectivity becomes too high. Also, one doesn't want many harmonics that fall into the band. If my cousin Jack were still alive, I could ask him. Among his patents are the VTVM and the ratio detector. http://tinyurl.com/2g8koyp 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. ���������������������������������������������������������������������
Reply by ●April 23, 20102010-04-23
On Apr 24, 8:24�am, Vladimir Vassilevsky <nos...@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!
Reply by ●April 23, 20102010-04-23
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. �����������������������������������������������������������������������
Reply by ●April 23, 20102010-04-23
On 4/23/2010 5:33 PM, HardySpicer wrote:> On Apr 24, 8:24 am, Vladimir Vassilevsky<nos...@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. �����������������������������������������������������������������������
Reply by ●April 23, 20102010-04-23
On Apr 23, 4:33�pm, "gretzteam" <gretzteam@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.
Reply by ●April 23, 20102010-04-23
HardySpicer <gyansorova@gmail.com> wrote: (snip)> The problem arrises when you want to do synchronous demod and the > carrier isn't there! What I mean by that is that when you have > double sideband supressed carrier. There is no power at the carrier > freq then and nothing to lock onto.> Solution...among otehr things you need to square the received waveform > and lock into twice the carrier then divide down (missing some other > crucial steps).As I understand it, commonly used by many modems doing anything except FSK. To make sure that there are enough transitions for carrier recovery, scramblers are commonly used on the bit stream. -- glen
Reply by ●April 23, 20102010-04-23
HardySpicer wrote:> On Apr 24, 8:24 am, Vladimir Vassilevsky <nos...@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
Reply by ●April 23, 20102010-04-23
Jerry Avins wrote:> On 4/23/2010 5:33 PM, HardySpicer wrote: > >> On Apr 24, 8:24 am, Vladimir Vassilevsky<nos...@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) VLV
Reply by ●April 23, 20102010-04-23
>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 theinput>> signal contains only a carrier, full scale, then the output of thelowpass>> filter is pretty much exactly 0.63 (2/pi), which is the average value ofa>> 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 withoutattenuation? Ok I'm way oversampled. I'm doing this to learn about it so I don't want to have the added difficulty of sample rate (just yet). Here is the current system - I should have posted this FIRST! parameters: fs = 4MHz carrier: 99kHz Currently, there is no noise, and no 'information' being modulated. Just a carrier sine wave:) One gotta start somewhere! I then bandpass using a 2nd order bandpass filter centered at the carrier. Then take the absolute value. Then lowpass filter using a 2nd order CIC filter all the way down to something ridiculous like 10-50Hz. The output matches surprisingly well the 2*A/pi formula depending on the A of the carrier. Now if I do a frequency sweep, using a full scale sine wave from 0 to 2MHz (fs/2), and plot the obtained average value after it settled, I get the shape of the bandpass filter! I guess this was to be expected, which is why I asked if 'method-1' was only dependent on the performance of the bandpass filter. I guess so far my 'information' is only at DC, but it works well. Now, ff I modulate a 2Hz signal, I also see it at the output, with a DC offset. And this is where I decided to post, since I didn't know how to measure performance of the system when there IS information. Does this make sense? Thanks!
