Coherent sampling with IF Freq
in [DSP]
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From: glen herrmannsfeldt on 24 Jul 2010 22:57 Tim Wescott <tim(a)seemywebsite.com> wrote: > On 07/24/2010 07:02 PM, Bendenj wrote: >> I have a question : >> Say that I'm trying to sample coherently an IF multi-tone signal. >> IF = From RF to some intermidate Freq. >> do I need to be coherent to the IF freq? or do I have to be coherent >> to the spacing between the tones in the signal? both? > I'm not sure that "coherently to the IF frequency" has any meaning. I think so (after reading the wikipedia page). I think you have to be a little lucky, though. Either that, or phase lock the IF to an appropriate multiple of the RF carrier. This reminds me of the complicated rules of the NTSC color subcarrier, selected such that different frequencies had the appropriate relationship with each other. In addition, it was necessary to have the appropriate relation to the difference between the video carrier and sound carrier. > You can be coherent to some signal that's in the IF, but > generally it's easier to have the ADC sample rate fixed, > and do any necessary rotation, phase locking, etc., in software. -- glen
From: Vladimir Vassilevsky on 24 Jul 2010 23:07 Bendenj wrote: > Hello All, > > I have a question : > > Say that I'm trying to sample coherently an IF multi-tone signal. > IF = From RF to some intermidate Freq. > > do I need to be coherent to the IF freq? or do I have to be coherent > to the spacing between the tones in the signal? both? To begin with, you need to develop coherent thought and speech. VLV
From: Eric Jacobsen on 25 Jul 2010 02:38
On Sat, 24 Jul 2010 21:02:05 -0500, "Bendenj" <betzerj(a)n_o_s_p_a_m.gmail.com> wrote: >Hello All, > >I have a question : > >Say that I'm trying to sample coherently an IF multi-tone signal. >IF = From RF to some intermidate Freq. > >do I need to be coherent to the IF freq? or do I have to be coherent >to the spacing between the tones in the signal? both? > >Thx, > >Josh. It's not completely clear what you mean, but if you mean that you're mixing the IF down via IF sampling, then it's difficult to not have the sampling rate be coherent to the IF frequency since the IF frequency and sampling are locked. If you mean to ask wether the sampling has to be coherent to an IF mixing frequency, including any frequency offset, in order to synchronize a multi-carrier signal, then the answer is no. Frequency offset can be removed numerically. A tricky thing to get right in a multi-carrier signal, however, is matching the sampling rate to the symbol rate. There are some caveats there that can make life difficult if not done carefully. If you mean something else that nobody has figured out, consider clarifying the question. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com |