Coherent sampling with IF Freq
in [DSP]
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From: Bendenj on 24 Jul 2010 22:02 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.
From: Nasser M. Abbasi on 24 Jul 2010 22:17 On 7/24/2010 7:02 PM, 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? > > Thx, > > Josh. hi Josh, I am a student, so not an expert :) but terminology wise, we are told in class that normally IF (intermediate frequency) is something preset in the hardware (may be by some comitte in the goverment who sets these things?, different IF's are defined for different purposes?, for example IF 455 Khz for audio devices). So, the receiver will convert down (via mixer) the input RF frequency to IF range before pushing the signal down for more processing. So, if one is doing demodulation, and need to do coherent demodulation, the frequency one is trying to detect is the carrier frequency in the RF signal itself. Nothing to do with the IF. So, my vote will be the detection of the spacing between the tones in the input signal. (I could be wrong ofcourse) --Nasser
From: Steve Pope on 24 Jul 2010 22:20 Bendenj <betzerj(a)n_o_s_p_a_m.gmail.com> wrote: >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? You don't have to sample coherently to the IF frequency, but if the IF frequency is not coherent with the symbol rate you will have to (at least) derotate after sampling to get a coherent result. Steve
From: Tim Wescott on 24 Jul 2010 22:26 On 07/24/2010 07:17 PM, Nasser M. Abbasi wrote: > On 7/24/2010 7:02 PM, 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? >> >> Thx, >> >> Josh. > > hi Josh, > > I am a student, so not an expert :) but terminology wise, we are told in > class that normally IF (intermediate frequency) is something preset in > the hardware (may be by some comitte in the goverment who sets these > things?, different IF's are defined for different purposes?, for example > IF 455 Khz for audio devices). The IF is usually decided by the system designer, although there are popular ones (like 455kHz for AM radio) that make it easy to get parts. You want to choose a really high IF to dodge problems with harmonic mixing and image reception. You want to choose a really low IF to make it easy to build the filters (or cheap to buy). Somewhere in the middle is a good number to use. > So, the receiver will convert down (via mixer) the input RF frequency to > IF range before pushing the signal down for more processing. It will convert from RF to IF _and filter_. > So, if one is doing demodulation, and need to do coherent demodulation, > the frequency one is trying to detect is the carrier frequency in the RF > signal itself. Nothing to do with the IF. > > So, my vote will be the detection of the spacing between the tones in > the input signal. > > (I could be wrong ofcourse) > > --Nasser -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: Tim Wescott on 24 Jul 2010 22:28
On 07/24/2010 07:02 PM, 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? I'm not sure that "coherently to the IF frequency" has any meaning. 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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html |