Phase/Amplitude detector in PLL
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From: Vladimir Vassilevsky on 12 May 2010 18:31 In the classic treatises on PLL, they consider phase detectors as purely phase detectors, i.e. devices which output the phase of the signal regardless of the instant magnitude of the signal. I wonder if there could be possible to improve the SNR of the PLL by considering the magnitude also. Do you know a book or article which talks about that? VLV
From: Greg Berchin on 12 May 2010 18:52 I haven't seen anything about using the magnitude, but it has long bothered me that PLLs seem to be most often used in places where the real goal is frequency lock, not phase lock. In other words, we are attempting to lock two signals by comparing their integrals. Granted, if two signals are phase-locked then they are also frequency-locked, but it seems like there would be some advantage to using a "frequency-locked-loop" when frequency-lock is the actual goal. I don't know off-hand how a "frequency-locked-loop" would be implemented. Greg
From: Tim Wescott on 12 May 2010 19:05 Vladimir Vassilevsky wrote: > > In the classic treatises on PLL, they consider phase detectors as purely > phase detectors, i.e. devices which output the phase of the signal > regardless of the instant magnitude of the signal. I wonder if there > could be possible to improve the SNR of the PLL by considering the > magnitude also. Do you know a book or article which talks about that? I have seen discussion in the context of carrier phase recovery from PSK, where the primary concern is that the loop gain changes with changing carrier strength. But that's not what you meant. If you treated the PLL as a Kalman filter wherein you wanted to make the optimal update each time, then you could certainly look at the magnitude of the signal for an indication of how much you should trust it's phase -- but I think that the amount you'd decide to trust its phase would then depend heavily on the expected channel characteristics. E.g. normally if you saw a huge signal you'd think "good! high SNR!". But in a channel that has impulse noise this situation would be much more likely to be a result of noise, not signal, and you may want to reject these outliers outright. I think this would be something that would depend so heavily on the expected channel characteristics that you couldn't make many global deductions. I suppose you could cover a broad range of RF applications by assuming Gaussian noise with the occasional 'event', or just Gaussian noise -- but I'm not sure that even that would be valid as equipment aged, etc. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
From: Eric Jacobsen on 12 May 2010 19:42 On 5/12/2010 3:31 PM, Vladimir Vassilevsky wrote: > > In the classic treatises on PLL, they consider phase detectors as purely > phase detectors, i.e. devices which output the phase of the signal > regardless of the instant magnitude of the signal. I wonder if there > could be possible to improve the SNR of the PLL by considering the > magnitude also. Do you know a book or article which talks about that? > > VLV Unless there's information in the magnitude that tells you something about the phase, I don't know how it would help if you're really trying to lock to the phase of the input signal. Magnitude and phase are generally orthogonal, so ignoring magnitude shouldn't have any effect on performance if the information that drives the PLL is in the phase. If that's not true, i.e., if there is some information in the magnitude that can affect the loop performance, then whatever the nature of that information might be would drive the changes to the phase detector. It's not unusual to have a PLL phase detector that must be able to handle changing signal magnitudes. QAM demodulators pretty much have to do this. -- Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com
From: Jerry Avins on 12 May 2010 20:25
On 5/12/2010 6:52 PM, Greg Berchin wrote: > I haven't seen anything about using the magnitude, but it has long bothered me > that PLLs seem to be most often used in places where the real goal is frequency > lock, not phase lock. In other words, we are attempting to lock two signals by > comparing their integrals. Granted, if two signals are phase-locked then they > are also frequency-locked, but it seems like there would be some advantage to > using a "frequency-locked-loop" when frequency-lock is the actual goal. > > I don't know off-hand how a "frequency-locked-loop" would be implemented. Some so-called phase-locked loops are actually frequency locked. The classic XOR detector develops a duty cycle that reflects the difference between the reference frequency and the LO's natural frequency. The duty cycle, in turn, is a measure of the phase error. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. ����������������������������������������������������������������������� |