Phase/Amplitude detector in PLL

Started by Vladimir Vassilevsky May 12, 2010
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
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
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
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
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. �����������������������������������������������������������������������

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.
Frequency locked loops are actually used quite often; you only have to lock the derivative of phase rather then phase. This decreases the order of the system by one. The dynamics is simpler then that of PLL. However, frequency is relative whereas phase is absolute; so there is 3dB loss in loop SNR. At low SNRs, there will be nasty threshold behavior. There are also mixed mode loops with phase and frequency feedbacks. VLV
On May 12, 11:31&#2013266080;pm, Vladimir Vassilevsky <nos...@nowhere.com> 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
Virtually all coherent, adaptive underwater comms employ these types of trickery which is perhaps an indicator of the difficulty of the situation. In particular, often times you have a fractional channel, a DFE, and a PLL all trying to jointly lock the phase down, then in DSSS systems you'd look at the output of the despreader and use that to guide the operation. All permutations are allowed and people have come up with all sorts of configurations changing PLL constants, adaptation updated, and the lot by feeding back a measure of SNR. Anyway, to answer your question: Yes, people have done that, mostly though it's still the stuff of research. -Momo

Tim Wescott wrote:

> 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've seen articles where they tracked QAM carrier while asigning the different "weights" to the phase measurements depending on the distance from the center of the constellation.
> 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.
The PLL with AWGN at high SNR is easy to analyse; however there is not much to gain as the SNR is good already. I wonder if something could be gained at marginally low SNRs, such as 3dB or below. VLV

Eric Jacobsen wrote:

> 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? >>
> 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.
OK, I've ran the numbers. At the SNR ~ 1, the gain in the loop jitter due to processing of the amplitude as well as phase could be ~2dB. The problem is related to the capacity of the channel, and the result is what could be expected. It is interesting to see that if the noise is Gaussian, then the huge values of the signal are more likely to be correct. The expected RMS error is decreasing with magnitude to some asymptotic value. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
>I've seen articles where they tracked QAM carrier while asigning the >different "weights" to the phase measurements depending on the distance >from the center of the constellation.
For PSK in AWGN condition into account the amplitude will not change anything. But not for flat fading channels, you can use the amplitude (before AGC) for the weight of the phase error.