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From: Vladimir Vassilevsky on 12 May 2010 20:35


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
From: Manny on 12 May 2010 20:43
On May 12, 11:31 pm, Vladimir Vassilevsky <nos...(a)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
From: Vladimir Vassilevsky on 12 May 2010 21:04


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



From: Vladimir Vassilevsky on 13 May 2010 00:37


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
From: Alexander Petrov on 13 May 2010 05:05
>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.
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