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From: Mark on 14 May 2010 12:08
On May 12, 6:31 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

I have seen a related discussion in the context of using PLLs for
threshold extension FM demodulation. What I recall is if the phase
detector is preceeded by a hard limiter, then the PLL FM detector
worked no better then a convention limiter discriminator, i.e. there
is no threshold extension. On the other hand, if the phase detector
is fed the signal without limiting, then the PLL can provide threshold
extension. The qualitative exlplination was that at low SNR the
limiter allows the noise to supress the desired signal. At higher SNR
of course, it makes little difference. I'm sorry I can't give you a
more qalitative answer but I suggest you investigate threshold
extension.

Mark




From: cfy30 on 15 May 2010 12:24
You can simply use an analog mixer as phase detector to extract both phase
and magnitude information. The mixer approach appears on textbook only and
is rarely used because it is undesirable to have the loop response
depending on magnitude of the PD output. Modern PLL always use digital PFD
to extract the phase difference and PFD is simply two D flip-flop in
principle.

By the way, TYPE I PLL is frequency locked only which is considered
obsolete and rarely used nowadays. TYPE II PLL with charge pump can achieve
phase lock.


cfy30

>>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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