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.
>
Reply by Mark●May 14, 20102010-05-14
On May 12, 6:31�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
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
Reply by HardySpicer●May 13, 20102010-05-13
On May 14, 12:28�am, Greg Berchin <gberc...@comicast.net.invalid>
wrote:
> On Wed, 12 May 2010 20:25:06 -0400, Jerry Avins <j...@ieee.org> wrote:
> >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.
>
> I'm having a little trouble getting my head around this. �If the XOR detector
> duty cycle represents frequency difference, then wouldn't the *integral* of the
> duty cycle represent the phase error? �And what would the integral of a duty
> cycle look like?
>
> Greg
A phase-locked loop as a phase-locked loop! You get out rate of change
of phase of course.
If you change the inner dynamics you still get the same output but it
will track better or worse.
Can't see what the argument is about.
Hardy
Reply by Manny●May 13, 20102010-05-13
On May 13, 6:51�pm, Eric Jacobsen <eric.jacob...@ieee.org> wrote:
> On 5/12/2010 9:37 PM, Vladimir Vassilevsky wrote:
>
>
>
>
>
> > 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 think I see what's happening. �It is often easy to exclude undesirable
> detector samples by setting the output to zero. �There are generally not
> ill effects from excluding occassional input samples unless one starts
> to approach a limit in jitter tolerance. �So it may be effective to just
> ignore inputs that are below some magnitude theshold. �Often this is
> nearly as good as some optimized algorithm.
>
> --
> Eric Jacobsen
> Minister of Algorithms
> Abineau Communicationshttp://www.abineau.com
Yes, this is also often done especially in the convergence-contention
algorithms I described earlier. You *bootstrap* your algorithm by some
uncool method that you just know to work. This also underscores a
broader ideological difference in how you go about doing things;
thorough analysis vs. empirical verification. Folks in comp.dsp always
make me feel bad about how shallow I seem to be in comparison. It's
just that the people I work with live an die by empirical data and
don't want you to dig deep in designing anything. The proof is in the
pudding they say! Thoroughness is traded for being more varied.
-Momo
Reply by HardySpicer●May 13, 20102010-05-13
On May 13, 10:31�am, 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
It will help but only if you have "special" problems with for example
FM ie co-cahannel or Multipath interference.
However, you don't interfere with the amplitude going into a PLL since
this will de-stabilise it and change the tracking properties. You use
a separate loop called an Amplitude-Locked Loop instead of a hard
limiter first. In this way when the amplitude of the FM goes to zero
at any time the ALL responds quickly and servo's out (as much as it
can) such changes without amplifying the noise (which a limiter does -
a limiter does no filtering of course).
Look here.
http://www.ampsysltd.co.uk/
Hardy
Reply by Eric Jacobsen●May 13, 20102010-05-13
On 5/12/2010 9:37 PM, Vladimir Vassilevsky wrote:
>
>
> 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 think I see what's happening. It is often easy to exclude undesirable
detector samples by setting the output to zero. There are generally not
ill effects from excluding occassional input samples unless one starts
to approach a limit in jitter tolerance. So it may be effective to just
ignore inputs that are below some magnitude theshold. Often this is
nearly as good as some optimized algorithm.
--
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com
Reply by Jerry Avins●May 13, 20102010-05-13
On 5/13/2010 8:28 AM, Greg Berchin wrote:
> On Wed, 12 May 2010 20:25:06 -0400, Jerry Avins<jya@ieee.org> wrote:
>
>> 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.
>
> I'm having a little trouble getting my head around this. If the XOR detector
> duty cycle represents frequency difference, then wouldn't the *integral* of the
> duty cycle represent the phase error? And what would the integral of a duty
> cycle look like?
It was late and I put it poorly. (Anyhow, *I* knew what I meant.)
If the LO's free-running frequency is the same as the reference, it
doesn't need to be pulled to achieve lock. The XOR's duty cycle will be
50%. As the reference moves*, the duty cycle (and hence the average DC)
will shift in order to generate the necessary control voltage for the
VCO. The frequencies are locked, but their phase offset is proportional
to how hard the LO has to be pulled. I.e., the frequency is locked, but
the phase is not. Locking phase requires an extra integrator.
Jerry
__________________________
* In the '50s, before AFC was universal in FM receivers, a listener
complained to a station manager that his station drifted. He told her,
"Nowadays, you can get a receiver that drifts with the station" and
referred her to my shop.
--
"I view the progress of science as ... the slow erosion of the tendency
to dichotomize." --Barbara Smuts, U. Mich.
�����������������������������������������������������������������������
Reply by Vladimir Vassilevsky●May 13, 20102010-05-13
Jason wrote:
> On May 12, 6:31 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?
>>
> Just a thought: don't some phase detectors that are often used have
> some degree of this built-in? One example I'm thinking of is a Costas
> loop where you might use I*Q as the phase error. If you scale the
> amplitude of the received signal, that scale factor (squared) is
> applied directly to the phase error. Of course, you might be able to
> squeeze out some more information based on intelligently considering
> the amplitude (i.e. by using some memory of the recent signal level
> instead of just on a sample-by-sample basis), but as you already know,
> many phase detectors already have some bit of amplitude sensitivity.
It is not obvious to me that the built-in sensitivity to the amplitude
is anywhere near optimal; it could be detrimental in some cases. Another
interesting question is the PLL behaviour during the acquisition.
There is clear correlation between the magnitude and the phase errors;
the consideration of the amplitude improves the loop by 1..2dB.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Jason●May 13, 20102010-05-13
On May 13, 8:30�am, Greg Berchin <gberc...@comicast.net.invalid>
wrote:
> On Wed, 12 May 2010 19:35:16 -0500, Vladimir Vassilevsky <nos...@nowhere.com>
> wrote:
>
> >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.
>
> Thanks, Vladimir. �I've never used a "FLL", and never even seen one mentioned in
> the literature. �But I admit, I'm a little out of my element here.
>
> Greg
One place where FLLs might be used is in a GPS receiver. In order to
make precise Doppler shift and carrier phase measurements used for
navigation, a PLL with a small noise bandwidth is desirable. However,
such a scheme has poor acquisition characteristics in the presence of
unknown frequency offset. During signal acquisition, you can use an
FLL (or a wider-bandwidth PLL) for fast acquisition, then transition
over to a tracking mode with a low-noise PLL.
Jason
Reply by Jason●May 13, 20102010-05-13
On May 12, 6:31�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
Just a thought: don't some phase detectors that are often used have
some degree of this built-in? One example I'm thinking of is a Costas
loop where you might use I*Q as the phase error. If you scale the
amplitude of the received signal, that scale factor (squared) is
applied directly to the phase error. Of course, you might be able to
squeeze out some more information based on intelligently considering
the amplitude (i.e. by using some memory of the recent signal level
instead of just on a sample-by-sample basis), but as you already know,
many phase detectors already have some bit of amplitude sensitivity.
Jason