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From: Jerry Avins on 13 May 2010 11:08
On 5/13/2010 8:28 AM, Greg Berchin wrote:
> On Wed, 12 May 2010 20:25:06 -0400, Jerry Avins<jya(a)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.
�����������������������������������������������������������������������
From: Eric Jacobsen on 13 May 2010 13:51
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
From: HardySpicer on 13 May 2010 15:15
On May 13, 10:31 am, 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

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
From: Manny on 13 May 2010 15:16
On May 13, 6:51 pm, Eric Jacobsen <eric.jacob...(a)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
From: HardySpicer on 13 May 2010 22:03
On May 14, 12:28 am, Greg Berchin <gberc...(a)comicast.net.invalid>
wrote:
> On Wed, 12 May 2010 20:25:06 -0400, Jerry Avins <j...(a)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
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