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From: Greg Berchin on 13 May 2010 08:28
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?

Greg
From: Greg Berchin on 13 May 2010 08:30
On Wed, 12 May 2010 19:35:16 -0500, Vladimir Vassilevsky <nospam(a)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
From: Jason on 13 May 2010 10:28
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

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
From: Jason on 13 May 2010 10:33
On May 13, 8:30 am, Greg Berchin <gberc...(a)comicast.net.invalid>
wrote:
> On Wed, 12 May 2010 19:35:16 -0500, Vladimir Vassilevsky <nos...(a)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
From: Vladimir Vassilevsky on 13 May 2010 10:50


Jason wrote:

> 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?
>>
> 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
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