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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Phase locked loops and confusion.
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 23 Nov 2002 07:41:37 -0000
NNTP-Posting-Date: Sat, 23 Nov 2002 07:41:44 GMT
John Larkin wrote:
> On Sat, 23 Nov 2002 01:06:45 GMT, Chuck Simmons
>> PLLs are fairly common and seem fairly simple but I found over the
>> past week that confusion abounds and since I have seen this
>> confusion before, I wonder if it is common.
>> Our digital team (yes, 2 engineers) has a need for a PLL but for
>> various reasons, an all digital loop could not be considered and,
>> therefore, I was dragged in as resident analog stuff guru. The y
>> showed me the loop they were proposing which consisted of digital
>> equivalents (pretty exact really) of a phase detector driving up and
>> down charge pumps dumping into a capacitor followed by a filter
>> followed by the VCO (which is analog). I was asked what the filter
>> needed to be and I mumbled that they had two integrators so it
>> should probably be a classic lead/lag and proceeded to draw response
>> curves on the whiteboard indicating what was needed.
>> The next day, I was told that all of the papers they could find on
>> the web used a low pass filter. My comment was an unenthusiastic
>> "Oh." I was correctly interpreted and a printed article soon
>> appeared on my desk. Suddenly I saw the confusion. The proposed loop
>> had a phase detector, charge pumps and capacitor which is Kp/s. The
>> article used a phase detector that was Kp. So the article loop has
>> Kp*Kf/s while the proposed loop has Kp*Kf/s^2. A low pass is
>> perfectly fine for the article loop but not workable for the
>> proposed loop.
>> I saw this confusion some years ago as well because the phase
>> detector, charge pumps and capacitor is used a lot in practice but
>> many articles don't mention this variant. This leads to confusion
>> about what sort of filter is actually needed. The charge pumps and
>> capacitor type has the advantage that it will drive the steady state
>> phase error to zero even when the desired frequency is greatly
>> different from the VCO center frequency.
> Yeah, the charge pump adds another integrator to the loop. I don't
> know why the charge pump thing is so popular;
>most of them have nasty deadbands.
This don't make sense to me. I don't see any reason for the charge pump
to have a dead band. A charge pump is nothing more then a switched
source and sink of current. The dead band is usually in the phase
detector, that is, if steps are not taking to correct it.
>A charge pump pd doesn't inherently drive the loop in the
> right direction when the freq error is large; it takes a bit of
> additional logic (like in the 4046) to do that.
The charge pump itself is not "supposed" to do that anyway. You put all
the frequency/phase detect logic in the frequency/phase detector.
> My favorite phase detector is a d-type flipflop operating in bang-bang
> mode: input signal on clock, reference osc on D. This is really
> interesting to analyze and to design a filter for. Gain is infinite,
> sort of.
But if you require low jitter, a bang bang is not very effective.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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