From: WarmSpgs@aol.com
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Date: Sun, 21 Jan 2001 02:13:33 EST
Subject: Re: LF: PLL inaccuracy
To: rsgb_lf_group@blacksheep.org
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In a message dated 1/20/01 4:27:03 PM Eastern Standard Time, 
akestelo@bellatlantic.net writes:

<< For simplicity's sake, let's assume that we have a phase  comparator that 
needs a  1 Hz difference between its two inputs before it creates a  DC 
correction signal  large enough to control the VCO (not a very good phase  
comparator, assuredly,  but just for demonstration purposes). >>

Ah, but that's the point.  It would be an extraordinarily bad phase 
comparator if it didn't begin to respond until there was a 360 degree/second 
error.  A PLL using such a comparator could probably never achieve lock.  At 
best, it would wander loosely around the hoped-for frequency.

There does not have to be a frequency error at all for a phase comparator to 
output a correction voltage...only for the oscillator being stabilized to 
have a _tendency_ to drift away from the desired frequency, which is 
inevitable.  Hence, once lock has been achieved, a more-or-less constant 
phase difference is maintained by the loop.  This is not frequency error.

Time is the difference between frequency and phase, as in Alan's analysis.  A 
_frequency_ difference between two signals means the _phase_ relationship is 
changing continuously in the same direction over the course of time.  If f1 > 
f2, the phase of f1 is constantly advancing relative relative to that of f2, 
for just as long as the frequency difference is allowed to exist.  This is 
the condition when lock has not been achieved.

When a PLL achieves lock, phase of the controlled oscillator is NOT allowed 
to move continuously in either direction.  There may be short-term variations 
around the center, but no continuing trend (thus, phase lock).  Without 
continuous phase change in a given direction, there is no frequency error 
relative to the reference.  The remaining short term variations around the 
desired phase relationship are simply that: phase noise, or jitter.

I hope this helps clarify the distinction.

73,
John