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Subject: Re: Phase locked loops and confusion.
References: <3DDED492.2B9873B0@webaccess.net> <3DDF16D8.90072F5@webaccess.net>
NNTP-Posting-Date: Sat, 23 Nov 2002 06:39:24 GMT
Organization: AT&T Broadband
Date: Sat, 23 Nov 2002 06:39:24 GMT
Chuck Simmons, John Larkin wrote in part:
>> 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. 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.
> We have an ironclad tested strategy for setting the frequency. There
> is a pattern we can look for that can tell us if the frequency is off
> by huge amounts well beyond the operating range. We get the frequency
> to about 0.1% before we attempt phase lock (overkill).
>> 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.
> We looked at an equivalent because the digital team considered that
> simple. I pointed out that it would limit cycle with larger amplitude
> than the deadband from somewhat more linear methods. The simulations
> of the bang bang were unimpressive given the fact that we are locked
> to edges coming at most at f/6 and on the average of f/12. We are
> essentially in harmonic lock but the harmonic varies from 6 to a
> maximum of 28 from edge to edge. Harmonic locking with the additional
> complication of a loop divider of around 200 makes it really
> interesting. The bang bang seemed to have trouble with that so we
> decided to change our approach.
What are you ultimately controlling with your circuit, an oscillator
for some sort of communications device? Am I correct in assuming that
the device frequency being controlled is quite high since you are both
dividing it by 200 (256?) and locking onto anywhere from the 6th to well
more than the 12th subharmonic of an already 200X reduced frequency?
Why do you need to lock onto subharmonics anyway? Are you doing
frequency hopping as part of an encryption scheme?
Could you please provide more background information and design
specifications for whatever it is you are up to. Thanks.
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