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Subject: Re: Phase locked loops and confusion.
X-Newsreader: Microsoft Outlook Express 5.50.4920.2300
Date: Sat, 23 Nov 2002 19:23:57 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 14:23:57 EST
Organization: Cox Communications
"Kevin Aylward" wrote in message
> John Larkin wrote:
> > 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.
I think you'll find that John's jitter is enviously low - we talked about
his PLL in a thread (about calculus, I think) a couple months back.
The difficult thing about the bang-bang phase detector is that the gain is
effectively infinite when there's no noise in the system; as the noise
increases, the gain decreases. This characteristic makes it rather difficult
to do traditional loop analysis, but if you analyze a noise free system with
an ideal bang-bang phase detector, it will exhibit a limit cycle whose
amplitude is a function of the other loop parameters.
The nice thing about the bang-bang phase detector in a digital system is
that the phase detector can be clocked by the same clock used to clock the
rest of the system. That means it can be synthesized (as in digital
synthesis tools) - a huge advantage in many digital systems.
-- Mike --
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