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From: firstname.lastname@example.org (Ken Smith)
Subject: Re: How to increase PLL order?
Date: Thu, 12 Dec 2002 00:50:52 +0000 (UTC)
Organization: a2i network
NNTP-Posting-Date: Thu, 12 Dec 2002 00:50:52 +0000 (UTC)
X-Newsreader: trn 4.0-test76 (Apr 2, 2001)
Originator: email@example.com (Ken Smith)
In article <firstname.lastname@example.org>,
>The disadvantage of an xor-type phase detector is its narrow seek
>range. If the VCO frequency is far off the reference, the pd output
>will be a high-frequency triangle that will be grossly attenuated by
>the loop filter. In that case, the VCO will just wobble a little but
>will not reliably walk toward lock. So there is a nasty, often
>untenable tradeoff between phase noise and lock range. For crystal
>oscillators, this is often not a problem, but with a wideband VCO this
>can be lethal.
There are 5 ways I know of to solve this problem:
If the PLL is being used for frequency multiplication of a fairly clean
signal, you can widen the bandwidth initially and then gradually narrow it
once the PLL has locked. This is what was done in early "direct reading"
Proton magnetometers. This method is quick but needs a good SNR to work
You can contrive the filter so that when there is no feedback around the
whole loop the filter is a low frequency oscillator. This causes the VCO
to drift back and forth until it stumbles over the correct frequency.
This method can be very slow and will lock to anything maybe not the
You and put in an explicit "not-locked" detector and switch to a edge
based PLL. This is usually done with tricky up-down counters inside of
CPLDs. The counters can give you some protection against losing or
gaining an edge here and there in a system that is corretly locked.
If you are building the PLL as part of a sync demod system, you can
monitor the output of the demodulator. If the output goes below zero, you
can slightly lower the gain of the filter a little while later. This
method locks to the strongest signal but it can take quite a while to do
it. It really is a softer version of (3).
I have to add this one because I've seen this done: Hire someone to
twiddle the knob until it locks.
email@example.com forging knowledge
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