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Subject: Re: How to increase PLL order?
Date: Sat, 14 Dec 2002 09:06:02 +1300
Organization: Attica Communications
References: <3DF99AEF.80FA3424@NAESPAM.yahoo.com> <%QhK9.firstname.lastname@example.org>
NNTP-Posting-Date: Fri, 13 Dec 2002 20:05:31 +0000 (UTC)
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Kevin Aylward wrote:
> tom wrote:
> > "Christopher R. Carlen" wrote:
> >> Kevin Aylward wrote:
> >>> A frequency/phase detector is a digital logic detector, of which
> >>> there are various implementations. In contrast to multipliers and
> >>> ex-or type that lock on at 90 deg for the center frequency only,
> >>> frequency/phase types always lock exactly in phase (ideally) no
> >>> mater what the centre frequency is.
> >> I think that depends on what the transfer function of the VCO is. Of
> >> course, we are used to Kvco/s, but for motors things go haywire.
> >> I am using the Z-state detector from a 4046, and it doesn't produce
> >> zero phase error with a motor, though it most certainly does with a
> >> normal VCO.
> >> The reasons are in the control theory, at which I am not yet good
> >> enough to explain why this happens. But I have observed it
> >> nonetheless. A crude attempt at my explaining it would go something
> >> like:
> >> The motor/VCO transfer function is wierd, so that the loop filter
> >> transfer function is not that of a simple low pass filter with a
> >> finite high frequency gain. Instead it is a "zero-pole" as I have
> >> OPed. This filter doesn't integrate. Phase error is thus some
> >> non-zero constant.
> > I have just seen this post and apologise for not reading all the
> > other posts but many of them. You seem to be adding a phase-lead at
> > unity gain which is quite common in closed-loop systems. What you
> > need to do is add a lag at low frequencies so you have a lag-lead
> > filter - like this
> Actually, this is one of those rare occasions that you might not really
> need to add a lag. In most cases one is usually stuck with:
> "The only guaranteed way to make a small fortune, is to start with a
> large one, and lose some of it."
> In most cases (e.g. amplifiers), you don't have any high frequency gain
> left to bring the system into a 20db/dec roll off at the unity gain
> point. So what you do is deliberately roll off the gain and then take it
> back out again. i.e. a lag-lead. However, in a motor control system the
> response is usually so slow that you can simple add an op-amp with
> sufficient BW to give a simple lead around the zero x-ing point, without
> having to throw away gain at low frequencies.
Not sure what you mean by throwing away gain at low frequencies. Usually
with a lag you increase the overall gain of the loop to maintain the same
unity gain crossover frequencies. In text books they make great play
of reducing gain at low freuencies with a lag but it is not comparing
oranges - for comparison you have to maintain the same bandwidth
each time you do a modification of the compensator (where possible).
It is always possible to squeeze a lag or integrator (P+i) into a loop
but at the expense of phase margin of course. That is where bandwidth is
- you must know the limiting factor on bandwidth.
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