Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DF99AEF.80FA3424@NAESPAM.yahoo.com> <%QhK9.firstname.lastname@example.org> <3DFA3DAA.E103628F@NAESPAM.yahoo.com>
Subject: Re: How to increase PLL order?
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Date: Sat, 14 Dec 2002 09:19:10 -0000
NNTP-Posting-Date: Sat, 14 Dec 2002 09:19:16 GMT
Chris Carlen wrote:
> Kevin Aylward wrote:
>> And this is exactly what I *explicitly* addressed above. You claimed
>> that one usually used a lag-lead filter, and this, of *course*,
>> rolls of the gain, i.e. limits the BW. To the contrary, I pointed
>> out that in slow systems, it is quite possible to have a lead on its
>> own that can stabilise the loop, with the advantage that it
>> obviously keeps the BW up, i.e. no lag.
>> Kevin Aylward
> So would the correct terminology for what I have been calling a
> "zero-pole" filter be a lead-lag?
Yes and no. There is no "correct" terminology here, both can be used
equally. I personally, always use describe things as poles and zeros. I
think this is more usually in amplifier design. In control (mechanical)
theory I think lead-lag is used more often.
> When you say lead-lag does that imply that the zero occurs at a lower
> f than the pole? Ie., would you call it a lead-lag just because it
> contains a zero and a pole or would you change it to lag-lead if the
> pole were at lower f than the zero?
Frankly, I never really describe compensation in this lag/pole zero/lead
way. I generally just describe in terms of the 20n.db/dec gain roll offs
The usually scenery is that the lag/pole comes first in frequency. This
reduces the gain right up to the x-ing point. The lead/zero is then
inserted around the o-xing point to bring the phase back up.
The *whole* point of the lag-lead is as I noted.
"The only guaranteed way to make a small fortune, is to start with a
large one, and lose some of it."
Learn his well.
If a gain stage in amplifier could be made to increases with frequency
at the zero-xing point, you would not need a lag at all. You would
simply add in the lead gain amp. All your trying to do is increases the
phase away from a 180 degrees (in a neg feedback system) at the 0-xing
point. This is obviously not likely, as the system is rolling off
because the amp components are maxing out already. So to get around this
you deliberately roll of the gain before the component limits kick in,
this means everything rolls of sooner, you can then un-roll off the bit
you rolled off prematurely. This results in the classic lag-lead.
But if you are fortunate to be dealing with a slow system, e.g. 1khz
type response, you can simple slap in a 100Mz op-amp and make a lead on
its own without having (essentially) he lag at all. I have actually done
this to stabilise a SMPS. Its rare to see this done becuse people get
into a mindset.
> Hmm, I just realized that a high pass is s/(s+wc), (I'm new at this
> so I don't have it all ingrained yet), so that means what I think you
> are calling a "lead" has an H(s) which contains both a pole and a
> zero at DC, ie., a simple high-pass.
Just about. Yes, in practise a lead is a single stage HP filter.
Technically, a pure S term, e.g. a differentiator is a lead, but a
practical one will always end up flattening off at HF.
> What would you call my filter with H(s)=(s+wz)/(s+wp) ?
That is a lag-lead, or a lead-lag, depending on what pole comes first.
Which is which is any bodies guess:-)
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