Subject: Re: How to increase PLL order?
Date: Sun, 15 Dec 2002 16:24:44 +1300
Organization: Attica Communications
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NNTP-Posting-Date: Sun, 15 Dec 2002 03:24:09 +0000 (UTC)
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Kevin Aylward wrote:
> 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
> or increases.
> 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:-)
> Kevin Aylward
> SuperSpice, a very affordable Mixed-Mode
> Windows Simulator with Schematic Capture,
> Waveform Display, FFT's and Filter Design.
(1+sT1)/(1+sT2) when T1>T2 this is a lead otherwise a lag.
For a lead-lag you need two of the above - one for the lead and one for the
Often this is quoted inccorectly ie a lag-lead is shown to be the above.
A phase lag starts flat then has a -6dB/octave roll off then flat again.