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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: How to increase PLL order?
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Date: Fri, 13 Dec 2002 09:34:47 -0000
NNTP-Posting-Date: Fri, 13 Dec 2002 09:34:51 GMT
> "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
>> 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.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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