From: "Christopher R. Carlen"
Subject: Re: How to increase PLL order?
Date: Tue, 10 Dec 2002 11:05:04 -0800
Organization: Sandia National Laboratories, Albuquerque, NM USA
NNTP-Posting-Date: Tue, 10 Dec 2002 18:03:16 +0000 (UTC)
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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.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA