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Subject: Re: How to increase PLL order?
Date: Fri, 13 Dec 2002 21:31:44 +1300
Organization: Attica Communications
NNTP-Posting-Date: Fri, 13 Dec 2002 08:31:15 +0000 (UTC)
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"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.
> Good day.
> Christopher R. Carlen
> Principal Laser/Optical Technologist
> Sandia National Laboratories CA USA
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
The phase - lag compensator acts as an approximate integrator at low
frequencies. Maybe you can try (1+sT)/s instead which will give you two
integrators (+ phase advance which you already have). Some books
call the PI (+ the D term which is your phase-lead).
You maximum bandwidth will be determined by the 2X carrier
feedthrough term and limited further by the structural resonance of the
The resonance will 'eat up' the phase margin so you need to be well clear
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