I don't have the 196, but I did build my own PLL's (discrete but not for musical purposes).
I think the output of the phase comparator is digital, in the sense that it is a square wave generated by digital logic. If the output is of the same frequency as the VCO free running freq (the one set by the offset) then the comparator out is of 50% duty cycle. If the output is higher, the duty cycle is higher (always assuming in lock). If you then lowpass filter this square wave, you'll get a DC voltage (with maybe a little ripple remaining). This voltage is fed back into the VCO.

You can build a PLL in a a100 without an a196. Use a VCO, a ring mod as a phase comparator and slew limiter as LPF. The LPF should be DC coupled, and the cut off frequency much below the normal LPF's. It does not work super good though. I think one of the problems is the slew limiter. A slew limiter is not an LPF, and an LPF should work better for an PLL.

So I agree with Göran. An input for the LPF would enable the use of the LPF for filtering CV's and it would enable to replace the a196's phase comparator with another (ring mod, for instance). But then, maybe it does not add so much in practise.

Christophe

--- On Wed, 2/25/09, Doug < dougc356@... > wrote:
From: Doug < dougc356@... >
Subject: 1 Re: A-196 lowpass in mod
To: Doepfer_a100@yahoogroups.com
Date: Wednesday, February 25, 2009, 12:32 AM












Hi Ingo, so the input to the VCO (the output of the LPF) is digital

That would certainly explain it. Not sure how I missed that in the

manual.



Regarding compromising the function of the PLL, of course I want to

compromise the function of the PLL! The poor thing is always the

object of abuse, isn't it



Doug



>

> i find that the a196 manual explains it quite well.

> the phase comparator in the a196 generates a digital

> signal, which is used to drive a linear vco.

> the lowpass filter is there to smooth out this

> digital signal and make it less jumpy, which leads

> to a more stable vco output. since other vco's are

> controlled in a different way (not by a digital signal,

> but a continuous analog voltage), you won't get the same

> results with "normal" vco's like a110 or a111. inserting

> a lowpass filter at a different position in the pll would

> just compromise the function of the module.

> somebody please correct me if i'm wrong here.

>

> best wishes

>

> ingo

>































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