[sdiy] SAW core VCO flyback time

Roman Sowa modular at go2.pl
Tue Sep 6 09:55:22 CEST 2016

It does not have to be calibratet out at all, that's the thing!
It can be almost as long as the full cycle and still does not influence 
The key is charging current always flowing via integrator cap, all the 
time, uninterrupted. 2nd key is that discharge is done by injecting 
precisely controlled amount of charge to the integrator, so reset phase 
is well defined as discharge current from VFC chip minus tuning current 
from expo converter.

Discharge current delivers the same charge that is sinked from the cap 
by expo converter during whole cycle. Since both must be equal and 
constant, defined by VFC current source and its timing RC network, 
integrator cycle depends only on charging current from expo converter, 
nothing else, there's no dead time involved.

With FET reset circuit, capacitor is discharged in uncontrolled manner, 
and that spoils hf tracking.

And 4151 datasheet:


W dniu 2016-09-06 o 08:17, Tim Ressel pisze:
> I was just going to comment on this. The reset time is a negative
> because it is unpredictable. If a fixed reset is generated on purpose
> then it is predictable and can potentially be calibrated out. The
> Chroma, using a 4151 IC (datasheet?) serves this function.
> --tr
> On 9/3/2016 4:42 AM, Mattias Rickardsson wrote:
>> On 30 August 2016 at 23:41, Donald Tillman <don at till.com> wrote:
>>> The ARP/Rhodes Chroma VCO had a unique charge pump VCO core that had
>>> a long reset time, but the reset didn't introduce a tuning delay in
>>> the cycle.
>> That's interesting, a neat way of getting rid of the time issue - but
>> why isn't this more widely used? Were there other drawbacks or
>> imperfections that made it less practical?
>> Speaking of the ARP/Rhodes Chroma,
>> if anybody has the service manual in a better PDF form than
>> one-PDF-per-page then I'd be more than happy to take a look. :-)
>> Impressive info site, however:
>> http://www.rhodeschroma.com/?id=servicemanual
>> /mr
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