[sdiy] VCO HF compensation - Rossum vs. Franco

Magnus Danielson magnus at rubidium.dyndns.org
Sat Jun 14 17:42:31 CEST 2014


Justin,

On 06/14/2014 05:15 PM, Justin Owen wrote:
> Anyone got any insights/thoughts/comments on the pros & cons of these two different HF compensation methods when used with an NPN expo converter running a Tri core VCO?
>
> Sims show pretty close frequency results - so far, maybe a little bit better with the Franco integrator R - but that's at the costs of some waveform deformation.
>
> For the Rossum diode I'm running the output of the OA with the first transistor in it's feedback loop, into the Cathode of the diode into a voltage divider and then into the non-inverting input of the exponential CV summing amp. Seen a few different variations on this - but this is the one that seems to work best.

They work on different aspects.

The Franco resistor aims to cancel the effect of the finite reset-time 
of the integrator. It's a clever solution as it will offset the voltage 
with the charge-current such that the charge-time will be shorter for 
higher frequencies, compensating for the reset-time which adds a fixed 
part of the time for every cycle. For higher frequencies, the reset time 
would have a larger part of the period-time of the signal, so the offset 
error would become severer, but as the loading current is supposed to be 
linear to the expected frequence, the series-resistor will shorten the 
load-period accordingly. The compensation forms a form of wave 
distorsion, but as this becomes apparent at many kHz up, their effect on 
overtones can usually be ignored.

The Rossum diode (as I recall it, so it's fuzzy), which occurs in 
various forms, aims originally to compensate for the bulk resistance in 
the expo-converter transistors. It used to have a significant effect for 
cheap transistors, but as transistor pairs made for good 
expo-conversions appeared, with thermal-gradient compensation patterns 
etc, the bulk resistance was made much lower.

Modern expos would suggest Franco-resistor.

Cheers,
Magnus



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