HFT

[Rene Schmitz]

At 20:56 29.12.99 -0000, Tony Allgood wrote:
>Hi all,
>
>Take one sawtooth VCO and standard expo. Whats the best way to get high
>frequency trim? The Moog method and others uses a 10K trimmer in series
>with the timing cap, seems OK but changes the output waveform. Rene's
>and Ian Fritz's method has a 1M trimmer & diode attached from the input
>of the expo to the output of the servo op-amp. Seems to alter the pitch
>at any frequency. And then there's Oberheim SEM method which takes a
>sniff of the expo current and passes that back to the CV input. But
>which is best for results and ease of setting up.

The 1M and diode arrangement that I "adapted" from Ian's, takes a sniff 
of the current as well - the voltage at the output of the servo opamp is
nearly 
proportional to the expo current. So this does basically the same thing as
Oberheims 
solution. One could argue that one can neglect the voltage drop across the
diode, 
and thus leaving it out. There is a description (including the maths) in
the archives. 
Also Rick Jansen had a drawing on his page of such a compensation using the
third transistor as the diode. It included some maths, and a reference to
S. Franco "Design with operational amplifiers and analog integrated
circuits". I never tried to find that book. You can find the Rbulk values
of some duals on my homepage.

In my opinion the feedback method has two advantages. First it can be used
with VCOs where the CCO core has two thresholds (i.e. schmitt trigger,
SCR), where the moog method would fail. It would shift both tresholds!
Second it avoids changeing the DC-level/amplitude of the waveform, which is
better for waveshapers. 

Bye
 René

Oh, and have a happy year 2000!






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