[sdiy] Question about my high freq compensation

[Oren Leavitt]

Hi Ray,

It looks like R13(2) will have to be connected to the output of the exponential servo (IC1-A) instead of that of the CV summer (IC1-B) for this to work correctly.

In looking at the VCO core, with an MPF102 JFET with a few hundred ohms of 'on' resistance and a 0.02uF timing cap, the discharge time may be quite large, requiring alot of HF compensation.
The discharge time, the pulse appearing at (syo), should ideally be less than a microsecond.

HTH,
Oren

-----Original Message-----
From: Ray Wilson <raywilson at comcast.net>
Sent: Dec 8, 2003 11:09 PM
To: Synth-Diy <synth-diy at dropmix.xs4all.nl>
Subject: [sdiy] Question about my high freq compensation

Hi List

I would greatly appreciate it if some of you really smart people would look
at my oscillator here:

http://www.musicfromouterspace.com/analogsynth/OctVCO_LM394_FixedSquare.html

and let me know if I have goofed on the high frequency compensation.

My explanation (which may be incorrect is this) is this:

Here is what I think is happening. I know the circuit looks wierd but  you
must remember that as the voltage gets higher on the inputs of the inverting
summer made up of IC1-B and associated resistors that the voltage on the
output of IC1-B gets lower and lower. I think the high freq compensation
works by adding a secondary path for current to pull the base of the top
transistor a little lower as the input voltage gets higher and subsequently
the frequency gets higher and higher so that the discharge time of the
integrating capacitor (which becomes more significant at higher frequencies)
is compensated for by causing the oscillator to oscillate a bit faster than
it would without the secodary current path.

Please let me know if I have goofed so I can fix it or if I got it right in
which case I can have a beer and celebrate.

Thanks and Cheers

Ray