[sdiy] Question about my high freq compensation

[Ray Wilson]

Hi List

I corrected my posted oscillator schematics thanks to help from Oren Leavitt
and Grant Richter. The PC board layouts had the correct connection for the
high freq compensation circuit so I am glad about that. I obviously fat
fingered the schematics.. all fixed.

Ray

----- Original Message ----- 
From: "Ray Wilson" <raywilson at comcast.net>
To: "Oren Leavitt" <oleavitt at ix.netcom.com>; "Synth-Diy"
<synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, December 09, 2003 7:46 PM
Subject: Re: [sdiy] Question about my high freq compensation


> Hi Oren
>
> Thanks for looking at the circuit. You are absolutely correct. The source
> for this circuit originated in Musical Applications of Microprocessors pg.
> 194 top schematic and you are 100% correct. I am going to have to correct
> all of my oscillator schematics and PCBs. All I can say is "Oy Yoy Yoy"
and
> thank you. Grant Richter also suggests that the diode also be in thermal
> contact with the transistor pair package.
>
> Thanks to both of you for checking it out.
>
> Ray
>
>
> ----- Original Message ----- 
> From: "Oren Leavitt" <oleavitt at ix.netcom.com>
> To: "Ray Wilson" <raywilson at comcast.net>; "Synth-Diy"
> <synth-diy at dropmix.xs4all.nl>
> Sent: Tuesday, December 09, 2003 1:08 PM
> Subject: Re: [sdiy] Question about my high freq compensation
>
>
> > 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
> >
> >
> >
> >
> >
> >
>