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Reverse sawtooth synthesizer vco now oscillates at low frequencies.
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Ryan OMalley:
Hi everyone, about a month ago I posted that my reverse-sawtooth vco wouldn't run below 96 hz. the fix was to add some hysteresis to the comparator by adding a 10k resistor between the non-inverting and inverting pins, and placing a 0.1uf capacitor across the jfet. the circuit now can oscillate between 16hz to 2900hz, which is a good range for the purpose i'm going to be using it for, an analog polyphonic synthesizer. The only problem that i'm having with it at the moment is that the voltage-controlled current sink is not responding fast enough to control voltage changes. I have tried adding resistors and capacitors in the negative feedback loop, but the circuit still doesn't respond fast enough. I will attach the Revision 2 schematic (in pdf form) below. any suggestions on how to improve the responsiveness of the voltage controlled current sink would be greatly appreciated.
Zero999:
You might get more help if you kept everything related to your project to one, single thread.
duak:
R O',

What are C? and R? for?  If the left side of C? is grounded or connected to a low impedance source, it will add a time delay to the frequency control input.  I would connect the right end of R? to the -input (pin 2) of U1A.

Will the left end of R7 ever go open circuit?

Why is C2 across J113 and not C1?  It is conducting any noise on the +12V into a critical node of the oscillator.  I also question the value of C2.  I seems very large for the frequency range you are after.

R4 has a value of 3M0.  I think this affect the upper frequency limit and would reduce the value to, say 47K.

I can't see how R12 materially affects the hysteresis of the comparator.  I would move it to connect U2B-7 to U3-3 and add a large resistor (3M0 or greater) between U3-3 and U3-1.  This gives some positive feedback.  If this affects the upper frequency limit, try reducing the relocated R12.  Note that this change requires a lower value for R4.

BTW, are all the VEE or - power pins of U1 & U2 connected to -12V?
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