Author Topic: Reverse sawtooth Vco won't oscilate below 96 hz  (Read 477 times)

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Offline Ryan OMalleyTopic starter

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Reverse sawtooth Vco won't oscilate below 96 hz
« on: February 08, 2020, 08:50:22 pm »
Hello everyone, I'm currently working on a reverse-sawtooth volts/hz  vco for an analog polyphonic synthesizer that I'm planning to build. I had initially hoped for a frequency range of 32hz to 2500hz, but the circuit will not oscillate below 96 hz. It is a comparator based design that charges 3 capacitors, turns off the input voltage once the capacitors reach 5 volts, then discharges the stored voltage in the capacitors through an npn transistor that is in series with 2 frequency range resistors to ground. the npn transistor is controlled by an op amp with a 0-5 v control voltage on its non-inverting input. I have the circuit built on a breadboard at the moment, and suspecting loose connections, I checked all of the positive, negative, and ground power supply connections and they were fine. My next step was to change the comparator reference to a higher voltage using a 9v battery and a 500k trimpot as a voltage divider. This resulted in stable oscillation at 32 hz, but this caused the circuit to stop oscillating at frequencies above 300hz, and also raised the amplitude to about 8.5 volts, which Is much higher than the 5 volt amplitude i want. I also tried adding more 2700pf capacitors in series with the 3 original ones shown in my schematic, and with 10 capacitors total and a capacitance of 27000pf, the circuit still won't oscillate correctly. I'm not sure what to do next at this point other than possibly adding either a single capacitor or multiple ones with a higher value. I've attached  my schematic for the circuit below, and I greatly appreciate any suggestions or ideas that will help me get this circuit to work correctly. thanks,  Ryan
 


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