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I've successfully built this VCO circuit using an LM13700 that works just fine if the VC (the 510K resistor) is not connected to anything. Needless to say, I'd like to be able to change the frequency of this VCO. However, when it's connected to a voltage source (potentiometer) it doesn't change the frequency but the amplitude. Tweaking the pot just makes the signal 'die'. I suspect the equations at the bottom has a lot to do with the frequency but I just cannot decipher them. What am I doing wrong? How can I change the pitch?

enter image description here

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  • \$\begingroup\$ Did you use an LM13700? \$\endgroup\$
    – jippie
    Commented Jun 22, 2013 at 13:23
  • \$\begingroup\$ Yes, indeed. Sorry, failed to mention. \$\endgroup\$
    – alkopop79
    Commented Jun 22, 2013 at 16:56
  • \$\begingroup\$ What value of potentiometer are you using to set Ic? Are you using 15V? \$\endgroup\$
    – user6972
    Commented Jun 25, 2013 at 23:45
  • \$\begingroup\$ I've been using 9 and 12V and a 1M pot. I've tried potentiometers of different values but doesn't seem to make much difference. There's a similar circuit in the datasheet that produces triangle and square wave that's 90% identical to this one. That works just fine, for some strange reason. \$\endgroup\$
    – alkopop79
    Commented Jun 26, 2013 at 16:52

2 Answers 2

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You have to change C not the R in the pot. In this case its 50pF, you either use a variable capacitor or varactor (to make it voltage controlled). In the last equation Fo it states that freq is determined by the main freq determining element C.

R changes amplitude not frequency this just an oscillator circuit, making C a varactor will convert this to a VCO.

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Set up a (say) 10k ohm potentiometer such that one end is connected to +15v and the other end to a small (say 680 ohm) resistor which in turn connects to -15v. Then connect the wiper of the potentiometer to your 510k ohm input resistor. This input supplies a bias current which then alters the transconductance of the LM13700 OTA, which in turn will vary the frequency for a fixed capacitance.

The reason for the 680 ohm resistor is because in my experience connecting the low end to the -15v rail can take the bias input voltage below the natural voltage of about -14v which sits on pin 1, and stop the system oscillating.

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