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I am following this VCO design I found from a tutorial and for some reason the frequency is way too low. It doesn't state in the design what it should be, but following these instructions it is just couple of Hz. I replaced C1 with 0.01u capacitor and now it is better (30-370Hz), but still that doesn't seem correct. I tried messing around with some of the resistance values, but only managed to change the duty cycles. Is it a problem with the design or am I doing something wrong?

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Updated version of my schematic:

enter image description here

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  • \$\begingroup\$ Why do you think these are incorrect? What frequency were you expecting from this circuit before you built it? Did you identify the time constants involved and what are they? What frequency range are you trying to achieve? \$\endgroup\$
    – user16324
    Commented Aug 3, 2020 at 10:17
  • \$\begingroup\$ It is supposed to be VCO for audio synthesizer. Usually these things operate in audible range so that would be 20Hz-20kHz. Sometimes even higher to use with other amplifiers or oscillators for modulation. \$\endgroup\$
    – somerandomusername
    Commented Aug 3, 2020 at 11:44
  • \$\begingroup\$ Yes it came from a syntheser (the choice of opamp hints at that). But this VCO was obviously designed for effects like vibrato, modulating amplitude or frequency of the audio frequency oscillator. I see no problem with it. \$\endgroup\$
    – user16324
    Commented Aug 3, 2020 at 11:47
  • \$\begingroup\$ I was following this tutorial - youtube.com/watch?v=SE9ZtG0E58g And at the end I see him measuring 3.7kHz square wave, so thats why I presume I might have done something wrong. \$\endgroup\$
    – somerandomusername
    Commented Aug 3, 2020 at 11:53
  • \$\begingroup\$ Not gonna waste time on a Youtube tutorial but are you sure C1 is 1u not 1n? \$\endgroup\$
    – user16324
    Commented Aug 3, 2020 at 17:15

1 Answer 1

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You built it well and measured the frequencies accurately. The value of C1 was originally probably 1nF. The signal is a bit asymmetric because the 57k resistors were originally probably 51k. 57k is not a standard value.

rcvco1

rcvco2

I suggest adding 4 more resistors to make the amplitude of the rectangle and the triangle the same.

rcvco3

With C1=1nF, the frequency can be set in the range of 200Hz ... 5kHz.

rcvco3a

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  • \$\begingroup\$ Oh wow! Thank you, this is above and beyond of the help I expected! I will add a switch to toggle between two capacitors, then I could have low and high frequency oscillator in one package. \$\endgroup\$
    – somerandomusername
    Commented Aug 4, 2020 at 9:52
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    \$\begingroup\$ The output voltage of your amplifier (NE5532) is 2V below the supply voltage range (2 ... 10V @ 12V). The control of the transistor is thus at the limit. Try placing a 22k resistor between the base and emitter of the transistor. \$\endgroup\$
    – csabahu
    Commented Aug 4, 2020 at 13:28
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    \$\begingroup\$ You may need an RR I / O amplifier (eg LMC6482) for this schematic, then you no longer need a B-E resistor. The value of this resistor can be critical (depending on the particular transistor).. \$\endgroup\$
    – csabahu
    Commented Aug 4, 2020 at 14:42
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    \$\begingroup\$ @somerandomusername The frequency range is proportional to the input voltage range. 1 ... 12V only causes a 12x frequency change. Levels lower than 1 V can only be performed with very good operational amplifiers (RRIO) and transistor replacement (MOSFET). Try replacing the transistor with a 2N7002 FET. Thus, the voltage can be adjusted between 100mV and 12V (120x). \$\endgroup\$
    – csabahu
    Commented Aug 21, 2020 at 12:41
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    \$\begingroup\$ Thank you. I will update later with my progress. I'm learning a lot with this and maybe someone else will find it useful. Now I'm playing around with adding/subtracting/inverting triangle and square to get interesting waveforms. \$\endgroup\$
    – somerandomusername
    Commented Aug 21, 2020 at 13:51

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