VCO Tuner

This is a utility module to aid in adjusting the 1v/octave response of VCO's. It uses the most sensitive of instruments - your ear's discrimination of pitch. I heard somebody on the Synth-DIY list mention this tuning method, but I don't remember who it was.

Status
  • Breadboarded and tested
    		•

    How It Works

    In the top half of the Reference and Clock schematic, U3:B and U3:C make up a simple schmitt trigger - integrator LFO, which feeds flip flop U5:A to give a nice two phase clock. These clock the analog switches. An LED gives a visual indication of the rate.

    On the bottom half, U1 provides a stable +5V (you can use other reference IC's besides the TL431(, which is inverted by U2:B to give -5V. This is fed to the CV ADJ pot, which is summed into summing amplifier U3:A, so it's output is adjustable from 0v to +5v. The -5v also feeds a voltage divider made of R6 and the 1V ADJ trimpot. This is adjusted to output a precise -1.000v. It is buffered by voltage follower U2:A. The analog switches U7:A and U7:B are arranged so that the voltage feeding the summing amplifier U3:A alternates between 0v and -1.000v as the clock switches. Therefore the output of U3:A will have a precise 1v step riding on top a level set by the CV ADJ pot. Feeding this into a VCO's CV input would give you an octave jump.

    The RUN/MANUAL switch selects the LFO clock (RUN) or a MANUAL control of the state. In MANUAL mode, the ZERO/+1 OCT switch controls the lo/hi state. An LED indicates when the state is Hi (in either clocked or manual mode).

    Refering to the Octave Divider schematic, the VCO output is fed into schmitt trigger U4 (which turns any waveform into a pulse. This feeds the other half of flip flop U5, which performs a divide by two frequency division. The analog switches U7:C and U7:D alternative select the "straight" VCO signal, and the octave divided signal, and feed it into summing amplifier U3:D. The analog switches are clocked such that the frequency divided signal is selected when the CV output is in the high state. This sums +1v into CV OUT, but the octave divider shifts the signal back down to the original frequency.

    Calibration

    Set the RUN/MANUAL switch to MANUAL, and the state switch to ZERO. Adjust the CV ADJ pot to its minimum (0v) position. Place a DVM on the CV OUT output. Adjust the OFFSET trimpot until you get a precise 0v out.

    Now switch the state to +1 OCT. Adjust the 1V ADJ trimpot until you get a precise +1.000v at U3:1 (CV OUT).

    Using the VCO Tuner

    1. Adjust the frequency controls on the VCO until they are outputting 0v. You want to do the initial tuning from the 0v point.
    2. Patch the CV OUT into the VCO's CV input

    3. Patch the VCO output into the VCO input on the tuner
    4. Run the VCO OUT on the tuner into an amplifier so you can listen to the output
    5. Switch to RUN mode
    6. You will hear the output alternate between two close tones as the clock switches. Adjust the RATE pot for about 2Hz
    7. Adjust your VCO V/OCTAVE scale trimpot until the two tones are equal
    If you do not have a HF Tracking adjustment on your VCO, you are done. If you do,
    1. Adjust the CV ADJ pot so you have a higher frequency - but not so high that you can't accurately here the frequency difference as the tuner switches state
    2. Adjust your HF Track trimpot until the two tones are the same frequency
    3. Often there is some interaction between the V/OCT scaling and the HF Track adjustment. Set the CV ADJ back down to minimum, and readjust the V/OCT scaling until the two frequencies are equal
    4. Repeat steps 1-3 until you are satisfied
    ©2002 Scott Bernardi