VCO High Frequency Tracking Correction

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z 1975, 2010, 2011 Nyle Steiner
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This PCB adds an adjustable correction to help with high frequency tracking. It takes pulses out of the VCO timing circuit, collects a charge on a capacitor, and feeds back a voltage into the VC summing node which increases with frequency. In this way, it corrects for the finite reset time of the VCO.



This module has one input and one output. The input is labeled RST. Connect this signal to the junction between R7, R10, and Q2. The output is labeled SUM. Connect this signal to the VC summing node; the junction of R37, R38, R42, R45, R46. R50, R35, and pin 2 of IC1.

This is a link to a PDF file which shows how it connects into the saw core.

Component Selection


This module was originally built with carbon core, 5% resistors with one or two 1% metal film resistors. So, you have a wide range of options here. I recommend using 1% tolerance, metal film resistors everywhere, but the critical resistors are R37 and R38, input summing resistors. These should ideally be hand matched or purchased to 0.1% tolerance to insure consistent response between the inputs.


There are probably a billion different ceramic capacitors at a place like Mouser. Pick a capacitor that can fit the hole easily, typically 0.1 inch on centers.


Nothing special, 1N4148s are fine.


Your choice for your panel. If you use the panel I laid out, the holes and spacing will work for the Alpha 12 and 16mm pots. You can probably use nicer BTI, Bourns, etc. 9mm pots with "pot chiclets"


Use good trimmers, please, really on this PCB use good trimmers. A good Bourns multi-turn trimmer like Bourns 3296Y series will fit the pad layout and works well.

The original modules used single turn, carbon trimmers. You can get a Bourns model that will fit the pad pattern if you want to. They cost about as much as the nicer multi-turn pots, though. It's a Bourns 3352H series has the appropriate pin spacing/configuration. The 3329H and 3329W series are sealed versions which will work, too.

Build Notes


I assume you know the basics of soldering. I like to insert the low lying parts first, like resistors, diodes, etc. After these, I install the IC sockets. Next capacitors, transistors, connectors. Use a good solder, either an organic flux, which you should wash regularly, or a no-wash flux.

Take a break every so often, wash off the flux if you are using a flux which required cleaning. Double and triple check orientations, pins, and solder joints.

VCO HF Compensation


This PCB has four holes to allow flexible mounting configurations. I suggest using a short standoff and mount the PCB near the VC summing resistors.

This is for the VCO type A, with all the outputs.

You need to connect three wires to the VCO board. (Photos will be up soon). First, connect a wire from the "Sum" pad on the HF PCB to one of the summing resistors on the side which connects to the LM741 input.

Second, connect a wire from the RST pad to the junction of R7, R10 and the base/collector of Q2. One of the resistor leads will be the easiest I think.

Last, you need to run a ground wire between the PCBs. Technically, if you use a metal standoff to mount the HF PCB, the grounds should connect through the standoffs. I suggest you still run a wire. I connected mine to the ground side of R32 , a 68 ohm resistor in the lower left corner of the VCO PCB. (It's part of the pulse wave shaper).

The procedure is comparable for the VCO type B VCOs. The component names will just be different.

In the download section, you will find the link for the schematic and layout for the PCB. You will also find two other schematics, VCO with HF Corr #1 and SteinerVCO with HF Corr #2. These two schematic show the VCO sawtooth core and how this compensation circuit hooks in. HF #1 is the implementation on this PCB.

HF #2 is an alternate way Nyle suggested.  This way involved cutting traces, and a fair amount of hacking. I include it here in case you want to try it, but you do so at your own risk. I didn't try this one, although Nyle said he tried it back when and it worked. HF #1 was implemented in the Steiner Microcon, too so I was pretty confident it would be a good option.

The photos at the right show my VCO with the PCB installed and where I choose to hook my wires. I suggest you make the "sum" wire as short as possible as it is directly feeding the input of the summing op amp and so will be more prone to pick up hum and noise.


The calibration only affects the V/Oct trimming, so do the other calibration first. Then, we just add a few steps to the V/Oct trimming.


First Step: SETUP

  1. You need a control voltage source which will give you a selectable voltage output and a range comparable to that which you use to play the synthesizer.
  2. Turn the HF correction trimmer fully counterclockwise. That is, to ground.
  3. Connect this voltage source to a VC input (not usually VC 3)
  4. Input a known voltage like 0 or 1 volt.
  5. Set the pulse width to something like a 50% duty cycle.
  6. Connect a frequency counter to the pulse output, or if you have a good ear, connect the output to headphones or a speaker.
Second Step: THE TUNING

  1. Press a "low" key, a control voltage around 0 to 1 volt, and tune the Frequency and Fine Frequency knobs to a desired pitch. A good start is around 100 Hz.
  2. Input a voltage twice or four times as great (following a 1 volt per octave scale). I suggest one or two octaves higher.
  3. Adjust R36 (on the PCB if used) or R33 (on the panel if used) to get the proper tracking.  Clockwise on R36 will increase the spread, CCW decreases it. That is, if your low frequency is 200 Hz and your high frequency is 796 Hz, turn R36 CW. Likewise, if your low frequency is 200 Hz and your high frequency is 807 Hz, turn R36 CCW. The oscillator frequency will change when you turn R36. Don't worry about it, this is normal.
  4. Re-input the first voltage. Now, you can either a) reset the Frequency / Fine Frequency to get your original pitch and repeat, or b) just note the new low frequency and multiply it by 2 for one octave, 4 for two octaves, 8 for three octaves, etc. to get the new target pitch for the high end.
  5. It should only take 4 or 5 repetitions or so to get pretty good tracking.

  1. Now, hit the highest note on your voltage source and see how close you are to the proper frequency (4 or 5 octaves higher is pretty good). You will almost certainly be low. Turn the HF correction trimmer clockwise. About half way is a good start. This will probably overshoot your target frequency. Go back to step 1 in the "Second Step", then repeat steps 1 through 5 for the V/Oct trimming because adjusting the HF Correction trimmer will affect the V/Oct. When you get good low frequency tracking, 1 or 2 octaves at about 100 to 200 Hz, then re-do the "Third Step" section until you get the desired HF tracking.
Repeat the "Second Step" and the "Third Step" until you get good low and high end tracking.

Doing this, I was able to achieve excellent tracking over 5 octaves at least. I ended up setting the HF trimmer a little more than half way.

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PCB Values

PCB Names


VCO with HF #1 as implemented

VCO with HF #2, alternate method