Synthesizer VCO Scaler

One of my least favorite jobs after a build or repair is scaling the VCO. It's typically an iterative process, especially if one is also adjusting a low frequency or high frequency trim. It takes too much time and candidly I forget which way I'm supposed to turn the trimmer. I do like the divide by two methodology and have that written up on my Modular Synth Tips as VCO Calibration. I have enough modules in my 5U system to implement this method but it only covers 1 octave.

I decided to design my own scaler. Six pushbuttons provide six octaves of CV. A switch selects 1V/Oct or 1.2V/Oct. A precision voltage reference is used to provide a 5.000 or 6.000 voltage standard. The VCO signal goes through a comparator to square it up and provide a uniform signal output. The switches tap different outputs of a divide by two chain.

I designed this several years ago but put off building it as I didn't want to spend time designing a PCB and wasn't looking forward to hand wiring it. I also couldn't find an IC to do the six pushbutton switches and finally wrote code for an ATTINY processor. I use this on the bench where I have access to a power supply, oscilloscope, and frequency counter. This could be made into a 4U or 5U module. In that case a built-in frequency counter might be a nice addition.

Chip at MEMS offered to lay out the PCB and I took him up on the offer. We haven't decided what the future for this is yet.



I call this a 6 octave scaler since there are six CV outputs but technically it is Root + 5 octaves. This video shows the results of scaling a 208 including the HF trim over 6 CVs/5 octaves. It was way faster, more accurate, and much much easier to trim and tweak the MO and CO. It's a bit hard to hear in the video but the root frequency is 65 Hz, so 5 octaves up should be 2080 Hz and I was measuring 2079 Hz, 1 Hz off, or less than 1 cent.