Tuning a voltage controlled oscillator VCO

Having recently completed building some voltage controlled oscillators I was eager to get them calibrated so I could start using them but after reading on the internet it didn’t seem as though it would be quite as simple as I had initially thought. There were a few articles but they seemed to me to make it sound difficult and it also looked like you needed lots of expensive equipment. One had an expensive frequency counter, complex oscilloscope an equally expensive voltmeter. Other suggested that it would take a couple of hours to tune a single VCO. I had eight to tune so I was a little worried. Sixteen hours work!

High frequency trim

I couldn’t quite understand how and why I’d be using the high frequency trim or where it should be set to start with. Anyway I managed in the end and it wasn’t as bad as I had expected. It took me a couple of hours to tune all eight in the end.

The initial idea was to use my digital tuner, midi keyboard and converter. This would have been a cheap, simple and accurate way of doing it but I realised early on that my midi converter wasn’t up to the job. That was after I’d starting worrying that it was the oscillators.

In the end I used a voltage source that I built switched between voltages 1-7 in one volt steps and used the frequency counter built into my cheap and cheerful pocket oscilloscope. Which proved again very useful for audio applications I have a review of the device here if you’re interested.

All of the stuff I read about the HF trim was about the fact that as the VCO got into high frequencies it would start to go “flat” due to various reasons and HF trim was there to correct it. It would also affect the volts per octave scaling once you tweaked it so it would be necessary to go between the two to get a compromise. Since the HF pre-set would sharpen the scale it seemed a fair bet that you should start with it so it produced no effect on the scale so I turned it to the end of its scale which produced the lowest oscillator pitch. I’m sure this was correct as I tried the whole procedure with it set to its other extreme and set in the middle but didn’t get very good results tuning the VCO.

Tips to help with tuning a VCO

Another huge tip I wish I could have given myself was to mark the pre-sets you need to be tweaking with masking tape that you can write on it, there are five pre-sets on my board and with eight boards it was too easy to get the wrong one on the wrong board and undo all the good work.

The second tip is to write down what the pre-set do to the frequency and what they should do. Maybe I’m just stupid but I got confused in the middle and lost the plot.

Third tip is to use on of the pre-set adjustment tools. You can use a small screwdriver but it’s a lot easier when you’ve got a lot of adjustments to make to use the proper thing as it doesn’t slip off the end like a screwdriver does. It can be quite frustrating using a screwdriver, they are also insulated so if you ever have to adjust pre-set capacitors you will be able to do it without your hand interfering with the result.

 

I tested the pre-sets and ended up with a piece of paper with,

“If octave is too small i.e. flat turn pre-set 2 anti-clockwise”

“If octave is too large i.e. sharp turn pre-set 2 clockwise”

I then knew exactly what I was doing from then on.

I also added

1v 220Hz

2v 440Hz

3v 880Hz

4v 1760Hz

5v 3520Hz

6v 7040Hz

7v 14080Hz

Having a voltage source that you are sure is producing 1 voltage increments also helped immensely. I started with 1 volt on the input and set the set the VCO frequency manually using the coarse and fine controls to 220 Hz. Then when you switch the voltage source to 2 volts it’s easy to see where you are. If it’s less than 440 Hz I needed to turn pre-set 2 anti-clockwise as I’d set the volts per octave pre-sets all to halfway all the oscillators were flat to begin with. After a while you get a feeling of how much you need to tweak it. Eventually you’ll get it as good as you can, don’t try to be too precise, things will drift slightly and everything in life isn’t perfect the closer you look also things won’t be perfectly linear.

After I had finished I’m sure I was closer with my oscillators than any of the analogue synths I had previously owned.

I noticed that at six and seven volts the frequencies were starting to get a little sharper so my range wasn’t perfectly linear. This happened for all the oscillators consistently and tweaking the high frequency adjust made the matter worse, this was contrary to what I had expected I had thought that they should be flat as that was a puzzle overall though it was pretty marginal. I started to recalibrate one of the oscillators with the HF pre-set starting in other positions but I never got as good results as to when I started with it low and leaving it there.

Power supply problems when tuning a VCO

Another problem I had initially was that after I first built the oscillators I had all eight connected to the same power supply. The frequencies were all over the place and continually fluctuating making setting up and calibration impossible. Not knowing what the problem was I disconnected them all and just started looking at one. On its own the frequency was perfect. I thought it might be a power supply problem, maybe it wasn’t stably enough or not supplying enough current. It turned out that it was capable of over 1 amp and the oscillators were only using a few milliamps. It was also perfectly stable so I was and still a to some extent confused. I found out that I could put four oscillators on the power supply but any more and the oscillator’s frequency started to bounce around. Maybe they were interacting with each other. I did pose the question on a synthesizer building forum but unfortunately didn’t get an answer so the easiest cure for me was to use two separate power supplies. That cured the problem whatever it was, I still don’t know. Worth bearing in mind if you encounter problems.

Linearity and MIDI to CV converters

With a calibrated VCO it was apparent how non-linear my midi to CV converter was. While two notes an octave apart were spot on some of the notes between were quite a bit off. I think this must be down to the digital to analog converter used in the circuit.