Calibrating CGS48 VCO

Audent · Joined: Dec 19, 2007 Posts: 24 Location: oh-hi-oh

I just finished stuffing my vco boards, applied power and all is well! Such a lovely sounding square coming out of this circuit.

So far so good...until i tried to calibrate the vco.

I used Ken's instructions from the CGS site....

so....first I measured pin 7 and adjusted the "Zero" trimmer until i hit 0V....at this point, the vco was somewhere in the 13K range.

next bit of business was to apply 1v to the v/oct input...and use the "Span" trimmer to set the oscillator to play an octave higer with the 1v input....

....now, unfortunately, i don't have the hearing range of a dog- so this was pretty impossible....so i grabbed the tune knob and took the oscillator down to about 500hz and tried to set the octave with "Span". Only the "Span" trimmer is acting more like an offset- it seems to tune the oscillator and CV with or without voltage applied- so my interval always stays the same. Its always about a half step off. Evil or Very Mad

i checked my soldering, connections- i don't see anything glaringly wrong...anyone here have any suggestions on where i might start on this?

much appreciated as always!

andrewF · Posted: Fri Jul 10, 2009 2:42 am Post subject:

I can't do it by ear either Very Happy

have done it with a 'scope and a calculator
but an easy way (for me)
is with a chromatic tuner and a multimeter.

Audent · Joined: Dec 19, 2007 Posts: 24 Location: oh-hi-oh

I just tried the calculator/MM method, and it worked pretty well! I found it difficult to get it PERFECTLY in tune, but this first VCO is pretty good over the 4 octaves i tested it over.

I guess I was running into trouble because I wasn't keeping 0V on pin 7 of IC1. i kept adjusting the coarse tune into a more audible range which, in turn, affected the tuning span (if that makes sense)

Thanks, andrew- your reply helped me quite a bit! Cool

zthee · Joined: Feb 20, 2008 Posts: 414 Location: Stockholm

Did you do this in the 13K range?

I've got the same problem. With 0V on pin 7 I see 9.821KHz on my frequency counter - and since it won't go beyond 10KHz I can't see what I'm doing.

Audent · Joined: Dec 19, 2007 Posts: 24 Location: oh-hi-oh

zthee · Joined: Feb 20, 2008 Posts: 414 Location: Stockholm

Thanks for the quick reply!

I'll just have to borrow a better meter from a friend Cool

druish · Joined: Jun 23, 2009 Posts: 91 Location: New Paltz NY

what is the calculator/multimeter method?

andrewF · Posted: Fri Sep 18, 2009 5:27 am Post subject:

i'm curious too Surprised

the multimeter is used to ensure you are putting 1V, 2V, 3V, etc into the CV input. I have used an oscilloscope to find the wavelength and then calculate the frequency. Much easier is an electronic tuner. set up the VCO on your favorite note ie - C, with nothing on the CV input. plug in 1V and see if the tuner indicator jumps to the next C, adjust the span trimpot until it does.

numbertalk · Posted: Fri Sep 18, 2009 5:36 am Post subject:

My Fluke MM has a frequency counter, which really comes in handy for calibrating V/Oct on a VCO, but I am also curious what the calculator is used for as well as if the MM as mentioned here is used to verify the V/Oct input voltage. More info greatly appreciated - always eager to learn new tricks that make calibrating the scaling on a VCO a little easier! It's really easy for me to get wrapped up and spend way too much time suffering over it, especially with the frequency counter and trying to get the frequencies to match up just right for every octave, which of course they won't, so I have to tell myself to be happy when it's off by such a small amount and more than close enough to sound in tune. I'm a little uptight like that Smile

andrewF · Posted: Fri Sep 18, 2009 7:03 am Post subject:

on the 'scope you can measure the period of the wave. Lets say Period = x.
So frequency = 1/x
easiest to do on a calculator Smile

of course a frequency meter will be quicker and more accurate, if you have one.

better explanation here

numbertalk · Posted: Fri Sep 18, 2009 7:20 am Post subject:

Ah right, of course. I do this all the time - duh.

druish · Joined: Jun 23, 2009 Posts: 91 Location: New Paltz NY

wow, just go dne tuning my cgs vco this way. Soo easy and I finally got to use the freq meter on my mm. Thanks.

rjd2 · Joined: Sep 02, 2007 Posts: 236 Location: philly

im still having trouble on this. hopefully someone can point me in the right direction? actions:

1-set coarse/fine pots to 12 oclock. trim ZERO to 0v. leave coarse/fine pots at 12.

2-apply 1V,2V, or 3V to CV in. measure V at pin 7 of IC 1.

when i trim SPAN to achieve 1V/oct, if i turn SPAN totally clockwise, the closest i can get is this:

input of 1=1.05V, or 2V=2.1V, or 3V=3.15V. unuseable, basically.

i have confirmed im sending exactly 1,2, or 3V in. even if i turn the coarse pot down to audible frequency, i get the same results. so the SPAN pot works as a span, but with the pot at 0 ohms, i still cant get down to 3V in=3V out at pin 7 of IC 1.

any thoughts?

druish · Joined: Jun 23, 2009 Posts: 91 Location: New Paltz NY

After setting the voltage to 0V at PIN 7, which you did, you have to connect your meter to the OUTPUT of the OSCILLATOR. This is NOT at PIN 7 of IC1. If I understand correctly this is what you are doing? Connect your meter to the square wave output jack. Then tune the SPAN based on those voltage values. Razz

rjd2 · Joined: Sep 02, 2007 Posts: 236 Location: philly

druish · Joined: Jun 23, 2009 Posts: 91 Location: New Paltz NY

Ah, I understand what you are doing. If you are trying to measure the voltage change per octave before the exponential converter, i.e. roughly 18mv per Volt of input, then you should attach the scope onto the end of the Tempco (not ground side) because the 56k resistor/Tempco combination is scaling down the voltage going to the linear to exponential current converter. This is the voltage that you want to measure if you want to do it that way.

rjd2 · Joined: Sep 02, 2007 Posts: 236 Location: philly

ahh, ok.

SO, if i bump the 100K R directly above IC1 down to 90k, i am able to scale to 1v/OCT; this puts me into a workable range using SPAN. finally!

i dont know if this is an artifact of being on +/-15V, or what, but it seems to be the case w/ all 3 CGS 48 VCO's ive built; they all seem to "top out" at about .9V/OCT in stock format. maybe i fudged a value here or there on an R, but it doesnt appear to be the case. all my R's before and after span are exactly to spec. whatever, it works!