Typically you`d look for one called "Sweep Function Generator": http://www.prime-electronics.com.au/dat ... erator.pdfhttp://www.ebay.com/itm/Protek-2-Mhz-Sw ... 2a25e98d8fhttp://www.ebay.com/itm/HC-Protek-B-801 ... 1c2d1fa2f6http://www.ebay.com/itm/Wavetek-FG3B-Sw ... 337d0ec8d6
The will output an oscillator signal within an extremly wide frequency range, like from seconds to highest audio freqs, so they can do sort of FM modulations also, like with the Doepfer MS 404 for example.
If you look for rates of like 30seconds and more, which is reasonable I think, you have to read the manuals before buying. Some of them or maybe most have a minimum rate at 0.2Hz or 0.1 Hz (5s, 10s).
You can`t draw waveforms, but they usually support square, triangle, Saw, Sine.
The nice thing is, with "Sweep" generators, you can set a start-frequeny and an end-frequency, and the time it should take to sweep between those 2, and whether that sweep is performed linearly or logarithmicaly! This is actually an awesome feature.
They are nice to use, as showing their freq on a nice display, and look super-cool-nerdy, haha!
It could be that on an mc-202, which is a digital device, that VCF input may not allow for audio rate modulation, I don`t know - it depends on the sampling frequency of the mc`s VCF input. It may also be you get weird sampling artefacts at high rates, but that may also be a cool feature :) You could try it first by putting some other synth`s-audio signal into that mc input and check if it works, in case audio rate mod is what you`re looking for.
What you should also know:
1) you have to create a cable on your own. I just used a normal audio cable, broke it up at one side, and somehow taped/clamped the wires onto the generators output. But better is to take a cable that matches the gen`s output, break that up and solder an audio cable onto it.
2) It`s a bit unclear to me if it always works, regarding proper groundings, to not have dc offsets for example. But I think it is very likely that it always does. At least on the MS20 is was just perfect.
I don`t think you can use the xoxbox outputs for anything here.