This is a circuit of my own, based on the XR-2206 VCO data sheet examples. Features:
- Coarse and fine frequency controls
- Low/High range switch
- Sine/Triangle switched output
- Square wave output
- Linear, AC-coupled FM input
- Internally normalized for cross modulation
My intended use of these VCOs is as modulation sources for linear FM of other VCOs, such as the Cynthia ZerOscillator and the Fritz Teezer. I was looking for a sine wave VCO for quite a while, but a 1U sine wave VCO in MOTM format is not commonly found. I happened across the XR-2206 in a discussion on Muff Wiggler’s forum and found this hobby store that sells a PC board.
Realizing that I did not need 1V/octave tracking or other high-quality music VCO features, I bought two boards and two XR-2206 chips. (Get the XR-2206 from Mouser for under $5.00 each).
After I got the chips I started breadboarding. Although I used the PC boards, I didn’t follow the original circuit but built my own. I added an FM input, set a fixed amplitude for the sine/triangle output, and selected capacitors for the low and high ranges. The XR-2206 is powered between ground and +15V, so the output is all above ground. The triangle is much larger than the sine wave. So I designed a buffer circuit, using a TL072 on a Tellun MUUB-2 PC board to provide +/-5V sine and triangle waves, and a +/-5V square wave obtained with the comparator circuit that I often use.
Here is the Dual XR-2206 Front Panel Express Panel.
There is a trimpot on the XR-2206 board for the sine shape, and trimpots on the MUUB board for the sine and triangle levels. The sine/triangle switch is DPDT. One side switches the wave selection and the other switches between trimpots on the MUUB, so that the output levels of the sine and triangle can be made the same. The FM input is AC-coupled by necessity, because the initial frequency is set by the combined resistance of the two frequency pots from pin 7 of the XR-2207 to ground. DC-coupling would be tricky to do, but not necessary in this application.
The sine/triangle output of each VCO is connected behind the panel to the switching lug of the other VCO’s FM input to facilitate cross-modulation without need for patch cords or multiples.
Here’s a demo recording of the module. It starts out showing the two frequency ranges of one oscillator, using the sine wave. (The low range is 8 Hz to 400 Hz, the high range 87 Hz to 4300 Hz.) Then the second VCO is brought in. The second then modulates the first, then the first the second, and then each other simultaneously. Then both are switched to triangle and knobs are twiddled a bit.