TheoryThere is a neat device for building a voltage, or rather current, controlled oscillator. It's a transconductance amplifier called LM13700. It has a positive and negative voltage input, but a current output. It also has a current bias input. The output current is proportional to the differential input times the current bias input. The LM13700 has a lot of applications such as voltage controlled -amplifier, -resistor and -filter and multiplier.
In the data sheet, page 16, there is a simple VCO, which works very similar to the VCO i built in an earlier blog post.
I did my own schematic of the VCO with pin numbers of the device:
|Voltage (or rather current-) controlled oscillator taken from the LM13700 datasheet.|
I want my oscillator to play between C2, 65.4064Hz, and C6, 1046.50Hz. The frequency of the VCO is: fOSC=IC2/(4CfIARA). The current for the lowest key, VC = 1V, was measured to 64μA, and current for the highest, VC = 5V, was 1.02mA. That means we have 4Cf IA RA = IC2fOSC, with the two values IC2fOSC = 64μA·65.4064Hz =979nC and IC2fOSC = 1.02·mA1046.50Hz = 975nC, which has a mean of 977nC. Our target will therefore be 4CfIARA = 977nC.
We also have that the output amplitudes of the oscillator is A=IARA, so that's a good place to start deciding our component values. If we go by the data sheet and set RA= 4.7kΩ (closest E12 resistor to 5.1kΩ) and we want 1V amplitude, we get IA=1V/4.7kΩ = 213μA. With a 39kΩ resistor connected to positive rail we get IA=231μA, close enough. We now only have to decide the value of Cf.
Cf = IC2fOSC /(4IARA)=977nC/(4·231μA·4.7kΩ) = 225nF. I found two 120nF caps in my junk box, so I'll use them in parallel to get 240nF.
|The LM13700 is hid under the decoupling cap. The black alligator clip is the current source.|
|The two waveforms at VC = 3V.|