The following figure shows the schematic diagram of voltage controlled oscillator (VCO). It has a good linearity and very compact. If this circuit is properly constructed, even the precision could be better than 0.01%. This schematic gives three different output waveforms : sawtooth, triangle and square which are quite important for measurement devices and music synthetizer :
This Voltage-controlled oscillator consists of a comparator with hysteresis U2 and a mixer (U1). The Field Effect Transistor Q1 will be conducting if U2 has positive output (+15V). When U2 has negative output, the FET will be open (the FET will function as an electronic switch). At pin 2 U1, the voltage will be kept constant at 1/3 Vin which is set by R3 R4 by op-amp feedback mechanism. The capacitor C1 will be charged from Vin to U1 out through R1 and R2 if Q1 is switched. Therefore, the voltage seen at U1 (pin 6) output will increase. Then pin 6 of U1 voltage will reach the upper threshold of U2 Schmidt trigger (the hysteresis is set by R6 R7), the U2 output will swing to V+ and then Q1 will be switched on. Now the current of the capacitor C1 reverse its direction, discharging, and the U1 output will be decreasing. After reaching the lower threshold level of U2 Schmidt trigger then U2 output will swing to V-, switch of the Q1, and then restart the cycle.
When SW1 is open, the sawtooth output will be a triangle wave with the same frequency as the square wave output. If the SW1 is closed, then the C1 discharging will be very fast and the triangle output will change the form into sawtooth with the frequency two times higher than before. The amplitude of sawtooth or triangle output will be more or less about 8.3 V and the square output will be around + – 15 V. Exceptions for R5 R9 and R10 which don’t need high precision, all resistors should be have tolerance on 1% or better.
And then, the output frequency will follow this equation : f =(Vin . R6)/(180.R7.R2.C1)
With the values shown in the circuit diagram, the voltage-frequency conversion rate will be 357Hz/Volt. To perform setting to the R11 potentiometer, short the negative and positive inputs of U1 to ground and adjust R11 to give zero volt reading at U1 output. [Source : Elektor magazine]