Voltage Controlled Oscillator Circuit Explanation

[johnnyblaz]

Hi there!
Attached below is a VCO circuit. Can anyone explain how the components work (especially the resistors and capacitors) and why the resistor and capacitor values are selected so. 
Any kind of help would be greatly appreciated. Thanks!!
P.S (This is not a homework question)

[Ammar]

Big question. You could start by looking at relaxation oscillators: https://en.wikipedia.org/wiki/Relaxation_oscillator

[danadak]

IC2, R5, R7 act as a comparator with hysteresis set by R5, R7.

Tool to determine values      http://sim.okawa-denshi.jp/en/compkeisan.htm

IC1, R1, C1 act as an integrator, to integrate the output of the comparator, to
change the comparator pulse into a ramp.

Q1, R4 used to reset the integrator, ramp it down.


Regards, Dana.

[Marco]

It doesn't really work like a relaxation oscillator.

The left opamp is an integrator, the right opamp is a schmitt trigger. The integrator charges the capacitor with a current equal to (Vin-Vin/2)/R1 when the output is low and discharges the capacitor with a current equal to (Vin*R4/(R1+R4)-Vin/2)/(R1//R4) (// meaning the parallel resistance, also I'm ignoring Q1 resistance for a moment ... it's about 1 Ohm when on). The voltage at the output of the integrator is Vin/2+Vc1, as that voltage reaches either 1/3*Vref or 2/3*Vref (the Schmitt trigger threshold voltages) the output changes from 0 to 1 and vice versa.

Why they say f is independent of R4 I don't know.

[ebclr]

The 1st stage is on integrator and the second stage is a comparator who triggers the FET for inverse integration.

The key componets is C1 and R1 ( they define the frequency

[JPortici]

but adjusting R1 means you have to adjust also R4 to mantain the simmetry of the waveform