Voltage controlled Oscillator Circuit explanation

Discussion in 'General Electronics Chat' started by johnnyblaz, May 27, 2016.

  1. johnnyblaz

    Thread Starter New Member

    Dec 10, 2015
    Hi there!
    Attached below is a VCO circuit. Can anyone explain how the components work 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)
    • VCO.pdf
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  2. Robin Mitchell

    Distinguished Member

    Oct 25, 2009
    Ok here is a quick explanation:

    There are two separate circuits here, an integrator (IC1), and a schmitt trigger (IC2). C1, R1, R2, R3 and R4 determine the rate at which the voltage input is integrated to give a triangle shaped waveform. The schmitt trigger (IC2) is used to discharge the integrator so that the system oscillates.

    1. Integrator output goes low
    2. Integrator output goes below the lower threshold for the schmitt trigger
    3. The schmitt trigger switches on (its an inverting type)
    4. This discharges the integrator so that the output now goes high (gradually)
    5. The integrator output crosses the upper threshold for the schmitt trigger
    6. The schmitt triggers output goes low and stops the discharging of the capacitor C1
    7. The integrator output now begins to go low and back to the beginning
  3. johnnyblaz

    Thread Starter New Member

    Dec 10, 2015
    thanks, but can i have some more clarification here? what's the purpose of C2 and the mosfet? And positive feedback of schmitt trigger? and the currents?
  4. wayneh


    Sep 9, 2010
    C2 helps isolate the power to IC2 from the rest of the circuit, helping to stabilize the reference voltage between R5 and R6, among other things.

    The MOSFET feeds back the state of IC2 to IC1, changing it from charging mode to discharging mode and vice versa.
  5. MrAl

    Distinguished Member

    Jun 17, 2014

    The positive feedback of IC2 is what makes that section a Schmitt Trigger. Without that it would not provide that action. Resistor R7 is the same as the other two resistors there, so the trip points are at 1/3 of VCC and at 2/3 of VCC.