# VCO explanation

Discussion in 'Electronics Homework Help' started by Achelius, Jun 20, 2013.

1. ### Achelius

2
0
Jun 20, 2013
Hello from Greece !!!

This is a Voltage-controlled oscillator (VCO). Αn oscillator that its frequency is controlled using a voltage signal. The VCO described here provide both triangular and square wave output. The control voltage can be varied between 5 mV to 5 V, to produce oscillation frequency from 10 Hz to 10 KHz.

Can someone please explain to me briefly the operation of the circuit ?

thank you

2. ### Harald KappModeratorModerator

11,517
2,652
Nov 17, 2011
The top opamp is an integrator (using R1, C1 - you can look up the basic circuit on lots of opamp related pages). The input voltage to the integrator is controlled by Q1 and Q2 to be either Vin (Q1 conducting) or GND (Q2 conducting). The switch Q1/Q2 is controlled by the square wave output of the lower opamp which in turn is operated as a schmitt-trigger circuit (feedback resistors R9, R10 - look up the basic operation of an opamp based schmitt-trigger). The schmitt-trigger is triggered by the triangular output of the upper opamp via D3 and D4, thus closing the feedback loop and enabling oscillation.
The other components are for compensating and offset-correcting the upper opamp.

To analyze the circuit, start with any constant input voltage and assume C1 is discharged. Assume further (for simplicity) that the output of the lower opamp is low.
- which of Q1 and Q2 is conducting?
- what will the output voltage of the upper opamp look like (Vout over time)?
- what happens to the lower opamp, when Vout(triangular) reaches the threshold of D3?
- as a consequence of the reaction of the lower opamp, what is the square wave output voltage?
- as a consequwence of the square wave output, what happens to Q1 and Q2?
- what follows for the input voltage of the integrator?
- how will the output of the integrator change in response?

Answer these questions and you will have analyzed the circuit yourself.

Then you will also be able to answer why and how the frequency is controlled by the input voltage. Hint: low voltage -> low frequency.

Last edited: Jun 20, 2013
3. ### Achelius

2
0
Jun 20, 2013
thank you sir. Can you explain me what is the purpose of C2 and R7 filter ?

4. ### Harald KappModeratorModerator

11,517
2,652
Nov 17, 2011
This is the standard frequency compensation for this IC. See datasheet, page 13, figure 38. That figure also shows the offset compensation via the potentiometer.