# VCO

Discussion in 'General Electronics Chat' started by Anoop Vishnu, Apr 4, 2006.

1. ### Anoop Vishnu Thread Starter New Member

Apr 2, 2006
5
0
Hi all,
I need general description about Voltage controlled oscillator. Please tell me how it function.What are all the different circuits of a VCO? If you can provide me block diagram,it would help me understanding the theory

Anoop

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
There are really only two pieces of a Voltage Controlled Oscillator. If you take any of the standard oscillators, like Colpitts, Heartly, or Pierce you will notice the presence of an amplifier like a tube or a transistor and some reactive components like inductors and capacitors. Negative feedback is used to stabilize amplifiers and positive feedback is used to sustain oscillations. In mathematical terms you want the roots of the characteristic equation to lie on the j-omega axis.

The voltage control part can be done with a varactor diode. This special diode changes its capacitence as a function of its bias voltage. The really cool part about this is that the capacitence affects the AC components of the signal, but not the DC component. So the bias voltage on the varactor diode can be changed and the frequency of oscillation will change.

There are still setups which use air variable capacitors and roller inductors to change the frequency of oscillation. In this case there is a motor which turns a shaft or moves a slider.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Hi,

I don't have a good description of it, but there's another possibility for a VCO. The old ICL8038 used an R-C network to set a base frequency, and a voltage input to shift the oscillator frequency upwards. The charge on a capacitor was monitored by an internal comparator. When it rose to a level, the current path was reversed symmetrically to pull charge off at the same rate. Another comparator revered the process as the capacitor voltage fell to the low level. The external voltage varied pass transistor conduction to speed up the rate of charge/discharge.

The 8038 is out of production, but datasheets are on the web. The device can be purchased from Mouser as an NTE846, identical in operation.