Introduction

In one of my future projects I will be needing a voltage controlled oscillator (VCO) that will have sufficient stability, low noise, operate up to 100 MHz and a tuning range of 1 MHz.

There are many VCO variants. There are probably good criteria for picking the right one for your application, so I did just that and picked randomly.

Colpitts oscillator

A Colpitts oscillator is an oscillator that has a capacitive voltage divider made of usually two capacitors in series across the inductor.

To change the resonant frequency we can change the inductance and or the capacitance. Usually it is easier to change the capacitance by using the properties of semiconductor junctions to change their junction capacitance with the change of the voltage applied to them.

In the above simulation we can see the changing resonance frequency with the change of the Vtune voltage. The only missing part is a transistor for providing positive feedback for the circuit to oscillate. Adding it, a voltage source and some resistors for biasing the circuit will start to oscillate. Note for new to LTspice: remember to set up Maximum timestamp in your Transient simulation or LTspice will optimize your oscillations to nothing.