Hi,
As part of an amateur radio project I'm trying to design a VFO with varactor tuning for the 80 m amateur band (3.5 - 3.8 MHz). I have a question which is perhaps very basic: In a common emitter Colpitts oscillator (for example) voltage swings comparable with the supply voltage occur across the tuned circuit. This is a little difficult if trying to tune the oscillator with a varactor which (to remain reasonably linear) needs to see small voltage swings - maybe of the order of a volt or less. Various practical ways round this can be dreamt up, but none of the published designs seems to take account of the problem. Am I missing something? Thanks, Chris.

 

When using a varactor to fine tune, both sides of it are isolated from the tank circuit by series capacitors. There is no DC connection.

 

The reason for the small voltage swing is to preserve the linearity of the waveform, particularly when the Varactors supplies most of the resinating capacitance.

What might help is to use the lowest power supply voltage to the oscillator, but if necessary apply a much higher tuning voltage.

 

When using a varactor to fine tune, both sides of it are isolated from the tank circuit by series capacitors. There is no DC connection.

Many thanks.

 

The reason for the small voltage swing is to preserve the linearity of the waveform, particularly when the Varactors supplies most of the resonating capacitance.

What might help is to use the lowest power supply voltage to the oscillator, but if necessary apply a much higher tuning voltage.

Many thanks.