[Elecraft] How varactors work - was K2 drif problems?

[Paul Saville]

My guess is that the "speed"of the varactor diode is too slow for it to
respond to bias voltage changes at RF frequencies. When the bias voltage
changes, electrons have to move around to make the depletion layer in the PN
junction thicker and thinner to change the capacitance, which takes a
certain amount of time.  I have seen varactors used to produce FM
modulation, so they must be fast enough to respond to audio frequencies, but
at RF the voltage change is just too quick for the diode to keep up with.
It's a bit like driving a car. When you apply throttle/accelerator it takes
a while for the car to respond and reach your desired speed (unless you are
driving a Holden Monaro / Pontiac GTO of course, in which case the response
is instantaneous).

Regards & Merry Christmas de Paul ZL3IN

----- Original Message -----
From: "Edmund H. Ramm" <ehramm@dk3uz.shnet.org>
>
>    ok, maybe I'll get an answer here. So far I completely fail to under-
> stand how a varactor in an oscillator's frequency determinant circuit
> can work without introducing a load of "dirt" effects.
>
>    A varactor's capacity is controlled by the (reverse) bias, the
> higher the voltage, the lower the capacity, right? The rf voltage
> across the tuned circuit's capacitor (said varactor) constantly adds
> or subtracts itself to/from the bias voltage once per half wave. To
> me this means a varactor controlled oscillator must output a whole
> frequency band, i.e. lots of noise sidebands.