[Elecraft] K2 drif problems?

Thu Dec 19 15:12:01 2002

Hi

I think you may be misunderstanding how a varactor works. The bias on the
diode causes the P-N junction to form a capacitor. The dielectric is the
semiconductor itself. Think of it as the higher the reverse bias the thicker
the dielectric so the smaller the capacitance or the lower the bias up to a
point the greater the capacitance. When added to a tank circuit in a
oscillator it acts just like any other capacitor in parallel or series
except by changing the bias you can change the capacitance. If the bias
voltage is held constant the capacitance is constant except for the change
due to temperature. This is where the drift is generated. If the bias
voltage can be compensated to match the change in capacitance due to
temperature then the oscillator will be more stable with temperature.

Don Brown
KD5NDB

----- Original Message -----
From: "Edmund H. Ramm" <[email protected]>
To: <[email protected]>
Sent: Thursday, December 19, 2002 1:28 PM
Subject: Re: [Elecraft] K2 drif problems?

> [email protected] writes:
>
> > [...]
> > Huf-n-puf does not introduce phase noise. It is simply a counter which
> > decides whether to gently increase/decrease voltage across a varacter
which
> > is in parallel with the LC cct of the VFO - in this case an xtal.
>
>    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.
>
>    Apparently it doesn't, or the K2 wouldn't perform as well as it
> does. So, please, what am I missing?
>
>    73, Eddi ._._.
> --
>        email: dk3uz AT darc DOT de  -  AMPRNET: [email protected]
>                Linux/m68k, the best U**x ever to hit an Atari!
>
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