Hi Paul,

I'm currently experimenting with VCO circuits (had an earlier thread about
varactors). My current prototype uses a MPF5179, a pair of BB909 tuning
diodes and a home made inductor. The inductor is about 4 turns on 0.25"
centre (air cored), spread out to about 0.4" in length, giving about 150nH.
This circuit operates to 350MHz and (if the tuning diode info is OK)
indicates strays of less than 1pF. This is built "dead bug" style, and is
really my first attempt at anything of this sort of high frequency.

I'm sure with a little care 450MHz is quite achievable.

James.

"Paul Burridge" wrote in message
...
Hi guys,

I've just been doing some calculations out of sheer curiosity. It
turns out that to make a tank circuit for 450Mhz (which isn't *that*
high a frequency by today's standards) would take 5pF || 25nF. These
are both *very* low values. 5pF is getting perilously close to being
seriously affected by circuit board stray capacitance and 25nF isn't
much better; little more than a couple of turns of wire, I guess.
What does one do in such circumstances? Should one be thinking in
terms of etching these values out of the PCB by the time one gets of
to these frequencies, or is it still acceptable to make them up out of
discrete components?

p
--

The day Microsoft make a product that doesn't
suck, they'll be making vacuum cleaners.