On Fri, 9 Jan 2004 16:31:56 -0800, "John Moriarity"
wrote:

One technique to improve the phase noise in wide-band VCO's was shown by
Ulrich Rhode. He uses several diodes in parallel in order to decrease the RF
current in each diode, reportedly lowering losses and improving noise. I
never tried it myself.

Or is it simply that when using multiple varactors in parallel, the
amount of capacitance needed from each varactor is reduced, hence a
higher tuning voltage must be used with the same inductance ? With a
high tuning voltage, the capacitance/voltage ratio is smaller, hence
the capacitance difference would be smaller on the separate RF half
cycles. This would reduce the phase noise.

If the diodes are the limiting factor, it can help.
The second diode seemed to do the most good,
the third, a little more, but any more didn't help
(in my designs).

Your observations would support my theory, since adding further
varactors would only increase the tuning voltage slightly with the
same inductance.

Paul OH3LWR