[eagle] SBD Varactor Help

Tom Clark, K3IO k3io at verizon.net
Sun Jan 6 20:25:05 PST 2008

Many of you have seen my idea of a voltage tunable phase shifter for
Eagle's S2 & C-band arrays. If you haven't, a copy of my paper from the
2007 AMSAT Space Symposium is available for download at
http://mysite.verizon.net/w3iwi/electronic_scanning_antennas.pdf. I
haven't had time to do much on the project the past few months, but in
the initial breadboard I found that conventional varactor diodes simply
create to be a discontinuity at the tap. I've done a little testing with
a resistor in series with the varactors, but I have no progress to report.

Matt Ettus and others have been providing me with a lot of reprints on
NLTL (non-linear transmission lines) used to generate picosecond
risetime pulses, and it is very interesting. The web has a lot of
relevant publications, including the voltage-steerable array described
at  http://tempest.das.ucdavis.edu/mmwave/NLTL/NLTL.html. At least these
publications show me that my idea is not totally screwy.

After researching the topic, I have come to realize that what we need
are not varactors used for tuning VHF radios, but rather some diodes
with MUCH smaller junction capacity. This seems to dictate the use of
Schottky Barrier Diodes (SBD) as varactors. Microwave SBDs are used as
parametric frequency multipliers and mixers up into sub-mm wavelengths.
In doing literature searches, I came across the MACOM 44700 series of
diodes (PDF attached) that look pretty good.

I wonder if we have some talent here in the EAGLE group who have
experience with low-capacity (range 0.1-2 pf or so) varactors who can
point the way to some suitable parts we might try.

73 & Happy New Year -- Tom
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