I was looking for varactors for UHF VCOs a couple of years ago. The lowest capacity parts that I found were GaAs varactors.
73,
John KD6OZH
----- Original Message ----- From: "Tom Clark, K3IO" [email protected] To: "AMSAT Eagle" [email protected] Sent: Monday, January 07, 2008 04:25 UTC Subject: [eagle] SBD Varactor Help
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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