[eagle] Re: SBD Varactor Help

John B. Stephensen kd6ozh at comcast.net
Sun Jan 6 22:16:32 PST 2008


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" <k3io at verizon.net>
To: "AMSAT Eagle" <Eagle at amsat.org>
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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