VFO

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From: Jim Kearman (jkearman@arrl.org)
Date: Fri Apr 23 1993 - 15:13:37 EDT


In response to my earlier posting about the GRC-109, Jeff Herman
<jherman@uhunix.uhcc.Hawaii.Edu> asked me:

>
>Jim,
> Could you give me the reference to the vfo you'll be building? Thanks.
>
>Jeff, NH6IL jherman@Hawaii.Edu
>
>
As this info may be of interest to other readers, I'm sure Jeff
won't mind my posting to the QRP group. In recent editions of
The ARRL Handbook, there are plans for a receiver by K5IRK and
W7ZOI. Actually, these circuits are a bonanza for the homebrewer!
You'll find optimized VFO and crystal oscillator circuits, mixers,
RF and audio amps--you name it. The VFO is one I have used many
times.

Actually, I could use the most-recent implementation right now.
I built a 5-MHz VFO and 12-MHz crystal oscillator based on the
ones in the article and mixed them for a 7-MHz output, mainly
to get some experience with heterodyning. I used an SBL-1 diode
mixer, followed by a few stages of amplification, then into an
IRF-510 MOSFET from Radio Shack (circuit from Three Fine Mice,
now in QRP Classics) for a neat 5-watt transmitter. I'm thinking
of stealing the 7-MHz RF from this transmitter to use as a VFO
but I'm afraid I'll NEVER build another one for the GRC-109!

The obsessive-compulsive side of me wants to build a multiband
heterodyne VFO for use with the GRC-109; the ham side of me says
make a bandswitched VFO. If done properly, it won't drift. When
I'm QRP I don't usually operate near band edges anyway, so who
needs perfect dial calibration? I'm planning to use a WW2 surplus
vernier drive to have an appearance like the 109. I think I'll use
a two-section variable cap, so I can have at least two tank
circuits that aren't switched. With some cleverness, I might be
able to cover 80 (3.5-3.7), 40 (7.0-7.1), 30 (5.5-5.6), 20
(7.0 x 2), 17 (4.5-4.6) and 15 (7.0 x 3) without much switching.

I could stick to three ranges, but the tuning rate would be too
fast if I multiplied from 3.5 to 21. By the time I learn to "fine
tune" the receiver, I'll be able to pick safes!

72,

Jim


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