|[AMRadio] Re: DDS VFO projects|
k4kyv at charter.net
Mon Apr 9 14:12:59 EDT 2007
> From: SBJohnston at aol.com
> Subject: [AMRadio] DDS VFO projects
> To: amradio at mailman.qth.net
>> In response to some inquiries, here's a webpage summarizing my DDS VFO
> Steve WD8DAS
You need to bypass the cathode of the 12BY7 to ground when the VFO is in
use. The stage will be more stable and possibly deliver better output.
Simply connect a 0.01 mfd capacitor, similar to C5 and C7, between the
cathode (pin 1 of the 12BY7) and the unused contact on S1.
On mine, I use a low impedance VFO about 10 ft. from the transmitter. The
VFO runs about 250 milliwatts into a 50-ohm load. I inject the rf directly
to pin 1 through a 0.05 mfd mica coupling capacitor, and short pin 2
directly to ground. I disconnected the wire from the junction of C3 and C4
that goes to the cathode. This converts the 12BY7 to a grounded grid buffer
stage. Your DDS VFO may not have enough output to do this without an
amplifier between the DDS unit and the 12BY7, so your circuit may work
better, but I would make sure the cathode is at rf ground. Turning S1 back
to the crystal positions in yours restores it to normal crystal control.
I disabled the crystal control function altogether in mine, and used one of
the holes that originally had C1 or C2, to hold remounted C9. As I recall,
I had to replace C9 with another capacitor with a longer shaft because the
original was screwdriver adjustable. But now I can adjust the tuning of
L3/C9 with a tuning knob from the front panel, without having to go inside
the transmitter, remove the cover of the oscillator unit and adjust the slug
in L3. I had to remove some turns from L3 to get it to resonate in the 160m
band. Then I adjusted the slug by setting C9 at mid range, and adjusting
the slug in L3 for resonance at 1900 kHz. I used trial and error when
removing the turns by removing just a few turns at time until I could bring
it into adjustment with the slug, with leeway in both directions of the
tuning range of the slug.
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