Markus L enscribed:
Am planning to build the GDO described by SM0VPO at
http://web.telia.com/~u85920178/use/gdo.htm using a BF245 FET (BC245
mentioned is a typo). What parameters determine the sensitivity of a GDO?
Will the sensitivity go up or down when increasing the gain in the
oscillator stage? Intuitively I would say the gain has to be just high
enough to guarantee a steady oscillation, but maybe I'm totally wrong.

Markus HB9BRJ



You are on the right track. As you probably know, the "grid" dip is a
result of the resonant circuit drawing energy from the oscillator. The
closer the oscillator is to stalling the more sensitive it will be to
dropping out when the dip occurs. The problem is that other factors,
such as proximity to metal objects, other spurious resonances, etc, can
also stop the oscillation. In practice the trick is starting with the
unit near the target circuit and keeping a high enough oscillator gain
to be well above the threshold and sweep the unit to see a small dip.
Then move the coil away from the circuit while reducing the gain and
sweeping to see that the dip frequency stays approximately the same. The
dip should become deeper. At some point the gain will be at the limit of
oscillation and this is the lightest coupling to the circuit, and the
most accurate.
Once you have it built you will find that it is really easy to get a
feel for the operation, more so than a few sentences can convey.
This reminds me that I haven't used a GDO for a few years, having
accumulated a bunch of nice test equipment, but somewhere in my shed is
an old Lafayette 6CW4-based GDO which over the years has helped me get a
load of homebrew gear on the air. That's a lot of QSOs thanks to a
simple piece of equipment!

73 Alf NU8I
Scottsdale AZ DM43an
(G4ABB/ON8KL/VK6GA/PZ5AG/etc 160-10Gigs)