Hi Paul,

ARRL QST had nice article on a modern GDO

May 2003 QST page 54 A Modern GDO--The "Gate" Dip Oscillator Bloom, Alan, N1AL


73 jimbo

Paul wrote:



I'm not using a Millen and this post isn't a troll as someone else
suggested. The meter I use started out life as a Tradiper (Japanese)
but because it was hopelessly outdated and used old germanium trannies
with enough lead inductance to tune a VoA transmitter, I decided to
rip its guts out and rebuild from scratch.The actual
chassis/meter/facia etc was quite high quality, so it made sense.
I got this nice circuit from the UK equivalent of the ARRL Handbook
and set about building it. It used 2 SK88 FETs and the output of this
oscillator could be adjusted to keep its impedence as high as poss for
each test, thereby giving really good dips when even quite heavily
loaded low Q circuits were tested *provided* they were physically big
enough to shove the sense coil into. The sense coils are about 3/4" in
diameter, which although fine for large, out-of-circuit component
measurements, is *hopeless* for getting in close on a circuit board
with subminature components a fraction of the size. That's the main
problem I face with all GDMs, though: they all seem to have relatively
huge sense coils relative to today's component sizes :-(

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill