"Paul Burridge" wrote in message
...

Hi gang,

I've never had a lot of luck with GDMs for some reason. Even with a
decent meter, it seems such a drag tuning across such a vast range
looking for a tiny, easily-missed dip which you have to screw out of
the meter by forcing the sensing coil so far into the circuit
concerned you practically break the circuit board. Am I alone in
finding this potentially invaluable device practically useless in
practice? Is there a more viable alternative?

p.
--

First rule is to get a good dip meter- the stuff made for the amateur
community is very poor- the Eicos, Heath Millen etc. Pick up a Measurments
model 59. With this meter you can take a 1/2 wave wire- say at 2M and hold
the meter a couple inches from the center and see a huge dip. Other meters
don't even respond when held to the wire. Dips on conventional L-C circuits
can easily be full scale.

Dale W4OP

THERE MUST HAVE BEEN SOMETHING WRONG WITH YOUR MILLEN. BILL T.

Yep...He probably bought it from you.

Tracy

On Sat, 15 Nov 2003 17:23:50 -0600 (CST), (Bill
Turner) wrote:

THERE MUST HAVE BEEN SOMETHING WRONG WITH YOUR MILLEN. BILL T.

GDOs are sort of dated by now. i build one last year when i was just
getting back into hamming. i found very little use for it over the
last 3 months that i have actively been homebrewing. let me explain
why ...

a gdo is primarily used to check for resonance of a tuned circuit. if
you knew the inductance and the capacitance, you could easily compute
the resonanating frequency youself.

as another poster mentioned, getting a dip is a fight. so, what i do
use is a combination of three things: an rf probe with a high
impedance voltmeter, a test oscillator and a frequency counter. all
these things are in themselves pretty useful. but i seldom go wrong in
getting properly tuned circuits.

i have a test oscillator (see the schematic
http://farhan.net.co.nr/testosc.gif). i plug in a coil with a 330pf
capcitance in series between the base of the vfo transistor and the
ground. and measure the frequency on the counter. that gives me a
pretty accurate (within 1%) measure of the coil's inductance. it
involves a bit of calculating, but once i have cast the values, there
is seldom need to change them. i tend to do simple maths in my head
using 10MHz as starting value for resonance (100pf with 2.5uH). I can
now scale up or down without resorting to a calculator.

As for peaking a circuit, it is best done by the ear or using an
oscilloscope. peaking by the ear is probably the best, if u can manage
it. now, i dont mean to be rude, but frankly very few people have the
ear to be able to tune for best fidelity rather than loudness. it
takes patience and care (i have very little of either). so, i depend
upon a scope. it is a little like knowing morse. it is the best mode
of communicating, but not all want to use it.

a poor man's alternative is using the RF probe. the RF probe will
never show distortions. But it can show clear peaks while tuning up a
circuit. be sure that you also terminate the output of the tuned stage
properly! otherwise you maybe be tuning away from the sweet point.

i would rather that you invested into building a simple PIC based
counter. It is pretty accurate, you will never need to caliberate it.
(I never got around to caliberating mine, it is off by 1.5KHz at
10MHz). That with the test oscillator, you would be completely
informed about your coils. An RF probe is a 10 minute project and if
you already have a good VOM, you might not need a High impedance
voltmeter. I brewed my own voltmeter to keep things completely
homebrewed.

the counter can always be used with all your projects as a standard
read out. The rf probe will the most useful tool in tuning up any
transmitter. The voltmeter is indispensable.

- farhan

Yep...He probably bought it from you.

Tracy

On Sat, 15 Nov 2003 17:23:50 -0600 (CST), (Bill
Turner) wrote:

THERE MUST HAVE BEEN SOMETHING WRONG WITH YOUR MILLEN. BILL T.

On Sat, 15 Nov 2003 17:23:50 -0600 (CST), (Bill
Turner) wrote:

THERE MUST HAVE BEEN SOMETHING WRONG WITH YOUR MILLEN. BILL T.

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

i use my GD-meter when building antennas and tuning
antennas and traps to frequency. Although
the calibration is quite inexact, it's always possible
to listen to the GD-meters frequency on the
receiver.

"Paul Burridge"
On Sat, 15 Nov 2003 17:23:50 -0600 (CST),
Turner) wrote:
THERE MUST HAVE BEEN SOMETHING WRONG WITH YOUR
MILLEN. BILL T.

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
:-(

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

james ) writes:
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:

But is it really modern, or just a rehash of what's come before?

I haven't seen the article, but in thirty years of reading the magazines
(and I've seen plenty of back issues from before that), there has been
very little change. Most of the articles are a small variant on a previous
article, with any real change being about coil forms ("I didn't have what
the previous article used", or "I noticed these things that would make a GDO,
so I built one around them") or variable capacitor.

Admittedly, when solid state devices came along, there had to be some change
since people did want to make use of them. The original ones were likely
pretty bad, using bipolar transistors, and of course there was the Tunnel
diode one from Heathkit. Once FETs came along, the GDOs were back to
basically a tube circuit, albeit with low supply voltage.

There have been the occasional outrageous scheme, switchable coils,
or making use of an existing signal generator or building a whole GDO along
such lines, but they never really held. Next time a GDO article was
published, it was back to simplicity.

If I was building one, I'd make sure it had a good reduction drive. I'd
certainly put in a buffer for an output, as a signal generator or to feed
a frequency counter. The latter then means the dial doesn't require much
effort, and the readout will be much much better than any GDO from before.
Maybe I'd even build a plug-in that has switchable coils, but the whole
thing is shielded, for those times when you just wanted a signal generator.

But I'd also be looking at the circuitry of the Millen Solid-state GDO,
from the early seventies. It was a more extensive design, but of course
it costs virtually nothing for those extra active devices. They found they
had to put a variety of chokes in the thing to isolate the oscillator from
the B+ line, so there weren't false dips.

The Heathkit from the eighties seemed rather interesting. Again, it
was a more complicated design. I can't remember what the extra circuitry
amounted to. That design did generate some similar home made circuits
at the time.

Michael VE2BVW

hi

This gdo uses three fet and runs off two aa batteries, nice project.
The coils are built with bnc connectors.

jimbo


Michael wrote:

james ) writes:
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:

But is it really modern, or just a rehash of what's come before?

I haven't seen the article, but in thirty years of reading the magazines
(and I've seen plenty of back issues from before that), there has been
very little change. Most of the articles are a small variant on a previous
article, with any real change being about coil forms ("I didn't have what
the previous article used", or "I noticed these things that would make a GDO,
so I built one around them") or variable capacitor.

Admittedly, when solid state devices came along, there had to be some change
since people did want to make use of them. The original ones were likely
pretty bad, using bipolar transistors, and of course there was the Tunnel
diode one from Heathkit. Once FETs came along, the GDOs were back to
basically a tube circuit, albeit with low supply voltage.

There have been the occasional outrageous scheme, switchable coils,
or making use of an existing signal generator or building a whole GDO along
such lines, but they never really held. Next time a GDO article was
published, it was back to simplicity.

If I was building one, I'd make sure it had a good reduction drive. I'd
certainly put in a buffer for an output, as a signal generator or to feed
a frequency counter. The latter then means the dial doesn't require much
effort, and the readout will be much much better than any GDO from before.
Maybe I'd even build a plug-in that has switchable coils, but the whole
thing is shielded, for those times when you just wanted a signal generator.

But I'd also be looking at the circuitry of the Millen Solid-state GDO,
from the early seventies. It was a more extensive design, but of course
it costs virtually nothing for those extra active devices. They found they
had to put a variety of chokes in the thing to isolate the oscillator from
the B+ line, so there weren't false dips.

The Heathkit from the eighties seemed rather interesting. Again, it
was a more complicated design. I can't remember what the extra circuitry
amounted to. That design did generate some similar home made circuits
at the time.

Michael VE2BVW