On Thu, 06 May 2004 04:12:58 GMT, (John Crighton)
wrote:

You are not following me here, I am talking about accuracy here.
Stick your ohm meter on your Marconi generators RF output
connector and click through the ranges. You will see a nearly
constant resistance reading. A small variation in resistance
reading is acceptable. No variation is prefection. A large variation
in resistance reading, such as short circuit or open circuit in any
attenuator switch position means it is stuffed. Just for interest
tell us what you measure.

Okay, well the vast majority of the positions are bang on 50 ohms.
About 7 are 51 ohms and after that point, the very end of the range
goes one ohm at a time up to 56 ohms as the worst instance. I guess
that's acceptable?

[cautionary tale snipped]

Fine. But why would you bother when you might be able to
pick up a ten times better sig gen for 25 quid. The Marconi
TF144H on ebay, near you, in your country, 2 hours away.
http://cgi.ebay.co.uk/ws/eBayISAPI.d...4746 090&rd=1
Still 0 bids.

How the hell do you find this stuff?
Don't tell me, that was a rhetorical question. :-)
The thing is, John, I rather like the *look* of my AVO. What's
stopping me from just hooking up an external, switched attenuator?
And you say the use of pots is cheapo, but this is an ex-military
piece of kit withe MOD record stamped on the front. The military don't
buy junk - well, apart from Nimrod, that is.

We are talking RF here!

Oh yeah. I'd forgotten about that. Plus the AVO goes up to 250Mhz,
too, so it's even more important. Not bad for its day, though!

If you examine your marconi attenuator you will see it is probably
a beautifully made cast aluminium assembly with a switch built
into it. It will be a top quality switch with beautifull silver
contacts. Each little resistor will have its own little compartment
in that beautifully machined solid block of aluminium. SHIELDING!
We are talking about shielding! A pot just can't compete
with an attenuator like that shielding wise.

Okay, okay.

Er, I *do* have a scope. Might that not be better or are you looking
for something a scope wouldn't show up?

We are talking microvolts here. Your scope, whatever it is,
will have a maximum sensitivity in millivolts. If you see millivolts
of RF by waving your scope probe near your sig gen then
your sig gen is useless. All you will see is a noisy 50 Hz
waveform.
The RF signal should come out of the signal generator
via the attenuator and RF connector and no where else.

It does. There's no leakage whatsoever from the case. Nonetheless, the
attenuator appears to be shot to bits. I either get full output or
nowt but jitter. But I can't bear to junk it so I'm going to (in due
time) try to replace the pots. You can't really blame me, can you?
It'd be a sin to chuck one of these onto the trash heap. It would also
make a very nice looking ornament for the mantlepiece; looks a lot
nicer in real life than that photo I took.

Two way radios, work down to one microvolt or less.
General purpose scopes don't.
If a radio can hear the signal generator and it is not
even connected to it on the desired channel frequency
or any of the IF frequencies. It means an RF signal is
leaking from the case of the generator. The attenuator,
should be the controller of all RF leaving the sig gen.
A leaky signal generator as a service instument for
measuring receiver sensitivity is useless.

It's not leaking one iota.

So before spending any money on the AVO sig gen,
check to see if it leaks RF. If it leaks forget it.

Er, it doesn't. See above.

I only mentioned the Marconi 144H on ebay because it
is easy to repair by hobbyists, performs well, is cheap,
has industry standard 50 ohms output Z. circuit is available,
the unit is for sale just up the M1 high speed motorway
near you.

Hey, "near you" in Australian terms, sure. But this is a very
overcrowded little island indeed! The M1 ain't no "high speed
motorway" except maybe at 3am. And to get on it in the first place I'd
have to circumnavigate the London orbital M25 "The Road to Hell" in
the Chris Rea record (and if you want to know why, just listen to the
intro to part 1 of the record). The only alternative is attempting to
drive straight across London, which would be even worse still.
Yeah, it's a great deal, but the guy won't deliver or post, so he
might as well be on the dark side of the moon as in Bedfordshire, I'm
afraid, notwithstanding it's less than 150 miles away. :-(

I thought I would be helping a beginner who is
struggling with filters and may need a second good sig gen.
I'll get out your hair now. :-)

I was a "beginner" 35 years ago. Obviously I'm not making much
progress. Thanks for the advice, anyway, it's always valued.

p.
--

The BBC: licenced at public expense to spread lies.