On Wed, 05 May 2004 23:21:02 +0100, Paul Burridge
wrote:

On Wed, 05 May 2004 18:45:36 GMT, (John Crighton)
wrote:

From an attenuator a large range is undesirable. The opposite
is what you want.

Oh, is that because the attenuator's maybe in parallel with the output
signal and earth?

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.

Someone may have transmitted into the generator or connected
it to a large DC voltage (wihout a DC blocking capacitor) when
testing a radio. That sort of thing happens all the time.

In one place I worked at there was an HP 8640B sig gen.
It had reverse RF protection and DC protection. You could
hear a distinctive loud click when the protection circuit
operated. The guy using the HP8640B would then look
sheepish and embarrassed because everyone in the work
shop, boss and all, knew he had shoved RF or DC up the
brand new (at that time) signal generator. All the other
signal generators were being silently zapped in that Racal
two-way radio factory. The man fixing the many signal
generators in that factory got the ****s after a while. He
insisted that an external pad be used. Meaning, all he
had to do was open up a tiny external box and replace
the burnt out resistors and not open up the sig gen.

The point I am making is all attenuators are suspect
because they are so easily damaged, I have done
it my self. Pushed some stuff on the bench to one
side and the mic button on a transciever connected
to the sig gen got pressed.


Lets just call the RF output Z for this unit 75 ohms Nominal. :-)
And you have crappy pots to put up with. Not good.

Not good but not serious! I can always just replace the pots with ones
of equal values to the old ones - and better quality too.

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.

This unit is a bit cheap in the attenuator department.
Pots for both course and fine controls. I don't like that idea.
I prefer a switched attenuator for course and a pot for fine.
I'd much prefer a switch for fine also but that would be
too expensive.

My Marconi uses two rotary-switched attenuator controls, actually.
What's the problem with pots? Switches can go noisy too...

We are talking RF here!
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.


Do a simple leakage test to see if this particular AVO
sig gen is worth keeping. On the bench beside the signal
generator place a radio and see if you can pick up the
signal from the generator. If a signal roars into the radio
then it is next to useless for serious or even hobby radio
work. Shield the RF output connector or short it.
( attenuator set to minimum output)
If it leaks RF and the frequency drifts a lot then you
might as well forget it.

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.

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.

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

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. 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. :-)

Regards,
John Crighton
Sydney