Measuring RF output impedance

[Paul Burridge]

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?

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.

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...

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?

Regards,

p.

 

[John Crighton]

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 shits 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.

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.dll?ViewItem&category=40004&item=3094746090&rd=1
Still 0 bids.

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.

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

 

[Paul Burridge]

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.dll?ViewItem&category=40004&item=3094746090&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.

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.

 

[John Crighton]

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?

Yes, that is fine. You are laughing with that unit. Great!

The thing is, John, I rather like the *look* of my AVO. What's
stopping me from just hooking up an external, switched attenuator?

Nothing! That is an excellent idea.

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.

Heh heh heh, I can see where you are coming from. Military
buying junk is a whole topic on its own.

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?

I like to see old stuff preserved and working too.

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.

Yes it would be a sin to chuck it. Just don't expect a lot from it.

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

I should have said helping a fellow beginner and hobbyist. I am
in the same boat as you, continually struggling. So I know what
it is like being in the dark and feeling my way.

Getting back to your external attenuator idea.
I often buy dud damaged boat anchor sig gens, for a few dollars
at ham meetings/trash and treasure buy and sells, in the hope of
finding a nice attenuator. I have humped a few monsters home
only to find rubbish inside. I have been lucky a couple of times.
There are plenty of ready made attenuator units for sale
usually for big dollars from a hobbyists point of view.

Have a look at this one.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=40004&item=3812959880&rd=1
How far is Farnam from you? Postage is 4 quid.
No information as to the maximum frequency but if you can
pick it up for a tenner it might be a good buy even if you use
it for low frequncies for something else. I am sure you spend
more that that at your local. Take a chance! Bid 10 max.

Regards,
John Crighton
Sydney

 

[Paul Burridge]

I should have said helping a fellow beginner and hobbyist. I am
in the same boat as you, continually struggling. So I know what
it is like being in the dark and feeling my way.

You could have fooled me. Still I guess everyone continues to learn at
some level or other, no matter how much they know.

Getting back to your external attenuator idea.
I often buy dud damaged boat anchor sig gens, for a few dollars
at ham meetings/trash and treasure buy and sells, in the hope of
finding a nice attenuator. I have humped a few monsters home
only to find rubbish inside. I have been lucky a couple of times.
There are plenty of ready made attenuator units for sale
usually for big dollars from a hobbyists point of view.

Have a look at this one.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=40004&item=3812959880&rd=1
How far is Farnam from you? Postage is 4 quid.
No information as to the maximum frequency but if you can
pick it up for a tenner it might be a good buy even if you use
it for low frequncies for something else. I am sure you spend
more that that at your local. Take a chance! Bid 10 max.

Amazing! I don't know how you find this stuff, John. It seems like an
excellent buy, but there's just one small problem: the Zout of my AVO
appears to be 75 ohms. I know that's 'getaroundable' but it takes the
shine off it. I'll try changing the pots first, then I'll borrow a VNA
and see what gives.

Regards,

Paul