Measuring RF output impedance

[John Fields]

---
Also, less commonly, a device used to match the impedance of a
generator to the impedance of a transmission line or a load, or the
impedance of a transmission line to the impedance of a load.

The impedance of the generator, line, and/or load are assumed to be
purely resistive, and there is always loss associated with the
transformation. Devices which exhibit the least loss are called
"minimum loss pads", and

http://www.maxim-ic.com/appnotes.cfm/appnote_number/972

gives a good description of the process involved in designing one.

The reference given to Bruno Weinschel and the ITT handbook (as we
old-timers call it) is particularly good, and if you're interested in
RF (or just about anything else...) and you can buy a copy of it you
should.

BTW, since a pad is usually rated in terms of the power it's supposed
to lose between its input and its output, the reference to "output
EMF" on your generator may be what it supposed to be with a 3dB pad on
its output.

Easy way to find out would be to build a 3dB and a 6dB pad to find
out. For a 50 ohm tee pad, here are the resistor values you'll need
for -3dB:

0dBIN>--[8.55]-+-[8.55]-->-3dBOUT
|
[141.93]
|
GND>-----------+----------->GND


and for -6dB:

0dBIN>--[16.61]-+-[16.61]-->-3dBOUT
|
[66.93]
|
GND>------------+----------->GND

The input goes directly to your generator, (assuming its output
impedance is 50+j0 ohms) a 50 ohm resistor gets connected across the
output of the pad, and you'll measure the voltage across the 50 ohm
resistor.

Use non-inductive resistors (carbon comp if you can get them) and keep
the leads short. That is, ********KEEP THE LEADS SHORT*********.

 

[John Jardine]

Hi,

I have a spare RF signal generator that has an unmarked output from a
type of socket I've never seen before. What's the simplest way of
establishing its output impedance? I've had a few ideas but no doubt
someone out there will know of something better, since I always seem
to end up making unnecessary work for myself.
Any suggestions?

p.

Those ex-military AVO's were really nice g.p. signal generators though could
never afford one. Seem to remember they were 50ohms out. The o/p socket and
plug were standard 50's military chic and still available.
regards
john

 

[John Crighton]

Thanks John (and others).

The model number ain't gonna mean a lot to anyone as this is a very
old piece of kit (1950s) that I keep mainly out of a sense of
reverence for the past. It's an old ex-RAF AVO. I have posted a
picture of the socket to a.b.s.e under this same thread title...
BTW, there's a 5p coin shown for scale, but since that won't mean much
to anyone outside Britain, the outer of the socket is approx. 1" in
diameter (which won't mean much to anyone in europe but it serves them
right for adopting the metric system.)

Go on, tell us the model number, there are a lot of us
still interested in boat anchor style test equipment.

Is this unit similar to your sig gen
http://www.thevalvepage.com/testeq/avo/sg3/sg3.htm
http://www.thevalvepage.com/testeq/testeq.htm
Unfotunately the connector has been removed and a
BNC type fitted.

I am guessing that you want to keep the original connector
on the sig gen and find a matching connector to make a
short test lead. Old Style connector to BNC.
My news group service doesn't allow me to see the group
a.b.s.e Pity. So I can not see the picture you posted.

So, what are the results of connecting your ohm meter
to the RF out put connector (with the unit switched off)
and measuring the resistance on all attenuator positions.

On one of the lower frequency ranges when you connect
up your cro via a "T" piece to the sig gen with say a 75
ohm load. Does the output level jump up to double
when the load is removed?

Repeat the experiment with a 50 ohm load. If the voltage
doubles from loaded to no load, you will be laughing.
But I don't think you will be that lucky.

I am guessing it will be closer to 75 ohm output impedance.

Try different value loads and let us know how you got on.

Cheerio,
John Crighton
Sydney

 

[Active8]

On Sun, 02 May 2004 09:04:33 -0500, John Fields

http://www.maxim-ic.com/appnotes.cfm/appnote_number/972

Once in a while, I can post a helpful link (I've got plenty of tech
info links, but most don't usually apply to the topic at hand
whatever that may be) , but some of you guys, like Spehro, You,
etc., ... it's like you can pull horshoe out of yer asses You
must do more digging than I do.

I got a kick out of this link because I have the opposite situation.
I don't need an MLP becuse I have an SLM tha *is* 75 ohfriggin'megas
and the link dealt with ... you know.

Use non-inductive resistors (carbon comp if you can get them) and keep
the leads short. That is, ********KEEP THE LEADS SHORT*********.

^^^^^^^^^^
advice that should be unsnipped. Er, snip the leads, though.

 

[Paul Burridge]

Go on, tell us the model number, there are a lot of us
still interested in boat anchor style test equipment.

Is this unit similar to your sig gen
http://www.thevalvepage.com/testeq/avo/sg3/sg3.htm
http://www.thevalvepage.com/testeq/testeq.htm
Unfotunately the connector has been removed and a
BNC type fitted.

Not really similar to any of them, I'm afraid. Model number is CT
378B. I did recognise that old Advance E2, though. Had one of 'em
about 25 years ago. It was pretty ancient then!

I am guessing that you want to keep the original connector
on the sig gen and find a matching connector to make a
short test lead. Old Style connector to BNC.
My news group service doesn't allow me to see the group
a.b.s.e Pity. So I can not see the picture you posted.

Pity. Hopefully some kind soul will post it onto a web site for you. I
can't as I have no FTP s/ware installed at present. :-(
The output is just under an inch in diameter and is co-axial as you
might expect. The centre pin is about 1/16th" thick and doesn't
protrude beyond the outer. It's pretty unremarkable apart from its
size; much larger than one usually sees.

 

[Paul Burridge]

Once in a while, I can post a helpful link (I've got plenty of tech
info links, but most don't usually apply to the topic at hand
whatever that may be) , but some of you guys, like Spehro, You,
etc., ... it's like you can pull horshoe out of yer asses You
must do more digging than I do.

Yup, John Chrighton's another one that springs to mind as being very
good at sniffing stuff out on the Web.

I'd often wondered what these "pads" were for. I'd been a bit
concerned to note that the output amplitude varied quite markedly when
sweeping the generator through it's frequency range when measuring the
'open circuit' voltage with a 'scope. I guess now I know why.
Thanks, guys.

 

[John Fields]

Not really similar to any of them, I'm afraid. Model number is CT
378B. I did recognise that old Advance E2, though. Had one of 'em
about 25 years ago. It was pretty ancient then!


Pity. Hopefully some kind soul will post it onto a web site for you. I
can't as I have no FTP s/ware installed at present.

 

[Steve Nosko]

My main sig gen states "output EMF using 6dB pad" next to the socket.
WTF is a "6dB pad"?
--

[...snip...]
For the calibration to be accurate on your generator it sounds like the
scale is calibrated so you need the 6 db pad after it. A 5 db pad will
reduce the power by a factor of 4 or a voltage by a factor of 2.

That last 5 should be a 6.
[/QUOTE]

 

[Steve Nosko]

Hi,

I have a spare RF signal generator that has an unmarked output from a
type of socket I've never seen before. What's the simplest way of
establishing its output impedance? I've had a few ideas but no doubt
someone out there will know of something better, since I always seem
to end up making unnecessary work for myself.
Any suggestions?
p.

One more thing to keep in mind, Paul. Over here in the US it is and has for
a long time been standard to display the LOAD level on the generator output
indicator. That is, the voltage (or power) at the load itself (assuming
matched). In the UK - but I don't know about recently, it was common to
show the generator EMF -- which meant the internal generator voltage. This
would be twice what would appear across the load if the load & source Z were
equal.

From your Sig you may very well be familiar with the EMF designation...

 

[Paul Burridge]

Oh yeah. I forgot about that!
Better ask here first though, as it's a binary and might not be
welcome.
I've also taken a shot of the whole gen., John. I can certainly e-mail
the pics to you if you're that interested in boat anchors (although
this one's only 25lbs so hardly qualifies). My Marconi, OTOH, has got
to be 3 times that much at least...

 

[John Crighton]

Oh yeah. I forgot about that!
Better ask here first though, as it's a binary and might not be
welcome.
I've also taken a shot of the whole gen., John. I can certainly e-mail
the pics to you if you're that interested in boat anchors (although
this one's only 25lbs so hardly qualifies). My Marconi, OTOH, has got
to be 3 times that much at least...

Hello Paul,
Yes, I would be interested to see the pictures of the AVO CT378B
What was the Marconi sig gen model number? I missed it.
No need for pictures of the Marconi unit. I might know it from
the model number.
Regards,
John Crighton
Sydney

 

[Paul Burridge]

Yes, I know that unit, I have a 2001
Repairing that unit is not dead easy for hobbyists.

Hey, I must be semi-pro by now, with all the info I've been given over
the last 2 years.
I'm screwed without a circuit diagram, anyway. Some people never seem
to need 'em but I've never understood how they manage. Given a
diagram, I'm actually surprisingly good at fault-finding.

I am assuming that AVO at Dover were useless with your
enquiries regarding information on the AVO CT378B

I just assumed they'd blank me on the age of it - like Tektronix - and
try to sell me one of their new ones (assuming they still make SGs,
that is).

If you wanted a good back up signal generator,
that is dead easy to repair. Here is one for sale.
Marconi TF 144H 50 ohms output Z. BNC connector.
10KHz to 72MHz
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=40004&item=3094746090&rd=1

Thanks, I'll take a look later 2day after I've downloaded the update
for this latest virus that attacks your system via web portals.

I like that model, for a number of reasons.
Very stable frequency wise (when warmed up).
It has that nice smell inside that you get from valve gear. LOL
Everything inside is getatable.

Yeah, but what if one of those hard-to-find toobz goes tits-up?

Big knobs and easy to read dials, good for failing eyesight.
When the attenuator gets smoked by accidentally pressing a
microphone push to talk button of a transceiver under repair
on the bench, it is very easy to replace the resistors in the
attenuator.
It was not that heavy that I couldn't hump it up the gangways
of ships, decades ago, to do radio service work but it is way too
heavy for me now. Heh heh heh...

You might like to check out how far away that unit for auction,
is away from you. I wouldn't pay any more than 45 quid.
Even 25 quid is right on the limit for me. But if no one bids
you could get lucky. I bought one here in Australia for $10
at a Ham fest.

I only mentioned the TF144H incase you were looking for a
good cheap sig gen to accompany your TF2002AS do some
serious radio receiver work. Both have good attenuators.
Having two good signal generators would help you out with
all that filter and interference work that has been plaguing you
over recent months. One sig gen as the desired signal, the
second sig-gen as the interfering signal etc.

Sounds like a great idea, actually. The problem is of course that even
here we're hardly spoiled for choice IRO older test equipment. Finding
one of those particular models for sale in this country would be
tricky, I'd imagine. Anyway, I'm about to mail you a pic of the AVO,
so keep an eye open in your inbox and don't delete me as spam!
Admire those classic 1950s lines.....

 

[John Crighton]

Hey, I must be semi-pro by now, with all the info I've been given over
the last 2 years.
I'm screwed without a circuit diagram, anyway. Some people never seem
to need 'em but I've never understood how they manage. Given a
diagram, I'm actually surprisingly good at fault-finding.


I just assumed they'd blank me on the age of it - like Tektronix - and
try to sell me one of their new ones (assuming they still make SGs,
that is).


Thanks, I'll take a look later 2day after I've downloaded the update
for this latest virus that attacks your system via web portals.


Yeah, but what if one of those hard-to-find toobz goes tits-up?


Sounds like a great idea, actually. The problem is of course that even
here we're hardly spoiled for choice IRO older test equipment.

Finding
one of those particular models for sale in this country would be
tricky, I'd imagine.

WHAT! YOU ARE KIDDING ME!
ARE YOU STILL ON THE PISS?
It is a bleeding Marconi. I thought they were made in
Chelmsford. That is just up the street from where you
are in London, by Australian standards of distance.

The unit I have been going on about is for sale in Beds UK.
Where is that? Bedfordshire maybe. Four streets away
by Australian distances. Current bid 0 bids. 25 quid.
You didn't even look. Cripes.....tearing hair out.
C'mon Paul, sober up.
Regards,
John Crighton
Sydney

 

[Paul Burridge]

WHAT! YOU ARE KIDDING ME!
ARE YOU STILL ON THE PISS?
It is a bleeding Marconi. I thought they were made in
Chelmsford. That is just up the street from where you
are in London, by Australian standards of distance.

The unit I have been going on about is for sale in Beds UK.
Where is that? Bedfordshire maybe. Four streets away
by Australian distances. Current bid 0 bids. 25 quid.
You didn't even look. Cripes.....tearing hair out.
C'mon Paul, sober up.

Whoa! Calm down there, John. I'm simply not taking the risk of doing
anything Interenet-Explorer related until I've got the patch for the
sasser worm that's flying around the world at record speed. Sure I'll
follow your link as soon as it's safe to do so. Gimme a break will ya?
;-)
BTW, I've just posted you the photo of the Sig gen. Took a bit longer
than I meant to as I've been faffing about with photo software trying
to enhance the image. Taking flash photos of shiny, black objects is
pretty tricky, I've discovered...

 

[John Crighton]

BTW, I've just posted you the photo of the Sig gen. Took a bit longer
than I meant to as I've been faffing about with photo software trying
to enhance the image. Taking flash photos of shiny, black objects is
pretty tricky, I've discovered...

Hello Paul,
I received the picture, thanks.
Those connectors are still easy to come by.
Here is a picture of a different sig gen but with a similar
style connector to yours.
http://www.thevalvepage.com/testeq/triplet/1632/1632.htm
That connector is a Pye connector.

Your connector is very similar but just slightly different.
When you contact AVO at Dover, ask them the name
of the RF connector that was used. Telling them it is
similar to a Pye should jog their memories.

If you are a purist
and you want to keep the original RF output connector then
you can find some at Ham radio trash and treasure field days
or if you are in a hurry, check out some radio service workshops,
like taxi two-way radio repair joints. Most technicians are horders
of old stuff. They might have an old lead with connector.

If you are not a purist
in keeping old gear original but only functional, open the unit
up and see if the old fashioned RF output connector can be
replaced with a BNC connector easily.

You still haven't told me what ohm meter readings you got
when you placed the meter prods from inner to outer on the
RF connector. (sig gen switched off)
Flick the attenuator switches to all positions. You are looking
for fairly consistent readings to check if the attenuator has
been damaged by a transceiver that transmitted accidentally
or the attenuator having being connected to a high DC voltage.

Tell us. Were the readings close to 50, 75 ohms or what?

Here in Sydney the Wireless Institute of Australia, a kind
of head office for hams keeps old manuals and circuits.
There would be an equivalent office for UK hams in London
with a library of old manuals for sure. Some UK hams
on this group will tell you who to contact.

You can still get those RF connectors and if you try hard
you will find a manual for the AVO CT378B.
Do the obvious first, phone or e-mail AVO.

"The Megger office in Dover
Archcliffe Road, Dover, Kent, CT17 9EN, England."
http://www.avointl.com/common/images/DoverMap.gif
The phone number is on this map.

There you are, they are a couple of hours down the road
from you. Nearly next door. Call in with your sig gen and
ask for a photo copy of the circuit, they will probably chuck
all sorts of stuff at you.

Regards,
John Crighton
Sydney

 

[Paul Burridge]

Hello Paul,
I received the picture, thanks.
Those connectors are still easy to come by.
Here is a picture of a different sig gen but with a similar
style connector to yours.
http://www.thevalvepage.com/testeq/triplet/1632/1632.htm
That connector is a Pye connector.

Certainly looks like the one shown, I must say. I'd always thought Pye
connectors, though, had three pins, like the one in the photo I sent
you in the top LH corner. Or maybe that's a Bulgin connector. Blimey.

Your connector is very similar but just slightly different.
When you contact AVO at Dover, ask them the name
of the RF connector that was used. Telling them it is
similar to a Pye should jog their memories.

Okay, John, just out of curiosity I will. I expect them to tell me to
get f*cked, like HP or Tek would, but we'll see what their "extended
aftersales care" is like. "What do you mean I ought to be a new model?
This one's only 50 years old!"

I'll post the results later today, along with answering your other
queries.

p.

 

[John Fields]

Okay, John, just out of curiosity I will. I expect them to tell me to
get f*cked, like HP or Tek would, but we'll see what their "extended
aftersales care" is like.

---
Excuse me???

I recently bought a surplus HP54659B module for my HP54602B DSO and
had trouble getting it to work, so I called Agilent for help and, even
though the scope and the module are no longer supported by Agilent,
they took the call, assigned me a case number and a pleasant
technician, and when she couldn't come up with a solution while we
were on the phone, went away to try to find some information she
needed to work on the problem. All for free. Above and beyond, I'd
say...

In the meantime, I continued to try to solve the problem and found it
was pilot error, so I called them back to let them know and to thank
them for their time. I also found out that even though one of their
instruments may become obsolete or break, they'll still do fee-based
repair and cal.

I hope you fare as well with Marconi.

 

[Paul Burridge]

Your connector is very similar but just slightly different.
When you contact AVO at Dover, ask them the name
of the RF connector that was used. Telling them it is
similar to a Pye should jog their memories.

Okay, I've just phoned them and drew the anticipated blank, I'm
afraid. It appears all the old info; manuals, spec sheets, data etc.
on all obsolete models got thrown out years ago. They only keep info
on current models, regrettably. Still, at least they didn't try to
sell me a new one.

If you are a purist
and you want to keep the original RF output connector then
you can find some at Ham radio trash and treasure field days
or if you are in a hurry, check out some radio service workshops,
like taxi two-way radio repair joints. Most technicians are horders
of old stuff. They might have an old lead with connector.

I think the obvious thing to do from my PoV is just to leave the front
as is for the sake of originality and just put a BNC socket at the
rear and feed the mains power in via a grommeted hole likewise at the
back. Having a dirty great power lead coming into the front panel
would have looked pretty gross anyway.

If you are not a purist
in keeping old gear original but only functional, open the unit
up and see if the old fashioned RF output connector can be
replaced with a BNC connector easily.

In the 1950s? I'd certainly have thought so!

You still haven't told me what ohm meter readings you got
when you placed the meter prods from inner to outer on the
RF connector. (sig gen switched off)
Flick the attenuator switches to all positions. You are looking
for fairly consistent readings to check if the attenuator has
been damaged by a transceiver that transmitted accidentally
or the attenuator having being connected to a high DC voltage.

Tell us. Were the readings close to 50, 75 ohms or what?

70-100 ohms depending on the positions of the coarse and fine
attenuator knobs, but not nice and smoothly; jumping around a lot (I
guess noisy pots that just want a squirt of contact lube. Does that
sound right to you? I'd have expected a much greater range...

[rest noted and snipped]

 

[Paul Burridge]

Sorry. Tek, then. I must have been thinking of M$ - Oh, perhaps not;
they don't even support their current range of 'products' :-(

 

[John Crighton]

70-100 ohms depending on the positions of the coarse and fine
attenuator knobs, but not nice and smoothly; jumping around a lot (I
guess noisy pots that just want a squirt of contact lube. Does that
sound right to you? I'd have expected a much greater range...

From an attenuator a large range is undesirable. The opposite
is what you want.
Lets just call the RF output Z for this unit 75 ohms Nominal.
And you have crappy pots to put up with. Not good.

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.

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.

A friend of mine did have a good use for a few old
crappy signal generators. He was into restoring really
old radio sets, record players with big horns and pianolas,
those pianos that play by themselves.
He was giving a demonstration of his gear at his house.
In another room he had the old signal generators being
externally modulated by modern tape recorders playing
old 1920's and 30s type music. The visitors were amused
and confused to hear the old style music coming from
the old style radios as they could tune into the local stations
also. The bullshit flew from those in the know.

Regards,
John Crighton
Sydney