Measuring RF output impedance
 

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.
--

The BBC: licenced at public expense to spread lies.

Paul,
The most obvious method would be to measure the open-circuit RF output
voltage, and then measure the output voltage with a known load. The output
impedance is a simple calculation of the ratio of the voltages.
However, depending on the vintage and quality of the generator, the
impedance is likely to change with frequency, and possibly even with output
level. For source-impedance-sensitive measurements, it's generally a good
idea to put a small amount of fixed resistive attenuation at the output
(6-10 dB) in order to "fix" the impedance.

Joe
W3JDR

"Paul Burridge" wrote in message
...
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.
--

The BBC: licenced at public expense to spread lies.

Paul,
The most obvious method would be to measure the open-circuit RF output
voltage, and then measure the output voltage with a known load. The output
impedance is a simple calculation of the ratio of the voltages.
However, depending on the vintage and quality of the generator, the
impedance is likely to change with frequency, and possibly even with output
level. For source-impedance-sensitive measurements, it's generally a good
idea to put a small amount of fixed resistive attenuation at the output
(6-10 dB) in order to "fix" the impedance.

Joe
W3JDR

"Paul Burridge" wrote in message
...
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.
--

The BBC: licenced at public expense to spread lies.

On a sunny day (Sat, 01 May 2004 14:19:59 +0100) it happened Paul Burridge
wrote in
:

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?

Load it with 50 Ohms, and measure output voltage, remove load and measure
again.
If it is half, it is 50, else do the math.
JP

On a sunny day (Sat, 01 May 2004 14:19:59 +0100) it happened Paul Burridge
wrote in
:

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?

Load it with 50 Ohms, and measure output voltage, remove load and measure
again.
If it is half, it is 50, else do the math.
JP

Paul Burridge wrote:
I have a spare RF signal generator that has an unmarked output from a
type of socket I've never seen before.

Would it be a car-type output socket (deep recessed centre pin)? Would
it be an old Advance generator with the rounded corners? If so, it's
almost certainly 75R.

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.

If it has a resistive output attenuator with no DC blocking capacitor,
you can switch to maximum attenuation (minimum output) and measure Zout
with an ohm-meter.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek

Paul Burridge wrote:
I have a spare RF signal generator that has an unmarked output from a
type of socket I've never seen before.

Would it be a car-type output socket (deep recessed centre pin)? Would
it be an old Advance generator with the rounded corners? If so, it's
almost certainly 75R.

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.

If it has a resistive output attenuator with no DC blocking capacitor,
you can switch to maximum attenuation (minimum output) and measure Zout
with an ohm-meter.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek

"Paul Burridge" wrote in message
...
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?


The generators are more of a voltage source. They will deliver their rated
output when loaded to the proper impedance. Load it with 50 ohms and see if
it gives the rated output. If not try 70 ohms, or another value. One of
the reasons for using a 6 db pad is that it helps isolate the impedance of
the generator and receiver.

"Paul Burridge" wrote in message
...
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?


The generators are more of a voltage source. They will deliver their rated
output when loaded to the proper impedance. Load it with 50 ohms and see if
it gives the rated output. If not try 70 ohms, or another value. One of
the reasons for using a 6 db pad is that it helps isolate the impedance of
the generator and receiver.

On Sat, 01 May 2004 17:03:26 GMT, Jan Panteltje
wrote:

On a sunny day (Sat, 01 May 2004 14:19:59 +0100) it happened Paul Burridge
wrote in
:

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?

Load it with 50 Ohms, and measure output voltage, remove load and measure
again.
If it is half, it is 50, else do the math.

---
Without doing a conjugate match and accounting for the impedance of
the voltmeter and the load, the best he'll be able to do is _assume_
that what he measures is what he's really got.

--
John Fields