[Test-Equipment] question on Polarad rms/dbm

[eugene at hertzmail.com]

Ok, I think I understand what you are saying. So I did some experiments
to see if I can make sense of it. Here's what I did.

1. Connected the polarad directly to the scope with no termination on
the scope input.
2. set the freq=1MHz, amplitude for 500mv as per the display on the
generator
3. I then looked at the unterminated output on the scope and this is
what I found.

When place the gen output impedance to:
5 ohms, scope shows 1.52Vp-p
50 ohms, scope shows 1.48Vp-p
600 ohms, scope shows 1.08Vp-p

All the while, the polarad gen says 500mv amplitude on its display
What does this tell us? So I am not sure it is showing the open circuit
output voltage, otherwise I would expect 500mv to be displayed on the
scope, no?

Then I did the same test, but terminated the input to the scope with 50
ohms (its all I have) Results were:

5 ohms, scope shows 1400mVp-p
50 ohms, scope shows 720mVp-p
600 ohms, scope shows 110mVp-p

Again, the polarad always says 500mv.

I then set the polarad to 7dBm output with the scope unterminated. The
scope displayed:

5 ohms, 1Vp-p
50 ohms, 3Vp-p
600 ohms, 7.4Vp-p

Then, with the scope terminated with 50 ohms, the scope reads:

5 ohms, 880mVp-p
50 ohms, 1400mVp-p  (this value looks correct for 50 ohms, 7 dBm, no?)
600 ohms, 800mVp-p

Again, I can't make sense of this except to say, that when displayed in
dBm and terminated with 50 ohms, the output on the scope matches the
dBm's displayed on the polarad. But in mV or V it still seems to show
2xVrms when terminated.

The unterminated experiments don't seem to have shed light, although I
may just be not seeing the light!

Anyone make sense of this?
Thanks again for taking the time.
Eugene

-----Original Message-----
From: test-equipment-bounces at mailman.qth.net
[mailto:test-equipment-bounces at mailman.qth.net] On Behalf Of Dave Brown
Sent: Sunday, January 27, 2008 4:50 AM
To: Discussion of Electronic Test Equipment
Subject: Re: [Test-Equipment] question on Polarad rms/dbm

Don't Panic, Eugene!  <G>......

As John and I have already pointed out- the Polarad is just reading 
out the source emf when it's switched to display the output in mV. You 
will probably get the same result for any of the voltage related 
display options. This is quite normal for some older equipment-there 
may even be some current equipment that does the same although I dont 
know of any.

Provided you are feeding a high impedance load, (i.e. the output is 
unterminated)  the indicated voltage will be correct. And thats 'high 
impedance' relative to 50 ohms-so any load over a couple of hundred 
ohms will see the indicated voltage.

DaveB, NZ