[Test-Equipment] question on Polarad rms/dbm

[Richard Knoppow]

----- Original Message ----- 
From: <eugene at hertzmail.com>
To: "J Forster" <jfor at quik.com>; "Discussion of Electronic 
Test Equipment" <test-equipment at mailman.qth.net>
Sent: Saturday, January 26, 2008 5:58 PM
Subject: RE: [Test-Equipment] question on Polarad rms/dbm


Hi John and the list-at-large.

I re-read my post and despite having spent some time trying 
to get the
explanation just right, it's clear I did not :)

For the moment, forget I have a scope. The issue is that the 
polarad
gives the user the choice of what units the amplitude is 
displayed in.
Very similar to my HP 8657A. You can choose for the 
amplitude display to
be in dBm, V, mV, or dbV. Just as in my HP, the units chosen 
as the
display do not affect the actual amplitude of the signal, 
the user is
merely switching between equivalent units (ie 0dBm = 
223mVrms =
0.223Vrms = etc).

On my HP, as I cycle through the various units, they are all 
equivalent,
one press shows 0dBm the next press shows 223mVrms, the next 
0.223Vrms,
the next, etc.

However, on the polarad, it seems to indicate that 0dbm is 
equivalent to
.447mV, not .223 as is expected. So despite the fact that 
the output is
really .223Vrms (According to my scope) the display is 
showing .447mV.

If I were to keep the polarad display on dBm at all times, 
then the
display would correspond to a real reading as per a scope. 
It is when I
wish to display in mV that the display is showing 2x the 
mVrms of what
it should using a dBm equivalent.

Does this clarify a little more?


    Its pretty certain the Polarad generator is calibrated 
for open circuit voltage. When terminated in its 
characteristic impedance the voltage will be half the marked 
value. Old General Radio signal generators, like the 1001, 
are the same way. The voltage marked on the attenuator is 
for open circuit so the actual voltage when terminated is 
half that value. 0.447 is double 0.223 with the last digit 
rounded off.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com