[Test-Equipment] question on Polarad rms/dbm

[eugene at hertzmail.com]

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?




-----Original Message-----
From: J Forster [mailto:jfor at quik.com] 
Sent: Saturday, January 26, 2008 6:18 PM
To: eugene at hertzmail.com
Subject: [Test-Equipment] question on Polarad rms/dbm

Hi,

Try taking this a step at a time. With the scope set up at 1 V/div with
a 50 ohms termination on the input, set each generator at 1 V output.
They should both read the same thing on the scope....  2.824V P-P = 2 *
SQR( 2.00)* Vrms.  Do they read the same and is the waveform centered
around Ground?

If yes, change the generators to 0 dBm. Does the scope now read   0.633
V P-P ?

0 dBm = 0.001W;     P = V*V / R;     so SQR (P*R) =Vrms = 2 *SQR(2.0)
*Vrms  P-P;  SQR ( .001 * 50 ) = 0.224 * 2.828 = 0.633 Vp-p

Sometimes dBm is referenced to 600 ohms in audio applications.  Maybe
the Polorad is like that.

FWIW,

-John




Hi all again,

I am working on learning about measuring and rms/Vp-p/dbm and I really
think I have it down! However, I have one piece of test equip that has
me scratching my head (aside from my 410B!).

I have been comparing the usage of two generators I have, one an HP
8657A recently calibrated. The other a Polarad SPN 1Hz-1.3MHz sine wave
generator.

Both have 50 ohm output (the SPN has 50 ohm,600 ohm and 5 ohm output
selectable, I have selected 50 ohm).  I am connecting each to my scope
using a tektronix passthru 50 ohm terminator.

The HP & scope combination seem to function exactly as I would expect,
meaning if I key the amplitude as 0 dbm and switch to V display the
display shows 224mv (I am pretty sure this means 224Vrms). And, when I
display this on my scope, I get 680mVp-p (/2 * .707) gives 240mV (pretty

close).

On the Polarad, things are strange. When I set its amplitude to 0 dbm,
the scope indicates also 680mVp-p (good) but when I change its display
from dbm to mV it indicates 447mV?! Switching back to dbm shows 0, but
mV is 447.

What is the Polarad indicating? Seems its displaying 2*Vrms ? Is this
some alternate standard of displaying amplitude? Could this be the
correct Vrms for a different output impedance other than 50 ohms?

I guess I'm trying to figure out if a) this is working correctly by
showing 2*Vrms when in mV display - I just don't understand why its
indicating this, or b) if this unit has a problem with it.

Can anyone tell me what's going on here?
Thanks
Eugene