[Test-Equipment] question on Polarad rms/dbm

[Dave Brown]

----- Original Message ----- 
From: <eugene at hertzmail.com>
To: "Discussion of Electronic Test Equipment" 
<test-equipment at mailman.qth.net>
Sent: Sunday, January 27, 2008 5:48 AM
Subject: [Test-Equipment] question on Polarad rms/dbm


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
PS anyone have a manual on the SPN? Or alternately, can anyone tell me
where to find the frequency adjustment? I'd like to correct the
frequency just a bit. Thanks!


It used to be quite common for signal generators (especially rf ones) 
to be calibrated in source emf-rather than the terminated output 
voltage.  In the transition phase-between the currently common 
practice of calibrating the output in terms of correctly terminated 
output power level (in e.g. dBm) and the earlier method using source 
emf, many generators had both available. I suspect the Polard falls 
into this category.

DaveB, NZ