[Test-Equipment] question on Polarad rms/dbm

[eugene at hertzmail.com]

Good suggestion. I just checked and there is no dc offset.

-----Original Message-----
From: test-equipment-bounces at mailman.qth.net
[mailto:test-equipment-bounces at mailman.qth.net] On Behalf Of Mike Manes
Sent: Saturday, January 26, 2008 2:59 PM
To: Discussion of Electronic Test Equipment
Subject: Re: [Test-Equipment] question on Polarad rms/dbm

Is there some DC offset or trace deviation from the zero graticule
during the Polarad test? Any dc will add to rms but won't show in p-p.
73 de Mike W5VSI

eugene at hertzmail.com wrote:
> 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!
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-- 
Mike Manes    mrmanes at gmail.com     Tel: 303-979-4899
"Things should be made as simple as possible, but not more so."
A. Einstein
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