[Test-Equipment] question on Polarad rms/dbm

[J Forster]

It sounds like the thing is displaying the 'volts behind 50 ohms **' or the
open circuit voltage. Since it's an audio generator, it may not be expecting
a 50 ohm termination. Put another way, the meter is reading the voltage of a
Thevenin Equivalent with the resistance being 50 ohms. If you pulled the 50
ohm termination off, I'd bet you see what the display says.

**  this comes from the telephone world where what you see is' 48 volts
behind 600 ohms'

-John



eugene at hertzmail.com wrote:

> 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