[Test-Equipment] question on Polarad rms/dbm

[James Duffer]

> Subject: RE: [Test-Equipment] question on Polarad rms/dbm
> Date: Sat, 26 Jan 2008 20:58:57 -0500
> From: eugene at hertzmail.com
> To: jfor at quik.com; test-equipment at mailman.qth.net
> CC: 
> 
> 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). 

Assigning a voltage value to 0dBm depends on the impedance that the 0dBm is being measured.  Could there be a change in impedance when you use the Polarad?
> 
> 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.  

Again has the impedance changed?
 
> 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?
>