[Test-Equipment] question on Polarad rms/dbm

[J Forster]

Well, first off, I'd do the experiment at 1 KHz, not 1 MHz where capacitive
reactance enters into the picture.

eugene at hertzmail.com wrote:

> Ok, I think I understand what you are saying. So I did some experiments
> to see if I can make sense of it. Here's what I did.
>
> 1. Connected the polarad directly to the scope with no termination on
> the scope input.
> 2. set the freq=1MHz, amplitude for 500mv as per the display on the
> generator
> 3. I then looked at the unterminated output on the scope and this is
> what I found.

OK. so far.

> When place the gen output impedance to:
> 5 ohms, scope shows 1.52Vp-p

You should see 2 * SQR (2.0) * 0.5 V = 1.141 V P-P

> 50 ohms, scope shows 1.48Vp-p
> 600 ohms, scope shows 1.08Vp-p

If you used 3' of RG-58 or something like it plus the scope, your load on
the generator is roughly 100pF

Xc = 1/ 2*PI*F*C = 1/ 6*10E6*10E-10 = 10E4 / 6 = 1600 ohms roughly

so, you have a reactive voltage divider of 600 ohm generator source and
1600 ohms from the C

No more mystery.

Best,
-John