dBm to Vrms - RF Cafe Forums

 Post subject: dBm to Vrms

Posted: Sat Oct 28, 2006 12:38 pm 

Dear all,

I generated a 50 MHz, -10 dBm sine-wave using a Rohde & Schwarz RF signal generator. I checked this using an Agilent spectrum analyser and, as you'd expect, I get my tone at 50 MHz with peak amplitude of -10 dBm. However, when I input my RF signal to a TDS 210 Tecktronix Digital Oscilloscope, the peak-to-peak voltage is 336 mV and the Cyc RMS voltage is 117 mV. Now, on the TDS210, Cyc RMS calculates the RMS voltage over one complete cycle of the waveform.

When I tried to calculate this I got a different answer:

P = -10 dBm, which is equivalent to 100 uW

Now, P = V^2 / R

I've taken R = 50 ohms

Therefore Vrms = sqrt(P x R)

That is: Vrms = sqrt(100 uW x 50 ohms)

So, Vrms = 70.7 mV

This is different to the 117 mV measured on the oscilloscope??

 Post subject:

Posted: Mon Oct 30, 2006 4:48 am 

3 possibilities come to mind

1) error in calibration of the scope, particularly at the measurement frequency. If the Y video amp was 3 dB down at the measuring frequency, there is a 3dB error immediately

2) the impedance was higher than 50 ohms.

3) presence of harmonics in the generated waveform.