dBm to Vrms

[RF Burns]

Dear all,
              I generated a 50 MHz, -10 dBm sine-wave using a Rhode & Schwarts 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??

 

[AN920]

Your scope will give 2 X the reading because it is not terminated into 50 Ohm

Your scope will also read lower because the 210 has a 60MHz BW meaning that the reading at 60 MHz will be up to 3dB lower. A scope of >= 100Mhz will give a more accurate reading.