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dBm to Vrms - RF Cafe Forums
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| dohagan
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Post subject: dBm to Vrms
Posted: Sat Oct 28, 2006 12:38 pm
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| Captain |
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Joined: Sat Mar 19, 2005
3:20 pm
Posts: 5 |
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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??
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nubbage |
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Post subject:
Posted: Mon Oct 30, 2006 4:48 am
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| General |
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Joined: Fri Feb 17, 2006
12:07 pm
Posts: 218
Location: London UK
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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.
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Posted 11/12/2012
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