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Chapter 7 Tutorial
Attributes of AC Signals
7
Note. If an average-reading voltmeter is used to measure the “DC
voltage” of a waveform, the reading may not agree with the DC Offset
setting of the function generator. This is because the waveform may have
a non-zero average value that would be added to the DC Offset.
You may occasionally see ac levels specified in “decibels relative to
1 milliwatt” (dBm). Since dBm represents a power level, you will need to
know the signal’s RMS voltage and the load resistance in order to make
the calculation.
dBm = 10 x log
10
(P/0.001) where P = V
RMS
2
/R
L
For a sine wave into a 50
Ω
load, the following table relates dBm to voltage.
For 75
Ω
or 600
Ω
loads, use the following conversions.
dBm (75
Ω
) = dBm (50
Ω
) – 1.76
dBm (600
Ω
)
= dBm (50
Ω
) – 10.79
dBm RMS Voltage Peak-to-Peak Voltage
+23.98 dBm 3.54 Vrms 10.00 Vpp
+13.01 dBm 1.00 Vrms 2.828 Vpp
+10.00 dBm 707 mVrms 2.000 Vpp
+6.99 dBm 500 mVrms 1.414 Vpp
0.00 dBm 224 mVrms 632 mVpp
-6.99 dBm 100 mVrms 283 mVpp
-10.00 dBm 70.7 mVrms 200 mVpp
-36.02 dBm 3.54 mVrms 10.0 mVpp
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