GW Instek GDM-8245 - Manual (Page 11)

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6.MEASUREMENT TECHNIQUES
6-1 dBm measurement technique
dBm is defined as above or below a 1mW reference. A voltage
measurement is converted to dBm using the following formula:
dBm=10*log
10
(1000*voltage value 2 /reference impedance.)
The standard reference impedance of the instrument is 600Ω.
For example, 0.7746V will be convert to 0 dBm.
6-2 True rms measurement
The true rms (root-mean-square) value of a waveform is equivalent
to dc value that causes the same amount of heat to be dissipated in a
resistor.
Since average-responding meters have been in use for so long, you
may have accumulated test or reference data based on them. Figure 6-
1 illustrates the relationship between ac and dc components for
common waveforms, and compares readings for true rms meters and
average-responding meters. Figure 6-1 will help you convert between
the two measurement methods.
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z Figure 6-1: Voltage Conversion