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3.5.6 Logarithmic (Log) Measurements (dBm and dB)
The Model 840-C can make logarithm (dBm and dB)
measurements referenced to either a 1 mW power level or
to any other observed power level with the use of the
REF-dB
key.
The basic procedure for using the Log mode is to select
autorange and press the
W-dBm
key (dBm annunciator turns
on). To get out of Log mode, press the
W-dBm
key again.
Because the logarithm of a negative number is undefined,
L0
is displayed if the logarithm of a negative number is
attempted.
NOTE
Log measurements should always be made on the
lowest possible range (without overranging). Read-
ings on high ranges will not allow optimum calcula-
tions of the logarithm. When in doubt, use autorange.
3.5.6.1 Log measurements: 1 mW Reference (dBm)
The Log measurement mode displays the absolute value
of 10 times the logarithm (base 10) of the input power,
referenced to 1mW. The following equation illustrates
this relationship:
dBm reading =
10 Log( Net Applied Power /
1mW)
=
10 Log((I-I
d
) / R) / 1 mW)
where
I =
detector input current
I
d
=
detector dark current
R =
responsivity of the detector (A/W)
(or detector with attenuator)
Using this reference power, the dBm reading span is from
-80.00 dBm to +10.00 dBm (10 pW to 10 mW), when the
detector responsivity equals 1.
To make dBm measurements (relative to 1 mW optical
power) with background correction, proceed as follows:
a.
Select autorange and a wavelength.
b.
Block the light to be measured.
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