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4.5.2.1 Logarithmic Measurements Using Reference Powers (dB)
The dB measurement mode displays the absolute value of 10 times the
logarithm (base 10) of the input power, referenced to a power level which
is selected by pressing the STOREF key. The following equation illustrates
this relationship:
dB reading = 10 * log [Net Applied Power/Net Referenced
Power(=STOREF)]
= 10 * log [(( I - I
z
)/R)/((I
STOREF
- I
Z
)/R)]
where
I = detector input current
I
z
= detector background current defined when
the ZERO key was pressed
I
STOREF
= referenced detector current defined when the
STOREF key was pressed
R = responsivity of the detector (A/W)
(or detector with attenuator)
To make dB measurements with background correction, proceed as follows:
a. Select the dB measurement mode, auto-range, and a wavelength.
b. Block the light to be measured.
c. Initiate background correction by pressing the ZERO key. Display
should read approximately zero, but will fluctuate depending upon the
variations in the amount of light reaching the detector and the detector
dark current fluctuations.
d. Illuminate the detector with the optical signal to be used as the
reference signal.
e. Press the STOREF key.
f. Illuminate the detector with the optical signal to be measured.
g. Take the reading from the display.
4.5.2.2 Logarithmic Measurements Using 1 mW Reference (dBm)
The dBm 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
z
)/R)/1 mW]
where
I = detector input current
I
z
= detector background current defined when
the ZERO key was pressed
R = responsivity of the detector (A/W)
(or detector with attenuator)
Using this 1mW reference power, the dBm reading span is from −90 dBm to
+10 dBm (1 pW to 10 mW), when the detector responsivity equals 1.
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