New Focus 1621, Руководство пользователя

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8 • Operation NEW FOCUS, Inc.
Conserving Signal Strength
If you have a low-power signal, 50
Ω
of transimped-
ance gain can lead to very small output voltages. If you
need to conserve signal strength, it is important to
focus your optical beam onto the photodiode. Adjust-
ing the photodiode position for optimal signal is best
performed as a two-step procedure:
1.
Set the load resistance to
10 k
Ω
or
Open
.
2.
Hook the detector output directly into an oscil-
loscope or other voltage-measurement device
with a large input impedance.
3.
Adjust the photodiode position (in both trans-
verse and focus directions) for maximum DC
voltage.
For optical signal levels above about 1 mW (at 10
k
Ω
), the voltage will saturate. Remember, the out-
put voltage can only rise up to the battery volt-
age—9 V when the battery is fresh. If you use the
1-M
Ω
input impedance of an oscilloscope, satura-
tion is even easier to reach.
4.
Select the 50-
Ω
resistor, or supply your own
small load resistance suitable to your gain and
frequency-response requirements. Make fine
adjustments to the photodiode position until
the maximum signal is achieved.
To compute the approximate output voltage for a
given input optical power use the relationship
V
out
= P
in
· R · G
,
where
P
in
is the input optical power in watts,
R
is the
photodetector’s responsivity in A/W (see page 6 for
typical responsivities), and
G
is the load resistance (V/
A). For example, with 10 mW of optical power, a
responsivity of 0.5 A/W, and with the load set to 50
Ω
,
the photodetector will produce an output voltage of
approximately (10 mW)
·
(0.5 A/W)
·
(50 V/A) = 0.25 V.
162x nanosec revB.fm Page 8 Friday, August 17, 2001 5:51 PM