Models 2051 & 2053
Frequency Response and Noise • 15
setting is 330 pW when operating at full detector
bandwidth.
You can further improve your signal-to-noise ratio by
using optical modulators or choppers with lock-in
amplifiers to limit the detection bandwidth. Using
such techniques you can reduce equivalent bandwidth
to 1 Hz or less.
Calculating Output-Voltage Noise
The output-voltage noise can be calculated from
G · R · NEP ·
,
where
G
is the gain (V/V),
R
is the photodiode response
factor (V/mW),
NEP
is the average noise equivalent
power, and
BW
is the bandwidth. This gives an output
noise voltage for the Model 2051 on the high gain
setting of
(3x10
4
V/V) · (0.35 V/mW) · (0.68x10
-9
mW/
)
·
=3 mV
rms
.
The Johnson noise at the input of a 100-MHz bandwidth
oscilloscope with 1-M
Ω
input impedance is 1.6 mV
rms
. This is
often the limiting factor in broadband measurements.
Summary
With the Model 2051 on the highest gain setting the
minimum NEP is 0.68 pW/
, and this yields an
output noise voltage of 3 mV
rms
. Viewed another way,
for operation at the peak responsivity wavelength of
900 nm and for the high gain setting, you will achieve a
signal-to-noise ratio of unity if the input power is
330 pW.
For the Model 2053 with an InGaAs photodiode, the
NEP at peak response wavelength of 1500 nm is
0.34 pW/
over the 150-kHz bandwidth. The full
BW
Hz
2
π
4
------
150 10
3
Hz
×
⋅
Hz
Hz
205x 10MHz Adj Rcvr revA.fm Page 15 Monday, January 10, 2005 9:29 AM
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