Large-Area Balanced Photoreceivers
Frequency Response and Noise • 11
Frequency Response and Noise
Measuring Bandwidth
The frequency response and noise characteristics of
the large-area photoreceiver depend on the selected
gain. The figures beginning on page 15 give the typical
frequency response and noise behavior for the
photoreceivers at each of the three gain settings—low,
medium, and high. The frequency response of the
transimpedance gain is plotted using the expression
20·log[Gain(ƒ)/Gain(0)]
,
where
ƒ
is the frequency and Gain(0) is the gain at DC.
The photoreceiver’s bandwidth is defined as the
frequency where the gain has decreased by 3 dB, or a
factor of
.
Measuring Noise
The photoreceiver noise is characterized using the
noise equivalent power (NEP), which is a measure of
the weakest optical signal that the photoreceiver can
detect. The NEP is the optical power which will
produce a signal-to-noise ratio of 1 in a 1-Hz
bandwidth. The minimum detectable optical power
can be found using the relationship
Minimum Optical Power =
NEP
·
,
where
BW
is the bandwidth. Note that NEP is a
wavelength-dependent quantity that changes with the
photodetector’s responsivity.
2
BW
23x7 LA Blncd Rcvr revA.fm Page 11 Tuesday, September 28, 2004 1:59 AM