Incoherent Detection
9006-008
9006-008a
The Carrier-To-Noise Ratio (CNR) in an incoherent (direct) detection system with an
Avalanche Photodetector (APD), is given by:
(MRiPr)2
CNR = ----------------------------------------
[2e{Ri(Pr + NbBo) + Id}M(2+x) + 4kTF/RL]Be
^ ^ ^ ^
. . . .
. . . .
. . . .
Signal . Dark Current .
Quantum Noise . Shot Noise .
. .
. .
Background Thermal
Shot Noise Noise
9006-008b
where:
M = avalanche current gain (400),
x = excess avalanche noise factor (0.25),
Ri = current responsivity (0.26 mA/mW),
Pr = received signal power (1.60 x 10-15),
Nb = background radiation (= 0),
Id = dark current at unity gain (0.3 nA),
k = Boltzmann's constant (1.38 x 10-23 J/K),
T = temperature (300 K),
F = noise figure of amplifier (6 dB),
RL = effective load resistance (10 Mohm),
Bo = optical bandwidth (1 Hz),
Be = electrical output bandwidth (1 Hz).
9006-008c
Shot noise limited detection is reached when the shot noise due to left hand side of
the denominator exceeds the thermal noise of the right hand side of the denominator, viz
2kTF
RL > -------------------------
e{Ri(Pr + NbBo) + Id}M(2+x)
In order to maintain shot noise limited detection when high bandwidths are required, a
high avalanche gain M and/or a transimpedance front-end amplifier can be employed (9008-028). The combination of avalanche photodetector and
transimpedance amplifier generally produces the most sensitive front-end. If M2RL
= 1000, the shot noise threshold is reached when the received power is about 1 mW. Note
that no account has been taken of 1/f noise which may be substantial at very low
frequencies, i.e., below 1 kHz.
In an ideal photon-counting receiver, the dark current and thermal noise sources may
be ignored. If the background radiation received Pb = NbBo
is negligible, then the sensitivity of the receiver is ideal.
9006-008d
For a front-end with the values given in parentheses, the shot-noise limit is reached
in an avalanche photodetector system with received power Pr = -90 dBm (10-12
W), when:
RL > 1 Mohm
Using the values given in parentheses:
CNR = 7 dB re 1 Hz
Under shot-noise limited conditions, the expression for CNR may be simplified to:
(MRiPr)2
CNR = ------------------------------
[2e{Ri(Pr + NbBo) + Id}M(2+x)]Be
9006-008e
Since Pr is very small, depending on the values of dark current, avalanche
gain and excess noise factor, the recovered CNR will be less than optimum. The background
radiation NbBo (Pb), which hopefully is mainly starlight
from the alien star, can be minimized by choosing a very narrow band optical filter. At 10
light years range for a received signal power of -118 dBm, the shot noise due to Pb
is only about 32 dB below the shot noise due to Pr alone if the optical
bandwidth is 1 Hz. This is the relative level of Planckian starlight at 10 light years if
the transmitter and star are not separately resolved in a 10 meter diameter telescope.
This ratio increases to about 72 dB for diffraction limited telescopes.
If the optical filter has a bandwidth as low as 1.6 kHz, the CNR will fall by 3 dB,
because Pb = Pr, and the shot noise would double. The occurs
irrespective of the electrical post-detection bandwidth. Such high Q optical filters are
very difficult to produce and tune for tracking of Doppler shifts and drifts (chirp).
Incoherent detection also means that the modulation format is restricted to be only one of
two basic forms; intensity or polarization modulation. A 1.6 kHz optical filter is
virtually impossible to construct, so we can eliminate this approach to filtering for
achieving the ultimate SNR. For a diffraction limited telescope, the corresponding optical
bandwidth is 16 MHz; a reasonable bandwidth for a Fabry-Perot interferometer.
9006-008f
A way to obtain increased sensitivity in incoherent receivers is to operate an APD in
the photon-counting mode. In this situation, dark current has a minor effect. As before,
if Pb is equal to Pr the recovered SNR will be degraded by 3 dB.
If all other sources of noise can be ignored, i.e., excess photodetector noise, dark
current noise and background radiation noise, the above equation may be further simplified
to:
eta.Pr
CNR = ------
2hfBe
This is 3 dB less than obtainable with a perfect optical heterodyne receiver.
The Columbus Optical SETI Observatory
Copyright (c), 1990
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