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Carrier-To-Noise Ratio In Incoherent Systems



Graph 9008-036 illustrates how it is not really possible to achieve the ultimate CNR in conventional direct detection optical receivers.  The ultimate CNR of 31 dB for the -118 dBm signal received from the 1 kW alien transmitter situated 10 light years away, is 3 dB less than obtainable with an ideal optical heterodyne receiver.  It is also 6 dB smaller than obtainable from an ideal optical homodyne receiver.

For the particular parameter values chosen for the Silicon Avalanche Photodetector (APD) system with a gain of 400, only a CNR of 7 dB is realisable.  This assumes that the load resistance RL = 10 Mohm, and that the unity-gain dark (leakage) current Id = 0.  Even this value of CNR is somewhat optimistic, since Id is always finite.  Very small area photodiodes, such as might be used to construct a two dimentional array, can help minimize Id.  No account is made of 1/f noise that will exist at frequencies close to zero.  The quantum-noise limited region of the characteristics are asymptotic about 6 dB below the dotted line, since this line is based on assuming that the APD excess noise factor x = 0.  The characteristics for the 1 Mohm and 10 Mohm loads and 0.2 nA dark current coincide, due to the high level of shot-noise produced by this current.  Clearly, if we need a moderate bandwidth with the very high value of load, a transimpedance front-end, as illustrated in 9008-028, will be required.

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