Carrier-To-Noise Ratio In Coherent Systems

9008-038

Graph 9006-017 illustrates how it is possible to achieve the ultimate CNR in heterodyne detection optical receivers.  A CNR = 34 dB is obtained for the -118 dBm (P_r = 1.60 x 10^-15 W) signal which is received from the 1 kW alien transmitter situated 10 light years away.  This is 3 dB more CNR than obtainable with the ultimate direct detection optical receiver.  It is also 3 dB smaller than obtainable with an ideal optical homodyne receiver.  For this heterodyne receiver with a load R_L = 10 kohm , the receiver is heavily shot-noise (quantum-noise) limited when the local oscillator power P_o is increased to 0 dBm (1mW).  For P_o = -10 dBm (100 µW), the CNR is just on the threshold of being shot-noise limited, and the CNR is 3 dB less than the ultimate.  Because we are dealing with a coherent optical receiver, the slope of the CNR characteristics are 10 dB/decade as long as P_o >> P_r and P_b.  This is independent of whether the receiver is or is not shot- noise limited.

Because the load impedance R_L is relatively low and photodetector RC time-constants (R_L.C_L) are small, it should not be necessary to use transimpedance amplifiers, such as illustrated in 9008-028, unless SETI signals are discovered that are wideband.  It would be prudent however, to use such amplifiers just in case the wider bandwidth is required, if only due to the practical aspects of tuning the local oscillator, and doing a wide radio frequency spectrum analysis of the beat signal to speed up the "search".