 dominate above 140MBit/s. The bandwidth of the integrating receivers is very small because the first pole is established by the high value load resistor and the 53.4.2 Analogue optical receivers input capacitance. This is of the order of 300kHz to 500kHz. However the bandwidth can be very large if the first pole is can- The SNR of an analogue receiver can be interpreted as either the celled by a zero. With this approach bandwidths of the order of SNR directly applicable to the information being received or as the 10GHz to 15GHz are possible, provided the first pole is at a few carrier to noise ratio (CNR) which should then be used as the input MHz making the compensation easy. Integrating receivers with SNR to the demodulator suitable for the modulation scheme em- 53/20 Fibre optic communications ployed. For example in a FM subcarrier modulation scheme the FM C7=Cy+Cx+Cu+C (53.68) SNR can be obtain from the CNR by standard FM detection ana- lysis. The SNR or the CNR is given by Equation 53.63 in which m (53.69) is the modulation index, <ina> is the amplifier mean square noise and A f is the receiver bandwidth. (mRM<Popi>)2 53.4.3 Digital optical receivers The design of a digital optical receiver is more complicated because first the performance index is the BER and secondly the receiver bandwidth depends both on the transmitted pulse and of receiver (53.63) output pulse. The BER of a binary digital optical receiver is given by Equation 53.70, where the parameter Q is related to the signal to The meaning of the other parameters can be found in the section noise ratio. on detectors. The details of the < in > depend on the representation of the amplifier noise. For optical receivers the best approach is to (53.70) BER = represent the amplifier, being a single device or a multistage ampli- fier, by two independent noise sources with white spectral density, a current source and a voltage source The detailed equations For Gausssina statistics and equal noise power for both ones and for these noise sources depends on t