. For example ceivers the bit error rate (BER) is used more or less universally. with a total capacitance of 1pF, a 50 ohms load and a 50 ohms The design of a complete receiver is a complex task and the amplifier input impedance the bandwidth is about 6.35GHz. Higher purpose of this section is to concentrate on the design of the optical bandwidth can be obtained if the load is reduced and since commer- front end which consists of the detector and the low noise amplifier. cially available amplifiers have bandwidths of the order of 15GHz to 20GHz optical receivers with bandwidths of similar order are 53.4.1 Receiver architecture feasible. The difficulty with this approach lies in the high noise the resistive load input receiver yields compared to the other ap- There are basically three classes of optical receivers; resistive load proaches. For this reason optical receivers designed for communi- input, high input impedance or integrating receivers and low input cation systems do not use the resistive load input configuration. Threshold detector Regenerator Optical receiver Post amplifier Equaliser Clock recovery Digital regenerator Figure 53.23 Digital optical receiver Optical receivers and transmitters 53/19 In a high input impedance or integrating receiver the photodetec- very large loads are used in low capacity systems such as 140MBit/s tor is feeding directly a high to very high ohmic load, as in Figure and below. 53.25. The signal developed across this resistor is fed to an active The low input impedance receivers use parallel input feedback at device of high input impedance such as a FET which acts as a the point where the detector drives the amplifier, as in Figure 53.26, voltage amplifier. After the first device there is subsequent amplifi- and they are also known as transimpedance receivers. In principle cation using amplifiers, with or without feedback according to there is no need for high value resistive loads and the bandwidth of requirements. The low frequency gain of the receiver is given by the rece