Operational Limitations of Charge Transfer Devices

According to a most significant theorem due to Shannon, 2 the maxim u m information transmission capacity CT of any channel is determined by the bandwidth and the signal/noise ratio of t h e channel. With a slight modification this theorem can be transformed into a theorem on the maximum information storage capacity C S of any channel, in particular of a C T D . This C, then places an upper limit on the number of bits of information which can be stored in an unregenerated section of a C T D . If we restrict consideration to codes in which the size of each charge packet is independent of preceding or subsequent packets, we can calculate a minimum error r a t e and specify an optimum detection scheme for digital signals. To obtain this result, some knowledge of the signal/noise ratio and the accumu- OPERATIONAL LIMITATIONS OF CTD's 1455 lation of residual charge owing to t h e incomplete transfer of portions of every charge packet is necossary. In this paper, in order to establish the minimum possible error rate and maximum possible storage capacities attainable with C T D ' s , it will be necessary to first review the way incomplete charge transfer determines the attenuation-versus-frequency characteristic and the total bandwidth of a C T D . The role of incomplete charge transfer in signal degradation will then be discussed. Once the dependence of the accumulated residual charge on t h e preceding signal is understood, it is possible to devise a dynamic detection scheme for simple digital signals and to compare this with the basic method of absolute-amplit u d e or fixed-threshold detection (static levels).