On Bandwidth Requirements for Multilevel PCM Encoders

Most receivers for multilevel pulse-code modulation (PCM) signals incorporate a sample-and-hold circuit in the analog-analog-to-digital converter. The sample-and-hold circuits cannot respond instantaneously in practice, and this paper considers the effects on system performance of their finite response time. To provide guidelines for system designers, the problem is analyzed in some generality and numerical results are obtained for a specific example. It is shown that a definite tradeoff exists between the bandwidth of the operational amplifiers in the sample-and-hold circuit and the follow time, and that system performance deteriorates rapidly if the bandwidth is reduced past a certain critical value. Unless the bandwidth is much larger than the critical value, there is a strong advantage in using follow times larger than onehalf the symbol interval. The bandwidth required for good eight-level transmission is about twice that required for binary transmission.