Investigation into synchronization for partial response signals and the development of a clock recovery scheme for 49QPRS signals

Abstract

Data communication is used increasingly in modern society. It is against this background that research is conducted worldwide toward the improvement of existing, as well as the development of new, improved communication techniques. Correlative encoding of data before transmission IS a very frequency-effective communication technique. The extent to which any communication technique is used, however, is dependent on a wide variety of factors. This study regarding the synchronisation of 49QPRS signals was undertaken with this in mind. Since digital signal processing (DSP) is used increasingly in modern communication systems, both a data transmitter and receiver were implemented by making use of this technique. Not only would this result in a system with all the desirable characteristics inherent to DSP, but, by making limited changes to the supporting software, the evaluation of a wide variety of alternatives became feasible. During the study a system making use of a pilot tone at one third the frequency of the carrier frequency was developed. The receiver recovers this signal by means of DSP techniques and its frequency is tripled. The phase of this recovered signal is crosscorrelated every 650 ~s in time with a locally generated signal of the correct frequency - and the phase of the locally generated signal is adjusted accordingly. It was found that the accuracy and stability of the locally generated signal were such that sufficient synchronisation was obtained in this manner. The quality of synchronisation is a function of the level of the pilot tone and if this tone should decrease to below a certain value, unacceptably large phase adjustments have to be made. This results in a senous degradation of the spectral purity of the recovered signal. However, the system as described exhibits extremely good noise immunity. During the development of the clock frequency recovery system, a baseband filter with a unique frequency response was defined. Making use of this, in conjunction with a limited amount of pre-processing, and an absolute value rectifier, recovery of the clock frequency becomes possible. In order to limit the amount of processing by the receiver, the baseband filter was implemented in its entirety in the transmitter. The recovered signal showed a moderate amount of amplitude variation, but an extremely stable synchronising signal could be derived from this. During the study both levels of synchronisation required by a hypothetical 49QPRS data communication system were therefore investigated fully and solutions found.