Tracking Systems Employing the Delay-Lock Discriminator

The delay-lock discriminator is a statistically optimum device for measuring the delay between two correlated waveforms. Its theory of operation and the performance of an experimental model have been described by Spilker and Magill.¹ The application of this device to range-and angle-measuring systems is discussed herein. Utilization of the delay-lock discriminator can lead to substantial simplification as compared to tracking systems relying exclusively on phase-lock loops. Such improvements result because the delay-lock loop is free of the ambiguities inherent in phase-lock measurements. The characteristics of presently available delay lines, an essential component of the delay-lock loop, and, in particular, the limited accuracy of which they are capable at present prohibits the immediate exploitation of the full capability of the delay-lock technique. For this reason, an attractive possibility for an immediate implementation is a hybrid system in which a delay-lock measurement is made to resolve ambiguities, while a phase-lock measurement determines the ultimate accuracy. A combined range- and angle-tracking system employing this hybrid concept is described herein, in which the delay-lock measurement is made on a low-pass noise waveform which, for transmission purposes, is applied as amplitude modulation to a stable carrier. The precise measurement is made by phase-locking onto the carrier. The phase-lock output is also used to synchronously detect the low-pass modulation. The characteristics of this system are discussed and expressions are given which show the dependence of threshold SNR and rms delay error upon system parameters. Its theoretical performance is illustrated in tracking a cooperative (i.e.