In modern communications systems where it is necessary to obtain maximum utilization of signal power, the phase locked loop is being designed into the system as a coherent detector. This loop has a threshold which occurs at very low signal to noise ratios, placing a lower bound on the range of useful signal to noise ratios. Since the loop operation becomes non-linear during threshold conditions, it is difficult to predict the threshold of a given loop during design stages. This paper presents experimental threshold data normalized to apply to a class of phase locked loops. A new definition of the optimum phase locked loop, and threshold are given, and a design method based on these definitions is described. In conjunction with this, the experimental set-up and method of measurement is described. The instrumentation is interesting because of the previously unexplored measurement technique.