A thorough investigation or existing publications was conducted to determine the etfeet on phase-locked loop operation of the pass band or the low-pass filter in the loop. From the findings, it was concluded that when the loop was out of lock, the pass band of the low-pass filter should be wide; however, when the loop was locked, the pass band of the low-pass filter should be narrow.

The transient analysis or the loop demonstrated that it was desirable to have a wide pass band for the filter in order to minimize the lock-in time of the loop. An analog simulation of the loop verified the transient analysis. The loop simulation required the synthesis of the low-pass filter with operational building blocks in such a way that the pass band could be controlled as potentiometer settings.

The desirability of a variable pass band and low-pass filter was established. The condition that the filter must be able to sense when the loop was either locked or out of lock and adjust its pass band accordingly was set as a prerequisite for the design of a filter. A filter was built and tested which could meet the above conditions. The limited range of the filter left a lot to be desired; however, the feasibility of the approach was demonstrated.