Phase-Locked Sampling Instruments

This paper describes a new approach to the design of RF instruments which are capable of measuring phase difference, level, distortion, and amplitude modulation of high-frequency signals from about 1 Mc/s to 1000 Mc/s or higher. The basic block consists of a modified phase-lock loop with a search and reset circuit. The input high-frequency signal is sampled, the frequency of the sampled output is locked to a reference frequency of a few kc/s, and the output waveform is a sampled replica of the input waveform. Using such a loop, measurements of RF signals can be made easily by a constant low-frequency readout circuit. Synchronization is obtained automatically for any input frequency without the need of a separate trigger circuit. Limitations as well as applications of the sampling phase-lock loop are briefly discussed.