Introduction

The phase locked loop (PLL) is used in a number of instrumentation systems. Best's text on PLLs [1] has a chapter on applications in which he lists tracking filters; modulators and demodulators for AM, FM and PM; the recovery of weak signals; frequency synthesis, a field on which there is another valuable text [2]; motor speed control; stereo decoding; subcarrier detection in colour television; and many others.

The original idea of the PLL probably came from the work of de Bellescize in 1932 [3], who used the PLL in AM receiver design. Later work on this application by Tucker and Ridgway provoked a considerable correspondence [4] which suggests that the PLL may be a case of multiple invention, as is so often the case in the field of electronic circuit design.

The simplest kind of PLL is shown in Fig. 9.1. A phase detector is used to give a measure of the phase difference, ϕ, between an incoming signal, v1 sin(ωt) and a VCO. This phase difference is used as an error signal in a closed loop control system in such a way that the VCO ‘locks’ on the incoming signal and follows its changes in phase, and thus in frequency, over some range determined by the designer.

This chapter begins with a very brief look at PLL theory, and this leads at once to an experimental programme using an easily available PLL integrated circuit.