Phase locked system design and measurement tutorial consisting of physical hardware and co-simulation environment

International Journal of Electrical Engineering Education, Oct 2004 by Burbidge, Martin John

Abstract

Phase locked loop based feedback techniques are used in a variety of system level timing, control and communication applications. Re-configurable hardware and associated simulation models have been developed with an emphasis towards teaching the fundamentals of phase locked loop systems. The material is hardware focussed and reinforces control system theory, characterisation, design, and modelling. The simulation part of the material can be used in an Internet based teaching environment.

Keywords behavioural system modelling; control systems; hardware verification; phase locked loops; phase locking

Phase locked loops (PLL) and associated phase locking theory are important and useful techniques that can be used in a variety of electronic communications, control and instrumentation applications. The techniques are widely used in larger systems that require precise timing or control reference signals that are directly synchronised to a system output. Typical applications include motor speed control1'2 and RF (radio frequency) carrier generation3. Various types of phase locked architectures exist, encompassing fully analogue, semi-digital, and fully digital types2 of PLL, respectively. It must be noted that although the term digital is used, the transient response of the PLL control parameters during acquisition is essentially an analogue phenomenon. In terms of popularity the semi-digital type of PLL or charge-pump phaselocked loop (CP-PLL) seems to be the architecture of choice for the majority of current hardware based phase locking applications. This type of PLL is readily and cheaply available as a stand-alone building block, and can be configured to have similar transient response to other types of PLL.

The PLL is essentially a closed loop feedback control system and can, depending on the implementation, be compared and contrasted with classic feedback control systems, such as proportional plus integral (PI) feedback loops. Many key PLL references rely heavily on classic control system theory and they often provide mechanical analogies to help explain PLL operation. All common control system design techniques - for instance, bode plots, step response plots, nyquist plots, and s-domain mapping4 - can be used to aid the engineer in realising a correct PLL system implementation. However, a PLL system differs in one significant aspect, which is that the inputs and outputs of the system are usually considered to be continuous periodic signals as opposed to a constant parameter. The PLL system essentially measures the phase difference (and sometimes frequency difference) between the two continuous signals and activates the feedback mechanism accordingly until the respective signals are phase aligned. This change from operating on an ideally constant parameter in the steady state (voltage, force, displacement, current, etc) to that of a constant parameter relating two continuously time varying signals (i.e. phase) can lead to confusion when attempting to explain phase locking theory and applications. In consequence, it can deter students from fully appreciating the processes involved.

Despite their widespread use in industrial applications, PLL system techniques seem to be only superficially covered in many general undergraduate electronic/ mechatronic courses. Structured practical examples and experimentation can be used to enhance theoretical understanding of this useful technique and also provide a good reinforcement of essential elements of classic control theory. Furthermore, linking of basic theory to experiments can provide potential practising engineers with a sound working knowledge of the system.

With the previous points in mind an attempt was made to see if any available material applicable to teaching applications was freely and cheaply available. Various simulation/modelling packages are available on the market and some of these include PLL models. However, it was a major objective of the envisaged course to include a significant proportion of practical experiments and measurements on physical hardware. Many of the available simulation models are generic in nature and are not wholly applicable to the specific task. Another problem was that of selecting a suitable hardware demonstrator. Many PLL manufacturers supply demonstration boards for their PLLs and also have excellent accompanying design tools.5,6 Unfortunately, due to many reasons, such as expense and complexity, many of the boards were unsuitable for general teaching applications.

The decision was made to develop a complete set of course notes, simulation models and hardware to allow students to carry out a set of measurements and experiments relating to PLL measurement and design. In the initial stages of course development a set of primary requirements and student objectives was decided upon. These are highlighted below.

* The course must be self-contained and include sufficient theoretical material to provide an initial working knowledge of phase locking theory.