This invention relates to line frequency synchronisation for use in
diagnostics for alternating current electrical circuits. The invention
provides a method of synchronising measurement system frequency with an
alternating current line frequency comprising the steps of: adjusting the
frequency of the measurement system frequency using a frequency locked
loop until the measurement system frequency is within a predetermined
range of said alternating current line frequency; and when the
measurement system frequency is within a predetermined range of said
alternating current line frequency adjusting the phase and the frequency
of the measurement system frequency using a phase and frequency locked
loop.