Phase sensitive detector and frequency component measuring apparatus

• Main Author: STEPHEN JOHN KEARLEY
• Format: Patent
• Language: English
• Published: 14.12.1983
• Subjects: MEASURING; MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES; PHYSICS; TESTING

A phase sensitive detector has a multiplier circuit (10-12) providing an output signal representing the product of an input signal and a predetermined switching waveform having a selected fundamental frequency. The detector includes switching means (13) arranged to form the switching waveform with 2n discrete levels (e.g. light levels), where n is greater than or equal to 2. The discrete levels are selected to minimise the amplitudes of a maximum number of lowest order harmonics in the switching waveform. A low pass filter (14) filters periodic components from said output signal. Apparatus (Fig. 3) for measuring the amplitude of a component at a predetermined frequency on an input signal comprises a pair of the above detectors, each being connected to receive said input signal as the input. The switching waveforms for said detectors have the same fundamental frequency equal to said predetermined frequency, but are in phase quadrature to one another, and a vector adder combines the output signals of the two detectors to produce a signal representing the amplitude of said component.