A TRA Bridge Technique for In-Circuit Impedance Measurement

A bridge technique is given for the measurement of impedances of two-terminal passive components in an arbitrary network without opening the circuit connections. It is ideally suited to the in-circuit measurements of passive elements in hybrid integrated circuits (HIC's) and printed circuit boards (PCB's). The method employs a new type of bridge, basically improvised from a transformer ratio-arm (TRA), a quadrature amplifier (QA), and two standard capacitors. All the measurements are in terms of the capacitors and the turns ratios of the TRA, and provide results with good accuracy. The range of measurement, together with the sources of error and their minimization, are discussed. Experimental results are found to be in good agreement with the theory.