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Absolute LRC Bridge for Picosecond Pulse Measurements

IP.com Disclosure Number: IPCOM000084151D
Original Publication Date: 1975-Sep-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Elliott, BJ: AUTHOR

Abstract

In the measurement of the properties of conductive materials and of fast circuits, it is frequently necessary to measure small inductive or capacitive effects in the presence of dominant resistive components, including changes in transmission line impedance at junction nodes.

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Absolute LRC Bridge for Picosecond Pulse Measurements

In the measurement of the properties of conductive materials and of fast circuits, it is frequently necessary to measure small inductive or capacitive effects in the presence of dominant resistive components, including changes in transmission line impedance at junction nodes.

One method, using time domain reflectometer, is effective and sensitive, but it does require a considerable quantity of equipment in order to achieve sufficient time position stability necessary for accurate measurements. Also indirect calibration is required in this method. Described is a simpler "bridge" technique that has intrinsic absolute calibration and is insensitive to time position instability (drift) in the associated oscillographic sampling circuits.

The basis of the present method is a modified Time Domain Reflectometer (TDR), which uses two time "windows" and an ancillary circuit to "time share" a commercial lock-in detector (LI), thereby comparing the two analogue sampled signals from the sampling oscilloscope of the TDR. Thus, referring to Fig. 1, the replicated, analog time, audio signal (typically 500 Hz) that is derived from the realtime signal reflected from the time window, W(A),(which contains the unknown impedance) is fed into one input channel of a synchronized lock-in detector 10, channel A, for half of the measurement time. For the other half, the earlier window, W(B), is viewed. The controlling switch 12, alternates the A and B inputs of the lock-in detector 10 at a slow rate (typically 5 Hz) while the time position biases (V(1) and V(2)) of the time-base circuit are alternated - with the aid of switch 14.

Usually the reference, realtime "standard" reflector is a coaxial short circuit, in window W(B). In this way the lock-in detector 10 is able to be used as a detector to "balance the bridge", by comparing the amplitude and phase of the sampled signal W(A) wit...