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# Measurement of an Unknown Impedance in an Automatic Tester

IP.com Disclosure Number: IPCOM000048046D
Original Publication Date: 1981-Dec-01
Included in the Prior Art Database: 2005-Feb-08
Document File: 2 page(s) / 27K

IBM

Reis, RJ: AUTHOR

## Abstract

The in-phase and quadrature responses of an unknown impedance are found by exciting the impedance with a known sinusoidal voltage, multiplying the resulting current sequentially by the in-phase and quadrature input signal, and measuring the resulting DC voltages.

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Measurement of an Unknown Impedance in an Automatic Tester

The in-phase and quadrature responses of an unknown impedance are found by exciting the impedance with a known sinusoidal voltage, multiplying the resulting current sequentially by the in-phase and quadrature input signal, and measuring the resulting DC voltages.

Fig. 1 shows a test system 10 for connecting an unknown impedance 11 to a reference AC voltage source 12 for generating Vr equals cosWT. Current-to- voltage converter 13 may use a high gain operational amplifier and feedback resistor Rf to produce a voltage like Vz=-IRf'cos(WT+theta) from the current I flowing through impedance 11. Analog multiplier 14 forms the product of Vz either with Vr or with Vq equals sinWT from quadrature generator 15, as selected by tester controlled switch 16.

The two product signals formed by multiplier 14 are:

Vpr equals -IRf'cos(WT+theta)cosWT equals -0.5

IRf(cos(2WT+theta)+costheta) and Vpq equals

-IRf'cos(WT+theta)sinWT equals -0.5 IRf(sin(2WT+0)+sin(-0)), depending upon the position of switch 16. Low-pass filter 17 then removes all AC components greater than WT, leaving the DC voltages Vmr=-0.5IRf'costheta and Vmq=0.5IRf'sin theta, corresponding respectively to the real and imaginary responses of impedance 11. These voltages can be analyzed in analog form or converted to digital form for finding the complex value Zeta of impedance 11. Vmp and Vmq frequently differ greatly in magnitude from each other, because Zeta...