Inductance and capacitance reactance measuring instrument having minimal inclusion of stray reactances

Claim 2

Electrical apparatus for producing a measurement of the reactive electrical impedance of a first circuit element, said apparatus comprising A. a pair of terminals for connection to said first circuit element, B. a second circuit element of known electrical impedance, C. current source and switching means connected with said pair of terminals and with said second circuit element, and producing a single alternating current with both a phase and a magnitude dependent in alternate time periods first on the reactive impedance between said pair of terminals and second on said known impedance of said second element, said current source and switching means producing said current with relative phases, in said first and second alternate time periods, dependent substantially only on the reactance of said first circuit element, and on the reactance of said second circuit element, respectively, D. signal transmission means providing a single information channel and coupled in circuit with said current source and switching means, and transmitting a signal corresponding to said single alternately-dependent current to signal-processing means separated therefrom, and E. detecting means receiving said signal corresponding to said alternately-dependent current and receiving a control signal identifying the switching of said current source and switching means between said alternate time periods, and producing in response thereto an output measurement of said reactive impedance between said pair of terminals relative to said known impedance.

Claim 3

Electrical apparatus as defined in claim 1 in which A. said reference circuit element has an impedance of known reactance, and B. said detecting means detects the phase of said alternately-dependent current which is dependent on said impedance between said pair of terminals terminals relative to the phase of said current which is dependent upon said known impedance to produce said output measurement.

Claim 4

Electrical apparatus as defined in claim 1 in which said current source and switching means comprises A. a source of alternating current, B. output terminal means at which said alternately-dependent current is developed, and C. double-pole, double-throw switch means alternately, in successive time periods, connecting said source current with a first relative phase of said current in circuit with said pair of terminals and said output terminal means and, connecting said source current with a reversal of said relative phase of said current in circuit with said second circuit element and said output terminal means.

Claim 5

Electrical apparatus as defined in claim 3 in which said current source and switching means further comprises circuit means of low reactance interconnecting said switch means and output terminal means with said pair of terminals and with said second circuit element, and develops said current at said output terminal means with phases, relative to the phase of said source, dependent substantially exclusively on the reactance of said first circuit element and of said second circuit element, respectively, in each of said alternate time periods.

Claim 6

A reactance-measuring circuit comprising A. means for modulating at least the phase of a radio frequency signal in a succession of time intervals alternately by essentially only the reactance of a first circuit element being measured and by essentially only the reactance of a second reference circuit element, B. single-channel signal transmission means for delivering said modulated radio frequency signal away from said modulating means, and C. means for receiving said modulated radio frequency signal from said Transmission means and for detecting at least the phase of the modulation of said signal by said reactance of said first circuit element relative to the phase of the modulation of said signal by said known reactance of said reference circuit element and in time synchronism with the alternation of said time intervals, and for providing an output signal responsive to the result of said detection, thereby to provide a measure of the reactance of said first circuit element.

Claim 7

A reactance-measuring circuit comprising A. means for modulating at least the phase of a radio frequency signal in a succession of time intervals alternately by essentially only the reactance of a first circuit element being measured and by essentially only the reactance of a second reference circuit element, B. single-channel signal transmission means for delivering said modulated radio frequency signal away from said modulating means, and C. means for receiving said modulated radio frequency signal from said Transmission means and for detecting at least the phase of the modulation of said signal by said reactance of said first circuit element relative to the phase of the modulation of said signal by said known reactance of said reference circuit element and in time synchronism with the alternation of said time intervals, and for providing an output signal responsive to the result of said detection, thereby to provide a measure of the reactance of said first circuit element.

Claim 8

A measuring circuit as defined in claim 5 in which said transmission means consists of a single conductive path interconnecting said modulating means and said receiving and detecting means and conducting said modulated signal therebetween.

Claim 9

A circuit for measuring the impedance of a first circuit element comprising A. a radio frequency source of alternating voltage at the frequency of measurement, B. a pair of first terminals for connection to said first circuit element, C. a second circuit element of known impedance, D. switch means having first and second switch conditions and, when in said first condition connecting said source voltage to said first terminals to develop therebetween a first current dependent in phase and magnitude on the impedance of said first circuit element connected therebetween, and when in said second condition connecting said source voltage to said second circuit element to develop therein a second current dependent in phase and magnitude on said known impedance, E. switch control means for changing said switch means successively between said first and second conditions, F. current summing means connected with said first terminals and with said second circuit element and developing a further current responsive in successive time intervals to said first current and to said second current, G. signal transmission means connected with said summing means and providing a single information channel for transmitting said further current to a location removed from said pair of first terminals, and H. synchronous detector means located removed from said pair of first terminals and receiving from said transmission means a signal corresponding to said further current from said summing means and receiving a control signal corresponding to the condition-changing operation of said switch control means for producing a first output signal responsive to the magnitude of a first component of said first current having a selected phase relative to said second current.

Claim 10

A circuit for measuring the impedance of a first circuit element comprising A. a radio frequency source of alternating voltage at the frequency of measurement, B. a pair of first terminals for connection to said first circuit element, C. a second circuit element of known impedance, D. switch means having first and second switch conditions and, when in said first condition connecting said source voltage to said first terminals to develop therebetween a first current dependent in phase and magnitude on the impedance of said first circuit element connected therebetween, and when in said second condition connecting said source voltage to said second circuit element to develop therein a second current dependent in phase and magnitude on said known impedance, E. switch control means for changing said switch means successively between said first and second conditions, F. current summing means connected with said first terminals and with said second circuit element and developing a further current responsive in successive time intervals to said first current and to said second current, G. signal transmission means connected with said summing means and providing a single information channel for transmitting said further current to a location removed from said pair of first terminals, and H. synchronous detector means located removed from said pair of first terminals and receiving from said transmission means a signal corresponding to said further current from said summing means and receiving a control signal corresponding to the condition-changing operation of said switch control means for producing a first output signal responsive to the magnitude of a first component of said first current having a selected phase relative to said second current.

Claim 11

A measuring circuit as defined in claim 7 in which said synchronous detector means further produces a second output signal responsive to the magnitude of a second component of said first current 90* out of phase with said first component thereof.

Claim 12

A measuring circuit as defined in claim 7 further characterized in that said switch means, said current summing means, and circuit means interconnecting said switch means and said summing means with said first terminals and with said second element are substantially free of reactance so that said first and second currents differ from each other in phase and magnitude essentially only by the difference between the impedance of said first circuit element between said first terminals and said known impedance of said second circuit element.

Claim 13

A measuring circuit as defined in claim 7 in which said synchronous detector means comprises A. a first detector for producing a reference direct voltage responsive to a signal corresponding to said second current from said current means, B. a phase detector receiving a first input signal corresponding to said sum of said first and second current from said current summing means, C. a reference signal source for receiving the output signal from said phase detector and producing a second input signal to said phase detector with a frequency and phase that null the phase detector output signal, and D. switch means connected for operation in timed sequence with said switch control means for applying to said first detector only the portion of said first input signal corresponding to said second current from said current summing means, and for applying the phase detector output signal to said reference signal source only during delivery to said phase detector of said portion of said first input signal corresponding to said second current from said current summing means.

Claim 14

A measuring circuit as defined in claim 10 further comprising output converter means for receiving said reference direct voltage and for receiving said phase detector output signal responsive to the portion of said first input signal corresponding to said first current from said current summing means, and producing in response thereto a measure of the reactance of said first circuit element relative to said known impedance.

Claim 15

A measuring circuit as defined in claim 7 in which said synchronous detector means comprises A. a multiplier receiving a first input signal corresponding to said sum of said first and second currents from said current summing means, B. delay means for receiving said first input signal to said multiplier and applying it to a second input of said multiplier with a time delay equal to the time interval during which said switch control means maintains said switch means in each of said first and second conditions, and C. a first detector receiving only the portion of said first input signal corresponding to said second current output from said summing means and producing a reference direct voltage in response thereto.

Claim 16

A measuring circuit as defined in claim 12 further comprising output converter means receiving said reference direct voltage and the output from said multiplier, and producing in response thereto a signal measuring the reactance of said first circuit element relative to said known impedance.