Improvements in the measurement of impedance ratios

Claim 2

Apparatus for measuring the ratio of first and second impedance means and producing a time ratio output proportional to the ratio of said impedances comprising: means (S 1 ) for supplying a voltage (V 1 ) amplitude modulated at a constant frequency to the first impedance (C 1 ): means (S 2 ) for supplying a voltage (V 2 ) to the second impedance (C 2 ) that is of opposite polarity to the voltage (V 1 ) is amplitude modulated at the same frequency as the voltage (V 1 ) and is of variable amplitude; comparator means (A l , C 3 , S 3 , A 2 , R 1 , C 4 ) responsive to the outputs of said first and second impedance means (C l , C 2 ) to supply an imbalance signal that is a function of (V 1 C l - V 2 C 2 ); and a variable squarewave waveform generator (G 2 , A3) whose mark/space ratio is modified by the imbalance signal from the comparator means and whose output waveform is fed through a low-pass filter (F 1 ) to produce a variable amplitude signal that is said voltage (V 2 ).

Claim 3

Apparatus according to claim 1, wherein the voltages (V 1 ) and (V 2 ) are derived from a common source (G).

Claim 4

Apparatus for measuring the ratio of first and second impedance means (C 1 , C 2 ) each connected at one side to an input to a first amplifier (A 1 ) that outputs a signal, means (G, S 1 , S 2 ) for supplying a repetitive waveform synchronously to the other side of the first and second impedance means (C 1 , C 2 ), a detector (S 3 ) driven synchronously with the waveform supplied to said impedance means (C 1 , C 2 ) and fed with the signal of the first amplifier (A 1 ) to generate a direct current output in response thereto, a second amplifier arranged as an integrator (A 2 , C 4 ) and fed with the output of the detector S 3 to output a voltage V 2 that determines the amplitude of the waveform fed to one of said first and second impedance means (C 2 ), the amplitude of the waveform fed to the other of said first and second impedance means being determined by a reference voltage (V I ) characterised in that the amplitude of the waveform fed to said one impedance means (C 2 ) is determined by the average level of voltage (V I ) supplied from a voltage source via first chopper means (S 4 ) controlled by a comparator (A3) supplied at one input with the output (V 2 , 11) from the integrator and at the other input with triangle or sawtooth waves (G 2 , 10) whereby said circuit reaches a balance such that t l/ t 4 = C 2/ C l wherein t 1 represents the period of the triangular waveform (10) fed to the comparator, t 4 represents the period for which the output of the comparator is at logic 1 and (C 1 , C 2 ) represent the values of the impedances being measured.

Claim 5

Apparatus for measuring the ratio of first and second impedance means (C 1 , C 2 ) each connected at one side to an input to a first amplifier (A 1 ) that outputs a signal, means (G, S 1 , S 2 ) for supplying a repetitive waveform synchronously to the other side of the first and second impedance means (C 1 , C 2 ), a detector (S 3 ) driven synchronously with the waveform supplied to said impedance means (C 1 , C 2 ) and fed with the signal of the first amplifier (A 1 ) to generate a direct current output in response thereto, a second amplifier arranged as an integrator (A 2 , C 4 ) and fed with the output of the detector S 3 to output a voltage V 2 that determines the amplitude of the waveform fed to one of said first and second impedance means (C 2 ), the amplitude of the waveform fed to the other of said first and second impedance means being determined by a reference voltage (V I ) characterised in that the amplitude of the waveform fed to said one impedance means (C 2 ) is determined by the average level of voltage (V I ) supplied from a voltage source via first chopper means (S 4 ) controlled by a comparator (A3) supplied at one input with the output (V 2 , 11) from the integrator and at the other input with triangle or sawtooth waves (G 2 , 10) whereby said circuit reaches a balance such that t l/ t 4 = C 2/ C l wherein t 1 represents the period of the triangular waveform (10) fed to the comparator, t 4 represents the period for which the output of the comparator is at logic 1 and (C 1 , C 2 ) represent the values of the impedances being measured.

Claim 6

Apparatus according to claim 3, wherein the first chopper means (S 4 ) controlled by the comparator comprises a pair of switches driven in antiphase therefrom, one switch connecting the said one impedance means (C 2 ) to the reference voltage source and the other to earth.

Claim 7

Apparatus according to claim 4, wherein the first chopper means (S 4 ) controlled by the comparator is connected via a low pass filter (F l ) to the said one impedance means (C 2 ).

Claim 8

Apparatus according to claim 4 or 5, wherein a dc reference voltage source is connected via second chopper means to said other impedance means (C 1 ) and via said first chopper means (S 4 ) and third chopper means (S 3 ) to said one impedance means (C 2 ), said first and third chopper means being driven synchronously.

Claim 9

Apparatus according to claim 6, wherein said first and said third chopper means (S 1 , S 2 ) each comprise a pair of switches driven in antiphase, one switch being connected to the voltage source and the other being connected to earth.

Claim 10

Apparatus according to claim 7, wherein said switches are field effect transistors.

Claim 11

Apparatus according to any of claims 6 to 8, wherein the source of triangle or sawtooth waves (G 2 ) is driven via a divider (D 1 ) from a carrier generator (G 1 ) that also controls the first and third chopper means (S 1 , S 3 ).

Claim 12

Apparatus according to any preceding claim, wherein said first and second impedance means (C 1 , C 2 ) are elements (C 1 , C 2 ) of a capacitative transducer.