Method of analog measurement of a capacitance and a capacitance meter for carrying out said method

Claim 3

A method according to claim 1, wherein oscillators are selected so as to ensure that their frequency is linearly dependent on the reciprocal of the capacitance connected thereto and linearly dependent on the control voltage.

Claim 4

A method according to claim 1, wherein: a fixed capacitance is connected in the circuit of a first oscillator and a fixed control votlage is applied to said first oscillator, the capacitance to be measured is connected in the circuit of a second oscillator, thereby modifying its frequency, the control voltage of said second oscillator is modified so as to bring the frequency of the second oscillator to the frequency of the first oscillator which is taken as a reference.

Claim 5

A method according to claim 1, wherein: the frequencies of the two oscillators are compared, an error voltage is generated and is proportional to the difference between the two frequencies, said error voltage is integrated, thereby supplying the control voltage for the second oscillator.

Claim 6

A method according to claim 3, wherein use is made of oscillators which deliver square-wave signals and wherein: the frequencies of said square-wave signals are compared, there is produced on the basis of the result of said comparison a rectangular-wave voltage having a level which is alternately +V and -V, said rectangular-wave voltage is employed as control voltage for the second oscillator, the mean value of said control voltage is measured and this gives the value of the capacitance connected in the circuit of the controlled oscillator.

Claim 7

An analog capacitance meter for carrying out the method according to claim 1 of the type comprising two oscillators whose frequencies are dependent on the capacitances which are present in the circuit of said oscillators, wherein said capacitance meter comprises: two oscillators each having at least two inputs, one of which is a voltage control input, and one output which delivers a signal whose frequency is dependent on the voltage applied to the voltage control input and on the capacitance connected to the capacitance control input, the capacitance connected to said capacitance control input being independent of the voltage applied to said voltage control input, a correspondence control circuit for controlling the frequency of either of the oscillators in dependence on the frequency of the other oscillator and connected to the outputs of both oscillators, the output of said circuit being intended to deliver a voltage which is the control voltage for the frequency-controlled oscillator, a voltmeter for measuring said control voltage which is supplied by said correspondence control circuit.

Claim 8

A capacitance meter according to claim 6, wherein: in the case of said first oscillator, the voltage control input is brought to a fixed potential and the capacitance control input is connected to a fixed capacitor, in the case of said second oscillator, the capacitance control input is connected to the capacitance to be measured and the voltage control input is connected to the output of said correspondence control circuit.

Claim 9

A capacitance meter according to claim 6, wherein each oscillator delivers a signal whose frequency is linearly dependent on the voltage and on the reciprocal of the control capacitance, the indication of the voltmeter being in that case linear as a function of the measured capacitance.

Claim 10

A capacitance meter according to claim 6, wherein said correspondence control circuit comprises on the one hand a phase comparison circuit which receives the signals emitted by the two oscillators and delivers a voltage which is proportional to the frequency difference between the two oscillators and on the other hand an integrating circuit which is connected to the output of said phase comparator and the output of which is connected to the voltage control input of the controlled oscillator.

Claim 11

A capacitance meter according to claim 7 wherein: a first capacitor having a value 2Co and connected to the capacitance control input of the first oscillator, the voltage control input of said first oscillator being connected to ground, a second capacitor having a value 2Co and connected to the capacitance control input of said second oscillator, the capacitance to be measured being connected in parallel with said second capacitor, the measurement rating of said capacitance meter being accordingly equal to Co.

Claim 12

A capacitance meter according to claim 8, wherein each oscillator is provided in addition with a third input which is brought to a fixed voltage and comprises an adjustable resistance, the period of the oscillator being linearly dependent on the value of said resistance.

Claim 13

A capacitance meter according to claim 10, wherein each of said first and second capacitors having a value 2Co is constituted by N capacitors in parallel which are selectable by means of a switch and define N measurement ratings of the capacitance meter.

Claim 14

A capacitance meter according to calim 6, wherein the voltage control input of the frequency-controlled oscillator comprises a variable resistor for adjusting the sensitivity of the control voltage.

Claim 15

A capacitance meter according to calim 6, wherein the voltage control input of the frequency-controlled oscillator comprises a variable resistor for adjusting the sensitivity of the control voltage.

Claim 16

A capacitance meter according to claim 6, wherein said oscillators are periodic function generators.

Claim 17

A capacitance meter according to claim 14, wherein said generators deliver a square-wave voltage.

Claim 18

A capacitance meter according to claim 9, wherein said integrating circuit comprises an operational amplifier associated with P switchable capacitors mounted in feedback and defining P values of the integration time.

Claim 19

A capacitance meter according to claim 7, wherein the oscillators are of the type which emit a square-wave signal and wherein said correspondence control circuit comprises a phase comparator for rectangular-wave signals, a shaping circuit which is controlled by said comparator and delivers a rectangular-wave voltage having a level alternately of +V and -V, said voltage being applied to the voltage control input of said second oscillator, said measuring voltmeter being a device for measuring the mean value and being connected to the output of said shaping circuit.