GB2242279A

GB2242279A - Measuring low capacitances

Info

Publication number: GB2242279A
Application number: GB9101869A
Authority: GB
Inventors: Jorma Ponkala
Original assignee: Vaisala Oy
Current assignee: Vaisala Oy
Priority date: 1990-02-07
Filing date: 1991-01-29
Publication date: 1991-09-25
Also published as: FR2657963A1; FI900592A0; IT1245137B; BR9100486A; GB9101869D0; FI84666B; ITMI910300A0; FI84666C; ZA91506B; DE4103433A1; ITMI910300A1; CA2035909A1; AU6949191A; FI900592A7

Abstract

A circuit for measuring low capacitances comprises an LC or RC oscillator (10) whose output frequency (f0) is a function of the capacitance connected between an input terminal (16) and ground (15). In use, a capacitance C1 is first connected between the input terminal (16) and ground and other capacitances (C2, C3) are connected (14) to the output (22) of a voltage follower (20) having its non inverting input connected to the oscillator input terminal (16). The capacitor connections are then varied. A shield (19a) may be used. <IMAGE>

Description

1 1:

1 Measurement coupling for low capacitances 2 7,>.4:2 --> The invention concerns a coupling for measurement of low capacitances, said coupling comprising an oscillator, such as an LC or RC oscillator, whose output frequency is a function of the capacitance to be measured and con nected between the Input terminal of the oscillator and the ground, the capact tances to be measured being connected alternatingly to the input terminal of said oscillator.

In prior art, a number of different oscillator couplings are known which Include a LC or RC circuit as a component affecting the frequency. In a way known in prior art, these oscillator couplings are used for measurement of capaci tance, because, by using the calculation formulas of each oscillator coupling, from the oscillator frequency it Is possible to determine the capacitance when the values of the other components in the coupling are known.

In prior art, various LC and RC oscillators are also known In which one terminal of their capacitance C is connected to the ground. In the case of measurement of low capacitances, the latter couplings, however, involve the drawback and problem that various uncontrolled stray capacitances are con nected in parallel with said capacitance C of the measurement oscillator, whereby the accuracy of measurement Is impaired. These problems are the more extensive, the lower the capacitances that are measured.

The measurement of low capacitances Is quite common in various detector techniques and In telemetry, e.g. in radiosondes, in which, for measurement of meteorological quantities, different capacitive detectors are used whose capacitance is relatively low, typically within the range of 1... 100 pF. In respect with these prior-art measurement methods and couplings, reference Is made to the following patents of the applicant: US 4,295,090, US 4,295, 091, 1 1, 2 The object of the present invention is to avoid the drawbacks that have come out, above and to provide a measurement oscillator coupling that is well suitable for measurement of low capacitances and also for measurement arrangements in which a change-over switch is employed by whose means different detector capacitances and reference capacitances are connected to the input of the measurement oscillator alternatingly in a suitable measure m6nt sequence.

An object of the present invention is to provide a measurement oscillator coupling by whose means low capacitances, typically of an order of 0... 100 pF, can be measured more accurately than in prior art, without disturbing effects from stray capacitances and equivalent.

It is a particular object of the present invention to provide a measurement coupling which is suitable for use in radiosondes in connection with capacitive detectors that measure meteorological quantities.

In view of achieving the objectives stated above and those that will come out later, the invention is mainly characterized in that, with the exception of the capacitance to be connected between the input terminal of the oscillator and the ground for measurement, the other capaci tances to be measured are connected, by one of their terminals, to a wire common of the capacitances to be measured and, by the other one of their terminals, to the output of the voltage follower, that the input of said voltage follower is connected to the input terminal of the oscillator coupling, and 3 that said voltage follower consists of an operation amplifier, whose output is connected to its inverting input, to which the capacitances that are not being measured are connected so that there are no voltage differences and, consequently, no charging currents in their terminals.

According to the Invention, when the input of the voltage follower, preferably the input of the operation amplifier, Is connected to the terminals of the capacitances that are not being measured, the opposite terminals of these capacitances will be at the same potential, so that, in the coupling, no such charging currents of said capacitances occur as could affect the output frequency of the measurement oscillator. In the coupling, various stray capacitances and DC leakage currents may occur within the scope of the properties of the operation amplifier or equivalent acting as the voltage follower.

is When the invention is applied, stray capacitances may also occur at the side of the common wire between the capacitances to be measured and the input of the measurement oscillator. The effect of these stray capacitances can be eliminated by on this wire employing a shield, which is connected to the output of the voltage follower. In said shielding, It Is In the other respects possible to apply the principles that are stated in the applicant's FI Patent No. 57319.

As a rule, a coupling in accordance with the invention is optimal when the measurement oscillator is a sine-wave oscillator, because in such a case the requirement of speed of the voltage follower is lowest. The amplifier of the voltage follower must amplify sufficiently, because in the contrary case the amplitude remains lower at the output than It is at the input, and in such a case stray capacitances begin to have a detrimental effect on the accuracy of measurement.

In the following, the invention will be described in detail with reference to some exemplifying embodiments of the Invention Illustrated in the figures in 4 the drawing, the invention being by no means strictly confined to the details of said embodiments.- Figure 1 shows a prior-art oscillator wiring which constitutes a starting point of the present invention.

Figure 2 shows a measurement oscillator wiring in accordance with the invention, by whose means three low capacitances are measured, said capacitances being alternatingly connected to the input of the measurement oscillator.

In Fig. 1 a RC oscillator in itself known is shown, in which an inverter 10 that has hysteresis is used. In respect of this coupling and of its application to the measurement of low capacitances and to sonde telemetry, reference is made to the applicant's FI Patents Nos. 54,664 and 57,319 and to US Patents Nos.

4,295,090 and 4,295,091. At the input terminal 16 of the oscillator shown in Fig. 1 and, thus, at the C-terminals of the capacitor, in relation to the ground, a voltage Ujt) is effective, whose wave form is drawn at Fig. 1.

The curve of the voltage U.W is composed of the parts of the discharge and charging curves of the capacitor C and of the stray capacitance C. connected in parallel with it, the time constant of said curve parts being tC, = 1/R (C+CH)' From the output terminals 18 of the oscillator wiring shown in Fig. 1, a rectangular wave Ul(t) is obtained, whose frequency f. = 1/T is a function of the capacitance C+CH connected between the input terminal 16 of the oscil lator coupling and the ground.

As comes out from the above, the stray capacitances CH have a direct effect on the output frequency f. of the oscillator coupling, which spoils the accuracy of measurement of low capacitances. These problems are accentuated further if a prior-art change-over switch is placed at the input side of a coupling in accordance with Fig. 1, by means of which change-over switch the various capacitive detectors and reference capacitances are alternatingly connected to i t 1 the measurement oscillator, whose output frequency is a function f. - F(CM) of the capacitance Cm to be measured.

Fig. 2 shows an example of a measurement coupling in accordance with the invention, by whose means the problems described above are solved.

The coupling shown in Fig. 2 comprises the capacitances Cl,C2 and C3, of which, for example, Cl Is a precisely known reference capacitance and C2 and C3 are capacitances to be measured, being, for example, detectors that measure the pressure (P), the temperature (T), andlor the relative humidity (U) in a radiosonde. One of the terminals of the capacitances Cl,C2 and C3 are connected together and, jointly by means of the wire 19, further to the Input terminal 16 of the inverter 10. The opposite terminals of the capacitances C11C2 and C3 are connected to the switches 11,12 and 13. Said switches 11, 12 and 13 have two positions, and in one of their positions the capacitances Cl,C2 and C3 are connected, by the intermediate of the contact 15, to the ground, and in the other one of the positions of the switches 11,12 and 13, the capaci tances Cl A and C3 are connected to the wire 21 or contact, which is con nected to the output terminal of the operation amplifier 20, which acts as a voltage follower. The + terminal of the Input of the voltage follower 20 is connected to the input terminal 16 of the Inverter 10.

In accordance with the, in itself known, mode of coupling of the voltage follower, the output of the operation amplifier 20 Is connected to the terminal of its input, which is Illustrated by the wire 22. It is a part of the properties of the voltage follower and of Its operation amplifier 20 or an equivalent component that the input resistance of the voltage follower Rin R; - and the output resistance Rt z 0 In an ideal case. In such a case, the output voltage of the voltage follower IS U2 = U According to Fig. 2, the capacitance Cl Is connected between the ground and the input terminal 16 of the inverter 10, in which case the output frequency of 6 the measurement oscillator is F. = F(C1). The capacitances C2 and C3 are connected, by the intermediate of the wires 14 and 21, to the output of the voltage follower 20, where the voltage U2 = U. prevails. Thus, at the terminals of the capacitances C2 and C3 there is no difference in voltage at all, so that there are no charging currents at same.

The switches 11,12 and 13 are controlled by a control unit 17 so that one terminal of each capacitance C11C2 and C3 IS, in its turn, connected to the ground and, thus, to act upon the input of the measurement oscillator, at which stage the other capacitances are connected between the + terminal of the input and the output of the voltage follower 20.

A stray capacitance may also occur at the side of the common wire 19 of the capacitances Cl C2,C3 to be measured. The effects of these stray capacitances can be prevented by providing the wire 19 with a shield 19a, which is con nected to the output of the voltage follower by means of a wire 23.

Above, the invention has been described in relation to such an application only wherein a RC oscillator is used. Within the scope of the invention, it is, however, possible to use any other suitable oscillator coupling in itself known, for example a LC oscillator, which is advantageous because of the good sine wave provided by it.

In the following, the patent claims will be given, and the various details of the invention may show variation within the scope of the inventive idea defined in said claims and differ from the details stated above for the sake of example only.

t 1 1 7

Claims

CLAIMS:

1. Coupling for measurement of low capacitances (C1,C2,C3), said coupling comprising an oscillator, such as an LC or RC oscillator, whose output frequency (f.) is a function of the capacitance to be measured and connected between the input terminal (16) of the oscillator and the ground, the capacttances (C11C21C3) to be measured being connected alternatingly to the Input terminal (16) of said oscillator, c h a r a c t c r 1 z e d in that, with the exception of the capacitance to be connected between the input terminal (16) of the oscillator and the ground for measurement, the other capacitances to be measured are connected, by one of their terminals, to a wire (19) common of the capacitances (C11C2X3) to be measured and, by the other one of their terminals, to the output of the voltage follower, that the Input of said voltage follower is connected to the input terminal (16) of the oscillator coupling, and that said voltage follower consists of an operation amplifier (20), whose output Is connected to its inverting input, to which the capacitances (C2, C3) that are not being measured are connected so that there are no voltage differences and, consequently, no charging currents in their terminals.

2. Measurement coupling as claimed in claim 1, c h a r a c t c r i z c d In that, for the time of the measurement, one electrode of the capacitance (C,) that Is being measured is connected to the ground (15) by the Intermediate of a connector switch (11).

3. Measurement coupling as claimed in claim 1 or 2, c h a r a c t c r i z c d in that, In the measurement oscillator coupling, an inverting amplifier (10) that has hysteresis is employed, which is feed-back connected by a resistor (R).

8 4. Measurement coupling as claimed in claim 1 or 2, c h a r a c t c r i z c d In that the measurement oscillator is a sine-wave oscillator.

L 5. Measurement coupling as claimed in any of the claims 1 to 4, c h a r a c t c r 1 z c d in that the common wire (19) of the capacitances (Cl,C2, C3) to be measured is provided with a shield (19a), for example with a protective sheath, which is connected to the output of the voltage follower (20).

6. Measurement coupling as claimed in any of the claims 1 to 5, c h a r a c - t e.r 1 z e d in that the measurement coupling comprises two or more capaci tances (Cl 1C21C3) to be measured, at least one of which is a reference capaci tance, whereas the others are detector capacitances to be measured.

7. Measurement coupling as claimed in claim 6, c h a r a c t e r i z c d in that the capacitances to be measured are capacitive detectors of a radiosonde which measure meteorological quantities, such as pressure (P), temperature (T), and/or relative humidity (U).

8. Measurement coupling as claimed in any of the claims 1 to 7, c h a r a c t e r 1 z e d in that the capacitances to be measured are of an order of 0... 100 pF.

J 11, 9 9. An apparatus for measuring low capacitances and comprising an oscillator operable to produce an output frequency dependent upon a capacitance to be measured, the oscillator having an input terminal for connection to a reference capacitance and to the capacitance to be measured, and an operational amplifier, the non-inverting terminal of which Is connected to the input terminal of the oscillator and the inverting terminal of which is connected to the output terminal of same and to switch means for connecting the inverting and output terminals of the operational amplifier either to the reference capacitance or to the capacitance to be measured.

'i 10. An apparatus for measuring low capacitance and substantially as hereinbefore described with reference to Figure 2 of the accompanying drawing.

Published 1991 at The Patent Office. Concept House. Cardiff Road. Newport. Gnment NP9 I RH. Further copies maybe obtained from Sales Branch. Unit 6. Nine Mile Point. CA-mfelinfarh. Cross Keys. Nm'port. NPI 7HZ. Printed by. Multiplex techniques ltd. St. Mary Cray. Rent.