Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Measurernent coupling for low capacitances
5 The invention concerns a coupling~for measurement of low capacitances, said
coupling comprising an oscillator, such as an LC or RC oscillator, whose
outpu~ frequency is a function of the capacitance to be measured and con-
nected between the input terminal of the oscillator and the ground, the capaci-
tances to be measured being connected alternatingly to the input terminal of
10 said oscillator.
In prior art, a number of dlfferent oscillator couplings are known which includea 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-
15 tance, because7 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, va~ious LC and RC oscillators are also known in which one
20 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. ~hese problems are the
25 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
- 30 capacitance is relatively low, typically within the range of 1...100 pF. In~u~r~ respect with theseiprior-art measurement methods and couplings, reference is
made to the following patents of the applicant: US 4,295,090, U~ 4,295,091,
FI 54664, FI 57319.
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The object of the present invention is to avoid the drawbacks tbat have come
5 -out -above and to provide a measureme:nt oscillator- coupling that is well
suitable for rneasurement 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-
10 ment sequence.
An object of the present invention is to provide a measuremen~ oscillatorcoupling 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
15 from stray capacitances and equivalent.
It is a particular object of the present invention to provide a measurement
coupiing which is suitable for use in radiosondes in connection with capacitive
detectors that measure meteorological quantities.
In view of achieving the objectives stated aboYe and those that will come out
later, the invention is mainly characteri~ed in
that, with the exception of the capacitance to be connected between the input
25 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,
30 that the input of said voltage follower~is connected to the input terminal of^- the oscillator coupling, and
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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, conse-
quently, no charging currents in their terrninals.
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
10 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 equi~alent acting as the voltage
follower.
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 measuremen~ oscillator. The effect of these stray capaçitances can be
eliminated by on this wire employing a shield, which is connected to the output
20 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 Fl 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
?5 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.
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- ~ In the following, the invention will be described in~-dètail~ith reference to
some exemplifying embo~iments of the invention illustrated in the figures in
~,03'~9~39
the drawing, the invention being by no means strictly confined ta the details
of said embodiments.
Figure 1 shows a prior-art oscillator wiring which constitutes a starting point
5 of the present invention.
F`igure 2 shows a measurement oscillator wiring in accordance with the inven~
tionj by whose means three low capacitan,ces 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 thathas 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 Fl Patents Nos. 54,664 and 57,319 and to US Patents Nos.
15 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 Uo(t) 3s effective, whose wave forrn is drawn at Fig. 1.
The curve of the voltage Uo(t) is composed of the parts of the discharge and
20 charging curves of the capacitor C and of the stray capacitance CH connected
in parallel with it, the time constant of said curve parts being to = l/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 fO = I/T is a function of
the capacitance C+CH connected between the input terminal 16 of the oscil-
25 lator coupling and the ground.
As comes out from the above~ the stray capacitances CH have a direct effecton the output frequency fO of the oscillator coupling, which spoils the accuracy
of measurement of low capacitances. These problems are accentuated further
30 if a prior-art change-over switch is placed at the input side of a coupling in
accordance with Fig. ~1r ~by meaDs of which~ change-over switch the various
capacitive detectors and reference capacitances are alternatingly connected to
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the measurement oscillator, whose output frequency is a function fO = F(CM) of
the capacitance CM to be measured.
Fig. 2 shows an example of a measurement coupling in accordance with the
5 invention, by whose means the problems described above are solved.
The coupling shown in Fig. 2 comprises the capacitances Cl ,Cz and C3, of
which, for example; C1 is a precisely known reference capacitance and Cz and
C3 are capacitances to be measured, being, for example, detectors that
10 measure the pressure (P), the temperature (T), and/or 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
Cl,C2 and C3 are connected to the switches 11,12 and 13. Said switches 11,12
15 and 13 have two positions, and in one of their positions tbe 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,C2 and C3 are connected to the wire 21 or con~act, which is con-
n~cted to the output terTninal of the operation amplifier 20, which acts as a
20 voltage follower. The * terrninal of the input of the voltage follower 20 is
connected to the input terininal 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 -
25 terrninal of its input, which is illustrated by the wire 22. It is a part of theproperties of the voltage follower and of its operation amplifier 20 or an
equivalent component that the input resistance of the voltage follower R~n ~
oo and the output resistance Rout ~ O in an ideal case. In such a case~ the
output voltage of the voltage follower is U2 = UO.
According to ~ig.-;2,:-thetcapacitance Cj~ is-connected between the ground and
the input terminal 16 of the inverter 10, in which case the output frequency of
the measurement oscillator is Fo = 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 = UO prevails. Thus, at the terminals
of the capacitances C2 and C3 there is no difference in voltage at all, so that
5 there are no charging currents at same.
The switches 11,1~ and 13 are controlled by a control unit 1~ so that one
terminal of each capacitance Cl,C2 and ~C3 is, in its turn, connec~ed to the
ground and, thus, to act upon the input of the measurement oscillator, at which
10 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 C1,C2,C3 to be measured. The effects of these stray capacitances
15 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.
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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,
20 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
~5 invention rnay 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.
