GB2220756A

GB2220756A - D.C. biasing apparatus for impedance measurement

Info

Publication number: GB2220756A
Application number: GB8915357A
Authority: GB
Inventors: Yoichi Kuboyama; Koichi Yanagawo
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1988-07-06
Filing date: 1989-07-04
Publication date: 1990-01-17
Anticipated expiration: 2009-07-04
Also published as: KR900002085A; KR0136791B1; JP2945015B2; GB2220756B; GB8915357D0; JPH0217458A; US5014012A

Abstract

In an impedance (2) measurement apparatus in which a measurement AC from source (11) and a DC bias (3A, 3B) are applied to impedance (2) the effects on the impedance measurement of the AC measuring current flowing in other components such as a guard line and a shield line are removed. An impedance unit DC bias (3A) is connected via a round trip line e.g. central and outer conductors of coaxial cable 4A, between terminal Hd of impedance (2) and a connecting line (5). A second impedance which may be a second DC bias (3B) is similarly connected via coaxial cable (4B) between the connecting line (5) and terminal (Ld) of impedance (2). <IMAGE>

Description

2220756 D.C BIASING APPARATUS The present invention relates to a D C

biasing apparatus in impedance measurement, and more particularly to an apparatus for applying a D C bias current in inductance measurement.

An inductor has a different inductance value depending on a D C bias current because of causes such as saturation of a magnetic core Accordingly, when inductance is measured under a similar condition as the actual employment state of the device, a D C bias power source is used together with an ordinary inductance meter A problem existing in such measurement is that an error is produced in impedance measurement because an A.C measuring signal undesirably flows through the D C.

bias power source which is connected in parallel to the device under test.

Here, a conventional method for measuring inductance under D C bias will be explained with reference to the drawings First, one of conventional examples is to connect an inductor 74 having sufficiently large inductance as compared with the device under test between a bias power source and the device under test as shown in Fig 7 (a) An impedance measuring equipment 71 is a general measuring instrument with which a value of a device under test is determined by applying an A C.

current to a device under test 72 from terminals HC and LC, measuring A C voltages at terminals Hp and HL and obtaining a vector ratio thereof Here, a current iac from the terminal HC is divided at the terminal Hd into a current id flowing in the device under test 72 and a current ie flowing to the bias power source The shunt ratio at this time is determined principally by the ratio of the impedance of the device under test 72 to the inductor 74 The current ie flows into the terminal LC of the impedance measuring equipment through the capacitor C and the power source E Since processing is performed in the measuring instrument 71, assuming that a current (id + ie) flows into the device under test 72, an error is produced in a measured value In order to reduce this error, it is just required to reduce ie.

For such a purpose, it is required to make the inductance of the inductor 74 very large compared with device under test For example, when it is desired to take the measurement within an error of 10 % in case the true impedance of the device under test is 1 KJ A (approximately 1 6 H) at the measuring frequency of Hz, the inductor 74 is required to be 16 H or more which is ten times as high as the true impedance.

Further, a current of the same value as the D C current which is desired to be applied to the device under test 72 flows in this inductor 74 Therefore, when the bias current is 10 A, for instance, the inductor 74 which has a large inductance value as described above becomes physically very large.

In another method a parallel resonance circuit 76 is provided in place of the inductor 74 as shown in Fig 7 (b) At the resonance frequency the impedance separating the D C bias source from the connecting point of terminals Hc and H 2 and the device under test becomes very large Therefore, highly accurate measurements can be made, but the measurement frequency is limited to the resonance frequency, thus the method is lacking in universality.

There is another problem in the D C biasing method.

Fig 8 (a) shows an external appearance of the system.

Said system 81 includes an inductance measuring unit 81, a D C bias power source unit 82, and a device under test connected to a measurement terminal 84 attached directly to, or in the vicinity of, the power source unit 82 so as to be measured.

However, it is desirable to extend and connect a probe 85 with cables as shown in Fig 8 (b) in order to improve workability in a measurement environment, but a new problem is caused by extension cables The bias current flows out of a power source 83 and flows in a bias cable 88 A as shown with an arrow mark I, and flows further in a bias cable 88 B through a device under test on the probe 85 and returns to the power source 83 again as shown with an arrow mark I 2 With this, two problems arise The first problem is that a capacitance is formed between bias cables 88 A and 88 B If it is assumed for instance that these two connecting cables are parallel conductors as shown in Fig 9 (a), the capacitance between conductors, viz, the capacitance C between bias cables is formularized approximately by the following expression.

TE o In (D/r) SF/m) EO = 8 854 x lo F/m The measured signal of the inductance meter expands the measurement error with the coupling (AC coupling) thereof through this capacitance C For example, the capacitance between cables when the bias cables 88 A and 88 B, each of length of 1 m, are held close to each other (D= 2 r) is approximately 40 p F, and this means that, when an impedance of 1 K JL is meaured at the measurement frequency of 1 M Hz, a stray impedance of approximately 4 K J is added in parallel, thus producing a measurement error of about 20 % Moreover, since this capacitance greatly depends on the distance between cables, the two lines of cables 88 A and 88 B have to be fixed in some configuration in order to improve measurement reproducibility, but which makes the workability worse.

The second problem is a magnetic field generated by bias currents I, and I 2 The magnetic field produced at a point at a distance a from bias cables is formularized as follows.

IJ Ol magnetic flux density = 2 a lWb/m 2 J In some cases, such apparatus does not satisfy the product standard because of the reason that magnetic field makes trouble for other instruments For instance, when a bias current of 20 A is applied, the magnetic field produced at a point at a distance of 1 cm from the cables is B = 4 x 10-4 Wb/m 2 per cable, viz 4 gauss Furthermore, the magnetic field is twice as high as the case of a line of cable, viz, 8 gauss is produced at the point P shown in Fig 9.

Against above mentioned problems, when respective cables 88 A and 88 B are shielded by a shield 91 as shown in Fig 9 (b), it is possible to prevent the generation of a capacitance between cables, but the magnetic field can not be prevented from being generated.

Also, if a coaxial cable 93 is used as shown in Fig 9 (c), the bias current flows in a core wire 93 a and a casing 93 b in the opposite directions, respectively.

Therefore, no magnetic field is produced on an outside of the cable On the contrary, however, the capacity between the core wire 93 a and the casing 93 b becomes large.

It is an object of the present invention to eliminate the necessity of using a large inductance for cutting off an alternating current, to aim at with this miniaturization and cost reduction of the device, and to improve the measurement accuracy greatly in impedance measurement under D C bias and particularly in inductance measurement It is another object to make it feasible to maintain high measurement accuracy without depending on the measurement frequency over all measurement frequencies It is still another object to make it feasible to extend from an impedance measuring equipment and a D C bias power source to a fixture by using cables without deteriorating the measurement accuracy Furthermore, it is another object to prevent the magnetic flux generated by a large D C bias current Moreover, it is another object to make it feasible to use an impedance measuring equipment in which an ordinary impedance measuring equipment is used, and among others, a four-terminal pair method which is superior as the connecting method may be used Also, it is another object to constitute a measuring system in which no modification is made on the impedance measuring equipment at all.

According to the present invention, there is provided a D.C biasing apparatus which applies D C bias to a device under test for measuring impedance thereof, characterized in that: there is provided at least one impedance unit connected between one terminal of said device under test and as connecting point and at least one impedance unit connected between said connecting point and another terminal of said device under test; at least one of said impedance units is a D C bias power source; and said connecting point is connected to a connecting line for removing, effects on impedance measurement of an A C measuring current which flows in components other than said device under test such as a guard line and a shield line.

In a preferred arrangement there is provided a first impedance unit connected to a first end portion of a first round trip line and a second impedance unit connected to a first end portion of a second round trip line; at least one of said first impedance unit and said second impedance unit is a D C bias power source; a second end portion of one line of said first round trip line is connected to one terminal of said device under test, and a second end portion of one line of said second round trip line is connected to another terminal of said device under test; and the second end portion of another line of said first round trip line and the second end portion of another line of said second round trip line are connected with each other, and connected further to a connecting line for removing effects on impedance measurement by an A C measuring current flowing in components other than said device under test such as a guard line, a shield line and the like.

Preferably, said first round trip line and said second round trip line are coaxial cables.

The present invention will be described hereinafter with reference to embodiments shown in the drawings wherein:- Fig 1 is an electric circuit diagram showing a D C bias application equipment according to the first embodiment; Fig 2 is a perspective view showing the external appearance of this first embodiment; Fig 3 is an electric circuit diagram of the final stage of the D C bias current source according to the first embodiment; Fig 4 is an electric circuit diagram for explaining the principle of the present invention; Fig 5 is an electric circuit diagram according to the second embodiment; Fig 6 is an electric circuit diagram according to the third embodiment; Fig 7 is an electric circuit diagram showing a conventional measuring method; Fig 8 is a perspective view showing an external appearance of the conventional example shown in Fig 7; and Fig 9 is a perspective view of cables for explaining problems of a conventional example.

First, the principle of the present invention will be explained based on a circuit diagram shown in Fig 4 An impedance measuring equipment 41 is the same as the impedance measuring equipment 71 which was described in a conventional example shown in Fig 7 Two sets of D.C bias power sources 43 A and 43 B that are examples of the impedance units are connected to an inductor 42 which is a device under test, and a D C bias current IDC is supplied by means of these D C bias power sources 43 A and 43 B On the other hand, a measurement signal current iac from a terminal H of the impedance measuring equipment is divided at the ratio of the impedance of the device under test 42 to the impedance of power sources 43 A and 43 B (mainly the impedance of an inductor L 1) at a terminal Hd, and a current id flows in the device under test and a current ib flows on the side of the power sources Then, ib flows through a capacitor C 1 or a voltage source El and is divided at a connecting point G into a current ibl which flows into a guard line 45 and a current ib 2 which flows to the side of the power source 43 B This ib 2 causes to produce an error in a measured value in the same manner as the conventional example shown in Fig 7 Here, when attention is paid to the shunt ratio of ibi to ib 2, this shunt ratio is determined approximately by the ratio of the impedance ZG of the guard 45 to the impedance of an inductor L 2, and ZG may be made essentially a value close to almost O A in a practical device Therefore, it is possible to set at ib 2/ibl = 0 (viz ib 2 = 0).

Accordingly, the current flowing into LC becomes (id + ib 2) = id, thus making it possible to realize bias application with very small error For example, when the impedance of the device under test is 1 K A, and the impedance of Li and L 2 is both 100 A at the measurement frequency, ib 2/ibl 10-3 is obtained even if the guide impedance ZG = 10 m Thus, the error caused by the D.C bias power source becomes 0 1 % Furthermore, since the guide impedance ZG does not depend on the frequency, (ib 2/ibl) = 0 is obtained over a wide frequency range.

Besides, it has been described in the explanation of the principle with reference to Fig 4 that the bias power source circuit is composed of an inductor, a capacitor and a voltage source, but it is not limited to this composition as a matter of course For example, such compositions that; a resistor is used in place of the inductor, a current source is used in place of the voltage source, or the capacitor is not used, or a current source only is used, may be considered as alternatives.

Furthermore, it has been described in Fig 4 that two bias power source circuits are connected in series.

However, this is not -limiting, but it is only required that at least one of two impedance units is a bias power source circuit For example, only an inductor and a resistor may be connected to terminals 48 a and 48 b in place of the bias power source 43 A Otherwise, a case in which the voltage of the voltage source El or E 2 is zero lv I may be considered What is important is that series connection is made and to obtain a sufficiently large shunt ratio at a common connecting point where the guard is connected between a plurality of impedance units in which at least one of them is a D C bias power source, and it will be apparent to those skilled in the art that various modifications are feasible However, the applicant considers that it is the best to connect current sources in series Therefore, a current source will be shown in embodiments hereafter.

Here, marks used in the accompanying drawings are mentioned in addition The components expressed with those marks do not necessarily mean ideal ones, for example a current source having an infinite impedance for a current source, but practical devices are expressed with simplification for the convenience of illustration Similarly, conductors in the accompanying drawings include a portion meaning zero resistance in the equivalent circuit expression and a portion meaning inclusion of N small impedance as symbols of actual wares and cables Such discriminibation will be understood easily by those skilled in the art from the whole intent of the present specification.

In the next place, the feature of the present invention will be explained in more details with reference to an embodiment shown in Fig 5 In the Figure, an impedance measuring equipment 51 and a device under test 52 that are connected by what is called five-terminal method are shown Cables for supplying current to the device under test 52 and cables for measuring the voltage of the device under test 52 are guarded by shields (guards 57 A through 57 D, respectively, and the current by a stray capacitance flows through these guards and has no influence on the measured value, which is effective for high impedance measurement Two sets of bias current source 53 A and 53 B are connected to the device under test 52 by coaxial cables 54 A and 54 B External conductors of coaxial cables 54 A and 54 B are connected with each other by a conductor 55, and these are connected further to the guard 57 by a conductor 58 A D.C bias current IDC passes one after another from a bias current source 53 A through, a core wire of the coaxial cable 54 A, the device under test 52, a core wire of the coaxial cable 54 B, the bias current source 53 B, a casing wire of the coaxial cable 54 B, the conductor 55, a casing wire of the coaxial cable 54 A and the bias current source 53 A.

-.

On the other hand, an A C measurement signal iac is divided into ib and id at the terminal Hd, and ib is divided further into a current ibl flowing to the side of the guard 57 and a current ib 2 flowing to the side of the power source 53 B through the conductor 58 In a similar manner as in the above mentioned explanation, this shunt ratio is determined by the impedance ratio of the circuit, thus ib 2 0 is obtained Since ib flows mostly to the guard 57, only the current id flowing in 1 S the device under test 52 is detected at the current detector of the impedance measuring equipment 51 Here, it is noticeable that, since the connection distance between the casing lines of coaxial cables 54 A and 54 B and the guard 57 may be shortened, that 'is, the length of the conductor 58 may be shortened, it is possible to make the shunt ratio of ibl to ib 2 very large.

In the next place, an embodiment which the applicant considers the best will be explained Fig 1 shows an application of the present invention in impedance measurement by a four-terminal pair method A measuring current flows to a device under test (DUT) 2 from an A.C power source 11 through a central conductor of a coaxial cable 6 A and passes an ammeter 13 External conductors of the coaxial cable 6 D and the coaxial cable 6 A form a return circuit of this current If the balance of currents in the onward circuit and the return circuit has been obtained, magnetic fluxes produced by the central conductor and the external conductor of the coaxial cable are negated, thus measurement errors caused by the mutual inductance between coaxial cables may be avoided When above said balance is lost, an error current flows in a zero potential detection amplifier 15 connected to a terminal Ld of the device under test 2 and fed back to the side of the'ammeter 13 until the amplified error current becomes zero With this, the potential difference between the terminal Ld and the external conductors of coaxial cables 6 C and 6 B becomes zero Therefore, the voltage between the terminal Ld and the terminal Hd is measured by a voltmeter 12 D C bias power sources 3 A and 3 B and the device under test 2 are connected in an almost similar manner as the embodiment shown in Fig 5 However, attention is to be paid to that 57 shown in Fig 5 is used as a shield wire and the conductor 58 makes connection with this shield, whereas the coaxial cable 6 A is for applying same currents in opposite directions to the central conductor and the external conductor, respectively, and the conductor 8 is for returning the current which flows out of the central conductor to the external conductor.

Fig 2 shows an external appearance of a system according to the embodiment shown in Fig 1 A test fixture 16 is provided with a contact 16 H for the terminal Hd and a contact 16 L for the terminal HL, which are connected to central conductors of cables 6 A through 6 D through D C.

cut off capacitors Also, the connection between the coaxial cable 4 A and the coaxial cable 6 A with a conductor 8 is made in the test fixture 16.

Fig 3 shows circuit diagrams of the last stage of the power source units constituting the power sources 3 A and 3 B 31 A and 31 B denote power-MOSFE Ts To control voltage input terminals 32 A and 32 B, control voltage having an equal absolute value and polarities opposite to each other are applied by a control voltage generating circuit (not shown) which may be constituted by prior art The amplitude of the D C bias current is obtained as follows.

DC = V 1/Rsense Next, the operation of the embodiment shown in Fig 1 and Fig 2 will be explained The D C bias current flows from the current source 3 A one after another through the path of a central conductor of the cable 4 A -a the device under test 2 -e a central conductor of the cable 4 B -' the current source 3 B the external conductor of the cable 4 B -da conductor 5 -a the external conductor of the cable 4 A the current source 3 A Since the amplitudes of the currents flowing respectively in central conductors and external conductors of the coaxial cables 4 A and 4 B are equal to each other, no magnetic flux is produced outside of the cables.

An A C measurement signal from the power source 11 is divided partially as a current ic at the terminal Ed, which flows to the central conductor of the cable 4 A.

The current which has returned to the external conductor of the cable 4 A through the reactance, such as the capacitance between the central conductor and the external conductor of the cable 4 A, is divided into a current icl of the conductor 8 and a current ic 2 of the conductor 5 However, the impedance on the side of the conductor 5 looking from the dividing point is very large as compared with that on the side of the conductor 8 That is, while the conductor 5 is connected to the power source 3 B, the conductor 8 is connected directly to the external conductor of the cable 6 A, and moreover, is connected in the fixture 16 Therefore, the conductor 8 itself is very short With this, the following expressions are obtained.

ic 2 = O cl = c Since only the A C measurement signal id flowing in the device under test 2 is measured with the ammeter 13, accurate measurement can be made without being affected by ic at all.

In that above mentioned embodiment, it has been described that a common connecting point between two current sources connected in series through two pairs of round trip lines is connected to the guard in the vicinity of the device under test However, it is not necessarily limited to such description, but it is possible to be extended comparatively long from the common connection point between two bias power sources Also, a coaxial cable may not necessarily be used for the cable for D C.

bias supply, bust a single wire or parallel wires may be used when generation of the magnetic field causes no problem Further, A D C cutoff capacitor is provided between the impedance measuring equipment and both terminals of the device under test in the above embodiment The location of this capacitor may be in the fixture in the vicinity of the device under test as shown in Fig 1, or outside of the impedance measuring equipment 41 or in the vicinity thereof as shown in Fig 4 Also, in impedance measuring equipment 51 provided with D C cutoff capacitors 56 A through 56 D as shown in Fig 5, these capacitors may be utilized.

However, the D C cutoff capacitor is not necessarily indispensable for an embodiment according to the present invention.

Further, another embodiment is shown in Fig 6 The same components as those in the embodiment shown in Fig 1 are affixed with same marks, and detailed explanation is omitted In this embodiment, a protective circuit 60 is provided in addition to the embodiment shown in Fig 1.

If any difference is produced temporarily in the amplitudes of currents of the D C bias power sources 3 A and 3 B, an excessive burden is imposed on the equipment, thus causing a fear to break or deteriorate the measuring equipment and so forth The protective circuit 60 is provided for the purpose of equalizing the D.C current flowing into the terminal Hd to the D C.

current drawn from the terminal Ld, and is composed of what is called active inductance circuit 63 denotes a control power source Its circuit diagram is shown in Fig 6 With this, it is arranged so as to control the current drawn from the terminal Ld corresponding to the D.C potential of the terminal Ld.

Embodiments of the present invention have been explained as described above, but it is needless to say that the present invention is not necessarily applied only to the case that extension is made with cables from the D C.

bias power source to the device under test For example, it will easily be realized that noticeable results are obtainable when it is applied to a system such as shown in Fig 8 (a).

Since the present invention is constituted and operated as described above, it is no longer required to use a large inductor for cutting off A C in impedance measurement and more particularly in inductance measurement under D C bias, thus the equipment is miniaturized and the measurement accuracy ii improved.

Also it is possible to maintain a high measurement accuracy without depending on the measurement frequency over all the measurement frequencies Furthermore, it is possible to extend -from the impedance measuring equipment and the D C bias power source to the fixture by using cables without lowering measurement accuracy.

Also, the magnetic flux is prevented from generating from cables supplied with D C bias current Also, the present invention may be applied to the application of the D C bias current in impedance measurement by a four-terminal pair method Moreover, a measuring system may be provided without modifying the impedance measuring equipment at all.

Claims

1 A D C biasing apparatus which applies D C bias to a device under test for measuring impedance thereof, characterised in that: there are provided;, at least one impedance unit connected between one terminal of said device under test and a connecting point, and at least one impedance unit connected between said connecting point and another terminal of said device under test; wherein at least one of said impedance units is a D C.

bias power source and said connecting point is connected to a connecting line for removing effects on impedance measurement of an A C measuring current which flows in components other than said device under test, such as a guard line and a shield line.

2 A D C biasing apparatus according to claim I wherein there is provided a first impedance unit connected to a first end portion of a first round trip line and a second impedance unit connected to a first end portion of a second round trip line at least one of said first impedance unit and said second impedance unit is a D C bias power source; a second end portion of one line of said first round trip line is connected to one terminal of said device under test and a second end portion of one line of said second round trip line is connected to another terminal of said device under test; and the second end portion of another line of said first round trip line and the second end portion of another line of said second round trip line are connected with each other, and connected further to a connecting line for removing effects on impedance measurement by an A C measuring current flowing in components other than said device under test such as a guard line a shield line and the like.

3 A D C biasing apparatus according to claim 2, wherein said first round trip line and said second round trip line are coaxial cables.

4 A D C biasing apparatus according to any of claims 1,2 or 3, wherein the connecting line for removing the effects on impedance measurement of the A C measuring current flowing in components other than said device under test is an external conductor of a coaxial cable in a four terminal pair connection method.

A D C biasing apparatus substantially as herein described, with reference to the accompanying drawings.

Published 1989 at The Patent Ofi ce, State House, 6 & 71 High Holborn, London W Cl R 4 TP P Further copies maybe obtamedfrom The Patent Ofce.

ales Branch, St Ma Ty Cray, Orpington, Kent BR 5 3RD Printed PY Multiplex techniques ltd, St Ma Iry Cray, Kent Con 1/87