GB2034998A - Waveform distortion correction - Google Patents

Abstract

Distortion of a bipolar signal due to passing through a transformer having an input winding 12 and output windings 15, 16, 17 is effected by including an amplifier 3 to which are applied positive and negative feedbank signals from further windings 13, 14. The operational amplifier (3) energises the magnetizing winding (12) and the negative feedback winding (13) is connected into a negative feedback circuit of the amplifier (3). A filter (7) is connected between the positive feedback winding (14) and an inverting input of the operational amplifier (3). The input is a bipolar reference voltage source (1) having a negligibly small dc voltage component at its output. <IMAGE>

Description

SPECIFICATION Precision inductive voltage converter The present invention relates to apparatus for measuring electric parameters, and more particularly to inductive precision voltage converters.

The present invention resides in a precision inductive voltage converter comprising an inductive voltage divider which has a magnetic circuit and a magnetizing winding, a negative feedback winding and output windings inductively connected to one another; an operational amplifier having a negative feedback circuit in which the magnetizing winding and the negative feedback winding of the inductive voltage divider are connected so that the operational amplifier acting upon the magnetizing winding adjusts the signal at the negative feedback winding so that the resultant input voltage approaches zero; a reference voltage source connected to an input of the inductive voltage divider; and an output unit intended to convert the output voltage of the inductive voltage divider and connected to an output of the same divider, wherein, according to the invention, the inductive voltage divider has a filter whose output is connected to an inverting input of the opera tional amplifier, and a positive feedback winding which connects the input of the filter with the output of the operational amplifier, the reference voltage source being made bipolar with a negligibly small dc voltage component at its output.

In accordance with the present invention, the most important advantages, namely, higher precision and simplified construction of the precision inductive voltage converter are achieved by the combination of a parametrical inductive voltage divider and an operational amplifier which compose an active induc tive divider, wherein the amplifier corrects the characteristics of the parametric inductive divider.

It is advisable that the converter have at least two inductive voltage dividers arranged in a cascade, each preceding inductive voltage divider being provided with an auxiliary power winding and an auxiliary measuring winding, and each successive inductive voltage divider being provided with a magnetic screen enveloping its magnetizing winding connected to the auxiliary power winding of the preceding inductive voltage divider, a correcting winding connected to the auxiliary measuring winding of the preceding inductive voltage divider, the correcting winding and the output winding of each successive inductive divider being adapted to envelope said magnetic screen, and the output windings of the inductive voltage dividers being connected in series.

It is essential that the voltage converter have a bipolar reference voltage source, a two-position switch and a control generator, an input of the switch being connected to potantial terminals of a dc voltage source, the output of the switch being connected to the input of the inductive voltage divider, and the control generator being connected to control inputs of the switch.

It is desirable that the output unit of the voltage converter comprise a differential amplifier and two two-position synchronized switches, each of said switches being provided with an earthed input and a potential input connected to the output of the inductive voltage divider, with an output being connected to one of the inputs of the differential amplifier, and with two control inputs being connected to outputs of the control generator whose phasing corresponds to the synchronous operation of the switches.

It is suitable that the output unit of the voltage converter be made as a voltage difference converter adapted to convert the difference between a dc voltage and voltage pulses to a dc voltage at the output of the inductive voltage divider, the voltage difference converter comprising a two-position switch, a storage unit, a voltage pulse converter for converting voltage pulses to a dc voltage, an amplifier and a resistance voltage divider connected in series with each other, the inputs of the switches being connected to the inductive voltage divider and to the resistance voltage divider, and the control inputs of the switch and control inputs of the pulse voltage converter being connected to the outputs of the control generator.

It is expedient that the input of the two-position switch and an input of the resistance voltage divider in the converter be connected to the dc voltage source.

It is advisable that the output unit of the converter have a capacitor whose one terminal is earthed and the other is connected to the output of the twoposition switch, a regulator adapted to regulate the voltage of the control generator, which regulator is connected to the output of the control generator and to at least one control input of the two-position switch.

It is important that the output unit of the voltage converter be provided with a circuit containing a capacitor through which the input of the twoposition switch is connected to the output of the inductive voltage divider, and with a switch whose one terminal is connected to said input of the twoposition switch and the otherterminal is earthed, a control input of the switch being connected to the output of the control generator.

The above circuit makes it possible to improve the precision and to simplify the construction of the converter by converting the bipolar voltage at the output of the inductive divider to a unipolar voltage having an amplitude which is equal to the peak-topeak value of the bipolar voltage at the output of the inductive divider and which is independent of the pulse duration ratio in each polarity.

Furthermore, the above circuit provides for an improved resolution of the voltage converter by suppressing low-frequency noise in the operational amplifier of the inductive voltage divider.

It is suitablethatthe voltage converter have an output unit provided with a source of a voltage correcting the output signal of the inductive voltage divider, via which source the switch is connected to earth.

It is efficient that the source of the voltage correcting the output signal of the inductive voltage divider be made as a resistance output voltage divider of the converter, adapted to convert the difference between a dc voltage and voltage pulses to a dc voltage at the output of the inductive voltage divider.

The invention will now be described in greater detail with reference to specific embodiments thereof, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a circuit diagram of a three-decade precision inductive voltage divider, according to the invention; Fig. 2 is a circuit diagram of six-decade precision inductive voltage converter incorporating inductive voltage dividers arranged in a cascade, according to the invention; Fig. 3 is a circuit diagram of the reference voltage source, according to the invention; Fig. 4 is a circuit diagram of the output unit converting bipolar voltage to a dc voltage at the output of the inductive voltage divider, according to the invention; Fig. 5 is a circuit diagram of the voltage difference converter adapted to convert the difference between a dc voltage and voltage pulses to a dc voltage, according to the invention;; Fig. 6 is a circuit diagram of an alternative embodiment of the voltage difference converter adapted to convert the difference between a dc voltage and voltage pulses to a dc voltage, said voltage difference converter including a reference voltage source, according to the invention; Fig. 7 is a circuit diagram of an alternative embodiment of the input portion of the voltage difference converter adapted to convert the difference between voltage pulses and a dc voltage to a voltage, said voltage difference converter being provided with a regulated control voltage, according to the invention;; Fig. 8 is a circuit diagram of an alternative embodiment of the input portion of the difference voltage converter adapted to convert the difference between voltage pulses and a dc voltage to a dc voltage, said voltage difference converter containing a converter adapted to convert a bipolar output voltage to a unipolar voltage, according to the invention; Fig. 9 shows waveforms at different points of the circuit diagram shown in Fig. 8, according to the invention; Fig. 10 is a circuit diagram of an alternative embodiment of the converter adapted to convert a bipolar output voltage of the inductive voltage divider to a unipolar voltage, said converter comprising a correction voltage source;; Fig. 11 is a circuit diagram of an alternative embodiment of the voltage difference converter adapted to convert the difference between voltage pulses and a de voltage to a dc voltage, said voltage difference converter being feedbackconnected, according to the invention Reference is now made to Fig. 1 showing a precision inductive voltage converter which comprises a reference voltage source, an active inductive voltage divider2 having an operational amplifier3 provided with a non-inverting input 4, and inverting input 5 and an output 6, a filter 7 containing a resistor 8 connected into the circuit between an input 9 of the filter 7 and the inverting input 5 of the operational amplifier 3, and a capacitor 10 which earths the inverting input 5.The converter also comprises a parametric inductive voltage divider whose core 11 has a mag netizing winding 12 wound up thereon, a negative feedback winding 13, a positive feedback winding 14, output windings 15,16,17, connected in the fol lowing way: the winding 12 is placed between the output 6 of the amplifier3 and earth, the winding 13 is placed between an output of the reference voltage source 1 and the non-inverting input 4 of the amp lifier 3, the winding 14 is placed between the output 6 of the amplifier 3 and the input 9 of the filter 7, the output windings 15, 16, 17 are connected in series via switches 18, 19, 20 and earthed via a low potential terminal of the winding 17 and connected to an output unit 22 via a high-potential terminal 21.

The main advantage of the present invention over the prior art resides in a higher precision of the voltage converter along with simplification of its design and manufacture. The construction is simplified through the use of small cores made of materials having a low magnetic permeability, in particular ferrites, and windings with a small number of turns, which, however, reduces the time constant of the magnetizing winding and, correspondingly, deteriorates the low-frequency electric characteristics of the parametric inductive divider.In addition, although the transformer-like construction of the output windings contributes to better precision and allows for various connections of the output wind ings, it decreases the resonance frequency of the output windings and limits the frequency response of the parametric inductive divider in the high frequency region of the response curve.

The above simplification of the construction of the parametric inductive divider allows its input resistance to be decreased and its output resistance to be increased. Thus, the advantages of the active inductive voltage divider over a parametric divider are achieved owing to the effect of the operational amplifier.

When a frequency-independent operational amp lifier is used, the time constant of the active inductive voltage divider whose gain is equal to K(o) increases K(o) times, and the resonance frequencies increase K(o) times, thereby permitting compensation for the above-mentioned deterioration of the electrical characteristics of the parametric inductive divider.

In accordance with the circuit arrangement (Fig. 1), wherein the negative feedback winding 13 is placed in series with the input 4 of the amplifier and is inductively connected to the windings 12, 14 there takes place an increase in the input resistance and a decrease in the output resistance of the active induc tive voltage divider with respect to the parametric divider.

From the foregoing it is clearthat better precision results from the increase in the gain of the opera tional amplifier. If the frequency-independent amp lifier is provided with a low-resistance magnetizing winding at its output, then even a comparatively low-- dc voltage at the input of the amplifier induces a high output current which disturbs the normal operation of the amplifier, magnetizes the inductive voltage divider and restricts the gain of such an amplifier.

The component parts of the proposed voltage con verter, namely, the amplifier 3 together with the filter 7 7 and the positive feedback winding 14, make up a frequency-independent amplifier whose dc gain is equal to unity. As the frequency of the signal increases, the gain in the feedback circuit drops due to the phase opposition of the winding 14 and the presence of the filter 7, which, correspondingly, results in an increase in the gain and in the appearance of a correcting effect of the amplifier. The windings 14 and 12 have an equal number of turns. Alternatively, the winding 14 may be provided with a greater number of turns and with a voltage regulator connectec to its output.

The connection of the windings 15, 16, 17, shown in the drawing makes it possible to form any number of mutually insulated windings, the connection of said windings being determined by a specific application of the converter. Of particular interest is the construction of windings wherein the number of turns corresponds to a specific code, such as a binary code.

The design of the converter permits the use of various switches of the output windings. If desired, the decade switches 18, 19,20 may be substituted by semi-conductor switches, which makes it possible to develop automatic measuring devices on the basis of the converter.

Alternatively, the converter may be made as a multi-channel one, various output devices being connected to each group of the output windings.

This being the case, the total combination of the output signals is scale-connected. In particular, an embodiment of the converter may have scaleconnected decoupled outputs.

Insofar as the converter features high precision in reproducing square pulses amplitude distortion A 10-5 + 10-), the square pulse in a time interval of its existence cannot be distinguished from direct current. With this feature in view and with due consideration for the above embodiments of the output windings and their switches, it is possible to use the converter for producing digital to analog converters, in particular those having fast response. High precision of such converters results from accurate reproduction of voltages atthe outputwindings of the converter and suppression of commutation noise due to the low output resistance of the inductive voltage divider.Owing to the above advantages, the converter allows for manufacturing analog-to-digital converters featuring high precision.

If the resolution of the converter is to be increased, it is necessary to arrange the inductive voltage dividers in a cascade (Fig. 2). With this in view, a preceding inductive divider (in this case, the active inductive divider 2 is provided with an auxiliary power winding 23 and an auxiliary measuring winding 24, and a successive inductive divider 25 has a magnetizing winding 26 wound on a core 27, a correcting winding 28, and output windings, 29, 30,31 wound on a magnetic screen 32 enveloping the winding 26, the windings 26, 28 being connected to the windings 23,24 respectively, and the output windings 29, 30, 31 being connected in series via switches 33,34,35, 18, 19, 20, to the output windings 15, 16, 17 of the active inductive divider 2.The inductive voltage divider 25 and each successive divider are provided with a power winding and with a measuring winding if a successive inductive divider is expected to be placed.

The measuring winding 24, the correcting winding 28, and the magnetic screen 32 made of a soft magnetic material form a correcting system which makes it possible, on the one hand, to increase the accuracy of transforming a voltage from a preceding inductive divider to a successive one and, on the other, due to a close inductive interconnection between the winding 28 and the output windings 29,30, 31,to decrease the output resistance of the successive inductive divider 25.

The winding 24 is included in a bundle of wires of the winding 17 which is a low-order decade of the inductive voltage divider 2 and, therefore, the voltage at the winding 24 exactly corresponds to that across the stages of the above decade. To increase the accuracy of correction, the circuit containing the windings 24, 28 is to be a low-resistant one. This can be achieved, in particular, by making the winding 24 of some wires of the winding 17, connected in parallel. The winding 28 and the output windings 29,30, 31 are practically evenly distributed over the magnetic screen 32.

In order to further enhance the resolution, the converter may be provided with a third inductive voltage divider whose construction is identical to that of the second inductive voltage divider 25. In this case, an auxiliary power winding is made, which is similar to the winding 23, and an auxiliary measuring winding from the wire bundle of the winding 35 is made which is similar to the winding 24.

The output windings of the third inductive voltage divider are connected in series to the output windings of the inductive voltage dividers 2,25.

If the inductive voltage dividers of the converter are cascade-connected, the numbers of turns are selected with consideration for scale coefficients.

The reference voltage source 1 is made bipolar with a negligible quantity of a dc voltage component at its output. In this case, the voltage at the output of the source 1 may be of an arbitrary waveform, which depends on the function of the converter. In particular, if the converter is expected to be used in dc devices, the reference voltage source I (Fig. 3) should produce a bipolar voltage in the form of square pulses. The reference voltage source I comprises two oppositely connected dc voltage sources 36,37 a two-position switch 38 and a control generator 39.

Inputs 40,41 of the switch 38 are connected to the potential terminals of the dc voltage sources 36,37, and an output 42 of the switch 38, which is the output of the reference voltage divider 1, is connected to the feedback winding 13. Control inputs 43,44 of the switch 38 are connected to outputs 45,46 of the control generator 39.

As switching elements, the two-position switch 38 comprises MOS-transistors controlled by the control generator 39.

Fig. 4 shows an embodiment of the output unit of the voltage converter which is efficient in converting a square-waveform bipolar voltage into a due voltage without intermediate storage, which makes it possible to increase its speed and to build a high-speed digital-to-analog converter on the basis of the above voltage converter.

The output unit comprises a differential amplifier 47 and two two-position synchronized switch 48 and 49 whose potential inputs 50, 51 are connected to the output 21 of the inductive voltage divider 2, inputs 52, 53 are earthed, outputs 54, 55 are connected, respectively, to the non-inverting input and the inverting input of the differential amplifier 47, and control inputs 55, 57 are connected to the control generator 39. The switches 48,49 are provided with switching elements made on the basis of MOStransistors.

Fig. 5 shows an alternative embodiment of the voltage converter comprising an output unit which converts the difference between voltage pulses and a dc voltage to a dc voltage. Such an embodiment makes it possible to considerably enhance the functional capabilities of the voltage converter. In particular, it enables the converter functioning as a dc voltage calibrator and as a means for measuring dc signals (potentiometer, voltmeter).

The output unit comprises the following components connected in series: the two-position switch 48, a storage circuit 58, a pulse voltage converter 59 adapted to convert voltage pulses to dc voltage, a dc amplifier 60 having its output 61 connected to an input of a resistance voltage divider 62. The input 52 of the two-position switch 48 is connected to an output 63 of the resistance voltage divider 62. Therewith, a control input 64 of the pulse voltage converter 59 together with the control inputs 56,57 of the two-position switch 48 are connected to the outputs 45,46 of the control generator so as to provide for a negative feedback in the closed loop of the output unit.

The following drawings show a portion of the circuitry of the output unit. This portion comprises the units 58,59,60 which are shown as a single unit 65 with the input 54 and the output 61.

Another embodiment of the invention shown in Fig. 6 comprises an output unit which is provided with an additional due voltage source 66 placed between the inputs 52,63 of the two-position switch 48 and the resistance voltage divider 62. The dc voltage source 66 makes it possible to shift the initial voltage level at the point 61, in particular to produce a bipolar dc voltage at the output of the converter. At the same time, when the converter is intended to be used as a potentiometer or a voltmeter, the dcvoltage source 66 also serves as a source of the voltage being measured.

An improved embodiment of the output unit shown in Fig. 7 makes it possible to prevent errors arising if the dc voltage sources have different inter nal resistances or if internal resistances ofthe dc voltage source 66 varies in the course of operation.

The output unit shown in Fig. 7 comprises an addi tional capacitor 67 earthing the terminal 54, and a control voltage regulator 68 adapted to adjust con trol voltages at the control input 57 andto be con nected by a terminal 69 to the control generator 39.

A distinctive feature of the output unit whose cir cuit diagram is shown in Fig. 8 is the provision of the output unit with a converter adapted to convert a bipolar voltage to a unipolar one at the output of the inductive divider, which makes it possible to improve the precision and resolution of the converter as a whole. To accomplish this, the output unit is provided with a capacitor70 placed between the input 50 of the two-position switch 48 and the output 21 of the inductive voltage divider, said capacitor 50 connecting the input 50 to earth by a switch 71 whose control input 72 is connected to the generator 39.

Further improvement of the output unit is shown in Fig. 10, wherein the switch 71 is earthed through a potential terminal 73 of an earthed source 74 which is intended to perform various functions. The source 74 may, in particular, be applied for setting the out put unit to zero.

The necessity to set the output unit to zero arises from the presence therein of amplifiers and switches. The embodiment of a zero corrector, according to Fig. 10 has the advantage that the signal of the zero corrector is applied with respect to earth, whereas the circuit of the corrector is not con nected to the circuits determining the accuracy of the output unit, for instance, a unit provided with a resistance voltage divider 62.

Fig. 11 shows an embodiment of the output unit, wherein the source 74 is formed by the output 63 of the resistance voltage divider 62, thus enabling the terminal 73 to be directly connected to the output 63 and, consequently, to provide the output unit with a common negative feedback.

The above-described precision inductive voltage divider operates in the following way.

With the aid of the feedback circuit comprising the filter 7 and the winding 14, the amplifier 3 is dc stabilized to a gain equal to unity. A dc voltage which may occur at the output of the amplifier 3 (several millivolts) due to a zero drift of the amplifier3 and the reference voltage source I does not affect the operation of the converter.

When a dc voltage is applied from the reference voltage source I to the negative feedback winding 13, the operational amplifier3 acts upon the magnetiz ing winding 12 and forms a magnetic flux in the core II. This magnetic flux generates a voltage in the winding 13, which is in phase opposition and equal to the voltage of the source I. Thus, the magnetic flux decreases to zero the voltage at the input of the amp lifier 3.

The voltage at the output windings 15, 16, 17 inductively connected to the winding 13 is propor tional to the voltage of the source 1. The voltage at the output 21 ofthe inductive divider is adjusted by the switches 18, 19,20 and applied to the output unit 22.

The possibility of improving the accuracy in reproducing pulses is illustrated by way of reproduc ing the horizontal portion of a square pulse.

When the square pulse is transmitted through the inductive divider, there is formed a slightly distorted pulse at the output of said divider. This distortion of the pulse manifests itself as a decrease in the output pulse amplitude with time. The decrease A in the pulse amplitude during the pulse determines the low-frequency passband of the converter. If the I inductive divider is used without the operational amplifier, then:

where: t,-pulse duration, time constant.

If the amplifier with frequency-independent gain K(o) is placed into circuit, then:

i.e., K(o) times as low, which is the ideal case. An important advantage of the proposed converter is that the accuracy in reproducing square pulses is as high as in the case where a frequency-independent amplifier is used, i.e., the distortion of the horizontal portion of the pulse is determined, according to the invention, by formula (10), which is indicative of wide capabilities of the converter in reproducing pulses along with dc stabilization of the amplifier 3.

The converter dynamics characterized by attenuation factor A (second-order system) is determined by the ratio:

where T is the time constant of thefi Ite r 7 and is regulated by the value of T at given , independently of the gain of the amplifier 3.

The following ratios illustrate the quantitative characteristics of the converter. If 7 = 0.1 sec, T = 1 sec, K(o) = 103, tu = 0.5 10-3, then the amplitude distortion A = 5 1 10-3 and the attenuation factor A 0.16.

The above advantages result, in particular, from the simultaneous action of both the winding 14 and the filter 7. It is expedient to single out the positive effect produced by the winding 14. In the absence of this winding (the number of turns is equal to zero), the amplitude distortion A is determined by the ratio:

and, with the parameters indicated above, increases 100 times as compared to the amplitude distortion in the case of simultaneous action of the winding 14 and the filter 7.

Although the amplifier3 of the converter has a dc gain equal to zero and, correspondingly, K(o) times as low as the signal gain, the value of the dc compo nent at the input of the amplifier3 should be considerably limited. The quantitative limitations are determined by the current-carrying capacity of the amplifier and by the construction of the inductive voltage divider. According to the data obtained for each particular construction of the converter, the permissible level of the dc component at the input of the amplifier 3 is a few millivolts. This does not present any specific problems in manufacturing the converter but requires that the reference voltage divider be made bipolar with a negligibly small due voltage component at its output.

The voltage converter, wherein the inductive voltage dividers are arranged in a cascade, operates in the following way.

The magnetizing winding 24 of the inductive divider 25 is energized through the power winding 23, and the voltage at the winding 24 is transformed by the windings 28, 29,30, 31. With the design of the inductive divider 25 being simplified and identical with that of the parametric inductive divider of the active divider 2, voltage transformation is character- ized by signal amplitude and waveform distortions.

Therefore, the voltage across the winding 28 does not coincide with that at the winding 24, which results in a correcting current through the circuit of the windings 24,28. The correcting current causes a correcting magnetic flux in the magnetic screen 32.

The correcting magnetic flux decreases said correcting current by generating an additional emf in the winding 28. The emf equalizes the voltages at the windings 24 and 28. The low-resistance correction system provides for a considerably small difference in the voltages at the outputs 24,28, which corresponds to the accurate transfer of the voltage from the active inductive divider 2 to the inductive divider 25. Thus, the output voltage of the windings 29,30, 31 is formed by simultaneous action of the main magnetic flux generated in the core 27 by the winding 26 and the correcting magnetic flux formed by the correction system in the magnetic screen 32.

With due accountforthe scale coefficients determined by the numbers of turns, the above output voltage exactly corresponds to the voltage at the output windings of the inductive divider 2. The close magnetic coupling of the output windings 29,30,31 and the winding 28 which is practically shorted by the winding 24 ensures a low output resistance of the inductive divider 25, which is essentially equal to the resistance of the output windings 29,30,31.

The sources 36,37 are alternately connected by their potential outputs to the output 42 of the switch 38 through the inputs 40,41 of the latter. For this purpose the control inputs 43,44 of the switch 38 are supplied with a voltage which makes the MOStransistors conductive. To rule out the dc component at the input of the amplifier 3, the ratio between the voltages of the sources 36,37 and the duration of shorting of the inputs 40,41 of the switch 38 to its output 42 are selected such as to provide for unequality of areas of the positive and negative half-ways of the bipolar voltage at 42. The pulse repetition frequency and duration are preset by the control generator 39.

To form a positive due voltage at the output of the differential amplifier 47 (Fig. 4), with a positive voltage at the output 21 of the inductive divider, the input 56 is supplied with a voltage from the generator 39, which voltage makes the MOStransistors conductive, thereby connecting the input 50 of the the switch 48 to the output 54 of the latter, and the input 53 of the switch 49 to the output 55 of the same switch. In this case, a positive voltage appears at the output of the amplifier 47. If the voltage at the output 21 of the inductive voltage divider is negative, then the voltage making the MOStransistors conductive is applied to the input 57.Due to this, the output 21 is connected to the output 55 of the switch 49 through the input 51 of the latter, and the input 52 of the switch 48 is connected to the output 54 of the same switch. The switches being in the above states, the voltage is supplied from the output 21 to the inverting input of the amplifier 47 and forms, at the output of the same amplifier, a voltage whose polarity coincides with that of the positive voltage half-wave.

To obtain a negative dc voltage at the output of the amplifier 47, the voltage polarity at the output 21 and the inputs 56, 57 is reversed as compared to that described above.

A distinguishing feature of the output unit shown in Fig. 5 is that the dc voltage at the input 52 of the switch 48, which input is one of those of the output unit, comes from the output 61 of the amplifier 60.

The operation of the two-position switch 48 is similarto that described above. As the input 50 is connected to the output 54 of the switch 48, the voltage of a particular polarity comes from the output 21 of the inductive divider to the memory unit 58 where its amplitude is stored. As the input 52 is connected to the output 54, the voltage is applied from the output 63 of the resistance divider 62 to the storage unit 58.

The polarities of the voltages applied to the inputs 50, 52 are selected so that in case of inequality of their amplitudes, there appears a voltage pulse at the output of the storage unit, which is converted by the unit 59 to a dc voltage in accordance with the pulses of the control voltage coming to the control input 64 of a switch incorporated in the converter 59.

Therewith, the phase of the control voltage is responsible for a negative feedback. The voltage amplified by the amplifier 60 comes through the output 61 to the output 63 of the resistance divider 62. The stable value of the voltage at the output 61 conforms with the equality of the amplitudes of the voltages at the outputs 21,63 of the inductive voltage divider and the resistance voltage divider, respectively. This being the case, the voltage amplitude at the output 61 is n times as high as the amplitude of the voltage at the output 21 of the inductive divider, where n is the divison factor of the resistance divider 62.If the voltage is set at the output 21 of the inductive divider, for instance, by the switches 18, 19,20,33,34,35, the dc voltage at the output 61 varies respectively, which makes it possible to use the converter as a multi-decade dc voltage calibrator.

Described below is the operation of the converter wherein the voltage source 66 is used to produce a dc bipolar voltage at the output 61.

The stable value of the voltage at the output 61 conforms with the equality of the voltage amplitude at the output 21 of the inductive divider with the sum of voltage U63 atthe output 63 of the resistance divider 62 and voltage U66 of the voltage source 66, i.e., U21 = U66 + Us,(5). In this case, voltage U63 is determined by the formula U63 = U21 - U66 (6) and its polarity depends on the relation between U and U66.1f U66 is the voltage being measured, then the value of this voltage can be determined by an instrument connected to the output 61 and serving as a null-indicator.

Voltage U6, can be decreased to zero by setting voltage U21 with the aid of the switches 18, 19,20,33, 34, 35, which is indicated by the null-indicator. In this case, U66 = U21, (7) which makes it possible to determine the value of voltage U66 by the position of said switches.

The above relations between the voltage amplitudes prove that the converter, if provided with the null-indicator, can be used as a potentiometer. It is known that when the switch based on the MOStransistors is controlled, through the inter-electrode capacitances connecting the control electrodes 56, 57 to other electrodes (drain source), there takes place a transfer of charges forming the current at the input 52. This current produces a voltage at the resistance of the source 66. With the above electrode capacitances, the value of said current is proportional to the value and frequency of the control voltage.To minimize the noise of the output unit and thereby to increase the resolution of the converter, it is necessary to control the switching elements of the switch 48 by means of voltages having sufficiently large amplitudes with increased frequency (-I kHz).

Therewith, the above current may be rather high (-10-8A), which causes serious errors (zero is shifted for Iji V when the resistance varies by 100 ohms).

Fig. 7 shows another embodiment of the output unit which allows the value of the above current to be considerably reduced (approximately 102 times).

The capacitor 67 is a capacitance trap which concentrates and holds charges coming through the interelectrode capacities. The concentration and holding of the charges is carried out with time constant T1 = C R, (8), where R, is the resistance of a switch in a conductive state, C is the capacitance of the capacitor 67. If the leading edge time constant of the control voltages is considerably less than CRI, exact compensation for the charges can be effected taking into account the opposite polarity of the charges passing through adjoining inter-electrode capacitances. The compensation is carried out by adjusting the control input at the control input 57 of the switch 48; Fig. 8 shows yet another embodiment of the out put unit incorporated in the converter. The operation of this converter is illustrated by the waveforms shown in Fig. 9, where 9a, b, c, d, are the waveforms of the voltages at the output 21 and inputs 72, 50, 56, respectively.

In describing the operation of the converter, the switch is assumed to be conductive and having a negative voltage at its control input.

With a negative control voltage at the control input 72 of the switch 71, the capacitor 70 is charged by a positive voltage half-wave at the output 21 of the inductive voltage divider. This causes the appearance of a positive voltage at the capacitor plate connected to the output 21, and the voltage at the potential output 50 is equal to zero. In synchronism with the appearance of a negative voltage at the output 21, the switch 71 is made non-conducting (the voltage at the control input 72 is equal to zero). The negative voltage is added up to the voltage stored in the capacitor 70 and the resultant voltage through the input 50 of the switch 48 comes to the output 54 thereof and is stored by the storage unit 58 (Fig. 5).

In accordance with the procedure described above, the storage device 58 is supplied with a voltage which is the sum of the positive and negative half-waves, i.e., equal to the peak-to-peak value of the voltage at the output 21. A better resolution is achieved by that the circuit incorporating the capacitor 70 and the switch 71 and operating in accordance with the procedure described above has a transfer coefficient which is proportional to the first signal derivative at the circuit input in time. As a consequence, the low-frequency noise of the amplifier practically does not reach the storage unit 58 and does not form any initial noise voltage at the output 61.

The output unit illustrated in Fig. 10 operates as described above. The only difference in the operation of this embodiment is that, when the switch 71 is conductive, the voltage of the source 74 is additionally stored in the capacitor 70. This voltage, along with the signal at the output 21 of the inductive divider, is applied to the storage unit 58. In the absence of a signal at the output 21, the voltage of the source 74 is adjusted so that the voltage at the output 61 is equal to zero.

The voltage converter comprising an output unit shown in Fig. 11 operates as a system with a negative feedback in much the same way as the embodiment shown in Fig. 5. The only difference lies in the transfer of the signal through the switch 71, as has been mentioned above while describing the circuit shown in Fig. 10. The steady-state value of the voltage at the output 61 of the converter corresponds to the formula: Us3 + U21t s ) = Us2l (9) where U2, s, , is the peak-to-peak voltage atthe out- put 21, Ub3, U52 are the voltage constants at 63, 52.

If the converter is intended for use as a calibrator, as described above according to one of the embodiments, U52 = 0 (input 52 is earthed), and U63 = U21( X . (10) If the proposed converter is used for measuring voltages, the earthed source of the voltage U66 being measured is connected to the input 52 by the potential input. With the aid of the indicator which functions as a null-indicator and is connected to the output 61, there is established an equality Ubb = U21( E )S (11) whereby U66 can be determined depending on the position of the switches 18, 19, 20, 33,34, 35.

As can be seen from the above description, the proposed precision inductive voltage divider, according to embodiments thereof, provides the above positive effect by the means disclosed in the main claims and supplementary claims.

In particular, the use of the present invention has made it possible to develop a portable potentiometer with accuracy grade 0.00005 over a temperature range from +10 to +30 C (the error is not more than 10-7U, + 0.4cm V at the measuring range limit of 10 V).

The resolution of the semiconductor amplifier incorporated in the potentiometer is 20nV with an input current of 5.10-'1A. The potentiometer comprises a seven-decade inductive voltage divider based on con ventional small-sized ferrite cores, which offers certain advantages in the manufacturing process.

On the basis of this converter, a seven-decade dc voltage calibrator has been developed, whose linearity can be determined by the above-given formula for the potentiometer.

Claims (11)

1. A precision inductive voltage converter comprising an inductive voltage divider having a magnetic circuit and the following inductively interconnected elements: a magnetizing winding, a negative feedback winding, a positive feedback winding, and output windings; an operational amplifier having a negative feedback circuit in which the magnetizing winding and the magnetic feedback winding of the inductive voltage divider are connected so that the operational amplifier provides for the resultant voltage being close to 0 at its input by acting upon the magnetizing winding and thereby adjusting a signal on the negative feedback winding; a filter whose input is connected to an output of the operational amplifier through the positive feedback winding, an output of the filter being connected to an inverting input of the operational amplifier; a bipolar reference voltage source with a negligibly small dc voltage component at its output, said source being connected to an input of the inductive voltage divider; an output unit adapted to convert an output voltage of the inductive voltage divider and connected to its output.

2. A precision inductive voltage converter as claimed in claim I, comprising at least two cascadeconnected inductive voltage dividers, each preceding inductive voltage divider having an auxiliary power winding and an auxiliary measuring winding, whereas each successive inductive voltage divider has a magnetic screen enveloping its magnetizing winding connected to the auxiliary power winding of the preceding inductive voltage divider, a correcting winding connected to the auxiliary measuring winding of the preceding inductive voltage divider, the correcting winding and output windings of each successive inductive voltage divider being adapted to envelop the magnetic screen, and the output windings of the inductive voltage dividers being connected in series.

3. A precision inductive voltage converter as claimed in claims 1,2, wherein the bipolar reference voltage source has a bipolar dc voltage source, a two-position switch, and a control generator, an input of the switch being connected to potential ter minals of the dc voltage source, an output of the switch being connected to the input of the inductive voltage divider, and an output of the control generator being connected to control inputs of the switch.

4. A precision inductive voltage converter as claimed in claim 3, wherein the output unit com prises a differential amplifier, two two-position synchronized switches each having an earthed input and a potential input connected to the output of the inductive voltage divider, an output connected to one of the inputs of the differential amplifier, and a control input connected to the output of the control generator.

5. A precision inductive voltage converter as claimed in claim 3, wherein the output unit is a difference voltage converter adapted to convert the difference between a dc voltage and voltage pulses to a dc voltage at the output of the inductive voltage divider, said difference voltage converter comprising a two-position switch, a storage unit, a converter for converting voltage pulses to a dc voltage, an amplifier and a resistance voltage divider interconnected in series, inputs of the two-position switch being connected to both the inductive voltage divider and the resistance voltage divider, and control inputs of the two-position switch as well as control inputs of the converter for converting voltage pulses to a dc voltage being connected to the outputs of the control generator.

6. A precision inductive voltage converter as claimed in claim 5, wherein the input of the twoposition switch and an input of the resistance voltage divider are adapted to be connected to the dc voltage source.

7. A precision inductive voltage converter as claimed in claims 5 and 6, wherein the output unit comprises a switch whose one terminal is connected to earth and the other one to the output of the twoposition switch, and a regulator of the output voltage of the control generator, connected to the output of the control generator and at least to one control input of the switch.

8. A precision inductive voltage converter as claimed in claims 5,6 and 7, wherein the output unit circuit includes a capacitor through which the input of the two-position switch is connected to the output of the inductive voltage divider, and a switching element whose one terminal is connected to said input of the two-position switch and the other terminal is connected to earth, a control input of the switching element being connected to the output of the control generator.

9. A precision inductive voltage converter as claimed in claim 8, wherein the output unit comprises a source of a voltage correcting an output signal of the inductive voltage divider, through which source the switch is connected to earth.

10. Aprecision inductive voltage converter as claimed in claim 9, wherein the source of the voltage correcting an output signal of the inductive voltage divider is a resistance divider of the output voltage of the converter adapted to convert the difference between a dc voltage and voltage pulses to a dc voltage at the output of the inductive voltage divider.

11. A precision inductive voltage converter substantially as described hereinabove with reference to, and as shown in the accompanying drawings.