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Publication numberUS3680013 A
Publication typeGrant
Publication dateJul 25, 1972
Filing dateFeb 16, 1971
Priority dateFeb 27, 1970
Also published asCA921137A1
Publication numberUS 3680013 A, US 3680013A, US-A-3680013, US3680013 A, US3680013A
InventorsPye Walter Arnold
Original AssigneeWelwyn Electric Ltd
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Film attenuator
US 3680013 A
Abstract
An electrical film attenuator or other film resistor network comprising an insulating substrate having adherently mounted thereon a single film resistive element, said element being provided with at least three terminals in electrical contact therewith and, on said element and in electrical contact therewith, an electrically conductive layer dividing up said element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of said terminals, one of which being an input and the other an output terminal, the third portion of said electrical resistive element being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.
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I United States Patent 3,680,0 13 Pye 1 July 25, 1972 [54] FILM ATTENUATOR 3,594,679 7/1971 Seay ..338/l 95 [72] inventor: Walter Arnold Pye, Morpeth, Northumberlandt England 3,521,201 7/1970 Veteran ..333/81 R [73] Assignee: Welwyn Electric Limited, Northumberland, England t Primary Examiner-Lems H. Myers Assistant Examiner-A. T. Grimley Filed: 1 1971 Allorne vMcDougall, Hersh & Scott [2i] Appl. No.. 115,244 ABSTRACT An electrical film attenuator or other film resistor network [30] Foreign Application Prhrny Data comprising an insulating substrate having adherently mounted Feb. 27, I970 Great Britain ..9,720/70 thereon a single film resistive element, said element being provided with at least three terminals in electrical contact 52 U.S. Cl ..333/81 R, 29/620, 323/94 H, therewith and, on said element and in electrical Contact 33 195 333 309 333 314 333/325 therewith, an electrically conductive layer dividing up said 51 Int. Cl. ..H01 1/22 oiomottt into three distinct portions two oiwhioh will have the 58 Field of Search... ..338/l95, 307, 308, 309, 31 1, Same electrical resistance value after removal of pan of Said 338/314, 325; 323/94 R, 94 H; 333/81 R 31 C 81 element at a locus midway between two ofsaid terminals, one 29/620 of which being an input and the other an output terminal, the third portion of said electrical resistive element being shaped [56] Reierences Cited so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in UNITED STATES E T electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal 3,599,l25 8/197] Yoshlda ..333/81 R or termina1s 3,329,921 7/1967 Badelt ..338/325 X 3,573,703 4/1971 Burks ..338/l95 X 8 Claims, 18 Drawing Figures PATENTED 3,680,013

SHEET 1 OF 4 A EJGJ CL V INPUT R3 OUTPUT INPUT R15 OUTPUT RB R14 SHEET 2 [IF 4 FIGZf.

PATENTEDJULZS I972 FIGZCI.

PATENTEU LZ I973 3,680,013

sum 4 BF 4 FILM ATTENUATOR This invention relates to electrical film resistor networks, especially electrical attenuators and a process for their production. Attenuators are combinations or networks of fixed electrical resistors arranged and interconnected in such a manner that, when inserted into the input line of an electrical device or circuit, they reduce the input power to a desired level while maintaining the normal characteristic impedance of the device or circuit.

Attenuators, on being incorporated in an electrical circuit, provide what is known as insertion loss" which is a relationship of electrical power and is measured in decibels or nepers (l neper 8.686 decibels); they may be used, for example, in communication systems and in systems where it is necessary to maintain a constant level of power when the load is altered.

Two basic kinds of attenuators are known and these are termed balanced and unbalanced attenuators respectively. An unbalanced attenuator is generally constituted by three electrical resistors arranged and interconnected in either Tee configuration or Pi configuration.

FIG. la of the accompanying drawings illustrates a known example of an unbalanced T-configuration, comprising resistors R,, R and R where R, R

FIG. lb of the drawings illustrates a known example of an unbalanced Pi" configuration, comprising resistors R R, and R where R R Balanced attenuators are generally constructed from four or five resistors; they may be arranged in a square (Balanced Pi") configuration (an example of which is illustrated in FIG. 1c of the drawings, consisting of resistors R R R R, where R, R and R R,,,) or in an H (Balanced T") configuration (an example of which is illustrated in FIG. 1d, consisting of resistors R,,, R R,,,, R,., and R all of which have the same resistance value).

In the manufacture of an electrical attenuator it is necessary to ensure that both the insertion loss and the input and output impedances correspond to the required nominal values for these parameters within stated tolerance limits.

When utilizing discrete resistors in the construction of attenuators, the resistors are manufactured with the desired values and tolerances and then interconnected in the required configuration, input and output terminations being provided. By means of this technique, the resulting network can be made to provide the correct insertion loss relative to the desired characteristic impedance with the necessary and stipulated degree of accuracy.

Attenuators may also be manufactured by so-called flat film techniques, whereby thick or thin film resistor elements and their associated interconnecting conductors are deposited on flat substrates be means of techniques such as electroless plating, vacuum evaporation, or screen printing of cermet materials which are subsequently fired at a high temperature. Attenuators of all the configurations described above may be produced in this way. Attenuators produced by these flat film techniques suffer from the great disadvantage that the adjustment of the resistance values of individual film resistors which is required has to be carried out on the network of deposited resistor films and it is difficult to adjust the value of a single resistor since measurements are normally made across the input or output terminations of the attenuator and the presence of more than one resistor causes confusion. Thus it is normal practice to break the attenuator circuit at some convenient point, adjust each resistor to the required value and finally repair the break in the circuit. Adjustment of each resistor is carried out by removing a portion thereof using, for example, a jet of air carrying particles of an abrasive material, a laser beam or any suitable grinding means. This procedure is slow since it is usually necessary to adjust at least three resistors. A further disadvantage exists, in that if one of the first resistors in an attenuator is incorrectly adjusted, the other resistors may have to be adjusted to other than the preferred optimum value for which the deposited film was originally designed.

It is a purpose of the present invention to minimize or overcome one or both the disadvantages of the previously known methods of making flat film attenuators.

For clarification of the theoretical background to the inven tion reference is made to FIGS.2a, 2b, 2c, 2d, 2e and 2f of the drawings.

If one considers a film D of resistive material in the form of a symmetrical Tee" having uniform electrical sheet resistance and provided with contacts A, B and C at each extremity as illustrated in FIG. 2a, then on connecting a pair of input lines to contacts A and C and a pair of output lines to contacts B and C, an unbalanced attenuator with equal input and output impedances has been constructed. The equivalent circuit, therefore, of such an attenuator is that shown in FIG. la and the resistance measured between contacts A and B will be represented by the sum of resistors R R,. In the embodiment represented by FIG. 2a, the region of the resistance film situated between contacts A and B and represented by a central line of current flow B will in large part be equivalent to the two resistors R, R but it should be noted that all other regions of the resistive film will contribute to the measured resistance between A and B. Similarly the resistance measured between contacts A and C as represented again by central lines of current flow (F F) will correspond to the sum of resistors R, R;,, and the resistance measured between contacts B and C, as represented by lines of current flow (G' G), will correspond to the sum of resistors R R;,. The shunt resistor R is represented to a large measure by the resistive material in the region of the central paths of current flow F and G with appropriate contributions from F and G. As paths E, (F F), and (G G) are idealized representations providing general indications of the direction of currents flowing in the overall resistive film, there is no absolute distinction or perfect separation between them and in particular it is not possible to equate areas of the resistive film to the equivalent resistors R,, R and R The electrical center of the equivalent circuit (i.e., the junction between R,, R and R in FIG. la) is not identified by any single point on the resistive film. Thus it is not possible to carry out an adjustment to the resistance value of those areas of the film which represent the sum of (R, R i.e., as represented mainly by the film in the region of path E, without at the same time affecting the resistance values of those areas of the film representing R For example, if one attempted to increase the resistance between contacts A and B by removing a strip of film from the edge of D between A and B as shown in FIG. 212, then this would have some measurable effect on the measured value of resistance between A and C and between B and C. Similarly the shunt resistance of the attenuator consisting largely of the current paths (F F) and (G G) cannot be increased in value by narrowing the limb of the resistive film in contact with C as shown in FIG. 20 without affecting the resistance measured between A and B.

If a region M of conducting material is deposited on film D as shown in FIG. 2d so that it covers the full width of that limb of D which terminates at C, then as a result a main line of current flow, represented by .I, can exist between C and M and two main lines of current flow, represented by L and (H K), can lie between A and B, and the attenuator of the present invention is produced. The isolated conductor region M is symmetrically situated with respect to contacts A and B and is substantially equivalent to the junction between resistors R,, R and R in FIG. la. M therefore defines the electrical center of the portion of resistive film lying between A and B (cf. the electrical center of resistors R, and R in FIG. la). Adjustment of the series resistance between A and B, which P comprises, may be carried out without affecting the value of the shunt resistance which N comprises and vice versa. The strip N of resistive material is conveniently deposited in such a pattern as to enable adjustment of the resistance value between C and M to be readily carried out. A top hat" configuration is shown for N in FIG. 2e, an increase in the, resistance value between C and M being achieved by removing part of the film in the direction shown as Q. This adjustment, shown in FIG. 2f, may

be effected without affecting the resistance value of the series path P. Referring to FIG. 1a, the attenuator of the invention has means for adjusting R without affecting R, and R If part of the resistance film P exactly midway between A and B is removed, or if a strip of film of constant width is removed from the whole length of P between A and B, as shown in FIG. 2f, then this will result in an increase in the total resistance of the series path P without altering the electrical balance of the two series resistors which P comprises and without affecting the value of the shunt resistance portion N. Thus, referring to FIG. la, the attenuator of the invention enables the series leg R, and R to be adjusted in resistance value while maintaining the balance between R, and R, and without affecting R It is seen that, in this way, only two simple adjustments are required for an unbalanced attenuator and one adjustment is not affected by the other. Two such attenuators, connected together by their common terminals, form a balanced attenuator. The balanced attenuator so produced has twice the characteristic impedance of the unbalanced halves. Only three adjustments are required for a balanced attenuator of the present invention since it is possible to combine the resistance path N of each unbalanced half" to form a single resistance path lying between two electrical center conductors M.

The invention is thus based on the finding that it is possible to make a film attenuator by dividing up a single film resistive element with an electrically conductive layer in electrical contact with said resistive element in a suitable position on the element; the conductive layer must be placed in such a way and the resistive element itself must be of a shape such that three distinct resistive element portions result, two of which have the same resistance value, said portions being capable of having their resistance value adjusted by removing part thereof. It will be appreciated that in some cases, i.e. when it is desired to produce a balanced attenuator, two additional distinct portions of the resistive element must be provided and we have found that this can be done by means of a further electrically conductive layer applied to the single film resistive element.

As will become apparent hereinafter, the above mentioned two portions of the single film resistive element which portions have the same resistance value, could be said to be a single resistor, but since they fulfill the function of the two resistors (in FIG. la these are indicated as R, and R of equal value of the classical film attenuator, elsewhere herein they are considered to be two portions which have the same resistance value, especially as the region of the single film resistive element concerned fulfills the function of two resistors of equal resistance value connected in series.

The present invention thus provides an electrical film attenuator or other film resistor network comprising an insulating substrate having adherently mounted thereon a single film resistive element, said element being provided with at least three terminals in electrical contact therewith and, on said element and in electrical contact therewith, an electrically conductive layer dividing up said element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of said terminals, one of which being an input and the other an output terminal, the third portion of said electrical resistive element being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electricallyconnected to the remaining terminal or terminals. The present invention further provides an electrical film attenuator or other film resistor network comprising an insulting substrate, adherently mounted thereon a single film resistive element, said element being provided with four terminals in electrical contact therewith and, on said element and in electrical contact therewith, two electrically conductive layers in such a way as to divide up the element into five distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between a pair of the terminals and two further of which will have the same electrical resistance value after removal of part of said element at a locus midway between the remaining pair of the terminals, said first pair and second pair of terminals each being an output and an input terminal, the fifth portion extending from one to the other of said two electrically conductive layers and being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said four terminals being associated with only one portion of said two sets of two portions having the same electrical resistance value.

The present invention also provides a method of producing an electrical film attenuator or other film resistor network, which comprises providing an insulating substrate, adherently mounting thereon a single film resistive element, providing said element with at least three terminals in electrical contact therewith and providing on said element and in electrical contact therewith an electrically conductive layer in such a way as to divide up the element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of the terminals, one of which being an input and the other an output terminal, the third portion being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.

The above method is suitable for the production'of both balanced andunbalanced attenuators. In the case of an unbalanced attenuator it is sufficient to provide three distinct portions of the resistive element and a total of three terminals, two of which will serve as separate input terminal and output terminal respectively, the third terminal serving as bothinput and output terminal. In the caseof a balanced attenuator, however, it is necessary to divide up the resistive element so as to provide two further distinct portions (i.e., making five distinct portions in all) and this is done by applying to the single film resistive element a further electrically conductive layer.

The present invention thus also provides a method of producing an electrical film attenuator or other film resistor network, which comprises providing an insulating substrate, adherently mounting thereon a single film resistive element, providing said element with four terminals in electrical contact therewith and providing on said element and in electrical contact therewith two electrically conductive layers in such a way as to divide up the element into five distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of the terminals and two further of which will have the same electrical resistance value after removal of part of said element at a locus midway between the remaining of the terminals, said first two and second two terminals being one out put and one input terminal, the fifth portion extending from one to the other of said two electrically conductive layers and being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said four terminals being associated with only one of said two sets of two portions having the same electrical resistance value.

From the above it is seen that the film attenuator of the invention comprises an insulating substrate having adherently mounted thereon a single film resistive element provided with at least three electrical terminals and at least one electrically conductive film area in electrical contact therewith in such a way as to divide up the resistive element into at least three distinctive portions. In the case of an unbalanced film attenuator, as will be seen from the above, there are only three electrical terminals and three distinct portions of the single film resistive element having thereon one electrically conductive layer. However, in the case of a balanced attenuator, there are four terminals, five distinct portions of the single film resistive element and two electrically conductive layers.

From another point of view, the film attenuator of the invention may be considered to have the allow following features:

a. A single film resistive element attached to a conductor pattern consisting of several distinct electrically conductive layers (including at least one layer within the area of the resistive element) on an insulating substrate; it is possible to apply the resistive element first and then the electrically conductive layers or vice versa.

b. The resistive element must be adapted to enable it to be made into a balanced or unbalanced attenuator having the desired insertion loss and characteristic impedance.

c. Termination layers, the number of which depends on whether the attenuator is balanced or unbalanced.

The precise geometrical arrangement of the pattern of all the portions of the resistive element and the conductive layers is dependent upon the insertion loss required and upon the total area available and the overriding requirement relative to power dissipation in the resistive element; for the sake of convenience, in general, the various portions of the resistive element will have the shape of geometrical figures having a line of symmetry, but in all cases said resistive element consists of:

i. For an unbalanced attenuator: three termination conductive layers constituting the input and output contacts of the attenuator, one of these layers being common to the input and output, and one further conductive layer in electrical contact with and situated within the resistive element in such a way that a line of symmetry can be drawn through it which is equidistant from the two termination layers neither of which is the common termination layer.

ii. For a balanced attenuator: four termination layers constituting two input and two output contacts of the attenuator, the further conductive layer mentioned at (i) and another, similar electrically conductive layer (also situated within the resistive element) through which a line of symmetry can be drawn in manner similar as mentioned at (i).

The single film resistive element, which is in contact with the three or four termination layers and the one or two said further conductive layers, provides:

1. One series resistance path between two termination layers, neither of which is the common termination layer, in the case of an unbalanced attenuator, or two series resistance paths, one between each corresponding input and output termination layer, in the case of a balanced attenuator.

2. A resistance path between the electrical center of the series resistance path and the common termination layer, in the case of an unbalanced attenuator, or a resistance path between the electrical center of each of the two series resistance paths in the case of a balanced attenuator, the said further conductor layer or layers definings the said electrical center or centers of the series resistance path or paths.

3. A first locus at which the resistance value or values of the series resistance path or paths may be adjusted without affecting the electrical balance of the series resistance path or paths and a second locus at which either the resistance value of the resistance path between the electrical center of the series resistance path and the common termination layer, or the resistance value of the resistance path between the electrical center of each of the two series resistance paths, may be adjusted without affecting the adjusted value of the series resistance path or paths.

When the film attenuator is processed to give it the required nominal values for the insertion loss and input and output impedances, an adjustment of the resistive film element is made; the first step of this adjustment comprises removing part of the resistive film midway between an input terminal layer and its corresponding output terminal layer or removing a strip of resistive film of uniform widthfrom the entire edge of the film between each input termination layer and its corresponding output termination layer, i.e., one operation for an unbalanced attenuator (because one of the three termination layers constitutes a common input/output terminal) and two operations for a balanced attenuator.

The second step in the process of adjustment consists of removing part of the resistive film to etfectively reduce the width and increase the length of the resistance path lying either between the electrical center of the series resistance path and the common termination layer, for an unbalanced attenuator, or between the electrical centers of the two series resistance paths, for a balanced attenuator. The important feature of this second adjustment step is that, in addition to making normal resistance measurement during the adjustment process, measurement of the actual attenuation may be made so that compensation for minor errors during the first adjustment step may be provided.

The attenuators of the invention have equal input and output impedance.

The attenuator of the invention requires adjustment of the single resistive film in only two regions, in unbalanced form, to provide the required electrical characteristics, compared with the adjustment of three separate elements required with unbalanced film attenuators of the prior art. In balanced form, the attenuator of the invention requires adjustment of the single resistive film in only three regions, compared with the adjustment of four or five separate elements required with balanced attenuators of the prior art. In the present invention each subsequent adjustment operation has no efiect of those previously carried out.

The geometry of the conductor pattern determines the ultimate insertion loss of the attenuator, while the electrical resistivity and shape of the resistor pattern determines the characteristic impedance. To manufacture an attenuator of a particular insertion loss and characteristic impedance, the conductor pattern appropriate to the insertion loss is first produced and then there is superimposed upon it a resistive pattern also appropriate to that insertion loss and having a resistivity directly related to the required impedance. The resistive pattern is designed to overlap the conductive pattern and the overlap provided is advantageous in that it eliminates the necessity for very precise registration between resistor and conductor elements which is required with film attenuators of the prior art.

Well known techniques may be used for depositing the conductive and resistive films, e.g., electroless plating, vacuum evaporation, or screen printing and firing of cermet materials; however, care must be taken that a resistive film is produced which has a lower resistance value than that corresponding to an ideal situation which would not need adjustment, since the adjustment steps described above only aloow an increase in resistance to be achieved. For this reason, resistive films will normally be deposited which are slightly lower in resistivity than the required value.

Two embodiments of the invention are now described with reference to FIGS. 3a, 3b, 3c, 3d and FIGS. 4a, 4b, 4c, 4d of the drawings in which:

FIG. 3a represents a plan view of an unbalanced attenuator according to the invention.

FIG. 3b represents a plan view of the conductive elements of the attenuator of FIG. 3a.

FIG. 3c represents a plan view of the single resistive element of the attenuator of FIG. 3a.

FIG. 3d represents, in schematic form, a circuit equivalent to the attenuator of FIG. 3a.

FIG. 4a represents a plan view of a balanced attenuator according to the invention.

FIG. 4b represents a plan view of the conductive elements of the attenuator of FIG. 4a.

FIG. 40 represents a plan view of the single resistive element of the attenuator of FIG. 4a.

FIG. 4d represents, in schematic form a circuit equivalent to the attenuator of FIG. 4a.

Referring to FIGS. 3a, 3b, Be, an unbalanced attenuator comprises three main conductive termination areas, 1, 2, 3 and one further conductor area 7, deposited on an insulating substrate 11. A single film resistive element (FIG. 30) covers the conductor 7 and also the main termination areas in regions 4, 8, 13, as shown in FIG. 3a.

As shown in the equivalent circuit in FIG. 3d, the attenuator consists of:

i. A resistive path 6, between terminations l, 2 with a conductor 7 constituting its electrical center.

ii. In parallel with the resistive path 6 is a further resistive path which is capable of being adjusted in value by removing part of it by cutting in the direction of the arrow 12.

iii. Between the electrical center conductor 7 and the third main termination area 3 lies another resistive path 10, also capable of being adjusted in value by removing part of it in the direction of arrow 9. The input to the attenuator is applied to terminals 1 and 3 and the output is taken from terminals 2 and 3.

The attenuator may be manufactured as follows:

Conductive patterns 1, 2, 3, 7 are deposited on an insulating (e. g., ceramic) substrate 11, by well known techniques such as those previously mentioned. Conductive patterns 1 and 2 constitute an input and an output terminal respectively, whereas conductive pattern 3 constitutes a combined input and output terminal. The single resistive film is then deposited on to the substrate by similar techniques so that it overlaps the conductor pattern as shown in FIG. 3a. The overlap is such that very accurate registration is not required between the conductors and the resistive film. Alternatively, the resistive film may be deposited first of all on to thesubstrate and the conductive patterns 1, 2, 3, 7 may then be deposited on top of it. Although not essential, the resistive film may be provided with indentations 14 and 15 to indicate the position where resistance adjustment has subsequently to be carried out. Indentation 14 is midway between terminals 1 and 2. Electrical connections are made to the terminals 1 and 2 and the resistance value between these terminals is monitored while a portion of the resistive film in region 5 is removed in the direction indicated by arrow 12, indentation 14 being a guide to the correct position for removing the film. The film is removed by well known techniques, e.g., a jet of abrasive particles, laser beam, or other suitable grinding or abrading techniques. The contacts of the resistance monitoring device are then placed across input terminal 1 and the combined input and output terminal 3. A portion of the resistive film in region 10 is removed by similar means to those previously employed, the film being cut away in the direction shown by the arrow 9, starting at the indentation 15. Monitoring in this case may either be by the normal method of resistance measurement or by the far more accurate method of actual attenuation measurement.

Referring now to FIGS. 4a, 4b, 40, a balanced attenuator comprises four main conductive termination areas, constituting two input and two output terminals 16, 17, 18, 19 and two further conductor areas 20, 21 deposited on an insulating substrate 22. A single film resistive element (FIG. 4c) covers the conductors 20, 21 and also overlaps the input and output terminals l6, 17, 18, 19 in regions 23, 24, 33, 34 as shown in FIG. 4a. As illustrated in the equivalent circuit in FIG. 4d, the attenuator consists of:

i. A resistive path 26, between input and output terminals 16, 17 with conductor constituting its electrical center.

ii. A resistive path 25 in parallel with path 26 and capable of being adjusted in value by removing part of it by cutting in the direction of the arrow 31.

iii. A resistive path 28, between input and output terminals 18, 19 with a conductor 21 constituting its electrical center.

iv. A resistive path 29 in parallel with path 28 and capable of being adjusted in value by removing part of it by cutting in the direction of arrow 32.

v. A further resistive path 27, lying between conductors 20 and 21, which is also capable of being adjusted in value by removing part of it by cutting in the direction of arrow 30.

The input to the attenuator is applied to terminals 16, 18 and the output is taken from terminals 17, 19.

The attenuator is manufactured up to the stage immediately prior to adjustment by the techniques described for the unbalanced attenuator. The pattern of conductors may be deposited first of all, followed by the single resistive film or the resistive film may be deposited first, with the conductors on top. Adjustment is then carried out as follows: the resistance value between terminals 16, 17 is monitored and part of the film 25 is removed by one of the methods previously mentioned until the desired value is obtained. The resistive film is cut away in the direction of arrow 31, an indentation 35 having been formed in the resistive film during the deposition process, together with indentations 36, 37 to indicate the midpoint of the resistive film between the conductors on either side. The resistance value between terminals 18, I9 is then monitored while part of the film in the region 29 is removed by cutting it away in the direction shown by arrow 32. It is necessary for the adjusted resistance value to be as close as possible to that previously measured between terminals 16, 17, depending upon the precision which is required in the attenuator. The resistance value between terminals l6, 18, or between 17, 19, is then monitored while part of the film 27 is removed as indicated by the arrow 30. As with the unbalanced attenuator, either resistance measurement or actual attenuation measurement may be made during this part of the adjustment process.

It is well known that the high frequency performance of known flat film attenuators is superior to that of attenuators comprising discrete resistors, as a result of the reduction of inductance and reactance, but the performance also tends to vary from pattern to pattern depending upon the precise layout of the conductors, the positioning of the termination areas and the amount of adjustment carried out on the resistive elements. In the present invention, it is envisaged that all attenuators of similar insertion loss will be constructed from an identical pattern designed to cover a wide frequency range with the minimum change of impedance or insertion loss. Thus the high frequency performance will be predictable and superior to that of previous designs.

The design also has application in continuously variable attenuator networks where a single substrate can carry decades of 0.1 dB, 1 dB and 10 dB attenuators, directly connected to switches, thus permitting rapid selection of any attenuation or insertion loss between 0.1 dB and 99.9 dB in 0.1 dB steps.

Although the above description is solely concerned with attenuators, the principle of dividing up a single resistive film element by one or more short-circuiting electrically conductive layers combined with removal of portions of the element may be applied to any networks in which a plurality of film re sistors would normally be used, e.g., in scaling networks or ladder networks.

Although the present invention is described herein with particular reference to specific details, it is not intended that such details shall be regarded as limitations upon the scope of the invention except insofar as included in the accompanying claims.

I claim:

1. An electrical film attenuator or other film resistor network comprising an insulating substrate having adherently mounted thereon a single film resistive element, said element being provided with at least three terminals in electrical contact therewith and, on said element and in electrical contact therewith, an electrically conductive layer dividing up said element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of said terminals, one of which being an input and the other an output terminal, the third portion of said electrical resistive element being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.

2. An attenuator or other film resistor network according to claim 1, in which a total of three terminals are provided, the third of which being a combined input and output terminal, and the third portion of the film resistive element is electrically connected to the third terminal, whereby an unbalanced attenuator results.

3. An electrical film attenuator or other film resistor network comprising an insulating substrate, adherently mounted thereon a single film resistive element, said element being provided with four terminals in electrical contact therewith and, on said element and in electrical contact therewith, two electrically conductive layers in such a way as to divide up the element into five distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between a pair of the terminals and two further of which will have the same electrical resistance value after removal of part of said element at a locus midway between the remaining pair of the terminals, said first pair and second pair of terminals each being an output and an input terminal, the fifth portion extending from one to the other of said two electrically conductive layers and being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said four terminals being as sociated with only one portion of said two sets of two portions having the same electrical resistance value.

4. A method of producing an electrical film attenuator or other film resistor network, which comprises providing an insulating substrate, adherently mounting thereon a single film resistive element, providing said element with at least three terminals in electrical contact therewith and providing on said element and in electrical contact therewith an electrically conductive layer in such a way as to divide up the element into three distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of the terminals, one of which being an input and the other an output terminal, the third portion being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said first two portions being in electrical contact with only one of said two terminals, the third portion being electrically connected to the remaining terminal or terminals.

5. A method according to claim 4, in which a total of three terminals are provided, the third of which being a combined input and output terminal, and the third portion of the resistive film element is electrically connected to the third terminal.

6. A method according to claim 5, which comprises the further steps of (i) removing part of the resistive film midway between said input and said output terminals, or removing a strip of resistive film of uniform width from the entire edge of said film between them, and (ii) removing part of the resistive film to effectively reduce the width and increase the length of the third portion of the resistive element, whereby it is possible to measure the attenuation achieved or the electrical resistance value of the network so as to enable compensation for minor errors produced by step (i).

7. A method of producing an electrical film attenuator or other film resistor network, which comprises providing an insulating substrate, adherently mounting thereon a single film resistive element, providing said element with four terminals in electrical contact therewith and providing on said element and in electrical contact therewith two electrically conductive layers in such a way as to divide up the element into five distinct portions two of which will have the same electrical resistance value after removal of part of said element at a locus midway between two of the terminals and two further of which will have the same electrical resistance value after removal of part of said element at a locus midway between the remaining of the terminals, said first two and second two terminals being one output and one input terminal, the fifth portion extending from one to the other of said two electrically conductive layers and being shaped so as to enable its electrical resistance value to be adjusted by removing part of it, each of said four terminals being associated with only one of said two sets of two portions having the same electrical resistance value.

8. A method according to claim 7, which comprises the further steps of (i) removing part of the resistive film midway between each of said first mentioned pair of terminals and said second mentioned pair of terminals or removing a strip of resistive film of uniform width from the entire edge of said film between each input and corresponding output terminals, and (ii) removing part of the resistive film to effectively reduce the width and increase the length of the third portion of the resistive element, whereby it is possible to measure the attenuation achieved or the electrical resistance value of the network so as to enable compensation for minor errors produced by step (i).

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Classifications
U.S. Classification333/81.00R, 338/309, 338/325, 29/620, 338/314, 338/195
International ClassificationH03H7/24, H01C7/00
Cooperative ClassificationH01C7/00, H03H7/24
European ClassificationH01C7/00, H03H7/24