US3786374A

US3786374A - Variable resistive film attenuator

Info

Publication number: US3786374A
Application number: US00266366A
Authority: US
Inventors: D Bergfried
Original assignee: Weinschel Engineering Co Inc
Current assignee: Lucas Weinschel Inc
Priority date: 1972-06-26
Filing date: 1972-06-26
Publication date: 1974-01-15
Anticipated expiration: 1991-01-15

Abstract

A continuously variable resistive film attenuator (often called ''''card attenuator'''' is provided, having distributed shunt and series film portions for use from D.C. above 8 GHz. It has a low minimum loss. It stays matched over its entire range. In one version it has substantially no incremental phase shift with changing attenuation. The resistive film is enclosed inside a slidable conductive sleeve which is moved to expose only that part of the attenuator which is inserted into the circuit, the unused part thus being in a field-free region where it can have no effect, while the inserted part of the shunt film is connected to ground through a continuous ground contact which is pressed down on the edge of the shunt portion by a multiple finger spring arrangement. In one version, the electrical length between input and output is maintained constant by shortening a conductive portion of the attenuator as the resistive portion is lengthened so that there is no phase shift between input and output as the attenuation is varied.

Description

ergiried Jan. 15, 1974 1 VARIABLE RESKSTIVE FILM ATTENUATOR [75] Inventor: Dietrich E. Bergiried, Potomac, Md.

[73] Assignee: Weinschel Engineering Co. Inc.,

Gaithersburg, Md.

[22] Filed: June 26, 1972 [21] Appl. No.: 266,366

Primary Examiner-Paul L. Gensler Attorney-Max L. Libman 5 7] ABSTRACT A continuously variable resistive film attenuator (often called card attenuator is provided, having distributed shunt and series film portions for use from DC. above 8 61-12. It has a low minimum loss. It stays matched over its entire range. In one version it has substantially no incremental phase shift with changing attenuation. The resistive film is enclosed inside a slidable conductive sleeve which is moved to expose only that part of the attenuator which is inserted into the circuit, the unused part thus being in a field-free region where it can have no effect, while the inserted part of the shunt film is connected to ground through a continuous ground contact which is pressed down on the edge of the shunt portion by a multiple finger spring arrangement. In one version, the electrical length between input and output is maintained constant by shortening a conductive portion of the attenuator as the resistive portion is lengthened so that there is no phase shift between input and output as the attenuation is varied.

7 Claims, 10 Drawing Figures PMENTEU JAH sum 1 BF 2 VARIABLE RESISTIVE FILM ATTENUATOR BACKGROUND OF INVENTION This invention relates to variable attenuators for use in high frequency coaxial lines. The attenuator consists of a resistive film which has distributed series and distributed shunt elements. An object of the invention is to provide a continuously variable attenuator which has a substantially flat frequency response over a wide range of frequencies from DC. to, for example 8 GHz. Other objects are to provide an attenuator which stays matched independent of attenuation position, has in one version small or no incremental phase shift with changing attenuation and essentially zero loss in the minimum attenuation position.

The basic problem with which the present invention is concerned can be illustrated by considering the limitations of some of the resistive variable attenuators now in use such as the variable T pad (U.S. Pat. No. 3,184,694) and the variable card attenuator (US. Pat. No. 3,157,846). The variable T-pad consists of three adjustable resistance elements in T formation, two of which are connected in series with the input and output center conductors and are therefore called series resistors. The third resistor is called the shunt resistor connected from the common point to ground. The attenuation of the T pad attenuator is varied by simultaneously moving contact points along each of the resistors thus changing the values of the three resistors in such a way that the T-network maintains a constant input and output impedance and at the same time pro vides the desired attenuation. However, the individual resistor elements will stay constant in value only below the frequencies where the wavelength is long compared to the physical dimensions of the resistor. When this condition fails, the value of the resistor and therefore the attenuation of the unit will change with frequency. This limits the usable frequency range and becomes especially important for values of attenuation greater than 20 dB. Depending on the attenuation setting, a shorter or longer part of the series resistors is connected between input and output, and since the length of this position is changing with the attenuation adjustment, the phase shift changes also in the simple version.

Variable card attenuators have an attenuation element which usually comprises a dielectric substrate supported within the coaxial structure and coated with one or more layers of resistive film. Such attenuators are usually subject to the following difficulties: (a) A minimum loss of at least 3dB because of shunting effect of the unused portion of the attenuator element in parallel with the output; (b) Not matched in all positions, only compromised match; (c) Phase shift with changing attenuation in the simpler version. The resistive film is designed to maintain the Heaviside relationship, wherein the ratio of series to shunt resistance is equal to the ratio of the series inductance to the shunt capacitance, which is the one case where the attenuation of the transmission line is not a function of the frequency.

SUMMARY OF THE INVENTION The attenuator element consists of an insulating substrate supporting one or more layers of resistive film which represents a lossy transmission line with distributed series and shunt losses. The resistive film may either be a symmetrical one in respect to ground as shown in FIG. 1 of US. Pat. No. 3,157,846 or FIG. 1 of U.S. Pat. No. 3,260,971 or FIG. 1 of US. Pat. No. 3,227,975. On the other hand, it also may be an unsymmetrical construction in respect to ground such as is shown in FIG. 5 of US. Pat. No. 3,157,846 or FIG. 9 of US. Pat. No. 3,260,971. The supporting insulating substrate can be flat or curved as long as the surrounding conductor is shaped to satisfy the requirements of the Heaviside relation which requires that the ratio of series to shunt resistance equals the ratio of series inductance to shunt capacitance. One end of this network is connected to the input connector through a conductive launching electrode, the other end over a conductive or capacitive contact to the sliding inner conductor grrd the output. A conductive contact is used if the frequency range includes DC. or very low frequencies where the series capacity of a capacitive contact would cause frequency sensitivity of attenuation and matchvT he resistive attenuator element is enclosed inside the inner conductor and, therefore, is in side a field-free region. Only the part that is desired in the respective attenuation position is inserted in the transmission line by moving it out of the enclosing inner conductor. A brush or capacitive contact connected to the movable inner conductor connects the series film to the inner conductor, thus making the connection input-output through the series film. At the same time, the inserted part of the shunt film is connected to ground through a continuous conductive or capacitive ground contact; in this case, over a series of contacts that are pressed down on the edge or edges of the resistor by a multiple finger spring. Adjusting the attenuator does two things simultaneously, first it connects a section of the series film between input and output, and second at the same time, an increasing number of brush contacts are connecting the inserted shunt area to ground so that a variable resistive attenuator element is being inserted in the transmission line. Since the unused part of the resistive element is in a field-free region, it is not connected in parallel with the output as in other types of variable card attenuators. This makes it possible to connect the movable inner conductor directly to the fixed input launcher in the zero attenuation position and to decrease the minimum loss to the loss of the transmission line between input and output with the resistive element completely out of the circuit. The sliding inner conductor makes contact with a conductive output transmission line portion, the effective length of which is varied in one version as the sliding center conductor is moved so as to maintain the total electrical length between input and output constant as will be explained below, and thus provides a constant electrical length so that no phase shift occurs irrespective of attenuation value.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings, in which:

FIG. l is a transverse sectional view through the attenuator, taken on line 1-1 of FIG. 3;

FIG. la is a schematic drawing used to explain the principle of the attenuator;

FIG. 2 is a view taken on line 22 of FIG. 1;

FIG. 3 is a view taken on line 33 of FIG. 1;

FIG. 4 is a view taken on line 44 of FIG. 2;

FIG. 5 is a bottom view of the edge contact sector of the attenuator;

FIG. 6 is an enlarged sectional view similar to that of FIG. 4, showing details of the spring finger contacts;

FIG. 7 is a plan view showing the slidng sleeve in place of the ceramic annulus;

FIG. 8 is a sectional view taken on line 88 of FIG. 7; and

FIG. 8a is an enlarged view of the sliding contact of FIG. 8.

Referring mainly to FIGS. 1 and 2, the attenuator is provided with a housing 2 having in its calibrated version a dial 4 on its face provided with appropriate markings or indicia to indicate the attenuation value which is controlled by the rotational setting of knob 6 carrying a conventional pointer or arrow inscribed on its face so that the rotational setting of the knob can be read in terms of the desired parameter to express the value of attenuation in the circuit as determined by the knob setting. Knob 6 turns control shaft 8 which preferably drives attenuator shaft through a suitable backlash-free reduction gear arrangement 9 so that a nearly full turn of the knob produces a much smaller turn of the shaft 10, since the full travel of the attenuator, as will be seen, is only a small arc of the entire circumference, thus providing, in effect, an expanded scale which can be readily read for greater accuracy.

Control shaft 10 extends through partition 12 into attenuator chamber 14, where it is squared off or otherwise arranged so as to rotatably carry attenuator elements l6, 18, 20, 22 and 23 but not the annular ceramic disc 24, which is supported independently and fixed to the housing 2 so that the ceramic disc cannot rotate within the housing. In some cases it may, however, be more practical to move or rotate the resistor substrate and keep the hollow inner sleeve stationary, since it does not matter whether the sleeve or the resistor is moved so long as there is relative motion between them.

Element 16 (FIGS. 2 and 4) is a conductive metal disc cut away about one-half the radial distance out from the center for about one-third of its circumference, leaving a circular center portion 30 and an arcuate circumferencial 32 of about 120 circumferencial extent. Sector 34, intermediate in radius between 30 and 32, extends for the remaining 120. The entire rotor assembly is fixed to the shaft 10 so that the entire assembly rotates with the shaft as a unit. This metal disc 16 serves as a portion of the grounded outer conductor.

Element 18 (FIG. 3) has a metal sector that extends for 240 and is fixed to sleeve 22 in any desired manner, shown as a thin insulating piece 23 fixed to both the sleeve and element 18, and element 18 is of sufficiently small diameter so that it fits loosely into the central aperture of ceramic disc 24, and it also is rotated by shaft 10. Its arcuate sector portion 18a corresponds to the

arcuate sections

32 and 34 of element 16.

Element (FIGS. 1, 4 and 6) also is carried by shaft 10 and at 21 has the same shape and size as the metal portion 32 of element 16 which it overlies. This disc serves as a portion of the grounded outer conductor. As shown in FIGS. 1 and 6, it has a downwardly extending rim 210 at its outer circumference, and extending back radially and inwardly from the rim are a number of spring fingers 38 each having a small piple 40 near its outer end which engages a dimple cut into the top of conducting contact block 42 so that the bottom of this block is urged into flat contact with the inner edge of the shunt portion of resistive film 44 on ceramic disc 24. This shunt film portion 44 extends from the inner edge of the ceramic disc towards the periphery where it meets the series resistive portion 46 (FIGS. 2 and 6), which may be a thicker layer of resistive material as shown in U.S. Pat. No. 3,157,846. This series portion 46 is connected to the center conductor of the input coaxial connector 28 by means of a highly conducting layer 48 of suitable material such as gold serving as a stripline conductor (FIG. 3). However, this device can also be constructed with a single resistive film which serves simultaneously as series and shunt resistance.

The effective length of the variable attenuator is in a practical device only a short arcuate distance of the total circumference, e.g., somewhat less than For another 120 or so the ceramic annulus is coated with a suitable conductive material such as a gold or

silver strip

48 or 49 providing a conductive stripline extension of the attenuator. The remote end of this conductive stripline is connected to the inner conductor of the output coaxial connector 50, so that the electrical inner path of the attenuator is from center conductor of input coaxial connector 28' to the center conductor of coaxial connector 50.

Element 22 (FIGS. 2 and 7) is a movable arcuate center conductor which has an elongated U-shaped cross-section (see FIG. 8) so that it slips over the inner edge of annular ceramic disc 24, and is a little longer than the arcuate length of the

resistive layers

44 and 46. At one end of element 22 is a dimple 52 which bears on a brush 53 similar to brushes 42, which in turn bears on the series resistive strip 46 and at the other end it has a number of spring fingers 54 which bear resiliently against the gold or silver stripline 49 which in turn is connected to the inner conductor of coaxial connector 50.

Element 22 rotates with element 20, from which it is separated only by a small gap. Thus, as the knob 6 is rotated to increase the attenuation, the dimple 52 moves clockwise away from the highly conductive gold strip 48 into contact with the resistive center strip 46 to insert series attenuation into the circuit between the inner conductors of

coaxial connectors

28 and 50. At the same time the spring fingers 38 of sector 21 move their contacts 42 into engagement with the inner edge of shunt resistive layer 44 to bring more and more of this shunt resistor into the circuit between ground and inner conductor while the opposite end of sleeve 22, through spring fingers 54, is bearing on the conductive strip 49. Thus, as the resistive portion of the attenuator is inserted into the circuit, the conductive portion represented by sleeve 22 and stripline 49 is being shortened so that the electrical length remains the same between the two coaxial connectors and the phase does not change due to a change in length of the circuit as the attenuation is added in. It will be noted that the unused portion of the resistive film is totally shielded by sleeve 22 and so has no effect on the circuit, unlike the usual case where it constitutes a parallel branch, though supposedly out of the circuit. Conversely, as the attenuation is reduced by turning the knob in the opposite direction, the sleeve 22 returns to enclose the unused portion.

The principle will perhaps be easier to understand by reference to FIG. 1a which is a schematic diagram of a linear version which is not as realistic in constructional detail and is used mainly to illustrate the principle of the invention. It depicts a straight-line model of the attenuator, rather than the rotary model shown in the remaining Figures, but the basic principle is the same in both cases, and the attenuator of the invention can be made either way. The same reference characters will be used for corresponding parts of the straight and rotary models where applicable, with the subscript added for the straight-line model.

The grounded outer conductor of the attenuator is represented at 2a and can be thought of as a tubular outer ground conductor of circular or rectangular cross section. The resistive attenuator portion is in the form of a thin film of resistive material deposited on a ceramic plate 24a and having a series portion 46a and a shunt resistive portion 44a, which may be a thinner layer of resistive material than 46a. Spring fingers 38a are carried by sleeve 20a and an increasing number of them make contact with the lower edge of the shunt resistive film 44a as the sleeve 20a is moved (by means not shown) to the right. Shielding sleeve 22a carrying contact 52a moves together with the sleeve 20a and maintains contact with conductive strip 49a (which serves as a conductive stripline between the resistive portion of the attenuator and the output connector a) by means of spring finger contacts 54a.

The linear version can also be constructed using a ceramic rod or tube as the substrate for the resistive film, in which the series portion corresponding to 460 is provided by a section of low resistivity distributed over an arcuate section ofabout one-sixth to one-third of the circumference, and the remaining portion of the circumference provides the shunt resistive portion which is wrapped around the tube and connected to ground at the side opposite to the series portion. The connection to ground can be made through suitable sliding contacts or through a sleeve or block in close proximity which provides a capacitive contact to ground. This construction has the advantage that the whole surface of the ceramic substrate is used to support the resistive film and not only one side as in the flat substrate. To provide the same surface area for the resistor the supporting substrate can, therefore, have a smaller cross section. This in turn requires a smaller enclosing sleeve inner conductor so that the outer conductor that provides thematched enclosing ground is of smaller cross section. Since the highest operating frequency of a TEM or quasi TEM structure is determined by the cross section, this construction provides a design for very high microwave frequencies.

In FIG. 1a, the total length between input and output is divided into three sections 11 1 and 1 11 is the length of the attenuator element inserted in the tramsmission path. This inserted attenuator element behaves like a lossy transmission line with an effective dielectric constant K The electrical length of this section is therefore, 1 I( 1 the transmission length of the sliding center conductor. K the effective dielectric constant, Vi -l, the electrical length of this section.

i transmission length of the wide stripline on which the movable center conductor slides back and forth. K is the effective dielectric constant. 1 is the electrical length of section 1 The total electrical length of the unit is, therefore,

VITA, VK l x l i l I is shortened by whatever amount ll is lengthened, when the attenuation is changed. The sum 1 1 is, therefore, constant. Making the effective dielectric constant of region 1 and 3 equal results, therefore, in a constant electrical length with varying attenuation which is W, X (1 1 :i- 1 Since this is constant, there is no phase shift when the attenuation is changed.

In a practical case it is desired to make the attenuation suitable for matching a 50 ohm circuit. The width of the stripline for 50 ohms determines the size of the enclosure around it. If the stripline is wider, the ground planes must be further removed or separated; where the stripline is wide, the ground planes are provided by the top cover and the housing, but in the region where the stripline is much narrower, the ground planes must be brought closer to maintain the 50 ohm impedance. Therefore, we provide sector-shaped rotor parts in the circular construction with a step where we change from narrow to wide, and this step rotates with the arcuate member, or moves with the linear member 20a in the linear version of FIG. lla. Resistive films can be manufactured which have no frequency sensitivity from DC. to beyond 26 GHz. It is therefore practical to design this device to cover a much higher frequency range than DC. to 8 GHz. However, since this device uses a quasi TEM mode in the resistive region and a TEM mode in the conductive regions, higher modes can be excited if the spacings between grounds are sufficiently large to permit their occurence. It will then be necessary to use two precautions: use such a construction as, e.g., symmetry which will not excite a higher order mode and install mode absorbing devices such as resistors or ferrites which will absorb the undesirable mode without interfering with the function of the device. However, by making the unit very much smaller, it is possible to use it at much higher frequencies.

It will be noted that the spacing of the ground planes above and below the center conductor as shown in FIG. 1 is different in FIG. 4. This is done to maintain the impedance constant in the two cases, since the conductive film at 49 (FIG. 3) is wider than at 48, and the sleeve 22 extends across all of this wide portion; it is therefore necessary to make the spacing to the ground conductor greater in the sleeve covered portion of the inner conductor, and this is provided by the cut-away portions of the conductive rotary elements 16, I8, and 20. It could also be done by adding insulation to adjust the effective capacitance between ground and the exposed portion of 44 and 46, but in any case, it is necessary, in order to maintain the impedance at the desired value (in practice, usually 50 ohms) to adjust the parameters (series inductance and shunt capacity) of the two sections, i.e., the effective exposed attenuator section and the sleeve-covered section, and this is provided for by making the entire variable portion, both center conductor and ground conductor, of the proper configuration and movable together as shown, so that there is no change in the desired matching condition as the attenuator is adjusted to various values, and there is no mismatch between the variable part of the center conductor and the rest of the center conductor.

I claim:

11. a. A variable resistive film attenuator of the type having a grounded outer conductor and an inner conductor with a resistive film portion lying on an insulattive portion to a portion which can be electrically connected to the grounded outer conductor,

0. a conductive inner conductor connecting one end of said resistive film to the inner conductor of an input circuit connector device,

(I. and a second inner highly conductive conductor adjacent to the other end of said resistive film and connected to the inner conductor of an output circuit connector device,

e. conductive shield means movable relative to said resistive film portion and closely surrounding the uninserted portion of the film attenuator which remains as a part of the film is inserted into the circuit, said shield means being electrically connected at one end to said second inner conductor,

. sliding contact means at the other end of said shield means in electrical contact with said series resistive film to insert an increasing length of said resistive film into the circuit as the shield means is moved relative to the resistive film to expose more of the series resistive film and thus to increase the attenuation,

g. and grounded slider contact means to electrically contact a variable portion of said shunt portion of the resistive film as the shield means is moved relative to the resistive film portion and thus insert a variable portion of the resistive film in shunt between the inner and outer conductors.

2. The invention according to claim 1, wherein the length between the input and output is physically maintained constant, for all attenuation values.

3. The invention according to claim 1, wherein the resistor is located on a flat disc.

4. The invention according to claim 1, wherein the resistor is located on a flat strip.

5. The invention according to claim 1, and outer ground conductor means movable with said shield means and having portions spaced differently from the inserted portion of the resistive film and from the shielded portion so as to maintain the same matched impedance along the two portions as the inserted attenuation is varied.

6. The invention according to claim 5, said grounded contact means comprising an array of individual conductive spring elements, each terminating in a single contact element so that more or less of these contact elements are brought into contact with the shunt resistive portion of the film as the contact means is moved to adjust the attenuator.

7. The invention according to claim 6, each said single contact element being a small block of conductive material having a flat bottom portion and having on top a pivotal connection with its spring element so that said bottom portion is maintained flat in contact with the attenuator film portion on which it slides.

Claims

1. A. A variable resistive film attenuator of the type having a grounded outer conductor and an inner conductor with a resistive film portion lying on an insulating support element, b. said resistive film portion including a series resistive film portion and at least one shunt resistive film portion extending away from the series resistive portion to a portion which can be electrically connected to the grounded outer conductor, c. a conductive inner conductor connecting one end of said resistive film to the inner conductor of an input circuit connector device, d. and a second inner highly conductive conductor adjacent to the other end of said resistive film and connected to the inner conductor of an output circuit connector device, e. conductive shield means movable relative to said resistive film portion and closely surrounding the uninserted portion of the film attenuator which remains as a part of the film is inserted into the circuit, said shield means being electrically connected at one end to said second inner conductor, f. sliding contact means at the other end of said shield means in electrical contact with said sEries resistive film to insert an increasing length of said resistive film into the circuit as the shield means is moved relative to the resistive film to expose more of the series resistive film and thus to increase the attenuation, g. and grounded slider contact means to electrically contact a variable portion of said shunt portion of the resistive film as the shield means is moved relative to the resistive film portion and thus insert a variable portion of the resistive film in shunt between the inner and outer conductors.