US3002165A - Variable attenuator - Google Patents

Description

P 1961 I D. R. AYER ET AL 3,002,165

VARIABLE ATTENUATOR Filed Nov. 17, 1960 2 Sheets-Sheet 2 I. I 1| "WW mk QZ Donald R. Ayer Arnold N. McDowell /N ENTORS ATTORNEY United States 3,092,165 VARIABLE ATTENUATOR Donald R. Ayer, Nashua, and Arnold N. McDowell, Amherst, N.H., assignors to Sanders Associates, Inc, Nashua, N.H., a corporation of Delaware Filed Nov. 17, 1960, Ser. No. 69,863 12 Claims. (Cl. 33381) This invention relates to transmission line attenuators. More particularly, it relates to a high frequency variable attenuator of the T or pi type in which the series and parallel resistances are adjusted in synchronism with each other. The attenuator, which is constructed in strip transmission line form, has a substantially constant impedance over the range of adjustment, and its operation is substantially independent of frequency.

In general, an attenuator is used to reduce the power level of an electromagnetic signal. This is done by inserting an impedance in the signal path between the source and the load to which the signal is delivered. Variable attenuators, which facilitate changing the attenuation introduced in the signal path, are particularly useful, for example, in development work and in testing and adjusting communications equipment. In many applications, it is desirable that the input and output impedances of a variable attenuator be independent of the attenuation adjustment. This may be accomplished, at low frequencies, by the use of both series and shunt impedances, adjusted in synchronism in a well-known manner.

A high fiequency variable attenuator well known in the prior art is a Waveguide section operated below its cutofi? frequency. The attenuation is changed by varying the length of the section. A disadvantage with this type of attenuator is that the attenuation also varies with frequency, especially when the wavelength of the signal approaches the wave guide cutoff wavelength. Moreover, waveguide devices are generally bulky, except at microwave frequencies.

Another well-known high frequency attenuator comprises a section of waveguide enclosing a resistive material to absorb energy. In this device, the attenuation is adjusted by varying the position or amount of resistive material in the guide. Variation of attenuation with frequency and the relatively large size of waveguide devices are disadvantages with this attenuator also.

Prior variable resistance attenuator networks of the type commonly used at low frequencies have not proved suitable for broad band operation at high frequencies. One reason for this is that the mountings and means for adjusting the various resistors result in a structure that appears highly reactive and, hence, is very frequently sensitive.

Accordingly, it is a principal object of my invention to provide an improved high frequency variable attenuator in which operation is relatively independent of frequency over a wide range of attenuation and frequency.

It is a further object to provide an attenuator of the above type in which the input and output impedances remain constant as the attenuation is adjusted.

Another object of my invention is to provide an attenuator having the above characteristics that is small and readily fabricated.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

In general, an attenuator embodying the features of my invention operates schematically in the same manner as a conventional low frequency resistive T or pi network.

Patented Sept. 26, 1961 I ice Thus, it comprises both series and parallel resistances which are adjusted in synchronism to vary the attenuation while maintaining the input and output impedances constant.

The attenuator is preferably constructed in strip transmission line of the type in which a ribbon-like inner conductor is sandwiched between two parallel ground plane conductors. The variable resistance in each branch of the attenuator is formed with a resistive section of inner conductor in telescoping, sliding contact with an essentially non-resistive section. Movement of one section over the other shortens or lengthens the effective length of the resistance section and thus alters its resistance. The resistance adjustments in the several branches are synchronized to provide constant impedance as the attenuation is changed.

Since the inner conductors in the strip line unit are bounded by the outer conductors on only two sides, they may readily be arranged for externally controlled movement with respect to each other without seriously affecting the field distribution within the unit. Furthermore, since the variable resistors are, in effect, portions of inner conductors, they do not present large reactance-causing discontinuities in the transmission line. Accordingly, the attenuation and input and output impedances are substantially independent of frequency over a Wide frequency hand.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a simplified perspective view of a section of a strip transmission line,

FIGURE 2A is a horizontal section of a variable T attenuator embodying my invention, showing the inner conductors in plan,

FIGURE 23 is a schematic representation of a resistive T network,

FIGURE 3 is a vertical section of the variable attenuator of FIGURE 2A,

FIGURE 4 is a plan view of the inner conductors of another T attenuator embodying my invention,

FIGURE 5A is a horizontal section of a variable pi attenuator embodying my invention and showing the inner conductors in plan,

FIGURE 5B is a schematic representation of a resistive pi network, and

FIGURE 6 is a vertical section of the variable pi net- Work.

In F IGURE 1 there is illustrated the field distribution in a typical strip transmission line. The line has: an inner conductor 10 situated between and parallel to a pair of outer or ground conductors 12 and 14. The conductors 10, 12 and 14 are fiat and may be quite thin. For example, they may be formed of foil bonded to dielectric material (not shown) filling the space between them. At an instant of time when the conductor 10 is positive with respect to the ground planes 12 and 14 and the current in the conductor 19 is in the direction of the arrow 15, the field distribution is the transmission line is as shown in FIGURE 1, with the solid arrows representing the electric field E and the dash lines representing the magnetic field H.

The field configuration of FIGURE 1 is indicative of the TEM propagation mode, more fully discussed in US. Patent No. 2,812,501 which issued November 4, 1957, to D. J. Somrners for Transmission Line. However, it is possible to transmit other modes on the line under certain conditions. For example, if the inner conductor 10 is offset from its nominal position midway between the ground planes 12 and .14, the ground planes will be at somewhat difierent potentials. This difference in voltage will support a parallel plate mode. Accordingly, the ground plane conductors are shorted together by a plurality of pins 16 spaced along both edges of the inner conductor. The pins impose an equipotential condition on the ground plane conductors and thereby suppress this mode. For effective suppression, the spacing of the pins in-the lengthwise direction of the line should be less than a half wavelength. Ordinarily, this spacing is on the order of one-eighth wavelength or less.

Another limitation on pin spacing results from the desirability of avoiding a resonant condition in any loop defined by the ground planes and a pair of adjacent pins. A resonant loop will distort the transmission characteristics of the line as well as facilitate radiation of energy therefrom. Resonance occurs when the length of the loop is an integral number of wavelengths, and, accordingly, the distance between adjacent pins should be considerably less than the spacing providing a wavelength loop.

If either of the transverse dimensions, i.e., ground plane to ground plane or pin to pin spacing is greater than a half'wavelength, a transverse electrical waveguide mode may be excited. Therefore, both these dimensions should -be less than a half wavelength. There is also a restriction on the length of the circumferential path around the inner conductor and passing; midway between the inner conductor and the ground planes 12 and 14 and pins 16. This path should be less than a wavelength. Otherwise, the line will support a higher order transverse electric transmission line mode.

As shown in FIGURE 2A, a T attenutator made according to my invention is embodied in 'a strip transmission line unit having an inner conductor generally indicated at 17. The conductor 17 includes, in sequence, a terminal section 1-8, a series resistance section 119, a bridging section 20, a shunt resistance section 21, a series resistance section 22 and a terminal section 23. The terminal sections 18 and 23 and bridging section 20 are of essentially non-resistive metallic foil, while the resistance sections 19, 21 and 22. are of conventional resistance materials suit-able for use at high frequencies.

Referring now to FIGURE 3, the T attenutator includes metallic housing members 24 and 2s disposed one above the other, with sliding contact between their opposing surfaces 24a and 26a. The housing members are formed with recesses 28 and 3%, whose surfaces 28a and 30a are the ground plane conductors for the strip transmission line inner conductor 17. The sliding contact between surfaces 24a and 26a performs the same function as the shorting pins 16 of FIGURE 1.

A pair of dielectric sheets 32 and 34, disposed in the recesses 28 and 30, have closely spaced opposing surfaces 32a and 34a. As shown in FIGURES 2A and 3, the terminal sections 18 and 23 and the shunt resistance section 21 of the inner conductor 17 are bonded to the surface 34a of dielectric sheet. 34. The series resistance sections 19 and 22 and the bridging section 20 are bonded to the surface 32a. The inner conductor sections bonded to the dielectric sheet 32 are in sliding contact with the sections bonded to the sheet 34. That is, the resistance sections 19 and 22 contact the terminal sections 18 and 23, respectively, while the bridging section 2! contacts the resistance section 21.

As shown in FIGURE 3, the shunt resistance section 21 is connected to the ground plane conductors by conductor 36. A coaxial connector, generally indicated at 42, facilities external connections to the terminal section 18. The connector d2 comprises an inner conductor 44- connected to the section 18 and coaxial outer conductor 46 mounted directly on the housing member 26. A similar connector (not shown) has an inner conductor connected to the terminal section 23. A knob 47, attached to the housing member 24, facilities movement of the latter along the housing member 26 to effectuate adjustment of the attenuator.

More specifically, the various sections of the inner conductor 17 of FIGURES 2A are schematically arranged as shown in FIGURE 23, resistors 19, 21 and 22' in the latter figure forming a conventional T attenuator. When the member 24 is moved to the right (FIGURE 3), the series resistance sections 19 and 22 of FIGURE 4 are effectively lengthened and the shunt resistance section 21 is effectively shortened. Thus, the resistances of the resistors w and 22' of FIGURES 2B are increased and the resistance of the resistor 21' is decreased. In a wellknown manner, this results in an increased attenuation (decreased amplitude) of signals passed through the unit but at an essentially constant characteristic impedance. Conversely, movement of the housing member 24 to the left decreases attenuation.

Referring to FIGURE 2A, the inner conductor terminal sections 18 and 23 are preferably colinear with the series resistance sections 19 and 22, respectively. Furthermore, the ends of the sections 18 and I9, and 22 and 23 are all preferably tapered in the region of overlap. This tends to minimize abrupt changes in characteristic impedance and thereby keep reflections at a low level. These tapers, along with a taper of the shunt resistance section 21, are such as to provide simultaneous changes in the resistances of the various resistance sections which maintain the input and output impedances of the attenuator constant over the range of adjustment. To minimize reflections introduced by shorting the shunt resistance section 21 to the ground plane conductors, the distance from the conductor 36 to the bridging section 20 should be substantially less than a quarter wavelength over the range of attenuation adjustment. In any case, reflected signals will largely diminish in the several resistance sections.

Referring to FIGURE 3, a spring 48 is disposed between the surface 28a and the dielectric sheet 32 to resiliently urge the inner conductor sections into more reliable contact while maintaining contact between the housing. members 24 and 26.

It is desirable that the spring 48 not introduce variations in the electrical spacing between inner conductor 17 and the ground plane conductor in the recess 28. Therefore, the spring is preferably made of a dielectric material to avoid such variations. Alternatively, a sheet of conductive material (not shown) may be interposed between the spring and the dielectric sheet 32 to serve as a ground plane conductor.

As shown in FIGURE 4, another T attenuator embodying my invention has an inner conductor, generally indicated at 49, comprising a terminal section 50, a series resistance section 52, a bridging section 54, a shunt resistance section 56, a bridging section 58, a series resistance section on and a terminal section 62. Inner conductor i9 is associated with ground plane conductors (not shown) in a structure similar to that of FIGURE 3. The resistive sections 52, 56 and 6t are bonded to one of the dielectric sheets, and the conductor sections are bonded to the other one.

The bridging section 54 and 5% of FIGURE 4 are preferably formed as one continuous conducting strip, indicated generally at d4, disposed across all three of the resistive sections and in sliding contact therewith. The shunt resistance section 56 is connected to the ground plane conductors by a con-ductor 63 similar to the conductor 36 of FIGURE 3.

The operation of the variable attenuator of FIGURE 4 will readily be understood by assuming movement .of the conductor strip 64 to the right along the several resistance sections. The series resistance sections 52 and 60 are thereby lengthened, and the shunt resistance section 56 is shortened, whereby the attenuation between the terminal sections is increased. Leftward movement of the strip .64 decreases attenuation. It should be noted that, in the attenuator of FIGURE 4, all the resistive inner conductor sections are bonded to one dielectric and all the. nominally non=resistive sections are bonded to the other dielectric sheet, thus making the attenuator somewhat easier to fabricate, in some cases, than the attenuator of FIGURES 2A and 3.

A pi attenuator embodying my invention is illustrated in FIGURES 5A and 6. As shown therein, an inner conductor indicated at 65 comprises a terminal section 66, a shunt resistance section 68, a series resistance section 70, a shunt resistance section '72 and a terminal section 74.

Referring to FIGURE 6, the pi attenuator is similar in construction to the T attenuators described above, with conducting housing members 76 and 78 disposed one above the other and formed with recesses 77 and 79. Parallel surfaces 77a and 7% are ground plane conductors forming a strip transmission line unit in combination with the inner conductor 65. A spring 80, attached to the lower horizontal surface 76a of housing member 76, is in sliding contact with the opposing surface 73a.

The resistance sections 63, 70 and 72 are preferably formed in a continuous resistance strip, generally indicated at 82, bonded to a dielectric sheet 84 disposed in the recess 77. The terminal sections 66 and 74 are bond ed to a dielectric sheet 86 disposed in therecess 79. They are angled toward each other, as shown in the drawing, and are in sliding contact with the resistance strip 82, intersecting the strip to divide it into the two shunt resistance sections 68 and 72 and the series resistance section 76. The ends 68a and 72a of the shunt resistance sections are connected to the ground plane conductors by conductors 88 and 90, respectively. The spring 80 on the housing member 76 provides a resilient contact between the housing members 76 and 73 and thus insures contact between the inner conductor sections as well as between the ground plane conductors.

Still referring to FIGURE 6, a connector indicated generally at 96 comprises an inner conductor 8 connected to the terminal section 66 and an outer conductor 100 connected to housing member 78. An identical connector, not seen in the drawings, has an inner conductor connected to the terminal section 74. A knob 102. is attached to housing member 76 to facilitate movement of the member 76 along housing member 78 for adjustment of the pi attenuator.

The operation of the pi attenuator may be more readily understood with reference to FIGURE 5B, which schematically indicates the arrangement of the several sections constituting inner conductor 65 (FIGURE 5A). Resistors 68', 70' and 72' form a conventional pi network. When housing member 76 is slidably moved to the left along the member 7 8, the resistance strip 82 move-s along the terminal sections 66 and 74, thereby shortening the shunt resistance sections 68 and 72 and decreasing the resistances of resistors 68 and 72. The series resistance section 70 is lengthened, increasing the resistance of resistor 70. Accordingly, the attenuation between the terminal sections is increased.

Referring now to FIGURE 5A, the terminal sections 66 and 74 and the resistance strip 82 are preferably shaped and arranged to maintain the input and output impedanccs constant as the attenuation is varied, To minimize the effect of the discontinuity where the shunt resistances sections are connected to the ground plane conductors, the electrical length of the shunt resistance sections 68 and 72 should be less than onequarter wavelength over the range of attenuation adjustment.

In summary, we have described a novel high frequency variable attenuator embodied in strip transmission line. The attenuator may take the form of a T or pi network in which each branch comprises an inner conductor section of a resistive material in sliding contact with a nominally non-resistive section. The attenuation is varied by changing the effective lengths of the resistive sections by sliding the resistive sections along the non-resistive sections. Attenuators thus formed are substantially less frequency sensitive than prior high frequency variable attenuators. At the same time, they are small and relatively simple in design. With the constructions disclosed above, the several resistances may be varied in synchronism and the series resistances changed in inverse relationship to the shunt resistances. In this manner, the input and output impedances of the attenuator can readily be maintained substantially constant as the attenuation is changed.

It will thus be seen that the objects set forth above, among those'apparent from the preceding description, are efficiently attained, and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, 'as a matter of language, might be said to fall therebetween.

What is claimed is:

1. A strip transmission line attenuator comprising, in combination, a transmission line outer conductor, a transmission line inner conductor having a first adjustable resistance section, a second adjustable resistance section, means connecting one end of said second section to said outer conductor, a movable section in sliding contact with said first and second sections, and means for moving said movable section in such manner as to affect the effective length of said first section inversely with respect to the distance between said movable section and said end of said second section.

2. A transmission line attenuator comprising, in combination, an outer conductor, an inner conductor, a first resistive element in series in said line, a second resistive element in parallel with said line, said resistive elements each comprising a resistive inner conductor portion in sliding contact with a substantially non-resistive portion, a movable member contacting one said portion of each of said resistive elements and means for moving said member to slide one of said portions of each of said elements along the other portion thereof and thereby change the effective lengths of said resistive portions in inverse relationship.

3. The combination defined in claim 2 including a third resistive element in series in said line, said second resistive element being disposed between said first and third resistive elements, said third resistive element comprising a resistive inner conductor portion in sliding contact with a substantially non-resistive portion, said movable member contacting one said portion of said third element, said third element being so arranged that movement of said member varies the elfective length of said resistive portion of said third element inversely to the change in length of said resistive portion of said second element.

4. The combination defined in claim 2 including a third resistive element in parallel with said line, said first resistive element being disposed between said second and third resistive elements, said third resistive element comprising a resistive inner conductor portion in sliding contact with a substantially non-resistive portion, said movable member contacting one said portion of said third element, said third element being so arranged that movement of said member varies the efiective length of said resistive portion of said third element inversely to the change in length of said resistive portion of said first element.

5. A strip transmission line attenuator comprising, in combination, a first ground plane conductor, an inner conductor disposed substantially parallel to said outer conductor, dielectric means insulating said inner conductor from said outer conductor, said inner conductor including a first series element and a first parallel element, each of said elements comprising a resistive portion in sliding contact with a substantially non-resistive portion, one end of said parallel element being connected to said ground plane conductor, a first member connected to one portion of each element, a second member connected to the other portion of each element, said members being movable with respect to each other to slide one portion of each element along the other portion thereof thereby to vary inversely the effective lengths of said resistive portions.

6. The combination defined in claim including a second ground plane outer conductor disposed symmetrically with said first ground plane conductor about said inner conductor, one of said ground plane conductors being structurally common with said first member and the other ground plane conductor being structurally common with said second member.

7. The attenuator defined in claim 5 including a second series element comprising a resistive portion in sliding contact with a substantially non-resistive portion, said first parallel element being. disposed between said first and second series elements, sm'd first member connected to one portion of said second series element, said second member being connected to the other portion of said second series element, and said second series element being arranged so that relative movement of said members varies the length of said resistive portion of said second series element directly with said resistive portion of said first series element.

8. The attenuator defined in claim 5 including a second parallel element comprising a resistive portion in sliding contact with a substantially non-resistive portion, said first series element being disposed between said first and second parallel elements, said first member connected to one portion of said second parallel element, said second member being connected to the other portion of said second parallel element, said second parallel element being arranged so that relative movement of said members varies the length of said resistive portion of said second parallel element directly with said resistive portion of said first parallel element.

9. A strip transmission line attenuator comprising, in combination, first and second electrically conducting housing members disposed one above the other and in sliding contact with each other along first surfaces thereof, said housing members having second surfaces forming strip transmission line ground plane conductors, a strip transmission line inner conductor disposed between said ground plane conductors, dielectric means insulating said inner conductor from said ground plane conductors, first and second adjustable resistive components in series in said inner conductor, an adjustable shunt resistive component having one end connected to said first and second series resistive components and the other end connected to said ground plane conductors, said adjustable resistive components each comprising a resistive conductor in sliding contact with a substantially non-resistive conductor, a member structurally connected to one said conductor in each of said resistive components, means for moving said member to vary the lengths of said series resistive components inversely With respect to the variation of the length of said shunt resistive component by sliding said resistive conductor of each said component along said non-resistive conductor thereof.

ll). A strip transmission line attenuator comprising, in combination, first and second electrically conducting housing members disposed one above the other insliding contact with each other along first surfaces thereof, said housing members having second surfaces forming strip transmission line ground plane conductors, a strip transmission line inner conductor disposed between said ground plane conductors, dielectric means insulating said inner conductor from said ground plane conductors, said inner conductor including first and second adjustable resistive components connected to said outer conductor, a third adjustable resistive component in series in said inner conductor between said first and second resistive components, said adjustable resistive components each comprising a resistive conductor in sliding contact with a substantially non-resistive conductor, a member structurally connected to one said conductor in each of said resistive components, means for moving said member to vary the length of said third resistive component inversely with respect to the variation of the lengths of said first and second resistive components by sliding said resistive conductor of each said component along said non-resistive conductor thereof.

ll. A strip transmission line attenuator comprising, in combination, a housing including first and second electrically conducting housing members disposed one above the other in sliding contact with each other along first surfaces thereof, each of said housing members having a recess in said first surface thereof, first and second dielectric members disposed in said respective recesses, said dielectric members having first and second opposing surfaces respectively, a strip transmission line inner conductor comprising in series sequence a first terminal section, a first series resistance section, a bridging section, a second series resistances section, and a second terminal section, a shunt resistance section connected between said bridging section and said ground plane conductors, said first and second terminal sections and said shunt resistance section being bonded to said first opposing dielectric surface, said first and second series resistance sections and said bridging section bonded to said second opposing dielectric surface, said first terminal section and said first series resistance section being in sliding contact, said second terminal section and said second series resistance section being in sliding contact, said shunt resistance section being in sliding contact with said bridging section, and means for moving said first housing members along said second housing member whereby the efiective length of said series resistance sections is changed in synchronism with and inversely from the effective length of said shunt resistance section.

12. A strip transmission line attenuator comprising, in combination, a housing including first and second electrically conducting housing members disposed one above the other in sliding contact with each other along first surfaces thereof, each of said housing members having a recess in said first surface thereof, first and second dielectric members disposed in said respective recesses, said dielectric members having first and second opposing'surfaces respectively, a strip transmission line inner conductor comprising first and second non-parallel terminal sections bonded to said first opposing dielectric surface, a resistive strip bonded to said second opposing dielectric surface and having each end thereof connected to the ground plane conductors, said resistive strip extending across said terminal sections and in sliding contact therewith, and means for sliding said first housing member along said second housing member, whereby said resistive strip is moved transversely along said terminal sections thereby changing the length of the portion of said strip intermediate said terminal sections inversely from the lengths of the portions of said strip between said terminal sections and the ends of said strips.

References Cited in the file of this patent UNITED STATES PATENTS 2,797,390 Kostriza June 25, 1957

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