Mode Suppressor For Strip Transmission Lines

Abstract

If strip transmission lines are enclosed in a conducting shield, the shield acts as a waveguide and spurious waveguide modes, which interfere with stripline transmission, are excited. A waveguide mode suppressing structure is disclosed which selectively suppresses the waveguide modes over a certain frequency range and also physically provides support for a dielectric substrate. The structure has at least one groove, having an approximate electrical depth one-quarter of the wavelength of the stripline operating frequency, positioned in the shield side wall. This structure provides suppression without restricting cross sectional dimensions of the channel, thereby allowing more circuits on a substrate and greater freedom in strip transmission line circuit design than in the prior art. Additionally, at least one resistive thin film may be deposited on the dielectric substrate in the vicinity of the groove to increase suppression capability.

Description

BACKGROUND OF THE INVENTION

This invention relates to strip transmission lines including stripline and microstrip lines and more specifically to mode suppression techniques for such lines. These lines are used for building passive networks and for interconnecting active devices in hybrid integrated circuits. As used herein, strip transmission lines are planar structures containing two parallel conductors; one conductor is called a ground plane and the other is called a conductor strip. A number of conductor strips may be used with a single ground plane to produce a plurality of circuits. A stripline is a strip transmission line in which the ground plane and dielectric substrate which supports the conductor strip are separated from each other by a material whose dielectric constant is less than that of the substrate. A microstrip line is a strip transmission line in which the ground plane and strip conductor are separated from each other only by the solid dielectric material upon which the strip conductor is mounted. Strip transmission lines are often shielded by a conducting channel which suppresses radiation from the transmission lines and reduces coupling between circuits. However, the surrounding channel and the ground plane form a waveguide-like structure and undesired waveguide modes may be excited above a certain cutoff frequency. These spurious waveguide modes can be suppressed in accordance with prior art techniques by dimensioning the channel width and height to be less than one-half of the electrical wavelength at the operating frequency of the strip transmission lines. However, this limitation on channel size imposes restrictions on the number of strip conductors which may be deposited on a given substrate. In addition, undesired waveguide modes excite spurious resonances which may also limit the performance of nonlinear devices associated with the strip transmission line. It is therefore desirable in order to minimize circuit losses to use large cross-section channels and to provide suitable mode suppression without restricting the channel dimensions.

SUMMARY OF THE INVENTION

The invention is directed in part to modifying the shielding structure which encloses the transmission lines by placing at least one groove of approximate electrical depth one-quarter .lambda., where .lambda. is the wavelength at the operating frequency of the transmission line, in the side wall of the channel. This groove runs along the entire length of the channel. It may also support the dielectric substrate to which the conductor strip is affixed.

The waveguide mode suppression technique disclosed is suitable for building compact, economical, solid-state sources and frequency converters in the microwave and millimeter-wave frequency range. The selective suppression of undesired waveguide modes permits substantial increase of channel dimensions thereby allowing great freedom in microstrip circuit design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section view of a shielded stripline structure found in the prior art;

FIG. 2 is a cross-section view of a stripline structure illustrating side wall grooves and resistive thin films in accordance with the invention;

FIG. 3 is a cross-section view of a microstrip structure in accordance with the invention; and

FIG. 4 is a cross-section view of a stripline structure with vertically oriented grooves in accordance with the invention.

DETAILED DESCRIPTION

FIG. 1 shows a cross-section view of shielded stripline structure known in the prior art. A rectangular conducting shield 100 encloses a dielectric substrate 101, a conductor strip 102 which is affixed to the substrate 101 and a ground plane 105. The substrate 101 is held in place by supports 103 and 104 in opposite side walls of shield 100. These supports perform no electromagnetic function and only provide physical support for the dielectric substrate. In the prior art, waveguide mode suppression is accomplished by restricting both the height h and width w of shield 100 to less than half an electrical wavelength at the operating frequency of the stripline.

In accordance with the invention, the shielding structure which encloses the transmission lines is modified by placing at least one groove of approximate electrical depth one-quarter .lambda., where .lambda. is the wavelength at the operating frequency of the transmission line in the side wall of the channel. This groove runs along the length of the channel and may support the dielectric substrate to which the conductor strip is affixed.

When a plurality of circuits are enclosed, each having different operating frequencies, a corresponding plurality of grooves or tapered grooves of appropriate depths may be used. For devices such as transmitter pump oscillators, fixed frequency oscillators, reciever local oscillators, path length modulators, the operating frequency is a single fixed frequency. For devices such as stable amplifiers or injection-locked amplifiers, where the circuit operates over a wide band of frequencies, the approximate center of the band is taken as the operating frequency of the circuit.

The groove stores electromagnetic energy. Electromagnetic fields are excited inside the groove and at a certain frequency, couple in accordance with Maxwell's equations, with fields outside the groove, causing the groove to present a high impedance to surface currents in the shield side walls. These currents and their undesired waveguide modes are thereby suppressed.

Suppression is directed primarily to the fundamental mode or the longitudinal section magnetic (LSM) mode. The fundamental mode is the lowest order mode which propagates in the channel in the absence of a dielectric other than air between the conductor strip and the ground plane. If a dielectric other than air is present, the lowest order mode propagated is called the longitudinal section magnetic (LSM) mode. The higher the operating frequency, the greater the number of possible waveguide modes which will be above their cutoff frequencies and which therefore may propagate, but for the range of frequencies in which communication systems work, it is usually necessary only to suppress either the fundamental or the LSM mode. Where the fundamental waveguide mode or the LSM mode is suppressed over a limited stop band, spurious resonances due to coupling of either of these modes to external circuits are also suppressed.

The groove acts as a resonator or impedance transformer and a thin film resistance can be used in conjunction with the resonator to increase the bandwidth of the device by increasing losses for undesired modes. The width of the stop band can also be increased by varying the depth of the groove along the channel length or by using a plurality of grooves each of a different uniform depth located at various places in the shield side walls.

Selective suppression of vertical currents occurs because the thin film affects the LSM or fundamental mode but does not affect strip transmission line modes since electromagnetic fields associated with these latter modes are between the conductor strip and ground plane and current associated with these modes is always perpendicular to the corresponding electromagnetic field. Accordingly, these latter modes have only longitudinal currents. The resistive thin film is therefore not lossy for the strip transmission line mode since this mode has a field pattern which is different than that of waveguide modes.

In FIG. 2 a conducting shield 200 comprises two side members 210 and 211, having grooves 201 and 202, a ground plane member 212 and a top member 213 parallel to the ground plane. The shield 200 encloses a dielectric substrate 203, the ground plane 212 and conductor strips 204 and 205 affixed to the dielectric substrate. The shield confines most of the electromagnetic energy to channel space 206 located between the dielectric and ground plane to prevent energy loss and to reduce interference and coupling with other circuits. Some electromagnetic energy is confined by the shield in channel space 207 located between the dielectric and top member 213. In addition, the shield protects enclosed circuits from external atmospheric influences such as humidity which causes corrosion and gives mechanical protection to circuit elements. If the shield is composed of insulating material coated with metal or is formed from an alloy which has a low expansion coefficient, dimensional stability is provided and this will make the enclosed circuits operate independently of the external ambient temperature. The metal coat provides the energy shielding property described above. To avoid leakage, the shield must be at least thicker than three skin depths (several micrometers) where one skin depth is the approximate depth of penetration of electromagnetic energy. Otherwise, the shield thickness is determined to give mechanical and structural strength to the shield structure. The shield 200 is conveniently manufactured in two separate pieces which may be bolted together at seams such as 215 and 216.

If the dielectric substrate extends into the grooves 201 and 202, the substrate dielectrically loads the grooves thereby producing an electrical depth of .lambda..sub.O /4.sqroot..epsilon..sub.R where .epsilon..sub.R is the relative dielectric constant and .lambda..sub.O is the free space wavelength. The dielectric confines and holds electromagnetic energy to a region within the channel spaces 206 which is normally between the ground plane and the conductor strip pattern.

Resistive structures which may be thin films or lossy dielectric material are used to enhance suppression capability. Resistive thin films 208 and 209 may be deposited on the substrate 203 and positioned in the vicinity of the groove 201 and 202. A portion of the thin film lies between the substrate and an edge of the groove. The thin film must extend from the groove beyond the shield side member into the channel space 206 but precise placement of the film is not critical. Typically the resistive thin films 208 or 209 may be made of metal material; they may be deposited by conventional techniques such as evaporation, sputtering or thick film processing. Alternatively, the resistive structure may be formed by doping a portion of the dielectric 203 with an ingredient such as boron or niobium which makes the dielectric lossy. If this dielectric is lossy in the vicinity of the opening of the groove into the channel space, it will function similarly to resistor 208 or 209 which it replaces. Typically, the lossy ingredient can be incorporated into the dielectric by diffusion or ion implantation.

FIG. 3 shows another embodiment of invention in which the grooves in the side members are placed adjacent the ground plane. The embodiment of FIG. 3 is otherwise similar to that of FIG. 2, and like functioning elements are identified by numbers having identical last two digits.

FIG. 4 shows a strip transmission line structure which operates similar to the structure of FIG. 2 and electrically like functioning elements are identified by numbers having identical last two digits. The structure has a shield 400 which consists of conducting structures 420 and 421 separated by dielectric 403. Grooves 401 and 402 extend vertically from dielectric 403 into the side walls of conductor 420. Their depths are chosen so that they behave as an impedance transformer with respect to undesired vertical currents in the channel walls and appear to these currents as an open circuit. In FIG. 4, the electrical depth of the vertical grooves 401 or 402 is one-quarter .lambda. and the grooves are located in the center of their respective side walls, which each extend an electrical distance .lambda. from the channel to the exterior boundary. The resulting distance of one-half .lambda. between the exterior boundary of the side wall and the center of the vertical groove establishes a short circuit. Alternatively, the electrical depth of the vertical groove is one-half .lambda. and the electrical distance from either side wall to the center of its associated vertical groove is one-quarter .lambda. . The dielectric 403 insulates the two parts of the shield from one another. This physical arrangement makes fabrication and assembly of the entire shielded strip line structure easy and inexpensive.

Conductor strips 404 and 405 are supported by the dielectric material. Two strips are shown for illustration only and it is understood that in FIG. 4, as in FIGS. 2 and 3, any plurality of conductors may be used. This is in contrast to the prior art where the limitation of the shielding channel dimension restricted the number of conductor strips which could be deposited. Resistive thin films 408 and 409 which are positioned between dielectric 403 and one of the shield conducting structures 420 and extend into channel 407, act to enhance suppression. It is understood that alternatively the dielectric may be made lossy to perform the same enhancement function in this or any other embodiment of the invention.

In all cases it is to be understood that the above described arrangements are merely illustrative of a small number of the many possible applications of the principles of the invention. Numerous and varied other arrangements in accordance with these principles may readily be devised by those skilled in the art without departing from the spirit and scope of the invention.

Claims

1. A transmission device comprising:

a ground plane member;

a dielectric substrate;

a conductor pattern supported by the dielectric;

a shield having two parallel side members, a top member and the ground plane member parallel to the top member, the shield being positioned to enclose the dielectric and the conductor pattern within a channel space;

means for suppressing modes associated with vertical currents, said means including a groove in at least one side member of the shield, said groove being dimensioned and positioned to appear as an open circuit to the vertical currents;

a resistive thin film being positioned between the dielectric substrate and shield and extending partially into the groove and partially into the

2. A device as described in claim 1 wherein the electrical depth of the

3. A transmission device comprising:

a ground plane member;

a dielectric substrate;

a conductor pattern supported by the dielectric;

a shield having two parallel side members, a top member and the ground plane member parallel to the top member, the shield being positioned to enclose the dielectric and the conductor pattern within a channel space;

means for suppressing modes associated with vertical currents, said means including a groove in at least one side member of the shield, said groove being dimensioned and positioned to appear as an open circuit to the vertical currents;

the shape of the groove being rectangular and the electrical depth of the groove being approximately one-quarter .lambda. where .lambda. is the

4. A device as described in claim 3 wherein the portion of the dielectric

5. A device as described in claim 3 wherein a resistive thin film is positioned between the dielectric substrate and shield and extends

6. A transmission device comprising:

a ground plane member;

a dielectric substrate;

a conductor pattern supported by the dielectric;

a shield having two parallel side members, a top member and the ground plane member parallel to the top member, the shield being positioned to enclose the dielectric and the conductor pattern within a channel space;

means for suppressing modes associated with vertical currents, said means including a groove in at least one side member of the shield, said groove being dimensioned and positioned to appear as an open circuit to the vertical currents;

the groove of approximate electrical depth one-quarter .lambda. being positioned vertically in the side member and said groove opening onto the dielectric substrate, said center of the opening of the groove being an electrical distance one-half .lambda., where .lambda. is the operating frequency of the transmission device, from the edge of the side member bounding the channel space and an electrical distance one-half .lambda.

7. A transmission device comprising:

a ground plane member;

a dielectric substrate;

a conductor pattern supported by the dielectric;

a shield having two parallel side members, a top member and the ground plane member parallel to the top member, the shield being positioned to enclose the dielectric and the conductor pattern within a channel space;

means for suppressing modes associated with vertical currents, said means including a groove in at least one side member of the shield, said groove being dimensioned and positioned to appear as an open circuit to the vertical currents;

the groove of approximate electrical depth one-half .lambda. being positioned vertically in the side member and said groove opening onto the dielectric substrate, the center of said opening of the groove being an electrical distance one-quarter .lambda., where .lambda. is the operating frequency of the transmission device, from the edge of the side member bounding the channel space and an electrical distance one-quarter .lambda. from the exterior edge of the side member.