GB2229322A

GB2229322A - Strip lines

Info

Publication number: GB2229322A
Application number: GB8927412A
Authority: GB
Inventors: David Bowman
Original assignee: Quantel Ltd
Current assignee: Quantel Ltd
Priority date: 1988-12-03
Filing date: 1989-12-04
Publication date: 1990-09-19
Anticipated expiration: 2009-12-04
Also published as: GB2229322B; GB8828281D0; GB8927412D0

Abstract

A strip-line in which the characteristic impedance Zo is increased by providing discontinuities in one, and preferably both, of the ground planes (16', 17'). The invention allows thin boards to be produced, using conventional glass fibre dielectrics (18'), having higher characteristic impedances. The strip-line has particular application in multi-layer boards for video signals requiring a characteristic impedance of 75 ohms. The discontinuities can be in the form of slots 30 or the conductive plane (40, Fig. 3) can be etched to form a mesh like structure (41). <IMAGE>

Description

STRIP LINES FIELD OF THE INVENTION The present invention relates to strip-lines.

BACKGROUND OF THE INVENTION A strip-line comprising a ribbon of conducting material positioned between two conductive planes'and an example of such a strip-line is disclosed in the MECL system design handbook written by William R.

Blood, Jr. and published by Motorola Semiconductor Products Inc, of the United States of America. This book also describes how the physical proportions of a strip-line may be modified to give desired electrical properties, such as for example particular values to the characteristic impedance Z0 or the propagation delay of the strip-line.

In this type of strip-line, the ribbon is supported centrally between the conductive planes by way of a suitable insulating material which also provides structural strength to the assembly. A commonly used material for the non-conducting portions of the strip-line assembly is a fibreglass/epoxy composite. This insulating material may also be used to provide over a conductive plane an additional layer upon which additional conducting surface tracks may be placed in order to make up a multi-layer circuit board or card incorporating a strip-line. In such a card pins can be placed in position through holes in the assembly to connect the ribbon and for the conductive planes to the surface tracks. The conductive planes may be used to carry power to active components connected to the surface tracks.

A multi-layer circuit board or card incorporating a strip-line is not an inexpensive assembly to produce and therefore in an electronic device the number of cards configured in this way should be minimised, with standard cards being used whenever possible in order to minimise costs. Thus, strip-lines tend only to be used when for example the characteristic impedance ZO (or another critical property) is crucial to the satisfactory operation of the circuit and are usually used to provide a transmission path which is compatible with external devices. When using a system including conventional cards and racks etc, it is a further requirement that any cards which include a strip-line should have physical dimensions similar to those of other cards so that the strip-line card will be compatible with the rack.If this further requirement is not met, then purpose built hardware will be required to support the strip-line cards, and this can be expensive.

In video processing systems, analogue video signals are transmitted (by modulating an RF carrier) over cables with a characteristic impedance of, typically, 75 ohms. In these systems, cards which incorporate strip-lines have hitherto typically been constructed using a glass fibre composite having a dielectric constant (Er) of 4.7 and using a ribbon which is typically made of copper 0.038mm (0.0015 inches) thick and 0.254mm (0.01 inches) wide. As will be explained in greater detail hereinafter, these values can be used to calculate the thickness of the strip, i.e. the distance between the conductive planes, required to give the desired characteristic impedance. However, hitherto it has not been possible to produce cards having a thickness which is less than or equal to the maximum suitable for conventional racks without using expensive specialist material.It has therefore been necessary to redesign racks, so as to take an oversize board, or to use expensive materials in order to overcome the problem of obtaining the required characteristic impedance on a board of standard thickness. In practice, an ideal board would be in the form of a strip-line providing a characteristic impedance of 75 ohms while having a thickness of 1.6mm (0.062 inches), which is the standard thickness, and using cheap insulating materials such as glass fibre.

It is an object of the present invention to provide such an improved strip-line.

SUMMARY OF THE INVENTION According to the present invention there is provided a strip-line comprising a conducting ribbon supported between two conductive planes by a dielectric material characterised in that at least one of said conductive planes is not continuous over a region in the vicinity of said ribbon.

By careful choice of the dimensions of the stripline, the characteristic impedance can be selected to be greater than 50 ohms, and typically can be selected to be greater than 75 ohms. Furthermore, this may be achieved whilst keeping the distance between the conductive planes to less than one tenth of an inch.

In one embodiment of the invnetion, the noncontinuous region includes a plurality of conductive slots formed in the at least one conductive plane.

The slots may be substantially rectangular and the longest edge of the rectangle may run substantially perpendicular to the length of the ribbon.

In another embodiment of the invention, the noncontinuous region is formed as a mesh in the at least one conductive plane. The mesh may be defined by substantially square holes etched in the plane.

The invention also extends to a multi-layer circuit board incorporating such a strip-line.

The above and further features and advantages of the invention will become clearer from consideration of the following detailed description of exemplary embodiments of the invention which are given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 shows a cross sectional view of a multilayer circuit board including a strip-line; Figure 2 shows a sectional isometric view of a strip-line with slotted conductive planes; and, Figure 3 shows an alternative type of conductive plane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A multi-layer circuit board 10 is shown in cross section in Figure 1 of the accompanying drawings. The circuit board 10 includes a strip-line formed by a copper ribbon 15 supported between conductive planes 16, which may be ground planes, and 17 by an insulating layer 18. Additional insulating layers 19, 20 are also adhered to respective outer surfaces of the ground planes 16, 17 and respective copper plates 21,22 are provided on the insulating layers 19,20 to enable conducting tracks to be formed by for example etching. Circuit elements (not shown) may be mounted to the tracks formed from the copper plates 20,21.

Electrical connections can be formed between the tracks 20,21, the ribbon 15, and the planes 16,17 by way of pins (not shown) connecting through plated holes (also not shown) in the insulating layers 18,19,20. The planes 16,17 can be used as power supply rails for active components mounted to the tracks formed by the copper plates 21,22 without affecting the electrical characteristics of the stripline.

It can be shown that the equation for calculating the characteristic impedance (ZO) of the strip-line shown in Figure 1 is as follows, namely

where b = the distance between the conductive planes W = the width of the ribbon t = the thickness of the ribbon and other symbols take their usually accepted meaning.

This, it will be appreciated by those skilled in the art that the characteristic impedance Z0 of a strip-line is related to the capacitance between the conductive planes and the ribbon.

The characteristic impedance (Z,) of the stripline should, for video applications, be of a value of 75 ohms. Of course it will be appreciated that in other applications a higher impedance may be required such as for example the 120 ohms usually used in the transmission of digital signals. However, a stripline made using conventional materials tends to have a characteristic impedance which is too small for video and other applications which require a characteristic impedance of typically 75 ohms or greater. From the above equation it will be appreciated that the characteristic impedance ZO can be increased by increasing the value of b, i.e. by increasing the distance between the planes 16,17, thereby reducing the capacitance between the planes and the ribbon.

However, standard rack mountings are arranged to accept board 1.6mm (0.062 inches) thick and if the thickness of the board is increased significantly beyond this, by increasing the distance b in order to increase the characteristic impedance Zo, the board will not be usable in a standard rack.

Another approach to increasing the characteristic impedance of a strip-line is to use an insulating material with a lower dielectric constant. Thus, instead of glass fibre, for which typically Er = 4.7, polytetrafluoroethylene (PTFE) with epoxy may be used.

This material has a dielectric constant Er of just above unity but bonding is more difficult using this material and the overall construction process therefore becomes much more expensive.

Another possible method for increasing characteristic impedance is to decrease the width W or thickness t of the ribbon in addition to, or instead of, increasing the distance b. Typically, the ribbon 15 is formed from 305g/m2 (loz/ftt) copper sheet providing a thickness t of 0.038mm (0.0015 inches) and it is difficult to reduce this further. Furthermore, using a ribbon with a width W significantly less than 0.25mm (0.01 inches) results in problems with the reliability etc of the strip-line. Accordingly, with known strip-lines, the only available options for increasing the characteristic impedance Z0 have hitherto been: (1) to increase the thickness of the insulating material 18, or (2) to use an insulating material, such as PTFE, with a lower dielectric constant.

An isometric view of a strip-line 10' embodying the present invention is shown in Figure 2 of the accompanying drawings. It will be noted that the strip-line 10' is shown for the sake of clarity without the outer layers equivalent to layers 19, 20, 21 and 22 shown in Figure 1 which make up a circuit board in which the strip-line may be incorporated.

Thus, the strip-line in Figure 2 comprises a conducting ribbon 15', of for example copper, supported by conductive planes 16',17' by an insulating layer 18'. The insulating layer 18' can be made from glass fibre which makes the construction of circuit boards relatively cheap. It should be noted that the planes 16'17' are not continuous as they were in the board 10 shown in Figure 1, but instead have a series of slots 30 at regions overlying the ribbon 15.

It will also be appreciated that since, generally speaking, capacitance is also related to the area of plates forming the capacitor, the effect of the slots 30 is to reduce the effective area of the planes. In this way, the characteristic impedance of the stripline is increased by reducing the capacitance between the planes 16',17', by effectively reducing the area of the capacitive plates that the planes form. This approach of providing slots 30 in one or each of the planes 16',17' enables a thin strip-line to be constructed, using a glass fibre insulator, while providing for characteristic impedances significantly above 50 ohm and typically 75 ohms or greater.

Each slot 30 has a width sw and a breadth sb, and is spaced from adjacent slots by a distance ss. These dimensions can be varied to achieve the require characteristic impedance zO. Furthermore, the stripline 10' can be incorporated into a multi-layer board and by varying the dimensions sw, sb and ss a multilayer board having the required characteristic impedance can be produced. For example, in order to produce a multi-layer board, such as that shown in Figure 1, and having a thickness of 1.6mm (0.062 inches) and a characteristic impedance of 75 ohms, a ribbon width w of 0.25mm (0.01 inches) can be used together with a slot width SW of 3.8mm(0.15 inches), a slot breadth sb of 0.64mm (0.025 inches) for each slot and a slot spacing of the same order so that the conductiv eplanes 16',17' cover approximately 50% of the ribbon area.Of course, it will be appreciated that these slot parameters may be varied to obtain the required characteristic impedance or to tune a circuit incorporating the strip-line for operation within a particular bandwidth. The above example is designed for video applications and therefore operates over a bandwidth up to twenty mega-hertz. For operation at higher frequencies the dimension sb should be smaller.

However, devices operating at these higher frequencies are less likely to require a high characteristic impedance and it may only be necessary to form slots in one of the conductive planes 16',17'.

An alternative embodiment of conductive plane is shown in Figure 3 of the accompanying drawings. In this embodiment, the conductive plane 40 is etched to create a mesh-like structure 41. The mesh covers substantially all of the plane and not just the areas above the strip-line. The direction of the strip-line is indicated by; arrow 43. In this embodiment, typically the pitch of the mesh is 0.38mm (0.015 inches) and the thickness of the conductor is 1.25mm (0.05 inches) in order to give electrical properties similar to those at the slotted configuration described with reference to Figure 2. An advantage of the mesh structure shown in Figure 3 over the slotted structure shown in Figure 2 is that it is easier to fabricate because it is more suited to an etching process.

Thus, the above described embodiments enable a strip-line having the required physical and electrical characteristics to be constructed using glass fibre as the insulating layer. Such a strip-line can be incorporated into a multi-layer board to provide a board having the required physical and electrical characteristics.

Having thus described the present invention by reference to preferred embodiments it is to be understood that the embodiments in question are exemplary only and that modifications and variations such as will occur to those possessed of appropriate knowledge and skills may be made without departure from the spirit and scope of the invention as set forth in the appended claims and equivalents thereof.

Claims

CLAIMS:

1. A strip-line comprising a conducting ribbon supported between two conductive planes by a dielectric material characterised in that at least one of said conductive planes is not continuous over a region in the vicinity of said ribbon.

2. A strip-line as claimed in claim 1, characterised in that the strip-line has a characteristic impedance greater than 50 ohms.

3. A strip-line as claimed in claim 1 or 2, characterised in that the distance between said planes is less than 2.5mm (0.1 in).

4. A strip-line as claimed in claim 1 or 2 or 3, characterised in that said non-continuous region includes a plurality of slots formed in said at least one conductive plane.

5. A strip-line as claimed in claim 4, characterised in that said slots are substantially rectangular and in that the longest edge of said rectangle runs substantially perpendicular to the length of the ribbon.

6. A strip-line as claimed in claim 1 or 2 or 3, characterised in that said non-continuous region is in the form of a mesh in said at least one conductive plane.

7. A strip-line as claimed in claim 6 wherein said mesh is defined by substantially square-shaped holes etched in said at least one conductive plane.

8. A multi-layer circuit board incorporating a strip-line as claimed in any one of the preceding claims.

9. A strip line substantially as described herein with reference to the accompanying drawings.