High Permeability Rolled Delay Line of the Coplanar Type

This invention relates to the field of electronic signal timing delay devices. More particularly, this invention relates to a new and improved electronic component suitable for use on a printed wiring board and which is capable of adjusting the arrival time of signals in high speed logic systems.

It is well known in the electronic circuitry art that for a digital network to function correctly, certain logic variables must change state at accurately controlled points in time relative to one another. As a consequence, the precise control of signals is an important concern in printed circuit board (PCB) or wiring board (PWB) design. This concern has become especially critical with the advent of high speed digital logic networks.

Time delay lines are used in the electronics industry to adjust the timing of electronic signals. As mentioned, signal timing is often critical for the proper operation of a system, particularly for high speed digital systems. In such systems, an integrated circuit may make decisions based upon two or more input signals. If all required inputs do not arrive at roughly the same time, the decision will be faulty. It is not necessary for the inputs to arrive at exactly the same time, but the arrival window becomes smaller and smaller as the speed of the system increases. As the system becomes more complex, the connections from circuit to circuit become longer and, as a result, the connection length for inputs to an integrated circuit may vary greatly. This variability in input connection length causes the transit time in each connection to vary, resulting in non-uniform arrival times. Since it is not possible to speed up the input signals arriving late, the signals arriving early must be delayed. Thus, a delay line is used to effectively increase the length of a travel of a signal, and therefore its transit time. The delay line's electrical characteristics should be as good as the circuit board on which the signals normally travel. Accordingly, it must have controlled impedance, low losses and minimal cross-talk, and it should not degrade the rise time of the signal.

There is a combination of factors present in a digital system which make it virtually impossible to design and build a digital logic network in which the signal propagation times will meet a precise time specification unless an appropriate compensation device, i.e., delay line, is provided. Moreover, because of inaccuracies in various elements of the logic system, the signal timing can only be set by tuning the network after it has been constructed. These inaccuracies are derived from, for example, gate propagation delays varying from the nominal, different lengths of printed circuit tracks, or uncontrolled delays in connectors and other segments of the digital signal paths.

It will be appreciated that the time it takes for an electronic signal to travel from one point to another in a printed circuit track is a function of, and is determined by, the physical length of the track, the line geometry and the substrate characteristics.

Typically, in high speed logic systems such as those employing Emitter Couple Logic (ECL) or gallium arsenide integrated circuits, the necessary signal timing adjustment is effected by the addition of a printed circuit track having a precise length and loaded in a given signal path. This extra length of printed circuitry track acts as a delay line whereby actual propagration delay is determined by the line length and line configuration or layout. This particular approach at controlling the timing of signals has at least two drawbacks. First, the extra length of circuitry track will use up valuable printed circuit real estate thereby incurring higher costs. Second, the characteristic impedance of the delay line is often difficult to control because of layout and construction problems.

Other time delay devices formed as discrete components and suitable for introduction onto a printed wiring board are also found in the prior art. These devices are generally of two types including lumped parameter and distributed parameter delay lines. Lumped parameter delay lines are made up of individual capacitor and inductor stages in series. The total delay is equal to the sum of the inductances multiplied by the sum of the capacitances. Multiple stages are used to smooth out the impedance as well as reduce the degradation in rise time caused by discrete inductors and capacitors. The higher performance the delay line, the more stages are required. Distributed parameter delay lines have distributed inductance and capacitance. Their performance is better than the lumped parameter type, but they are bulkier and restricted to short delays. It will be appreciated that no matter what construction is utilized, the rise time of the above two delay lines is no better than one nanosecond for even short delays. There are also wide variations in impedance within the components which distort high speed signals.

In U.S. patent application Ser. No. 691,193 to Carey and Brumbaugh, now abandoned, assigned to the assignee hereof, all the contents of which are incorporated herein by reference, a new and improved electronic signal timing delay device is disclosed which is comprised of a microstrip flexible circuit rolled up into a compact strip line form. The benefits of this improved delay line of U.S. patent application Ser. No. 691,193 include excellent impedance control, compact package size, better rise times than found in the prior art and reduced distortion of high speed signals.

While suited for its intended purposes, there is a perceived need to further reduce the size of the electronic signal delay device of Ser. No. 691,193; and there is also a perceived need to further reduce the cost of manufacturing a signal time delay device of the type disclosed in application Ser. No. 691,193.

The above discussed and other problems of the prior art are overcome or alleviated by the electronic signal timing delay device of the present invention. In accordance with the present invention, a novel signal path delay device is provided by forming a laminate of highly conductive metal bonded to thin, flexible dielectric film. The metal is deposited or etched so as to produce a pattern consisting of a signal line in a ground shield. The signal line is serpentine (i.e., zigzags) and makes one or more passes back and forth on the dielectric film. A ground plane is also provided via the conductive metal and surrounds the signal line, separated thereby by a small gap on both sides of the line. Two pads or other means are provided at the ends of the signal line to interconnect the same with the circuit in which it is used. This coplanar flexible circuit is then rolled up tightly into a cylindrical shape. Significantly, the serpentine pattern of the signal line must be designed so that when the flexible circuit is rolled up, the signal line will overlap the ground plane of the next layer (not the signal line of the next layer). While there will be some overlap of the signal lines, such overlap should be at right angles and with a minimal break in the ground shield. The rolled circuit should use adhesive to hold it together and to stabilize the effect of the dielectric. Thereafter it may be packaged and marked by a number of well known methods.

In accordance with the present invention, the delay of the delay line is substantially increased (without increasing the line length of the circuit) by utilizing a dielectric and/or adhesive having high permeability. The use of a high permeability dielectric and/or adhesive will minimize the size, cost and resistive losses of the time delay device.

The signal delay device of the present invention has many advantages and features over both currently used delay lines as well as over micro strip flexible circuit delay lines of the type disclosed in patent application Ser. No. 691,193. These several features and advantages will be discussed in more detail hereinafter.

Accordingly, the signal delay device of the present invention will provide a standard electronic component to be used on high speed logic boards, which will provide an accurate fixed time delay for high speed electronic signals. In accordance with the present invention, this time delay will be provided with minimum distortion and degradation of the delayed signal. Additionally, it will be of compact size and extremely economical to manufacture in high volume production.

The above discussed and other advantages of the present invention will be apparent to and understood by those skilled in the art from the following detailed description and drawings.

Referring now to the drawings, wherein like elements are numbered alike in the Several FIGURES:

FIG. 1 is a schematic view of a printed circuit track delay line in accordance with the prior art;

FIG. 2 is a perspective view of a two staged lump parameter delay line in accordance with the prior art;

FIG. 3A is a perspective view of a distributed parameter delay line in accordance with the prior art;

FIG. 3B is a enlarged perspective view, partly in cross-section, of a portion of the distributed parameter delay line of FIG. 3A;

FIG. 4 is a plan view of one embodiment of a circuit laminate used in forming an electronic signal timing delay device in accordance with the present invention;

FIG. 5 is a plan view of a portion of the circuit laminate of FIG. 4 subsequent to being rolled and in accordance with the present invention;

FIG. 6 is a cross-sectional elevation view along the line 6--6 of FIG. 5;

FIG. 7 is a cross-sectional elevation view along the line 7--7 of FIG. 5;

FIG. 8 is a plan view of another embodiment of a circuit laminate used in forming an electronic signal timing delay device in accordance with the present invention;

FIG. 9 is a perspective view of the circuit laminate of FIG. 8 subsequent to being rolled;

FIG. 10 is a plan view of a portion of still another circuit laminate used in forming an electronic signal timing delay device in accordance with the present invention;

FIG. 11 is a perspective view of the circuit laminate of FIG. 10 partially rolled;

FIG. 12 is a perspective view of the circuit laminate of FIG. 10 subsequent to being rolled;

FIGS. 13A-13C are examples of final packages used for through hole mounting of an electronic signal timing delay device in accordance with the present invention;

FIGS. 14A and 14B are examples of packaging used for surface mounting of an electronic signal timing delay device in accordance with the present invention;

FIG. 15A is a cross-sectional elevation view of the delay line of the present invention having a high permeability adhesive layer prior to rolling;

FIG. 15B is a cross-sectional elevation view of the delay line of FIG. 15A subsequent to rolling;

FIG. 16A is a cross-sectional elevation view of the delay line of the present invention having a high permeability dielectric layer prior to rolling; and

FIG. 16B is a cross-sectional elevation view of the delay line of FIG. 16A subsequent to rolling.

Referring first to FIG. 1, a printed circuit track delay line in accordance with the prior art is shown generally at 10. In FIG. 1, "A" and "B" are logic variable inputs (signals). As signal A is faster than signal B, it may be desired to slow down or delay signal A. If signal A is delayed sufficiently, it will take the same amount of time for both signal A and signal B to travel between the logic gates identified at 12 and 14 respectively, and the logic device 16.

As discussed earlier, the electronic signal or logic variable speed is a function of printed circuit track length, track layout or geometry and substrate material. Accordingly, the particular length and geometry of the tracks within box 10 will be such as to slow down the time of signal A to equal signal B, this difference being defined as .DELTA.t. It will be understood that the solid track 18 is representative of a first layer of the printed wiring board while the broken track 20 is indicative of a second PWB layer. The two tracks 18 and 20 are connected at the respective feed thru positions identified at 22 and 24.

In summary, there is a propagation delay for the signal A as it travels through the logic gate 12. The result is a faster signal A(t.sub.1) relative to signal B(t.sub.2). Signal A(t.sub.1) is then sent through a PC track delay line 10 whereby it is slowed by .DELTA.t. Finally, .DELTA.t is determined so that A(t.sub.4) will be equal to B(t.sub.3). This will result in the A logic variable matching the B logic variable.

As mentioned, the above described prior art method of delaying electronic signals suffers from many enumerated drawbacks including the use of valuable and needed PWB real estate and the lack of precise control in the characteristic impedance of the delay line.

Referring now to FIGS. 2, 3A and 3B, two presently used time delay devices formed as discrete components and suitable for introduction on printed wiring board are shown. In FIG. 2, a lumped parameter delay line is generally shown at 26. Lumped parameter delay line 26 is of the two stage type and is made up of individual capacitor and inductor stages in series. Thus, two multi-layer chip capacitors 28 are electrically connected to a pair of inductors 30, the inductors surrounding a plastic core 32. The total delay time is equal to the square root of the sum of the inductances multiplied by the sum of the capacitances or .sqroot.(.epsilon.L)(.epsilon.C). Multiple stages are utilized to smooth out the impedance as well as to reduce the degradation in rise time caused by discrete inductors and capacitors. In order to achieve higher performance of the delay line, more stages are required. IN, OUT and GROUND terminals are provided to the capacitor and inductor stages as shown in FIG. 2 and the entire delay line is than encapsulated or otherwise packaged as schematically shown at 34 in FIG. 2.

In FIGS. 3A and 3B, a distributed parameter delay line having distributed inductance and capacitance is shown generally at 36. Typically, distributed parameter delay lines are comprised of a glass rod 38 having a silver ink coating 40 thereon which forms a ground plane. Insulated wire 42 is then wrapped about silver coating 40, the insulation 43 from the wire forming the necessary dielectric (see FIG. 3B). As in the lumped parameter delay line, IN, OUT and GROUND terminals are provided as needed and the entire component is then encapsulated or otherwise packaged as schematically shown at 44 in FIG. 3A. It will be appreciated that the performance of the distributed parameter delay lines are better than the lumped parameter type, however the former is bulkier and restricted to only short delays. As mentioned earlier, there are several deficiencies and drawbacks associated with both prior art delay line components. For example, both constructions shown in FIGS. 2 or 3 will not provide a rise time any better than one nanosecond for even short delays. There are also wide variations in impedance within the components which distort high speed signals.

Turning now to FIG. 4, a circuit laminate used in forming an electronic signal time delay device in accordance with the present invention is shown generally at 50. Laminate 50 is comprised of a highly conductive metal bonded to a thin flexible dielectric film or substrate. The metal is deposited or etched so as to produce a circuit pattern consisting of a signal line 52 and a ground shield 54. An important feature of the present invention is that the signal line is serpentine (i.e., zig zags) and makes one or more passes back and forth on the dielectric substrate 56. The ground plane or shield 54 is provided over almost the entire surface of the laminate and is separated from the signal line 52 by a small gap 58 on both sides thereof. A pair of terminal pads 60 and 62 are formed at the two ends of the signal lines. Pads 60 and 62 will interconnect the delay device of the present invention with the circuit in which it is to be used.

Turning now to FIGS. 5-7, the electronic signal delay line of the present invention is formed by tightly rolling up the coplanar flexible circuit 50 of FIG. 4 into a cylindrical shape. A portion of the rolled coplanar flexible circuit is shown in FIG. 5. It is important that the serpentine pattern of the signal line 52 be designed so that when the flexible circuit is rolled up, the signal line 52 overlaps the ground plane 54 of the next layer; and not the signal line. Of course there must be some overlap of adjacent signal lines from adjacent overlapping layers, but these signal lines should be preferably overlapped at right angles with respect to adjacent signal lines and with a minimal break in the ground shield. The preferred orientation of the rolled serpentine signal lines of the present invention is best shown in the cross-sectional FIGS. 6 and 7.

Subsequent to flexible circuit laminate 50 being rolled, it will be appreciated that the inner layers of the circuit laminate will become effectively a strip line construction while the first and last layers will be of micro strip construction. Preferably, the areas of micro strip construction i.e., first and last layers, should have a signal line width which is approximately twice as wide as the width of the signal line in the strip line section. The wider width of the signal line in the micro strip section of the circuit laminate 50 is shown at 61 in FIG. 4.

In an alternative embodiment of the present invention, the necessity for varying the line width of the innermost and outer-most layers is precluded by eliminating the micro strip region in the inner most and outer most layers. Thus, in FIG. 8, a flexible circuit laminate is shown generally at 63 and comprises a nonconductive flexible substrate 64 having a conductive circuit layer thereon consisting of ground plane 66 and serpentine signal line 68. However, unlike the circuit laminate of FIG. 4, the flexible circuit laminate 63 of FIG. 8 includes an overlap ground area 70. It will be appreciated that this overlap ground area 70 will be the first layer in the roll. As a result, all subsequent layers will be of the strip line type. An additional extension of the base substrate material 64 shown at 72 may be used in conjunction with the overlap ground area 70 to form a protective or insulative covering for the ground thereby making the inner packaging complete. In packaging the delay line, an outer cover area shown at 74 which is simply an extension of base material 64 may be used. Thus, after the active portion of circuit laminate 62 has been rolled up, the outer cover area 74 may be rolled on as a protective cover layer, leaving only the pads 76 and 78 exposed (FIG. 9). It will be appreciated that logos and delay values could be printed on this layer either before or after rolling as desired.

In FIGS. 10-12, still another alternative embodiment of the present invention is shown which also makes the outer most circuit layer to be of the strip line construction. Thus, in FIG. 12, a ground plane extension area having no signal line formed therein is provided at 80 and extends past terminal pads 76' and 78'. Ground plane extension 80 may be wrapped around the outer circuit layer to form a strip line construction. The company logo and markings may also be etched on the ground plane. Preferably, a window 82 is provided in the base substrate material extension 74' to form openings for the three connector pads 76', 78' and ground plane 66'.

The rolled circuit laminate of the present invention preferably utilizes an appropriate adhesive to bond the individual layers together and to help stabilize the effect of the dielectric constant thereof. Thereafter, the rolled laminate may be packaged as desired. For example, in FIGS. 13A, 13B and 13C, molded, dipped, and back filled packagings are shown respectively at 84, 86 and 88. As is well known, these packages are well suited for effecting through hole mounting of the electronic signal time delay device of the present invention.

Alternatively, the signal pad and ground pad patterns may be designed to accommodate surface mounting. Such surface mounting packages are shown respectively at 90 and 92 in FIGS. 14A and 14B.

It will be appreciated that the present invention may include any other suitable packaging scheme such as injection molding, shrink tubing and metal containers, and should not be limited to the particular packaging schemes shown in FIGS. 13 and 14.

The signal and ground patterns formed on the circuit laminate of the present invention may be manufactured by either additive or subtractive processes and may be of any geometry required to maintain a given impedance, control cross-talk, improved rise time and reduced signal line overlap. The signal line width, as well as the gap separating it from the ground plane, may vary. Similarly, the laminate material may be varied as necessary. Fillers may be used to change dielectric constant, permeability or physical properties. Also, composite films may be used in conjunction with the circuit laminate of the present invention. As mentioned, adhesives may be used as required to hold the circuit together as well as to improve electrical properties. As will be discussed in greater detail below, high permeability/permitivity dielectrics and/or adhesives may be used to increase the delay of the device. Finally, the delay time and impedance for each delay line can be set at any appropriate value.

It will be appreciated that reducing the size, cost and resistive losses of the delay lines described in the foregoing FIGURES is highly desireable. It has been found that the delay of the delay line can be substantially increased without increasing the line length of the circuit by utilizing a dielectric material having high permeability. By way of background explanation, the propagation velocity within the circuit is determined using the following relationship: ##EQU1## where V.sub.p =propagation velocity

C=speed of light

E.sub.r =relative dielectric constant

.mu..sub.r =relative permeability

Therefore, delay time is calculated as shown below: ##EQU2## T.sub.d =delay l=line length

Clearly, then, if line length is to be minimized to reduce size, cost and resistive losses; E.sub.r or .mu..sub.r must be increased. Boosting E.sub.r alone causes an increase in capacitance and, therefore, a decrease in impedance. To counteract this, the line width would have to be reduced making manufacture exceedly difficult and line resistance unacceptable.

By increasing .mu..sub.r and E.sub.r simultaneously, and holding their ratio constant, impedance can be held in the specified range using typical line widths while delay time is increased. This can be accomplished by two methods including (1) doping the adhesive with a high permeability material or (2) doping the entire dielectric with such a high permeability material. These two constructions are respectively shown in FIGS. 15A-B and 16A-B.

In FIG. 15A, a portion of a raw circuit (prior to rolling) is generally shown at 100 and comprises a dielectric substrate layer 102 having a conductive circuit trace 104 and ground plane 106 thereon with a high permeability doped adhesive layer 108 therebetween. After a second layer of adhesive 108 is deposited over ground plane 106, circuit traces 104 and exposed adhesive surfaces 110 and 112, circuit 100 is rolled on itself to form the rolled circuit laminate delay device of FIG. 15B.

While FIGS. 15A and 15B disclose a delay line incorporating a high permeable adhesive layer, in FIGS. 16A and 16B, a delay device utilizing a circuit having a high permeability dielectric layer is shown generally at 114. Delay device 114 comprises a high permeability dielectric layer 116 having a ground plane 118 and circuit trace 120 thereon. As in the earlier described embodiments, ground plane 118 and trace 120 are separated by gaps 122 and 124 to expose a portion of the dielectric surface. Prior to rolling, a high permeability adhesive material is provided over ground plane 118, trace 120 and gaps 122, 124 whereupon circuit 114 is rolled as shown in FIG. 16B.

Preferably, the high permeability adhesive and/or dielectric materials utilized in the embodiments of FIGS. 15 and 16 comprise a low loss type material having a relatively high dielectric constant. Such materials are presently used in microwave applications and include ferrites and garnets. A preferred method of providing the necessary high permeability material to the delay line circuit would be to dope the entire dielectric with the high permeability material (as shown in FIGS. 16A and 16B) so as to provide the greatest delay times and minimized dielectric interfaces. It will be appreciated that any of the embodiments shown in FIGS. 4-14, or in prior U.S. patent application Ser. No. 691,193 may be used in conjunction with the high permeability dielectric and/or adhesive layers as is described above in conjunction with FIGS. 15 and 16.

The electronic signal timing delay line of the present invention includes many features and advantages over prior art delay lines. These advantages include improved impedance control and uniformity; improved rise times to delay ratio for short delays (less than 2 to 5 nanoseconds); reduced packaging size for short delays; reduced signal distortion; adaptability for surface mounting; low manufacturing and fabrication costs; and the ability to provide delays in the sub-nanosecond range.

Moreover, the electronic signal time delay device of the present invention also includes certain features and advantages over the micro strip flexible circuit delay line disclosed in U.S. patent application Ser. No. 691,193. For example, the present invention permits reduced manufacturing costs because there is no separate ground layer and cover film as is necessitated with the delay line of the prior application. Also, for the same signal line width, the outer package diameter is significantly reduced, especially for longer delays. Similarly, for the same package diameter, the signal line can be twice as wide and the dielectric twice as thick. As a result, manufacturing variations in tolerance will have less impact upon electrical characteristics. This is a particularly significant feature as the typical line width would be increased from 2-3 mils to 4-6 mils with the delay device of the present invention.

Yet another feature of the present invention is that single sided access to input, output and ground pads is provided. It is also conceivable that the present invention provides some improvement in electrical performance over the delay device of prior patent application Ser. No. 691,193 by virtue of the "semi-coaxial" construction. Thus, because there are top and bottom ground shields as well as shields on both sides of the signal line, this construction may actually improve electrical performance relative to the earlier disclosed micro strip flexible circuit delay line.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention