Active solid-state devices (e.g. – transistors – solid-state diode – Transmission line lead
Reexamination Certificate
2001-06-01
2002-07-16
Quach, T. N. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Transmission line lead
C257S758000, C438S622000
Reexamination Certificate
active
06420778
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the transmission of electrical signals and, more particularly, it relates to the design and fabrication of transmission lines on multiple layer circuit structures such as printed circuit boards.
2. Discussion of the Prior Art
Printed circuit boards (PCBs) are employed as a foundation for the mounting of various electronic components making up a circuit or system. With the short signal transition times and high clock rates of modem digital circuitry, the electrical properties of the electrical conductors carrying signals between devices become increasingly more important. High-speed logic families such as FAST, ECL and GaAs circuits have required the PCB designer to model the aforementioned electrical signals as transmission lines. Indeed, PCB layout is now deemed very critical to maintaining signal integrity, such as preserving signal edges and reducing distortion due to reflections and crosstalk. Proper impedance control and impedance termination are required in order to achieve desired signal integrity.
Essentially, the impedance of a printed circuit board (PCB) conductor is controlled by its configuration, dimensions (trace width and thickness and height of the board material) and the dielectric constant of the board material. When a signal encounters a change of impedance arising from a change in material or geometry, part of the signal will be reflected and part transmitted. Such reflections are likely to cause aberrations in the signal which may degrade circuit performance (e.g., low gain, noise, and random errors). Each transmission line on a PCB is generally formed by two conductive traces. If one of the two conductors of the transmission line coupling the circuits is used as a grounding connector, the circuit is referenced as single-ended and the transmission line is a single-ended or unbalanced transmission line. Otherwise, the two non-grounded conductors form a differential or balanced transmission line for a differential circuit. Equivalent examples of differential transmission lines are shown in
FIG. 1A
in which the two conductors
2
a
and
2
b
are implemented as wire rods separated by a dielectric material
4
within a grounded sheath
6
—and in FIG.
1
B—in which the two conductors are
2
a
and
2
b
are implemented as conductive traces formed by metallizations on dielectric layers
8
a
and
8
b
between ground planes
10
a
and
10
b.
Although all electrical circuits depend on a difference in voltage for their operation, differential signaling refers to where two signals are sent and received with the information conveyed by their different in voltage or current rather than by those individual quantities with respect to a common ground. A differential signal is applied across the two conductors, as voltage varying signals −V and +V applied to conductors
2
a
and
2
b
in
FIGS. 1A and 1B
, by a differential generating circuit (not shown), the signal travels down the transmission line to a differential receiving circuit (not shown), and the received signal is measured as the difference between the voltage or current in conductors
2
a
and
2
b
. In other words, a differential circuit generates or receives a pair of complementary signals in a phase inverted relation with each other, known together as a single differential signal.
The chief advantage of differential signaling is higher noise immunity, achieved by eliminating common-mode influences picked up by the environment. Provided its conductive traces are close enough together to be exposed to nearly the same environmental influences, the differential transmission line is desensitized to voltage drift because the induced noise in each line rise and fall in tandem in the complementary signals and cancel each other. The inherently superior noise immunity of differential circuits allows signal levels to be reduced, with consequent improvements in switching speed, power dissipation and noise. Another advantage lies in the area of ground bounce—since differential pairs always sink and source the same amount of current (into a resistive load), there is no bounce and, in theory, no bounce-induced crosstalk. And since the currents are complementary there is little net magnetic flux from a differential pair, so EMI is dramatically reduced as well.
Despite the aforementioned benefits of configuring PCB traces interconnecting circuit devices as differential transmission lines, complications do arise in the application of this technique to electrical systems in which many signals need to be conveyed wherein it is desired to employ multiple sets of differential lines. An illustrative electrical board
12
embodying the approach is shown in
FIG. 2
, with three differential transmission lines indicated generally at
14
,
16
and
18
, being constructed using multiple planes of metallizations over corresponding dielectric layers
20
and
22
. Upper ground plane
24
is spaced the same distance above upper conductors
14
a
,
16
a
and
18
a
of transmission lines
14
,
16
and
18
as lower ground plane
26
is separated from lower conductors
14
b
,
16
b
and
18
b
, so as to have equal impedances to ground. As between a first differential transmission line as differential conductor pair
14
a
and
14
b
and a second differential transmission line as differential conductor pair
16
a
and
16
b
, the problem of crosstalk can arise in which a signal on the first line can induce a voltage on the second line and vice versa.
It is well known that the most effective way to improve local PCB crosstalk between adjacent differential transmission lines is to move the affected traces further away. Assuming one is presented with solid power and ground planes, crosstalk between aggressor and victim traces falls off as the square of increasing distances. Thus, for example, doubling the distance cuts crosstalk to one-fourth. While placing the two traces of a line pair closer together also helps reduce crosstalk, the greatest effect is achieved by generally increasing the separation between aggressor and victim. Unfortunately, however, space is generally limited on printed circuit board structures and design considerations tend to require that the differental transmission lines be placed as closely together as practicable. Accordingly, a need arises for a technique for realizing differential transmission lines as conductive traces on a PCB in which space on the board is used more efficiently than has heretofore been possible without unacceptable degradation in performance due to crosstalk. A further need arises for a PCB which may be fabricated using conventional manufacturing techniques and equipment.
SUMMARY OF THE INVENTION
The aforementioned needs are addressed, and an advance is made in the art, by a multilayer circuit structure which includes first and second superposed planar dielectric layers. On the first layer, first and second conductor lines are formed, and on the second layer, third and fourth conductor lines are formed. The first and third conductor lines, and the second and fourth conductor lines, respectively, form at least a portion of corresponding first and second differential transmission line. The first conductor line extends in parallel with the third conductor line but is both vertically spaced and horizontally offset therefrom such that a first plane orthogonal to and extending through the first and second planar layers extends through one, but not both of the first and third conductor lines (i.e., a non-overlapping region). Similarly, a second plane orthogonal to and extending through the first and second planar layers extends through a non-overlapping region of the second and fourth conductor lines.
The dimensions and electrical properties of the first and third conductor lines are selected so that the characteristic impedance of the first transmission line is matched to the output of a differential transmission circuit which is operable to generate and supply to the first and third conductor lines a
Aralight, Inc.
DeMont & Breyer LLC
Quach T. N.
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