U.S. patent application number 10/984293 was filed with the patent office on 2005-03-24 for impedance-matching electrical connection apparatus for high-speed data communications system.
Invention is credited to Aronson, Lewis B..
Application Number | 20050062556 10/984293 |
Document ID | / |
Family ID | 28044692 |
Filed Date | 2005-03-24 |
United States Patent
Application |
20050062556 |
Kind Code |
A1 |
Aronson, Lewis B. |
March 24, 2005 |
Impedance-matching electrical connection apparatus for high-speed
data communications system
Abstract
This disclosure concerns systems and devices configured to
implement impedance matching schemes in a high speed data
transmission environment. In one example, an electrical connection
system is provided that includes a circuit board upon which are
disposed a one or more signal contact pads, each of which is
configured to communicate with a complementary element of an
external electrical device such that a respective shunt capacitance
is defined. Also disposed on the circuit board are one or more
inductive traces, each of which is connected to a respective
contact signal pad, and each of which has an associated
compensating series inductance configured to substantially offset
the corresponding shunt capacitance. As well, one or more signal
lines are likewise disposed on the circuit board such that each
signal line is connected to a respective inductive trace. The
circuit board finally includes one or more ground contact pads and
power contact pads.
Inventors: |
Aronson, Lewis B.; (Los
Altos, CA) |
Correspondence
Address: |
ERIC L. MASCHOFF
WORKMAN, NYDEGGER & SEELEY
1000 Eagle Gate Tower
60 East South Temple
Salt Lake City
UT
84111
US
|
Family ID: |
28044692 |
Appl. No.: |
10/984293 |
Filed: |
November 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10984293 |
Nov 9, 2004 |
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10285772 |
Nov 1, 2002 |
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60366092 |
Mar 19, 2002 |
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Current U.S.
Class: |
333/33 ;
439/60 |
Current CPC
Class: |
H05K 2201/0373 20130101;
H05K 1/0216 20130101; H05K 2201/09681 20130101; H05K 2201/094
20130101; H01R 23/688 20130101; H05K 2201/093 20130101; H01R
13/6585 20130101; G02B 6/43 20130101; H05K 2201/09727 20130101;
H01R 12/721 20130101; H05K 1/117 20130101 |
Class at
Publication: |
333/033 ;
439/060 |
International
Class: |
H01P 005/02; H01R
012/00 |
Claims
What is claimed is:
1. An electrical connection system suitable for use in connection
with high-frequency communication signals, the electrical
connection system comprising: a circuit board; a plurality of
signal contact pads disposed on the circuit board, each of the
plurality of signal contact pads configured to interact with a
complementary element of an electrical device such that a coupling
between the signal contact pad and complementary element is defined
that has an associated shunt capacitance; a plurality of inductive
traces disposed on the circuit board, each inductive trace
connected to a respective contact signal pad of the plurality of
signal contact pads, each inductive trace having an associated
compensating series inductance configured to substantially offset a
portion of the corresponding shunt capacitance; and a plurality of
signal lines disposed on the circuit board, each signal line
connected to a respective inductive trace of the plurality of
inductive traces.
2. The electrical connection system as recited in claim 1, wherein
the electrical connection system is compatible for use with one or
more high-speed differential signal paths.
3. The electrical connection system as recited in claim 1, wherein
the electrical connection system comprises a portion of a card edge
connector.
4. The electrical connection system as recited in claim 1, wherein
at least one signal contact pad defines at least one open portion
through which signals cannot pass.
5. The electrical connection system as recited in claim 1, wherein
a total area defined by at least one signal contact pad includes at
least one open portion through which signals cannot pass, the at
least one open portion having an area in a range of at least about
fifty percent to at least about seventy five percent of the total
area.
6. The electrical connection system of claim 1, wherein, for one or
more predetermined data rates, each inductive trace and associated
signal contact pad collectively implement an impedance that
substantially matches an impedance associated with the
complementary connector to which that inductive trace and signal
contact pad are electrically connected.
7. The electrical connection system of claim 1, wherein, for one or
more predetermined data rates, each inductive trace and associated
signal contact pad collectively implement an impedance that
substantially matches an impedance of the signal line to which that
inductive trace and signal contact pad are electrically
connected.
8. The electrical connection system of claim 1, wherein the
compensating series inductance associated with at least one of the
inductive traces is a function of a length and a width of that
inductive trace.
9. The electrical connection system of claim 8, wherein the width
of the at least one inductive trace is no greater than about thirty
five percent of a width of the signal line connected to the
inductive trace.
10. The electrical connection system of claim 8, wherein the width
of the at least one inductive trace is no greater than about twenty
five percent of a width of the signal line connected to the
inductive trace.
11. The electrical connection system of claim 1, wherein a
coupling, between a contact pad and a complementary connector,
cooperates with the inductive trace connected to that contact pad
to collectively implement an average impedance value based in part
on the offset portion of the shunt capacitance, and the average
impedance value substantially matches a nominal impedance of an
external transmission line of the electrical device.
12. The electrical connection system of claim 1, further comprising
at least one ground contact pad disposed on the circuit board.
13. The electrical connection system of claim 12, further
comprising a trace connected to the ground contact pad and disposed
on the circuit board.
14. The electrical connection system of claim 13, wherein the trace
is configured so as to have an associated inductance that
facilitates a reduction of shunt capacitance.
15. The electrical connection system of claim 1, further comprising
at least one power contact pad disposed on the circuit board.
16. The electrical connection system of claim 1, further comprising
at least one signal contact pad directly connected to a
corresponding signal line and disposed on the circuit board.
17. The electrical connection system of claim 1, further comprising
a ground plane mounted on an opposite side of a dielectric layer of
the circuit board from the plurality of contact pads and the
plurality of inductive traces, wherein the ground plane is
configured and arranged to affect a strength of a capacitive field
associated with at least one of: at least one inductive trace; and,
at least one signal contact pad.
18. An electrical connection system suitable for use in connection
with high-frequency communication signals, the electrical
connection system comprising: a circuit board; a plurality of
signal contact pads disposed on the circuit board, each of the
plurality of signal contact pads configured to interact with a
complementary element of an electrical device such that a coupling
between the signal contact pad and complementary element is defined
that has an associated impedance discontinuity; a plurality of
inductive traces disposed on the circuit board, each inductive
trace being connected to a respective signal contact pad of the
plurality of contact pads, and each inductive trace having a
geometry configured to implement a reduction in the impedance
continuity; and a plurality of signal lines disposed on the circuit
board, each signal line being connected to a respective inductive
trace of the plurality of inductive traces.
19. The electrical connection system as recited in claim 1, wherein
the electrical connection system is compatible for use with one or
more high-speed differential signal paths.
20. The electrical connection system of claim 18, wherein each
inductive trace has a width no greater than about thirty five
percent of a width of the signal line connected to that inductive
trace.
21. The electrical connection system of claim 18, wherein each
inductive trace has a width no greater than about twenty five
percent of a width of the signal line connected to that inductive
trace.
22. The electrical connection system as recited in claim 18,
wherein the electrical connection system comprises a portion of a
card edge connector.
23. The electrical connection system of claim 14, wherein each
inductive trace is associated with a corresponding compensating
series inductance, and the impedance discontinuity associated with
each inductive trace is susceptible to reduction by use of the
compensating series inductance to offset a portion of a shunt
capacitance associated with the corresponding coupling.
24. The electrical connection system as recited in claim 14,
wherein at least one signal contact pad defines at least one open
portion through which signals cannot pass.
25. The electrical connection system as recited in claim 14,
wherein at least one signal contact pad defines a plurality of open
portions through which signals cannot pass.
26. The electrical connection system of claim 25, wherein the
plurality of open portions collectively define at least a portion
of a cross-hatch pattern.
27. The electrical connection system of claim 25, wherein plurality
of open portions comprises a plurality of openings, each of which
is generally in the shape of a parallelogram.
28. The electrical connection system of claim 25, wherein the
plurality of open portions collectively define at least a portion
of a zigzag pattern.
29. The electrical connection system of claim 14, further
comprising at least one ground contact pad disposed on the circuit
board.
30. The electrical connection system of claim 29, further
comprising a trace connected to the ground contact pad and disposed
on the circuit board.
31. The electrical connection system of claim 30, wherein the trace
is configured so as to have an associated inductance that
facilitates a reduction of shunt capacitance.
32. The electrical connection system of claim 14, further
comprising at least one power contact pad disposed on the circuit
board.
33. The electrical connection system of claim 14, further
comprising at least one signal contact pad directly connected to a
corresponding signal line.
34. The electrical connection system of claim 14, further
comprising a ground plane mounted on an opposite side of a
dielectric layer of the circuit board from the plurality of contact
pads and the plurality of inductive traces, wherein the ground
plane is configured and arranged to affect a strength of a
capacitive field associated with at least one of: at least one
inductive trace; and, at least one signal contact pad.
Description
BACKGROUND OF THE INVENTION
[0001] In designing high-speed communications system devices, such
as high-speed optoelectronic transceivers, signal integrity is a
paramount issue. With extremely fast signals and corresponding
short bit periods, such as those utilized in a 10 Gb/s
optoelectronic transceiver, even slight degradation of the signal
can render the system unuseable. This is problematic because even
relatively small transmission-line discontinuities can lead to
significant signal degradation in a high-speed communications
system.
[0002] One particularly problematic location for transmission line
discontinuities in a high-speed communication system is the
interface between two different electronic components, such as the
interface between a plug-in module having an edge-card connector
system and the edge-card connector on a host printed circuit board
(PCB), or the interface between a surface-mounted chip and the PCB
on which the chip is mounted. These types of interfaces tend to
introduce discontinuities in the transmission path of the
communication signals.
[0003] For example, an edge-card connector system typically uses a
series of spaced-apart contact pads on a plug-in card or module
that engage and electrically couple to a series of spaced-apart
complementary connectors in a connector unit on a separate PCB. An
example of a connector unit is shown in FIG. 1, and an example of a
plug-in card or module is shown in FIG. 2. This type of connection
can be used with, for example, an optoelectronic transceiver that
plugs into a host PCB. The edge-card connection mechanism,
well-known in the art, makes mounting electronic cards and devices
onto a host PCB solder-less and easy. It also tends to introduce
discontinuities in the signals that pass through the complementary
connectors into the contact pads and onto the card or module
containing the contact pads. These discontinuities are likely
caused by several features of the physical design of such a
system.
[0004] In particular, some of the physical features used by an
edge-card connector unit for structural support and connectivity
purposes likely cause discontinuities in the signal transmission
path. As illustrated in FIG. 1, external electrical device
connector 100, also referred to as a connector unit, is typically
comprised of a series of spaced-apart complementary connectors,
also referred to herein as contact points. (In the side plan view
of FIG. 1, only one complementary connector 102 is depicted,
however). Each complementary connector has a connector-arm portion
(i.e., the top-most curving part of the complementary connector)
that electrically couples with a contact pad on a card or module
having an edge-card connector system. To mount complementary
connector 102 in a perpendicular fashion to host PCB 110, the
complementary connector must lock against connector body 108. This
is typically accomplished using a mounting feature 106, which is
shown in FIG. 1 as an integral mounting pin that fits into a
mounting hole on the connector body. Other types of structural
interlocking mechanisms in addition to a pin and hole pair are also
commonly used.
[0005] The purpose of locking complementary connector 102 to
connector body 108 is to provide additional structural stability
for the complementary connector 102. However, this design also
likely adds undesirable electrical effects into the transmission
path of electric signals conducting through the complementary
connector. Because complementary connector 102 is a conductor,
mounting feature 106 is also typically a conductor, due to
manufacturing requirements. Thus, mounting feature 106 adds
increased planar surface area to the transmission path. This, in
effect, adds excess shunt capacitance to the transmission path,
relative to the nominal capacitance of the path. (A transmission
path has a nominal impedance per unit length--e.g., 50 ohms in a
single-ended transmission path--and, relatedly, a nominal series
inductance and shunt capacitance per unit length). This excess
shunt capacitance is also referred to as parasitic capacitance.
This undesirable capacitance results from interaction with other
portions of the path, such as with the connector arm portion of
complementary connector 102, and with the grounding plane (not
shown in FIG. 1).
[0006] Another possible source of unwanted discontinuities is the
connector arm portion of complementary connector 102. When a
plug-in card or module is slipped into the opening shown on the
right side of external electrical device connector 100, the contact
pads on the top of the card will couple with the complementary
connector 102, and the contact pads on the bottom of the card will
couple with the bottom-edge connector contacts 104 (typically used
for low-speed communications). To ensure that complementary
connector 102 reliably and easily mates with a contact pad, the
connector arm portion presents a convex surface relative to the
contact pad. Using this particular design, the contact pads of a
plug-in card or module can easily be slid along the connector arm
portion of the complementary connector 102, providing an electrical
coupling without impeding the movement of the plug-in card or
module. Depending on the angle of the complementary connector 102
relative to the plane of the plug-in card, the complementary
connector can also provide additional support and resistance
against extraneous movement by the plug-in card.
[0007] However, this layout also causes part of the complementary
conductor, and hence the transmission path, to extend past, and up
and away from, the actual point of contact with a contact pad
(i.e., because of the "hooked" end). Like with the mounting feature
106, this causes parasitic shunt capacitance to appear in the
transmission path in excess of the nominal shunt capacitance
associated with the transmission path.
[0008] Referring to FIG. 2, a typical edge-card connector 200 is
shown. Printed on a circuit board are a series of spaced-apart
contact pads. As shown in FIG. 2, the contact pads may include
ground contact pads 202-1, 202-2 . . . 202-n, signal contact pads
204-1, 204-2 . . . 204-n, and power contact pads 208-1, 208-2. The
ground contact pads are typically longer than the signal contact
pads to allow for hot-plugging of the edge-card connector, by
ensuring that the system is grounded before power and signal
connections are made. The ground contact pads generally connect to
a trace extending a short distance beyond the ground contact pad,
which then connects to a connector going downward to a (typically)
internal ground plane, separated from the top surface by a
dielectric material.
[0009] The signal contact pads of the edge-card connector connect
to a signal line, such as signal lines 206-1, 206-2 . . . 206-n.
The signal contact pads are typically wider than the signal wires.
The purpose of having contact pads with a wide surface area is to
facilitate an easy coupling with a complementary connector, because
the connector need only contact any portion of the wide surface
area to establish an electrical connection. However, it is likely
that this extra surface area also increases the shunt capacitance
between each contact pad and the ground plane, as well as between
the contact pads themselves, resulting in excess, parasitic shunt
capacitance that causes the aforementioned transmission-line
discontinuities.
[0010] Edge-card connector 200 will also typically include at least
one contact pad for power, such as power contact pads 208-1 and
208-2, at a predetermined voltage level. The length of the power
contact pads tends to be intermediate the length of the ground
contact pads and the length of the signal contact pads, again to
allow for hot-plugging by ensuring that the edge-card connector
system is powered before signaling connections are made (but only
after grounding has been achieved). Of course, the card containing
the edge-card connector will also include various electrical
components, such as resistors, capacitors, potentiometers,
inductors, diodes, etc. These components will typically be
interconnected through various traces and signal lines, and are
typically utilized in the transmission path as well. The card may
also contain, or comprise, other specialized components, such as
optoelectronic components, processors, ASICs, and the like.
[0011] Regardless of the purpose of the plug-in card or module on
which an electrical connection system is mounted, it would be
desirable to provide an electrical connection system that reduces
discontinuities in the transmission path through the interface
between the plug-in card and the host PCB.
BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION
[0012] In general, exemplary embodiments of the invention are
concerned with systems and devices configured to implement
impedance matching schemes in a high speed data transmission
environment. In one example, an electrical connection system is
provided that includes a circuit board upon which are disposed a
one or more signal contact pads, each of which is configured to
communicate with a complementary element of an external electrical
device such that a respective shunt capacitance is defined. Also
disposed on the circuit board are one or more inductive traces,
each of which is connected to a respective contact signal pad, and
each of which has an associated compensating series inductance
configured to substantially offset the corresponding shunt
capacitance. As well, one or more signal lines are likewise
disposed on the circuit board such that each signal line is
connected to a respective inductive trace. The circuit board
finally includes one or more ground contact pads and power contact
pads.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Additional objects and features of the invention will be
more readily apparent from the following detailed description and
appended claims when taken in conjunction with the drawings, in
which:
[0014] FIG. 1 is a schematic of a prior art connector unit having a
connector arm for coupling with the contact points of an edge-card
connector.
[0015] FIG. 2 is a schematic of a prior art edge-card connector
having contact points that couple with the connector arms shown in
FIG. 1.
[0016] FIG. 3 is a graph of a typical differential time-domain
reflectivity measurement, displaying impedance versus time,
measured at the interface between a prior art connector unit and a
prior art edge-card connector system.
[0017] FIG. 4 is a schematic demonstrating an edge-card connector
with inductive traces, in accordance with one embodiment of the
present invention.
[0018] FIG. 5a is a schematic of an edge-card connector having
cross-hatched signal contact pads in accordance with a second
embodiment of the present invention.
[0019] FIG. 5b is a schematic of an edge-card connector utilizing
both cross-hatched signal contact pads and inductive traces in
accordance with yet another embodiment of the present
invention.
[0020] FIG. 6a is an exemplary embodiment of the present invention
utilizing zig-zag-patterned signal contact pads.
[0021] FIG. 6b is another exemplary embodiment utilizing both
zig-zig-patterned signal contact pads, as well as inductive
traces.
[0022] FIG. 7 is a schematic showing a cross-section of a circuit
board having elements of one embodiment of the present invention,
including a shortened ground plane.
[0023] FIG. 8 is another graph of a differential time-domain
reflectivity measurement, similar to FIG. 3, but demonstrating the
effects of utilizing an embodiment of the present invention.
[0024] FIG. 9 is a schematic of a connector unit, similar to FIG.
1, and an edge-card connector having features of the present
invention plugged into the connector unit, in accordance with one
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] Referring to FIG. 3, a time-domain reflectivity graph is
depicted that demonstrates the effects of the discontinuities of an
existing edge-card connector system, described above. This graph
was obtained by transmitting a sharp edge with a very fast rise
time, 40 ps, across a transmission path including an interface
between an edge-card connector and a connector unit of a host PCB.
A very fast rise time was utilized because the discontinuities
described above are only problematic and significant in high-speed
systems, i.e., in systems communicating at an extremely fast data
transmission rate, such as 5 Gb/s and above. (In fact, these
discontinuities may not even be seen in systems operating at 1 or 2
Gb/s, or may only be seen as a single discontinuity that cannot be
spatially resolved into separate discontinuities at the lower
transmission rates). The particular edge-card connector tested was
part of a plug-in card or module containing an optical transceiver
device. An example of an optical transceiver utilizing an edge-card
connector design is the Small Form Factor Pluggable (SFP)
transceiver, developed under a Multisource Agreement (MSA) and
manufactured by the present assignee, Finisar Corporation, among
others.
[0026] The output pulse and return signal measurements were made by
a time-domain reflectometer (TDR). The system measured a
differential transmission path having a 50 ohm nominal impedance
path on each line, resulting in the 100 ohm impedance seen in the
graph. The actual time-domain reflectivity measurement was made by
measuring a voltage return signal as a function of time, but in
FIG. 3 it has been converted to, and is displayed as, impedance (in
ohms) 302 against time (in picoseconds) 304. The measurement may
also be viewed as impedance versus distance (position) in the
transmission path, because the time value reflects the amount of
time the signal traveled before being reflected and measured,
therefore indicating a distance from the point of measurement on
the transmission path.
[0027] As shown in FIG. 3, the measured differential impedance is
100 ohms starting at time zero, representing the fact that the
impedance measurement is made far enough down the transmission path
so as not to reflect the initial transmission path impedance
ramp-up at the point the test voltage is applied to the path. In
other words, the nominal impedance of the differential transmission
path is meant to be a constant 100 ohms. However, a first
discontinuity 306 undesirably reduces that impedance by about 20%,
to approximately 80 ohms. One possible source for such a
discontinuity would be the mounting feature 106 described for FIG.
1. This discontinuity causes reflections that result in signal
degradation, as the impedance in the transmission path is no longer
impedance-matched at this location.
[0028] Beyond the source of this first discontinuity, the impedance
returns to the nominal 100 ohms, but then dips down again at a
second discontinuity 308, again by about 20%, to approximately 80
ohms. A possible cause of the second discontinuity might be the
extension of the complementary connector 102 beyond the point of
coupling with a contact pad, as described for FIG. 1, or it may be
the geometry of a signal contact pad, described for FIG. 2, or
possibly a combination of both. In any case, the second
discontinuity 308 can be considered to be associated with the
coupling between a contact pad and a complementary connector. Like
with the first discontinuity, this second discontinuity causes
further signal degradation.
[0029] In FIG. 3, the first and second discontinuities are
approximately the same magnitude. However, depending on the
specific parameters of the system being utilized, the
discontinuities may be of unequal magnitudes, and may be spaced
further apart, or may be closer together. In addition, there may be
more than two distinct discontinuities. For example, the second
discontinuity 308 may actually be two separate discontinuities, of
equal or unequal magnitude, spaced close enough together to be
partly merged. This result may reflect the possible different
sources of a second discontinuity, such as the extension of the
complementary connector beyond the coupling point, and the
increased surface area of a signal contact point. Again, for
purposes of this document, the second discontinuity is considered
to be associated with the general coupling between a contact pad
and a complementary connector.
[0030] One possible way to reduce at least the second discontinuity
in the transmission path is shown in FIG. 4. In this embodiment of
the present invention, an edge-card connector having inductive
traces 400 is shown. (This embodiment, as well as other related
embodiments, are also referred to herein as an impedance-matching
electrical connection system, or a discontinuity-reducing
electrical connection system.) An inductive trace is a portion of a
trace that has a narrower width than a contact pad or a signal
line, similar to a bond wire. Due to the narrow width of an
inductive trace, each inductive trace has a higher series
inductance relative to the nominal series inductance of its
associated transmission path (and, relatedly, a higher impedance
than the nominal impedance of the associated transmission path).
This increased series inductance can be used to substantially
offset the undesirable (i.e., excess) shunt capacitance from the
coupling between a contact pad and a complementary connector. In
this sense, the series inductance of an inductive trace wire is
used as a compensating series inductance.
[0031] Preferably, the substantial offset of the undesirable shunt
capacitance by the inductive traces, either alone or in combination
with other embodiments of the present invention, is at least 50%,
and preferably is at least 75%. In other words, if excess shunt
capacitance is reducing the connector impedance by 20 ohms (for
signals in a predetermined frequence range), from 100 ohms to 80
ohms, then the offset produced by the inductive traces is at least
10 ohms, producing a net impedance of at least 90 ohms, and
preferably producing a net impedance of at least 95 ohms.
[0032] As shown in FIG. 4, inductive traces 408-1, 408-2 . . .
408-n connect to a respective signal contact pad 404-1, 404-2 . . .
404-n. The "trick" to offsetting the excess shunt capacitance of a
coupling with the compensating series inductance of an inductive
trace is to place the inductive trace close enough to the coupling
to cause the decrease in impedance from the excess shunt
capacitance, and the increase in impedance from the series
inductance, to effectively cancel each other out. This is
accomplished by placing an inductive trace (e.g., 408-1) adjacent
to the a signal contact pad (e.g., 404-1). Also, the inductive
trace cannot be too long, or its compensating series inductance
will not overlap the excess shunt capacitance, and thus will not
effectively cancel out the excess shunt capacitance associated with
a coupling. This effect would be revealed in a time-domain
reflectivity graph as a third discontinuity--a bump (as opposed to
the dips of the other two discontinuities) in the graph reflecting
an increased transmission line impedance from the unmatched
compensating series inductance of the inductive trace.
[0033] To achieve optimal discontinuity-reduction and yet avoid
overcompensation, both the width and the length of the inductive
traces are configured appropriately. In one embodiment, a typical
inductive trace is 4 to 6 mils (i.e., 0.004 to 0.006 inches) wide,
and less than 40 mils long, while the signal traces are typically
17 to 23 mils wide. Or, from another perspective, the width of each
inductive trace is no greater than 35% of the width of the signal
line to which the inductive trace is connected, and preferably, is
no greater than 25% of the width of the signal line to which the
inductive trace is connected.
[0034] The inductive traces 408-1, 408-2 . . . 408-n also connect
to the signal lines 406-1 . . . 406-n. Note that not all types of
contact pads utilize an inductive trace. For example, power contact
pads 410-1 and 410-2, which are actually linked together, do not
utilize inductive traces. Ground contact pads 402-1, 402-2 . . .
402-n may link to traces, which may or may not be used to reduce
excess shunt capacitance. In other embodiments (not shown), one or
more of the signal contact pads may not utilize an inductive trace,
and may instead connect directly to a signal line, such as in the
case of a lower-speed data transmission line, for which the use of
an inductive trace may be unnecessary or unhelpful. The invention
encompasses using inductive traces intermediate all connections
between signal contact pads and signal lines, or using a
combination of intermediate inductive traces and no inductive
traces (i.e., a direct connection between contact pad and signal
line).
[0035] Referring to FIG. 5a, another embodiment of the present
invention is shown. In this embodiment, the undesirable shunt
capacitance associated with a coupling between a contact pad and a
complementary connector is reduced by reducing the total surface
area of a signal contact pad. However, instead of directly reducing
the outside dimensions of the contact pad, which would reduce the
ability of a contact pad to reliably couple with a complementary
connector, the surface area of a contact pad is reduced via certain
surface area geometries. For example, as shown in FIG. 5a,
cross-hatched signal contact pads 502-1 . . . 502-n are utilized.
By removing four "windows" (i.e., parallelograms) of surface area,
the original outline of the contact pad is maintained (i.e., the
original width and length), but the total surface area is reduced,
because the remaining surface area is less than the product of the
width and the length. In one embodiment, at least 50% of the total
surface area of the signal contact pads are removed (there is no
reason to adjust the ground contact pads, as they do not constitute
part of the signal transmission path). By reducing the surface area
in this manner, the excess shunt capacitance is reduced, while
sufficient surface area is maintained for the complementary
connector to couple with the contact pad.
[0036] This embodiment can be further enhanced by combining it with
the inductive trace embodiment described in conjunction with FIG.
4. This is depicted in FIG. 5b, which includes a cross-hatched
signal contact pad 506-1 connected to an inductive trace 504-1. By
utilizing both techniques to reduce the excess shunt capacitance
associated with the coupling between the contact pad and a
complementary connector, even greater discontinuity reduction, or
improved impedance matching, in the transmission path is
achieved.
[0037] For example, in the external electrical device containing
the complementary connectors, at least one of the complementary
connectors will connect to an external transmission line (e.g.,
signal lines on the host PCB). This external transmission line(s)
will have a nominal impedance. Similarly, the cross-hatched signal
contact pad 506-1 and inductive trace 504-1 shown in FIG. 5b will
have, jointly, an average impedance, which is based at least in
part on the excess shunt capacitance associated with the coupling
of the contact pad 506-1 and the complementary connector, and the
compensating series inductance of the inductive trace 504-1. The
average impedance of this combination is ideally configured to be
substantially equivalent to the nominal impedance of the external
transmission line on the host PCB, and preferably is within 10% of
this nominal impedance.
[0038] Another way to consider the impedance-matching functionality
of the present invention is to consider that each inductive trace,
and the contact pad connected to it, are configured to provide
substantial impedance matching to a coupling complementary
connector. Again, preferably the impedance-matching is within 10%
of the nominal impedance. However, in addition to configuring the
inductive trace and contact pad connected thereto to
impedance-match the complementary connector, these components must
also be configured to provide substantial impedance matching to the
signal line connected to the inductive trace. Otherwise, while
improving one discontinuity in the transmission path, the inductive
trace and modified contact pad could cause another. In one
embodiment, these types of impedance matching are achieved at data
transmission rates of 10.0 Gb/s and higher, and more generally
above 5.0 Gb/s and higher, because the discontinuities in
transmission line impedance are most typically seen at these
high-speed transmission rates.
[0039] Other patterns for the surface area geometries of the signal
contact pads are possible. For example, as shown in FIG. 6a,
zig-zag patterned signal contact pads 602-1 . . . 602-n can be
utilized. Again, the net effect is to reduce the total surface
area, preferably by at least 50%, and more preferably by least 75%,
while maintaining a substantial surface profile for easy coupling
with a complementary connector. Also, as shown in FIG. 6b, a
zig-zag patterned signal contact pad 606-1 can be combined with the
inductive trace 604-1 technique to further reduce discontinuities
by improving impedance matching.
[0040] As mentioned above, a typical circuit board containing the
electrical connection system of the present invention will also
contain a grounding plane that runs parallel to the top surface of
the circuit board. This is shown in FIG. 7 as a PCB card with
electrical connection system and ground plane 700. The actual
grounding plane 714 is generally on an opposite side of a
dielectric layer (e.g., dielectric layer-1 710) from the contact
pads 702, inductive traces 704, and signal lines 706, and is
typically internal to the circuit board (e.g., encapsulated by
dielectric layers 1 and 2, 710 and 712). (By keeping the ground
plane proximate to the contact pads and signal lines, the features
of the transmission path can be kept at a reasonable width while
still achieving the desired nominal transmission line
impedance).
[0041] As shown in this figure, another method to reduce the excess
shunt capacitance associated with the coupling between the contact
pads and the complementary connector is to have the ground plane
714 not extend into a region below at least a subset of the set of
contact pads 702. This decreases the capacitive field between
contact pads 702 and ground plane 714 through dielectric layer-1
710. Preferably, the ground plane 714 does not extend into a region
below any of the signal contact pads of the card. In another
embodiment, the ground plane further does not extend into a region
below at least a subset of the set of inductive traces 704. This
further decreases the strength of the capacitive field surrounding
the inductive traces, and hence increases their inductance.
[0042] In the embodiment shown in FIG. 7, the ground plane extends
only under the signal lines 706. While the ground plane is shown
ending directly at a plane perpendicular to the intersection of the
signal lines 706 and the inductive traces 704, in various
embodiments the ground plane may extend somewhat past this line, or
may end somewhat before it. The actual configuration can be
adjusted to achieve maximum discontinuity-reduction, or
impedance-matching, in conjunction with either the specialized
surface area geometry techniques, or the inductive trace
techniques, or both, of the present invention.
[0043] The overall improvement in discontinuity reduction, or
impedance matching, from these techniques is depicted in the
impulse response graph 800 shown in FIG. 8. This graph was
generated under the same conditions as the graph in FIG. 3, and
uses the same axes. However, the electrical connection system
tested for FIG. 8 employed the inductive traces and the specialized
contact pad surface area geometries of one embodiment of the
present invention. As a result, the second substantially-reduced
discontinuity 808 is within about 5% of the transmission
impedance--about 95 ohms (the nominal impedance is 100 ohms, based
on using differential signaling in a nominal 50 ohm transmission
path). Clearly, the excess shunt capacitance associated with the
couplings between the contact pads and the complementary connectors
has been substantially offset by the compensating series inductance
of the inductive traces, and reduced by the specialized surface
area geometries of the contact pads and the shortened ground
plane.
[0044] In one embodiment demonstrating one particular use of the
present invention, the invention is an optoelectronic device for
use in a high-speed communications system. The host PCB on which
the electrical connection system described above is mounted
includes at least one optoelectronic component, e.g., an
optoelectronic transceiver, and a housing at least partially
enclosing the structure, creating a pluggable transceiver.
[0045] One particular advantage of using the present invention in a
pluggable transceiver is that the entire device can be kept very
small. This is because high-speed serial interconnects, such as the
couplings of the present invention, reduce the need for power- and
space-intensive multiplexers, such as those used in the larger
optoelectronic transponders. However, high-speed interconnects are
also prone to the transmission-path discontinuities described
herein, and therefore benefit greatly from the use of the
discontinuity-reduction technology of the present invention. In
this sense, the discontinuity-reduction of the present invention
makes the use of optoelectronic devices without multiplexors much
more feasible. Thus, in one embodiment, the physical dimensions of
the optoelectronic device of the present invention are as follows:
width, 3 cm or less; length, 6.5 cm or less, and height, 1 cm or
less. In an alternate embodiment, the physical dimensions of the
optoelectronic device are: width, 0.54 inches or less; length, 2.24
inches or less; and height, 0.34 inches or less.
[0046] Another advantage of the present invention for a pluggable
optoelectronic transceiver is improved signal integrity in a
high-speed communications system operating at data transmission
rates of 5.0 Gb/s and above, and preferably 10 Gb/s and above.
Preferably, the pluggable optoelectronic transceiver uses one or
more high-speed differential signal paths.
[0047] An example of a coupling between a pluggable optoelectronic
device and an external electrical device (such as a host PCB) is
shown in FIG. 9. Specifically, FIG. 9 depicts the connection
between a contact pad and a complementary connector 900. (There
would actually be a series of such connections between a connector
unit and an edge-card connector system). The same features of the
external electrical device (i.e., connector unit) of FIG. 1 are
shown in FIG. 9. In addition, optoelectronic device card 904 has
been inserted into the slot formed on the right-hand side of the
connector unit. Contact pad 902 (not explicitly shown as a raised
surface) on the optoelectronic device card 904 is electrically
coupled to the complementary connector 102. Also, if there are
bottom contact pads on the optoelectronic device card 904, a bottom
contact pad is electrically coupled to the bottom-edge (low speed)
connector contacts 104.
[0048] Contact pad 902 may have one of the specialized surface area
geometries described above, in accordance with an embodiment of the
present invention. In addition, or in the alternative, contact pad
902 may also connect to an inductive trace (not shown explicitly as
a raised surface). Furthermore, the shortened ground plane shown in
FIG. 7 may be utilized in optoelectronic device card 904. The
effect of a specialized surface area geometry for contact pad 902,
and/or a connected inductive trace, and/or a shortened ground
plane, would be similar to the results shown in the graph at FIG.
8.
[0049] The electrical connection system of the present invention is
suitable for use with many types of electrical and electronic
devices other than optoelectronic devices, including processors,
ASICs, electrical interfaces, discrete electronic components, and
the like. It is particularly advantageous in both single-ended and
double-ended transmission paths.
[0050] While the present invention has been described with
reference to a few specific embodiments, the description is
illustrative of the invention and is not to be construed as
limiting the invention. Various modifications may occur to those
skilled in the art without departing from the true spirit and scope
of the invention as defined by the appended claims.
* * * * *