Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2000-01-21
2002-02-12
Gaffin, Jeffery (Department: 2841)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C174S250000, C174S261000, C333S012000, C333S246000, C333S247000, C710S120000
Reexamination Certificate
active
06347041
ABSTRACT:
BACKGROUND
1. Technical Field
This patent application relates, in general, to suppressing electromagnetic radiation in and around data processing systems.
2. Description of the Related Art
Data processing systems generally include electronic components (e.g., integrated circuits, resistors, capacitors, etc.) which are typically mounted on or integrated within printed circuit boards. A printed circuit board is a board made of non-conducting material, such as plastic, glass, ceramic, or some other dielectric on which electronic components are mounted. A printed circuit board has metallic tracings to provide electrical connections between various electronic components mounted on the printed circuit board.
During operation of a data processing system, one or more electronic components typically send electronic signals over one or more conductive paths (e.g., metallic traces) of one or more printed circuit boards. Such electronic signals often result in electromagnetic energy being radiated.
Electromagnetic radiation can interfere with data processing system operation (in which case the electromagnetic radiation is referred to as electromagnetic interference (EMI)). Accordingly, efforts are made within the art to suppress electromagnetic radiation emission from and around printed circuit boards and/or their associated electronic components.
One way in which electromagnetic radiation emissions are conventionally suppressed is via the use of what is known in the art as Low Voltage Differential Signaling (LVDS). LVDS is illustrated in FIG.
1
.
With reference now to
FIG. 1
, shown is a partially schematic diagram depicting an example of LVDS via use of differential driver
100
which is contained within one or more printed circuit board
550
. Illustrated is that differential driver
100
produces first voltage waveform
102
, shown for sake of example as a square wave ranging between 0 and +5 volts which typically would have a period of around 2 nano-seconds (e.g., a 500 MHz waveform, very commonly used in microprocessor systems, has a period of 2×10
−9
seconds), on first printed-circuit-board trace
104
. Also shown is that second voltage waveform
106
, which is depicted as an exact inverse of first voltage waveform
102
, and which is illustrated for sake of example as a square wave ranging between 0 and −5 volts which also typically would have a period of around 2 nano-seconds (e.g., the period of a 500 MHz waveform), on second printed-circuit-board trace
108
, which is the exact inverse of first voltage waveform
102
in voltage and which is exactly synchronized with first voltage waveform
102
in time.
There are at least two intents involved in LVDS. A first intent is to increase the accuracy of any data processing system in which LVDS is used, since the difference of the received signal will give a signal of twice the magnitude of either of the differential signals taken alone. For example, if second waveform
106
is subtracted from first waveform
102
, resultant signal will be a square wave waveform of twice the magnitude of first voltage waveform
102
.
A second intent of LVDS is to reduce the amount of electromagnetic energy radiated by voltage waveform
102
and voltage waveform
106
. This is illustrated by resultant radiated energy graph
110
. The intent of resultant radiated energy graph
110
is to show that at some distance from printed-circuit-board traces
104
and
108
, the energies respectively radiated by voltage waveform
102
and
106
substantially cancel each other. While those skilled in art will recognize that the energies do not typically cancel to exactly zero as indicated by resultant radiated energy graph
110
, those skilled in the art will appreciate that the cancellation LVDS does tend to be substantially effective.
There are certain instances, described below, in which the differential signals, or waveforms, used in LVDS get out of phase (i.e., become un-synchronized in time). Such a lack of synchronization can create radiated energy in a fashion such as that illustrated in FIG.
2
.
Referring now to
FIG. 2
, depicted is differential driver
200
, substantially analogous to differential driver
100
, in which second waveform
206
, output by differential driver
200
on first printed-circuit-board trace
208
, is slightly out of phase (or time synchronization) with first waveform
202
output by differential driver
200
on second printed-circuit-board trace
204
. Illustrated by resultant radiated energy graph
210
is that at some distance from printed-circuit-board traces
204
and
208
substantial energy is radiated by voltage waveform
202
and
206
, in that the phase-mismatch, or time slipping, of one waveform in relation to the other indicates that they do not cancel each other.
There are numerous ways that initially time-synchronized LVDS signals can become un-synchronized (i.e., develop a phase differential) when traveling within separate printed-circuit-board traces. One example showing how initially time-synchronized LVDS signals can become un-synchronized is illustrated in FIG.
3
.
With reference now to
FIG. 3
, illustrated is an example of how LVDS can get out of phase. Shown is differential driver
100
driving first and second printed circuit traces
300
and
302
. Depicted are that printed circuit traces
300
and
302
are formed in circular arc fashion, about a point P, with a “radius,” R
1
, of about 2 units for inner trace
302
and a “radius,” R
2
, of about 3 units for outer trace
300
, with the arc of printed circuit trace
302
arranged inside that of the arc of printed circuit trace
300
. Here, theta, in radians is roughly &pgr;/2. Those skilled in the art will recognize that the formula for circular arc length is approximately R*&thgr;. Thus, the distance traveled by a signal on inner trace
302
is about &pgr;/2 units less than that traveled by a signal on outer trace
300
. Consequently, insofar as that the electrical energy in both inner trace
302
and outer trace
300
travels at about the same velocity, signal
102
traveling on outer trace
300
will arrive at the endpoint of outer trace
300
time-delayed relative to signal
104
(initially time-synchronized with signal
102
) traveling on inner trace
302
. To illustrate this, shown in
FIG. 1
is a sample calculation with units set to inches. The units are expressed in inches for ease of illustration, but those skilled in the art will recognize that in typical printed circuit boards, units of length are usually expressed in terms of “mils” (standing for thousandths of an inch). The calculation used in
FIG. 3
essentially states that a distance, d, traveled by a point on an electromagnetic energy waveform (e.g., a point on a square wave) multiplied by (1/(velocity of the electromagnetic energy waveform in a particular medium)) equals the time it takes for the point on the electromagnetic waveform to traverse the distance d. Illustrated is that the different-length paths followed by inner trace
302
and outer trace
304
result in loss of time synchronization between signal
102
and
104
, which will accordingly give rise to electromagnetic energy radiation analogous to that described in relation to FIG.
2
.
In printed circuit board layout design, it is not uncommon for printed-circuit-board traces carrying paired LVDS signals to take slightly different paths, such as those illustrated in relation to
FIG. 3
(albeit on a much smaller scale), along and through a printed circuit board. Such varying paths which quite often result in a phase difference between respectively paired LVDS waveforms. This fact has long been recognized, and it is conventional within the art to build in some architecture at the end of the trace taking the shorter path on and/or through the printed circuit board in order to delay the signal on the shorter path such that the signal on the longer path has a chance to “catch up” with the signal on the shorter path, thereby allowing the LVDS signals to be “differenced” to achieve one of the intended bene
Hailey Jeffrey C.
Simon William M.
Dell USA L.P.
Gaffin Jeffery
Skjerven Morrill & MacPherson LLP
Vigushin John B.
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