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FIELD OF THE INVENTION
The present invention is related to a component useable for terminating
transmission lines so as to control EMI while maintaining signal integrity
and in particular to a component which combines a ferrite bead with
resistance, preferably in a surface mount package.
BACKGROUND OF THE INVENTION
Many electronic devices are advantageously configured to reduce or
substantially eliminate electromagnetic radiation which can potentially
cause interference, so called electromagnetic interference (EMI). Examples
of such electronic devices include computers, network devices such as
routers, bridges and the like. A major source of electromagnetic
interference is signal or data transmission lines which, depending on how
the lines are terminated, can effectively operate as antenna, transmitting
electromagnetic signals which result in interference. Typically, the
transmission lines are used for carrying signals which, for proper
operation of the electronic device, must be configured to avoid distorting
or changing the waveshape percent of the signal and must, in general,
substantially maintain signal integrity. Accordingly, it would be useful
to provide a termination device for transmission lines which is
configurable to simultaneously reduce or eliminate EMI and to maintain
signal integrity. In particular, resistors are commonly used as
transmission line terminators, but may not be completely satisfactory,
especially when both signal integrity and reduction of EMI are important.
Accordingly it would be useful to improve on resistors, as transmission
line terminators.
One type of transmission line commonly found in computers and other
electronic devices, and believed to be responsible for a major portion of
EMI are clock signal lines. Accordingly, it would be advantageous to
provide a device for terminating clock signal lines which both reduces or
eliminates EMI and maintains the waveshape or otherwise maintains signal
integrity of clock signals.
Typically, transmission line terminators will be provided on one or more
printed circuit boards (PCBs) of an electronic device. In general, many
electronic devices are designed to be as small as feasible so that the
circuit boards therein typically cannot be configured with additional
components which are large in size and/or numerous. Accordingly, it would
be useful to provide terminators for transmission lines which require
relatively little space on PCBs. Preferably, the terminator components are
configured so as to be relatively low in cost and relatively easy to
manufacture and assemble on the PCB.
SUMMARY OF THE INVENTION
The present invention provides a terminator device which effectively
reduces or eliminates EMI from a transmission line while substantially
maintaining signal integrity. In one aspect, the invention is an
improvement over resistors which, while effectively terminating
series-terminated transmission lines in many situations, typically do not
properly handle the higher frequency contents of many waveforms (e.g.
typical clock signal waveforms). In this context, higher frequency content
can include frequency content starting at about the "knee" frequency,
1/(.pi..tau..sub.r).
According to one aspect, the termination device includes a ferrite bead
combined with a resistance in such a way that the device (or the
equivalent circuit to the device) provides such resistance in series with
the ferrite bead. Adding ferrite material (e.g. inside a surface-mount
resistor package) will attenuate higher-frequency contents before they can
propagated down the transmission line, thus reducing EMI. The resistance
is selected, in light of the characteristics of the transmission line and
the signals it will carry, to provide substantially constant ("flat")
frequency response in lower frequency ranges while the ferrite bead
presents a large impedance at higher frequencies. A substantially flat
frequency response in lower frequencies assures signal integrity and acts,
in combination with the high frequency impedance from the ferrite beads,
to effectively reduce or eliminate EMI, while preserving signal integrity.
Preferably, the values of the resitance and ferrite are selected such that
the impedence created by the ferrite becomes relatively significant at and
beyond the "knee frequency" 1/(.pi..tau..sub.r).
Preferably, in order to achieve this functionality without unduly consuming
surface area of the circuit board, the ferrite bead-resistance combination
is provided in a surface mount package, preferably a relatively small
surface mount package. Although it is possible to provide the ferrite bead
and the resistance as two discrete components within the surface mount
package, in one embodiment, the ferrite bead is itself configured to have
a desired low frequency resistance.
According to one aspect of the invention a ferrite bead is placed in series
with a terminating resistor on a transmission line such as a clock
transmission line. In lower frequency ranges where the ferrite bead is
substantially not effective, the transmission line will be effectively
terminated by the resistor. In higher frequency ranges where the resistor
is substantially no longer effective, the transmission line will be
terminated by the ferrite bead. In one aspect, the resistor and ferrite
bead are provided in a single surface mount package.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram depicting a source-terminated transmission
line on a PCB according to previous devices;
FIG. 2 is a graph of typical clock pulses showing amplitude versus time;
FIG. 3 is a frequency-domain diagram of the spectrum of the clock waveform
of FIG. 2;
FIG. 4 is a schematic diagram of an equivalent circuit for a surface mount
resistor;
FIGS. 5A and 5B are diagrams of typical magnitudes of impedance as
functions of frequency for a typical surface mount resistor are a typical
ferrite bead, respectively;
FIG. 6A is a schematic diagram of an equivalent circuit for a transmission
line terminator according to an embodiment of the present invention;
FIG. 6B is a perspective view of a PCB, partially broken away, including a
transmission line with source termination according to an embodiment of
the present invention, and
FIG. 7 is a diagram of the equivalent frequency response of impedance for a
transmission line terminator according to an embodiment of the present
invention.
DETAILED DESCRIPTION
The present invention includes a recognition of the nature of certain
problems in previous devices. Both ferrite beads and resistors have been
used individually for various purposes. However, neither component, by
itself, addresses the problems solved by the present invention, such as
reducing unnecessary high-frequency currents on a transmission line (e.g.
to avoid or reduce EMI) while preserving the waveform of signals (e.g. on
a clock signal line or other type of transmission line).
Ferrite beads have been used in connection with controlling EMI e.g. by
positioning ferrite beads on lines exiting the chassis, particularly
adjacent to a chassis connector, e.g. to prevent high frequency currents
going beyond the chassis by absorbing their magnetic fields and
dissipating the energy as heat. However, a ferrite bead, by itself, has
insufficient low-frequency impedance to operate effectively as a
transmission line source terminator. Indeed, it is believed that,
previously, ferrite beads were specifically designed to provide low
frequency impedance which was as small as possible.
Certain types of transmission lines, e.g. as illustrated in FIG. 1, are
provided with a resistor, such as a surface mount resistor 112 between a
source 114 and a transmission line 116 leading to a load 118. This
configuration may be suitable for certain types of transmission lines 116
in which there is, e.g. less concern with electromagnetic interference. In
a configuration of FIG. 1, the resistor 112 is selected such that its
resistance, plus the output impedance of the driver 114 is approximately
equal to the characteristic impedance of the transmission line (Z.sub.0)
116. A typical value the resistor 112 might be, e.g. about 33 ohms in
connection with driving a 55 ohm transmission line 116.
FIG. 2 shows a typical, if somewhat idealized, clock pulse for transmission
on a clock line, depicting the amplitude A as a function of time T. In the
depicted embodiment, the clock pulse has a typical characteristic
trapezoidal shape 212.
FIG. 3 depicts the frequency-domain spectrum 310 of the clock waveform of
FIG. 2. For a pure, idealized resistor (with substantially no capacitance
or inductance) the driving waveform (FIG. 2) is voltage-divided by the
series-termination resistor 112 and the Z.sub.0 of the transmission line
116 before it propagates down the line. A typical load is about 8 pF and 4
nH, caused by the package and silicon parasitics of the load. Under ideal
conditions, the reflection coefficient at the end of the line, i.e. at the
load 118, is +1 and, accordingly, the waveform doubles and reflects back
towards the source 114. The termination resistor 112 (plus the source
resistance) being equal to the Z.sub.0 of the line 116, the waveform is
dampened. The frequency 312, equal to 1/(.pi..tau..sub.r) is the frequency
below which most of the energy concentrates. For a rise/fall time of one
nanosecond, frequency 312 corresponds to about 318 MHZ. In order to ensure
that the waveform of the signal, such as the clock signal of FIG. 2,
travels to the load 118 undistorted (and thus does not cause signal
integrity violation such as excessive clock skew, threshold errors and the
like), it is useful to ensure that the frequency response to the
transmission line is relatively constant at frequencies less than the
1/(.pi..tau..sub.r) frequency 312. In one embodiment, it is desired to
provide a configuration in which the transmission line source terminator
has substantially constant resistance between DC and about 1 GHz.
Unfortunately, resistors used in actual circuits can depart significantly
from an idealized pure resistor. A typical surface mount resistor provides
a certain amount of capacitance and inductance, with the equivalent
circuit for a surface mount typical resistor being depicted in FIG. 4. A
typical surface mount resistor provides an amount of inductance 412,
generally considered lead inductance and an amount of capacitance 414
referred to as parasitic capacitance. This leads to behavior, in the
frequency domain, illustrated (somewhat simplified) by the diagram of
impedance 515 as a function of frequency, shown in FIG. 5. An isolated
surface mount resistor typically is capable of terminating a transmission
line in the range of frequencies where signal integrity is important, such
as in the lower frequencies (below the knee frequency). Thus it is
generally desirable to improve on a resistor, as a transmission line
terminator, to address higher-frequency EMI concerns, without
substantially degrading the type of lower-frequency signal integrity
acheivable with a resistor. Without wishing to be bound by any theory, in
general, at higher frequencies (beyond the knee frequency), the resistor
no longer effectively blocks current from the driver, and no longer
terminates effectivley terminates the transmission line as the reflected
wave comes back toward the drivers (the reflection coefficient
.GAMMA..noteq.1, resulting in reflection). The higher frequency currents
are allowed to bounce back and forth on the transmission line, until they
attenuate, resulting in EMI.
Accordingly, the behavior of an ideal resistor and, to an even greater
degree, the behavior of an actual surface mount resistor, departs
significantly from the desired substantially constant impedance between DC
and about 1 GHz. Accordingly, while a surface mount resistor, in
isolation, may be suitable as a line terminator for many purposes, when
used in connection with a clock line or similar transmission line where it
is important to avoid EMI (without sacrificing signal integrity or
waveform integrity, which can otherwise result in excessive clock skew,
threshold errors and the like), a surface mount resistor may be less than
ideal.
FIG. 5B depicts the frequency domain impedance 514 (on a log-frequency
scale) of one example of a ferrite bead. Although the frequency-domain
behavior of various ferrite bead components differ, in general, as
illustrated in FIG. 5B, a ferrite bead also typically fails to provide a
substantially constant impedance between DC and 1 GHz.
According to an embodiment of the present invention, a transmission line
such as a clock signal line or similar transmission line, is
source-terminated by a combination of a resistor and a ferrite bead. A
series-combination of a resistor and a ferrite bead is believed to
correspond to equivalent circuits similar to that of FIG. 6A. In the
configuration of FIG. 6A, the combination not only provides the lead
inductance 612 and parasitic capacitance 614 associated with the resistor
616, but also inductance 618 arising at least from the ferrite bead. As
shown in the frequency domain diagram of FIG. 7 (depicting impedance from
the resistor 716, ferrite bead 718 and the ferrite bead-resistance
combination 722 of the equivalent circuit of FIG. 6A), preferably the
resistor and ferrite bead are selected substantially so that the high
frequency impedance attributable to the ferrite bead 718 increases
substantially in the same frequency region where the impedance of the
resistor component begins to drop off 716. Accordingly, the impedance of
the combination 722 avoids the type of rapid mid-frequency decrease in
impedance shown in FIG. 5A. The frequency responses shown in FIG. 7 are
intended to illustrate, generally the relative contributions of the
resistor and ferrite bead, as a function of frequency. The actual
frequency responses for particular embodiments of the present invention
may differ from the illustration of FIG. 7. Without wishing to be bound by
any theory, it is believed that this mid-frequency behavior of the
combination, as depicted generally in FIG. 7, is largely responsible for
preserving wave shapes and avoiding signal integrity violations of, e.g.,
clock signals on source terminated transmission lines using the present
invention. In the frequency region where the impedance of the transmission
line (as a function of frequency) should be constant (i.e. frequencies
less than about 1/.pi..tau..sub.r) the impedance of the combined R-L is
preferably designed to be equal to Z.sub.0 -Z.sub.out (driver impedance).
In the region above the 1/.pi..tau..sub.r, the impedance of the combined
R-L is preferably designed to be in the range of about 500 ohms to 750
ohms, preferably about 700 ohms, to dissipate high frequency currents
believed to contribute to EMI.
In order to provide a resistor-ferrite bead device, as described, which is
practical for use in modem electronic devices, it is particularly
advantageous to provide the combination in a single surface-mount package,
preferably in a relatively small package such as having a footprint of
less than about 5 mm.sup.2, preferably less than about 4 mm.sup.2.
Although, as noted above, ferrite beads typically have been fabricated so
as to provide low frequency impedance which is as small as possible, in
one embodiment the present invention is implemented by configuring a
ferrite bead to provide a substantial low-frequency resistance (such as by
fabricating a ferrite bead having a constricted region and/or doping with
materials having less conductivity) so that a single device provides both
the high frequency impedance function of a typical ferrite bead and the
low frequency resistance features desired. Alternatively, a resistor and a
ferrite bead can be provided as separate components and combined in series
to form a transmission line source terminator, according to an embodiment
of the present invention.
In light of the above discussion, a number of advantages of the present
invention can be seen. The present invention is able to provide a
transmission line terminator, such as a transmission line source
terminator, which can reduce or avoid EMI while substantially preserving
integrity of signals, such as clock signals and the like. The present
invention provides for a generally improved transmission line source
terminator in a relatively small component, preferably a surface mount
component, such as providing a terminator with a width 632 less than about
1.5 mm and/or a length 634 less than about 2.5 mm and/or a footprint
(length times width) less than about 5 mm.sup.2. The present invention
substantially avoids significant drops in impedance at frequencies below
about 1/.pi..tau..sub.r, preferably at frequencies below about 1 GHz. The
present invention provides for substantial impedance such as impedance of
greater than about 500 ohms, at higher frequencies, such as frequencies
greater than about 100 MHZ, e.g. for reducing or avoiding EMI. The present
invention provides a generally improved transmission line source
terminator in a fashion that is economical to design, fabricate, assemble
and use.
A number of variations of modifications of the invention can be used.
Although the invention has been described as being particularly useful in
connection with clock signal lines, the invention can also be used in
connection with other types of transmission lines, such as control lines
and other series terminated transmission lines. It is possible to use some
features of the invention without using others. For example, it is
possible (although not necessarily economically desirable) to provide a
transmission line terminator as described herein by providing a resistor
and a ferrite bead in series, without providing the terminator as a
surface mount package.
The present invention, in various embodiments, includes components,
methods, processes, systems and/or apparatus substantially as depicted and
described herein, including various embodiments, subcombinations, and
subsets thereof. Those of skill in the art will understand how to make and
use the present invention after understanding the present disclosure. The
present invention, in various embodiments, includes providing devices and
processes in the absence of items not depicted and/or described herein or
in various embodiments hereof, including in the absence of such items as
may have been used in previous devices or processes, e.g. for improving
performance, achieving ease and.backslash.or reducing cost of
implementation. The foregoing discussion of the invention has been
presented for purposes of illustration and description. The foregoing is
not intended to limit the invention to the form or forms disclosed herein.
Although the description of the invention has included description of one
or more embodiments and certain variations and modifications, other
variations and modifications are within the scope of the invention, e.g.
as may be within the skill and knowledge of those in the art, after
understanding the present disclosure. It is intended to obtain rights
which include alternative embodiments to the extent permitted, including
alternate, interchangeable and/or equivalent structures, functions, ranges
or steps to those claimed, whether or not such alternate, interchangeable
and/or equivalent structures, functions, ranges or steps are disclosed
herein, and without intending to publicly dedicate any patentable subject
matter.
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