U.S. patent application number 13/100965 was filed with the patent office on 2012-11-08 for intra-pair skew cancellation technique for differential signaling.
This patent application is currently assigned to Texas Instruments Incorporated. Invention is credited to Robert F. Payne.
Application Number | 20120280720 13/100965 |
Document ID | / |
Family ID | 47075452 |
Filed Date | 2012-11-08 |
United States Patent
Application |
20120280720 |
Kind Code |
A1 |
Payne; Robert F. |
November 8, 2012 |
INTRA-PAIR SKEW CANCELLATION TECHNIQUE FOR DIFFERENTIAL
SIGNALING
Abstract
A method for deskewing a differential signal is provided. A
common-mode voltage of a differential signal and an average for the
common-mode voltage of the differential signal are measured. A
difference between first and second portions of the differential
signal is determined, and deskew information is derived from the
common-mode voltage and the average. The deskew information can
then be combined with the difference to deskew the differential
signal.
Inventors: |
Payne; Robert F.; (Lucas,
TX) |
Assignee: |
Texas Instruments
Incorporated
Dallas
TX
|
Family ID: |
47075452 |
Appl. No.: |
13/100965 |
Filed: |
May 4, 2011 |
Current U.S.
Class: |
327/72 |
Current CPC
Class: |
H04L 25/0276 20130101;
H03K 5/003 20130101 |
Class at
Publication: |
327/72 |
International
Class: |
H03K 5/22 20060101
H03K005/22 |
Claims
1. An apparatus comprising: a measuring circuit that is configured
to receive a differential signal and that is configured to
determine a common-mode voltage; an averaging circuit that is
configured to receive the differential signal and that is
configured to determine an average for the common-mode voltage; and
an output circuit having that is coupled to the measuring circuit
and the averaging circuit and that is configured to receive the
differential signal, wherein the output circuit determines a
difference between first and second portions of the differential
signal, and wherein the output circuit determines deskew
information from the common-mode voltage and the average, and
wherein the output circuit generates a deskewed differential signal
from the deskew information and the difference.
2. The apparatus of claim 1, wherein the output circuit further
comprises: a first differential amplifier that is configured to
receive the differential signal; a second differential amplifier
that is coupled to the measuring circuit and the averaging circuit;
and an adder that is coupled to the first and second differential
amplifiers.
3. The apparatus of claim 2, wherein the first and second
amplifiers have first and second gains, respectively, wherein the
second gain is twice the first gain.
4. The apparatus of claim 1, wherein the measuring circuit further
comprises a voltage divider.
5. The apparatus of claim 1, wherein the averaging circuit further
comprises: a first resistor that is configured to receive the first
portion of the differential signal and that is coupled to the
output circuit; a second resistor that is coupled to the first
resistor and that is configured to receive the second portion of
the differential signal; and a capacitor that is coupled to the
first and second resistors.
6. A method comprising: measuring a common-mode voltage of a
differential signal; measuring an average for the common-mode
voltage of the differential signal; determining a difference
between first and second portions of the differential signal;
determining deskew information from the common-mode voltage and the
average; and combining the deskew information with the difference
to deskew the differential signal.
7. The method of claim 6, wherein the step of determining the
deskew information further comprises determining a difference
between the common-node voltage and the average.
8. The method of claim 7, wherein the step of determining the
difference between the first and second portions of the
differential signal further comprises applying a gain to the
difference between the first and second portions of the
differential signal.
9. The method of claim 8, wherein the gain further comprises a
first gain, and wherein the step of determining the deskew
information further comprises applying a second gain to the
difference between the common-node voltage and the average.
10. The method of claim 9, wherein the second gain is at least
twice the first gain.
11. An apparatus comprising: a first terminal that is configured to
receive a first portion of a differential signal; a second terminal
is configured to receive a second portion of the differential
signal; a measuring circuit that is coupled to the first and second
terminals and that is configured to determine a common-mode
voltage; an averaging circuit that is coupled to the first and
second terminals and that is configured to determine an average for
the common-mode voltage; a first difference circuit that is coupled
to the first and second terminals and that is configured to
determine a difference between the first and second portions of the
differential signal; a second difference circuit that is coupled to
the measuring circuit and the averaging circuit and that is
configured to determine deskew information from the common-mode
voltage and the average; and a combiner that is coupled to the
first and second output circuits.
12. The apparatus of claim 11, wherein the first difference circuit
further comprises a differential amplifier having a gain.
13. The apparatus of claim 12, wherein the differential amplifier
further comprises a first differential amplifier, and wherein the
gain further comprises a first gain, and wherein the second
difference circuit further comprises a second differential
amplifier having a second gain.
14. The apparatus of claim 13, wherein the combiner further
comprises a node.
15. The apparatus of claim 13, wherein the second gain is at least
twice the first gain.
16. The apparatus of claim 15, wherein the measuring circuit
further comprises: a first resistor that is coupled to the first
terminal and the second differential amplifier; and a second
resistor that is coupled between the second terminal and the first
resistor.
17. The apparatus of claim 16, wherein the averaging circuit
further comprises: a node that is coupled to second differential
amplifier; a third resistor that is coupled between the first
terminal and the node; a fourth resistor that is coupled between
the second terminal and the node; and a capacitor that is coupled
to the node.
Description
TECHNICAL FIELD
[0001] The invention relates generally to deskewing circuitry and,
more particularly, to intra-pair deskewing circuitry.
BACKGROUND
[0002] Turning to FIG. 1, an example of a conventional
communication system 100 can be seen. This system 100 generally
comprises a transmitter 102, a transmission medium 104, and a
receiver 106. Typically, the transmission medium 104 is comprised
of a pair of transmission lines that are configured to carry a
differential signal from the transmitter 102 to the receiver 106.
This type of system 100 is used in a wide variety of applications
ranging from transmitting information over long distance (through
cables) to on-chip communications. One issue with system 100 is
that there is usually a length mismatch between the transmission
lines in medium 104 that can lead to skew between the portions of
the differential signal carried by the medium 104 (known as
"intra-pair skew"), and an example of intra-pair skew can be seen
in FIG. 2, where the transmission medium 104 for this example
introduces 1 unit interval (UI) of skew. For low frequency signals,
intra-pair skew can be largely ignored, but, for high frequency
signals (i.e. >1 Gb/s), intra-pair skew can significantly impair
communications.
[0003] To address intra-pair skew, several solutions have been
proposed. These proposed solutions approach intra-pair skew as
being a propagation delay issue, and an example of such a proposed
solution can be seen in FIG. 3. As shown, skew compensator 200
(which is included within receiver 106) uses several delay elements
202-1 to 202-N coupled in series with one another to delay each
portion of the differential signal accordingly to compensate for
the intra-pair skew. Adjustments to the delay elements 202-1 to
202-N are made through adjustment of the control voltage VCNTL, but
it can often be difficult to adequately adjust the relative delays
to compensate for the intra-pair skew. Therefore, there is a need
for an improved skew compensator.
[0004] Some other conventional circuits are: U.S. patent
application Ser. No. 12/948,757; U.S. Pat. No. 6,335,647; U.S. Pat.
No. 6,909,980; U.S. Pat. No. 7,729,874; U.S. Patent Pre-Grant Publ.
No. 2005/0099216; U.S. Patent Pre-Grant Publ. No. 2006/0244505;
U.S. Patent Pre-Grant Publ. No. 2009/0174448.pdf; Zheng et al., "A
5 Gb/s Automatic Sub-Bit Between-Pair Skew Compensator for Parallel
Data Communications in 0.13 .mu.m CMOS," 2010 Symposium on VLSI
Circuits/Technical Digest of Technical Papers, Jun. 16-18, 2010;
Olisar, Robert, "Unbalanced Twisted Pairs Can Give You the
Jitters!," Maxim Engineering Journal, Vol. 64, September 2008, pp.
5-12.
SUMMARY
[0005] An embodiment of the present invention, accordingly,
provides an apparatus. The apparatus comprises a measuring circuit
that is configured to receive a differential signal and that is
configured to determine a common-mode voltage; an averaging circuit
that is configured to receive the differential signal and that is
configured to determine an average for the common-mode voltage; and
an output circuit having that is coupled to the measuring circuit
and the averaging circuit and that is configured to receive the
differential signal, wherein the output circuit determines a
difference between first and second portions of the differential
signal, and wherein the output circuit determines deskew
information from the common-mode voltage and the average, and
wherein the output circuit generates a deskewed differential signal
from the deskew information and the difference.
[0006] In accordance with an embodiment of the present invention,
the output circuit further comprises: a first differential
amplifier that is configured to receive the differential signal; a
second differential amplifier that is coupled to the measuring
circuit and the averaging circuit; and an adder that is coupled to
the first and second differential amplifiers.
[0007] In accordance with an embodiment of the present invention,
the first and second amplifiers have first and second gains,
respectively, wherein the second gain is twice the first gain.
[0008] In accordance with an embodiment of the present invention,
the measuring circuit further comprises a voltage divider.
[0009] In accordance with an embodiment of the present invention,
the averaging circuit further comprises: a first resistor that is
configured to receive the first portion of the differential signal
and that is coupled to the output circuit; a second resistor that
is coupled to the first resistor and that is configured to receive
the second portion of the differential signal; and a capacitor that
is coupled to the first and second resistors.
[0010] In accordance with an embodiment of the present invention, a
method is provided. The method comprises measuring a common-mode
voltage of a differential signal; measuring an average for the
common-mode voltage of the differential signal; determining a
difference between first and second portions of the differential
signal; determining deskew information from the common-mode voltage
and the average; and combining the deskew information with the
difference to deskew the differential signal.
[0011] In accordance with an embodiment of the present invention,
the step of determining the deskew information further comprises
determining a difference between the common-node voltage and the
average.
[0012] In accordance with an embodiment of the present invention,
the step of determining the difference between the first and second
portions of the differential signal further comprises applying a
gain to the difference between the first and second portions of the
differential signal.
[0013] In accordance with an embodiment of the present invention,
the gain further comprises a first gain, and wherein the step of
determining the deskew information further comprises applying a
second gain to the difference between the common-node voltage and
the average.
[0014] In accordance with an embodiment of the present invention,
the second gain is at least twice the first gain.
[0015] In accordance with an embodiment of the present invention,
an apparatus is provided. The apparatus comprises a first terminal
that is configured to receive a first portion of a differential
signal; a second terminal is configured to receive a second portion
of the differential signal; a measuring circuit that is coupled to
the first and second terminals and that is configured to determine
a common-mode voltage; an averaging circuit that is coupled to the
first and second terminals and that is configured to determine an
average for the common-mode voltage; a first difference circuit
that is coupled to the first and second terminals and that is
configured to determine a difference between the first and second
portions of the differential signal; a second difference circuit
that is coupled to the measuring circuit and the averaging circuit
and that is configured to determine deskew information from the
common-mode voltage and the average; and a combiner that is coupled
to the first and second output circuits.
[0016] In accordance with an embodiment of the present invention,
the first difference circuit further comprises a differential
amplifier having a gain.
[0017] In accordance with an embodiment of the present invention,
the differential amplifier further comprises a first differential
amplifier, and wherein the gain further comprises a first gain, and
wherein the second difference circuit further comprises a second
differential amplifier having a second gain.
[0018] In accordance with an embodiment of the present invention,
the combiner further comprises a node.
[0019] In accordance with an embodiment of the present invention,
the second gain is at least twice the first gain.
[0020] In accordance with an embodiment of the present invention,
the measuring circuit further comprises: a first resistor that is
coupled to the first terminal and the second differential
amplifier; and a second resistor that is coupled between the second
terminal and the first resistor.
[0021] In accordance with an embodiment of the present invention,
the averaging circuit further comprises: a node that is coupled to
second differential amplifier; a third resistor that is coupled
between the first terminal and the node; a fourth resistor that is
coupled between the second terminal and the node; and a capacitor
that is coupled to the node.
[0022] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter which form the subject of the claims
of the invention. It should be appreciated by those skilled in the
art that the conception and the specific embodiment disclosed may
be readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes of the present
invention. It should also be realized by those skilled in the art
that such equivalent constructions do not depart from the spirit
and scope of the invention as set forth in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0024] FIG. 1 is a diagram of an example of a conventional
communication system;
[0025] FIG. 2 is a diagram illustrating intra-pair skew for the
system of FIG. 1;
[0026] FIG. 3 is a diagram of an example of a conventional skew
compensator;
[0027] FIG. 4 is a diagram of an example of a skew compensator in
accordance with an embodiment of the present invention; and
[0028] FIGS. 5A-8 are diagrams depicting the operation of the skew
compensator of FIG. 4.
DETAILED DESCRIPTION
[0029] Refer now to the drawings wherein depicted elements are, for
the sake of clarity, not necessarily shown to scale and wherein
like or similar elements are designated by the same reference
numeral through the several views.
[0030] Turning to FIG. 4, an example of a skew compensator 300 in
accordance with an embodiment of the present invention can be seen.
This skew compensator 300 is generally included in a receiver
(i.e., receiver 106) so as to perform intra-pair skew compensation,
and it generally comprises an averaging circuit 302, a measuring
circuit 304, and an output circuit. The output circuit generally
uses two difference circuits (which are typically differential
amplifiers 306 and 308) that generate differential signal
information and deskew information on a main signal path and a
deskew path, respectively. This differential signal information and
deskew information can then be combined by combiner 310 to generate
an output signal VOUT that should generally match the desired,
deskewed differential signal. Typically, differential amplifier 306
(which is in the main signal path) is coupled to the input
terminals of the skew compensator 300 (which carry the portions INM
and INP of the differential signal), while the differential
amplifier 308 (which is in the deskew path) is coupled to the
averaging circuit 302 and measuring circuit 304 so as to receive a
common-mode voltage VCM and an average of the common-mode voltage
VCMA. The common-mode voltage VCM is usually generated by the
measuring circuit 304 through the use of a voltage divider (i.e.,
resistors R3 and R4), while the average VCMA is generated by using
a voltage divider (i.e., resistors R1 and R2) to continuously
measure the common-mode voltage VCM and a memory device (i.e.,
capacitor C1) to average it over a long period of time.
[0031] To illustrate the function of skew compensator 300, signals
propagating through system 100 (which includes skew compensator
300) are shown in FIGS. 5A and 5B. In this example, transmitter 102
outputs a differential signal, where the portions INM and INP can
have a value of +1 or -1 (assuming differential amplifier 306 has
unity gain). As the differential signal propagates across medium
104, 1 UI of intra-pair skew is introduced, which significantly
distorts the differential signal. It can, however, be recognized
that the intra-pair skew converts the differential signal to
common-mode, since there is a difference between the common-mode
voltage VCM and the average VCMA (which for this example is 0).
When the difference between portions INM and INP (i.e., INP-INM) is
taken by differential amplifier 308, this difference can have
values of +2, 0, and -2, and the difference does not match the
desired (deskewed) output from transmitter 102. Because the
difference between the common-mode voltage VCM and the average VCMA
can have values of -1, 0, or +1, differential amplifier 308 (for
this example) applies again of 2 so as to generate the deskew
information (i.e., 2*(VCM-VCMA)). This deskew information can then
be combined with the difference between the portions INM and INP by
combiner 310 (which can, for example, be an adder or node) to
generate the output signal VOUT, which generally matches the output
of the transmitter 102. It should also be noted that the gains of
differential amplifiers 306 and 308 are typically different, and
that the gain of differential amplifier 308 will typically be at
least twice or double the gain of differential amplifier 306.
[0032] Turning now to FIG. 6-8, other examples of the operation of
the skew compensator 300 can be seen. In FIG. 6, there is no
intra-pair skew, so the skewed and deskewed signals generally match
(but are shown with different scales). In FIG. 7, 1/2 UI (which is
about 50 ps for this example) of skew is introduced, and the skew
compensator 300 is able to fully recover the eye. In FIG. 8, there
is 1 UI (i.e., about 100 ps for this example) of skew (which
significantly distorts the differential signal), and the skew
compensator 300 is able to recover the differential signal.
[0033] Having thus described the present invention by reference to
certain of its preferred embodiments, it is noted that the
embodiments disclosed are illustrative rather than limiting in
nature and that a wide range of variations, modifications, changes,
and substitutions are contemplated in the foregoing disclosure and,
in some instances, some features of the present invention may be
employed without a corresponding use of the other features.
Accordingly, it is appropriate that the appended claims be
construed broadly and in a manner consistent with the scope of the
invention.
* * * * *