Download Classification of Data Streams Using Adaptive Naïve Bayes

Common Mode Noise Suppression in Differential
Right Angled Bend Using EBG Technique
Ashish Lohana, Jyoti Varavadekar & Sulabha Ranade
E-mail : ashishlohana@gmail.com, jyotivaravdekar@yahoo.co.in, sulabha.r4@gmail.com
Abstract – In recent years there has been a tremendous
increase in density of digital circuits’ layouts using printed
transmission lines at high frequency for high speed circuits
. Signal degradation caused due to various discontinuities
and external couplings, in the form of common mode noise
in such transmission lines cause loss of signal integrity.
EBG(electronic band gab) structures can be used to
suppress the common mode noise and improve differential
mode signal propagation.
I.
to, and the opposite polarity from, the first and ideally
ground carries zero current; this is called differential
signaling. when external noise couples to such a system
it gets removed at the receiver. Since a pair of lines are
required for differential signaling, microstrip coupled
lines are used for high speed data transmission at
microwave frequencies excited differentially.
INTRODUCTION
Printed transmission lines are widely used, they
provide circuits that are compact and light in weight.
The microstrip line is a transmission line geometry with
a single conductor trace on one side of a dielectric
substrate and a single ground plane on the opposite side.
There are numerous such transmission lines present on
PCB leaving little space between the two lines, thus
increasing the coupling between the two lines carrying
different data signals. This acts as external additive
noise, which once added cannot be removed and
degrading signal quality. A very high coupling
coefficient caused by a coupled microstrip lines is
therefore to form serious crosstalk coupled to the other
victim lines. Degradations also occur in printed
microstrips due to reflections from discontinuities
present in the form of bends, change in step widths, T
junctions, etc. When single transmission line is used for
propagation of signal, it forms single ended signaling
and an equal current returns through the ground plane
Fig.1 : Differential Signaling [1]
The differential amplifier at the receiver subtracts
the inverted version from the normal signal to yields a
signal twice of original signal:
II. USE OF DIFFERENTIAL LINES
(V + n) – (-V + n) = 2V
The coupling effect causes crosstalk between
nearby lines adding noise to the signal, which is not
removable. To reduce or suppress such noise use of
differential lines is made. When two lines are used for
signal propagation, one trace carries the positive signal
and the other carries a negative signal that is both equal
Where ‘n’ is the additive noise.
(1)
The noise coupled to the differential line is assumed
to be equal and thus same amount of noise travels
through the pair and thus called common mode signal.
For the lines to be perfectly differential:
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
For the voltages to be equal and opposite, as needed
for balance, the following features must exist:
III. STUDYING THE DIFFERENTIAL RIGHT ANGLED BEND
DISCONTINUITY
1.
The amplitude in both circuits must be identical
2.
The load impedance must be identical
3.
There can be no skew between rising and falling
edges
4.
The rise and fall time must be identical
In printed circuit boards various bends occur in the
transmission lines introducing discontinuities. one of the
most prominently occurring bend is the right angled
bend. since use of differential lines is made, a pair of
line will be used to suppress the common mode noise
occurring due to external coupling.
5.
The physical trace routes must be not only equaled
in length overall, but also balanced along their
entire length
6.
Coupling to any other conductors must be equal [2]
A differential right angled bend is as shown in
figure 3
Advantage of differential signaling is, it uses lower
voltage levels than single ended signals because the
threshold in differential receiver is better controlled than
in single ended due to high noise immunity. The lower
voltage swing leads to faster circuits and reduction in
power consumption, thereby increasing the bandwidth.
Fig. 3 : Differential right angled bend with inner line length
shorter than outer line length
However for the lines to be perfectly differential the
must be equal in length. as seen from the figure the
outer line is longer as compared to the inner line. this
introduces a skew in the signal and thus differential
mode gets converted to common mode signal. The sharp
right angled bend causes discontinuity which further
converts differential mode to common mode signal.
To make the length of the two lines to be equal,
detour technique can be used. [4]
Fig 2 : Differential signal at receiver with and without skew
Fig 4: Differential right angled bend with Detour
The below figure 5 shows the comparison of
common mode voltage applied to right angled bend and
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
right angled bend with detour, and it can be seen that for
the detour technique the common mode voltages nearly
cancel each other since the line lengths are equal and no
skew is present.
From figure 6 it is easily visible that mode
conversion from differential to common mode is less for
detour technique by 5dB that for simple right angled
bend, this due to the removal of skew by making inner
and outer lines of same length.
Fig 5 : Differential output voltage of skewed and detoured
coupled lines
To study differential lines, use of mixed mode S
parameters is made.[3]
Fig 7 : Common mode signal propagation for with and without
detour
From the above figure 7 it is seen that common
mode signal propagation has higher losses for 90' bend
at higher frequencies above 5GHz and can easily
propagate for detour technique.
From figure 6 and figure 7 we can say that
differential to common mode conversion is reduced for
detour technique which was desired. But the undesired
thing is once the common mode signal enters the system
it gets easily propagated in detour technique. Thus using
EBG technique.
Sdd = the differential S-parameters,
Scc = the common-mode S-parameters,
Sdc = the mode conversion that occurs when the device
is excited with common mode signal and the
differential signal is measured, and
IV. EBG TECHNIQUE
Scd = the mode conversion that occurs when the device
is excited with a differential- mode signal and the
common mode response is measured.
The Electromagnetic bandgap (EBG) structure is a
periodic structure that can prevent electromagnetic wave
to transmit. The EBG structure seizes attention owning
to its ability of blocking electromagnetic mode
transmission and radiation in microwave and millimeter
waves. At the first start, it is mainly applied to improve
the antenna design, for instance, to suppress the
crosstalk between antennas, to improve the impedance
matching of the low- profile antenna and to enlarge the
gain of the antenna. Afterwards, the EBG structure is
drawn on electromagnetic compatibility (EMC) area. It
is embedded in the power/ground plane for improving
the power integrity as well as electromagnetic
interference (EMI). It gives a significant effect of
suppressing ground bounce noise (GBN) and
simultaneous switch noise (SSN).
Fig 6 : Differential to Common mode conversion of with and
without detout technique
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
Fig. 8 : Mushroom Structure EBG [5]
Mushroom structure is a typical electromagnetic
bandgap (EBG) structure which is shown in Figure 8 Its
unit cell is composed of a patch and a via that connects
to the ground plane. The unit cell is placed periodically
on the ground plane. The mushroom structure is also
called high impedance surface (HIS) structure because
the surface impedance will become very high while
applying the mushroom structure to the metal plane. The
HIS structure is like an LC resonator in the specific
frequency range. In the specific frequency range, the
surface impedance becomes very high and also the
electromagnetic wave cannot transmit.
Fig 11: Equivalent of Mushroom structure EBG [6]
(2)
(3)
Fig 9 : Substrate layout for mushroom EBG
Microstrip Coupled line having width = 1.3mm,
spacing = 1.8mm, substrate heights are as shown in
Figure 10, with mushroom hat size of 7mm * 3.2mm
and spacing between the two mushroom = 0.18mm were
simulated in Advanced Design System.
From the below figure 12 it is seen that differential
to common mode conversion to common mode
conversion is highly attenuated in the frequency range 3
GHz to 6.5 GHz using EBG technique.
Fig 12 : Differential to Common mode conversion comparison
Fig 10 : Mushroom EBG for Differential right angled bend
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
VI. REFERENCES
[1]
H. Johnson, M. Graham, High-speed signal
propagation Advanced black magic Prentice Hall,
2002.
[2]
Bruce Archambeault, Associate Editor, Member IEEE,
Design Tip – EMC Considerations for Differential
(Balanced) Lines
[3]
Garth Sundberg, Member of the Technical Staff
Technology, Research, and Development, Grasp The
Meaning Of Mixed- Mode
S-Parameters.
Microwaves and RF, vol. 40, pp. 99-104, May 2001.
[4]
Guang-Hwa Shiue Dept. of Electr. Eng., Nat. Taiwan
Univ., Taipei Wei-Da Guo ; Chien-Min Lin; RueyBeei Wu, Noise reduction using compensation
capacitance for bend discontinuities of differential
transmission lines; IEEE Transactions Aug. 2006
[5]
Chung-Hao Tsai Dept. of Electr. Eng., Nat. Taiwan
Univ. (NTU), Taipei, Taiwan; Tzong-Lin Wu, A
metamaterial-typed differential transmission line with
broadband
common-mode
suppression,
Electromagnetic Compatibility - EMC Europe, 2009
International Symposium June 2009
[6]
Ying-Chun Lai, A Development of a Common-Mode
Filter Using an EBG Structure in High Speed Serial
Links, Stockholm, Sweden 2012, XR-EE-ETK
2012:012.
Fig. 13 : Common mode propagation
From the above figure we can see that common
mode signal propagation is highly reduce in the
frequency range of 3 GHz to 5.9 GHz using EBG
technique.
Thus from the above two figures 12 and 13 it is
clear that the designed EBG technique can be efficiently
used in the frequency range of 3 GHz to 5.9 GHz.
V. CONCLUSION
Skew in the differential line was reduced from
0.22V to 0.02V using detour technique thus reducing the
common mode noise. Differential to common mode
conversion was reduced by 5dB as compared to right
angled skew bend, by detour technique, but failed to
provide attenuation of common mode propagation. The
use of EBG provides advantages of very low differential
to common mode conversion and high attenuation of
common mode signal along in a band of 3 GHz to 5.9
GHz. Thus providing high bandwidth for utilization.
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