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. 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.
I. I NTRODUCTION
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
II. U SE OF D IFFERENTIAL L INES
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
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:
(V + n) – (-V + n) = 2V (1)
Where ‘n’ is the additive noise.
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:
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
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]
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.
III. S TUDYING THE D IFFERENTIAL R IGHT A NGLED B END
D ISCONTINUITY
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.
A differential right angled bend is as shown in figure 3
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
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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
ISSN (Print) : 2319 – 2526, Volume-2, Issue-4, 2013

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]
S dd
= the differential S-parameters,
S cc
= the common-mode S-parameters,
S dc
= the mode conversion that occurs when the device is excited with common mode signal and the differential signal is measured, and
S cd
= the mode conversion that occurs when the device is excited with a differential- mode signal and the common mode response is measured.
Fig 6 : Differential to Common mode conversion of with and without detout technique
97
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.
IV. EBG T ECHNIQUE
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).
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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 10 : Mushroom EBG for Differential right angled bend
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Fig 12 : Differential to Common mode conversion comparison
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International Journal on Advanced Computer Theory and Engineering (IJACTE)
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. C ONCLUSION
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.
VI. R EFERENCES
[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; Ruey-
Beei 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.
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