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ysaclibabal.tk - Signal Integrity: Applied Electromagnetics and Professional Practice
One example of this "memory" nature is the capacitive effect. Due to capacitive effect, each transition has a finite charge or discharge time. If the transition happens such that the next transition occurs before the previous transition reaches the designated level, deviation of both time and level occurs for the current bit.
Such an effect can be cascaded.
The ISI effect is shown in Figure 1. Any pulse-width broadening or spreading effects cause ISI, and dispersion is a known physical phenomenon that causes a traveling pulse to be broadened or spread.
As such, ISI is expected to occur in a fiber-based communication system too. Those different modes have different propagation times. The spread of the propagation times in multimode fiber cause the pulse to spread at the other end of the fiber. For a single-mode fiber, the dominant spread mechanism is the dispersion effects, including chromatic dispersion CD and polarization mode dispersion PMD.
The physical reason for CD is that the refraction index of the fiber material is wavelength-dependent. Therefore, the group velocity of the wave propagation inside the fiber is wavelength-dependent. Both laser source and modulation waveform have some spread in their spectrum. The combined spread spectrum of the input optical waveform, coupled with the CD effect, causes the optical pulse train to spread in the time domain, resulting in both timing and amplitude ISI. PMD is due to the birefringence, in which the refraction indexes along the two orthogonal axes are different, causing different propagation velocities.
Again, the two different velocities for the two orthogonal modes of PMD eventually cause pulse train at the other end of the fiber to spread, resulting in ISI. Two types of crosstalk are discussed here. One is associated with copper cables, and the other is associated with optical fibers.
Crosstalk is basically an interference phenomenon. Crosstalk is generally involved in a parallel channel system in which signals are propagated concurrently and affect each other. For copper-based communication channels, crosstalk is caused by electromagnetic coupling. For integrated circuits ICs where the geometry and space between connects is relatively small, the capacitive coupling is the dominant mechanism.
For board-level circuits where the geometry is relatively large, inductive and capacitive coupling are both important. Inductive coupling follows Lentz's Law, in which changing the magnetic field flux generates an electrical field, and that electrical field, coupled with electrical charge, causes voltage fluctuation.
In general, the effect of crosstalk can be modeled primarily as the voltage fluctuation or noise. However, it can affect the timing jitter directly as well. When two transmission lines are coupled capacitively, and when digital transitions occur simultaneously on two lines from the same end the near end , the slew rate of the signals at the other end the far end is larger if the two transitions at the near end are in phase have the same polarity or is smaller if the two transitions are out of phase have the opposite polarity.
The crosstalk due to the simultaneous steps' response with opposite polarities slows down the slew rate of the step signals at the far end.
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From the definitions of mutual capacitive and inductive constants C m and L m , the voltage noises due to capacitive and inductive coupling can be calculated according to the following equation:. For inductive-induced voltage noise, we have. You can see that crosstalk is proportional to the voltage or current slew rate. As the date rate or frequency keeps increasing, the rise time of the digital signal becomes smaller. Therefore, the slew rate and crosstalk-induced noise increase. As mentioned in previous sections, timing jitter due to crosstalk can be estimated through division of appropriated far-end signal slew rate.