Learn the fundamentals of signal integrity
(GHz). The three short stubs under the green checked box have a length of 0.025 in. Their resonant frequency is ten times higher or 60 GHz. Both spectral responses are shown in the spectrum analyzer plot in the upper left. The red spectrum is the response of the 0.25 in. stub, while the green trace is that of the 0.025 in. stub; the 0.25 in. stub shows a “suck out” response centered at 6 GHz with a very low amplitude. The eye diagram in the upper right corner overlaps multiple-bit sequences of 011, 001, 100, and 110 to produce a graphical SI measurement. As long as the eye remains open, the transmission is
successful. Vertical eye closures are due to noise, reflections, and crosstalk. Horizontal eye closures are related to timing issues like jitter. The 6 GHz resonance results in the collapsed eye due to the loss of signal amplitude. SI in specifications for interconnection components Interconnection components that support AI processors in data centers include coaxial and twisted pair cables, connectors, and pc boards (Figure 9). These components are usually specified in terms of characteristic
while far-end crosstalk (FEXT) appears on the receiver side. Crosstalk can be reduced by increasing the distance between adjacent transmission lines, decreasing the path length, using differential lines that cancel noise common to both lines, keeping traces on adjacent pc board layers perpendicular, and incorporating integral grounding and electromagnetic interference (EMI) shielding. Resonance Resonance occurs when a signal’s path is a multiple of a quarter of the signal’s wavelength. At such points, the reflected signal overlaps the incident wave and either amplifies or attenuates the transmitted signal. The frequencies corresponding to these wavelengths are called resonances. Resonances can cause noise or distort signals, and arise due to unterminated lengths of transmission lines, called stubs, in the signal path or non-ideal ground returns. Figure 8 shows resonance effects due to stubs of several kinds with two different lengths on a 12 gigabit per second (Gbps) channel. The stubs highlighted with red boxes have a length of 0.25 inches (in.), resulting in a resonant frequency of about 6 gigahertz
Figure 5: The transmitted signal is distorted by the reflected components summed with time delays proportional to the propagation delays of the reflection’s path. (Image source: Amphenol)
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