Learn the fundamentals of signal integrity
the junctions. The receiver sees the sum of the transmitted signal and all the reflections. Note that the received signal has nonuniform top and bottom levels due to the addition of the reflections. If the reflection amplitudes are high enough, errors can occur when reading the data. One of the critical goals of SI is the reduction of reflection anomalies. Return loss and insertion loss Transmission lines are characterized in both the frequency and time domains. Reflections are measured as return loss (RL) in units of decibels (dB) in the frequency domain (Figure 6). The portion of the incident power that fails to reach the load is characterized by the insertion loss (IL), also measured in dB. A lower insertion loss translates to a better connection. The parameter describing insertion loss in bulk coaxial cable is attenuation per unit length specified as dB per foot (dB/ft.) or dB per meter (dB/m). Noise Noise is any undesired signal that appears on a transmission line. Reflections can be viewed
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)
Figure 6: The return loss measures reflected power in the frequency domain, while the insertion loss measures the power received at the load. (Image source: Art Pini)
reflection coefficient determines the amplitude and phase of the reflection. The signal at the receiver is the sum of the transmitted signal and the time- delayed reflections. The junction of Z 2 and Z 3 reflects part of the incident signal back toward the transmitter while the bulk of the incident energy
continues to the receiver. The reflected signal encounters a mismatch in the reverse path and is partially reflected back toward the receiver. The edges of the signal are reflected with a polarity dependent on whether the impedance across the junction increases or decreases. The timing of the reflections depends on the physical distance between
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