THE PHYSICS OF HIGH-SPEED SIGNAL INTEGRITY Three key tools used in measuring source-to-load power losses in interconnect data links.
INSERTION LOSS: The Insertion loss of a link, or a portion of a link is a measure of the circuit resistance, or simply the amount of power lost during the transmission. For a section of a cable, the power lost follows an exponential decay per unit length. It’s important to distinguish between power and voltage, because the rules for addition for power and voltage are different. Power is a real number. AC voltage is a real number times a phase: 1 Watt plus 1 Watt is always 2 Watts But AC voltage at a given frequency: 1 Volt plus 1 Volt is anything between 0 and 2 depending on how the phases line up. Most link budgets are estimated using power, because it’s simple. But in complex links with multiple connectors, the effects of phases must be considered, resulting in insertion loss being expressed in logarithmic form: IL= Log10(power_in/power_out)
RETURN LOSS: When power is lost in a link, it can be either due to the circuit resistance (insertion loss), or it can be lost because some portion of the signal was reflected back into the link (in optical fiber transmission the same phenomenon is often referred to as back-reflection). Losing the signal to ohmic loss or to reflections has a different impact on the quality of what we receive, therefore both numbers are important when considering the quality of a connector or a cable. The cause of return losses is a change in impedance along the link, often at the connectors, but potentially originating in the cable due to a kink, sharp bend, or construction defect. Intermittent defects in a cable (for instance due to restrictions from taping or braiding) can create strong return-loss reflections at certain frequencies, as if a mirror had been inserted into the system.
This measurement of insertion loss from a HD Stacker high-speed board-to-board connector shows -3dB insertion loss at 5.6 GHz for an effective electrical bandwidth of 12 Gbps
This measurement of return loss from a HD Stacker high-speed board- to-board connector shows -3dB insertion loss at 5.6 GHz for an effective electrical bandwidth of 12 Gbps
QwikConnect • July 2020
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