Differential Signaling

Abstract

In exploring transmission lines and signals, it is clear that signals that are close to each other have both capacitive and inductive coupling between them. It turns out that this is both good news and bad news. In this chapter, we will start with the good news and introduce differential signaling. In the next chapter, we will break the bad news and discuss crosstalk. One common method for better signal integrity is to move to differential signaling. This seemingly simple change, using both a positive and negative waveform in a matched set, eliminates the need for an intact ground plane, takes care of return current, manages ground bounce, and generally improves signal integrity.

Homework

Construct a SPICE simulation of two coupled dual transmission lines with LEN = 1, L = 100 nH, C = 10 pF, and LM = CM = 0. Connect the lines serially and connect a voltage source (800 ps risetime) at one end and a single terminating resistor across the outputs of the lines. (See the diagram below.)

• 1. Calculate the Z ₀ of each line. For R _S = 0 (i.e., no source resistance) and R _L = 2Z ₀, simulate the signal and show that the signal has “perfect” signal integrity.

• 2. Add a 2 nH inductor in series and a 5 pF shunt capacitor (to ground) to one of the signal lines at the midpoint between the two sets of lines. Which is affected more – the differential signal or the signal of a single line?

• 3. Restore the setup of the first problem (i.e., eliminate the L and C) and instead create 200 ps of clock skew between the two voltage sources. Do this by making one of the differential signals switch 200 ps after the other one. What effect does it have on the differential signal? On each of the individual signals?

• 4. Restore the setup of the first problem and instead change LM to 10 nH and CM to 5 pF. What effect does this have? How do you explain the result?

Example: Schematic of Problem 1 and Problem 4

Plot the voltage at each source, the differential voltage across the inputs (measured at the input to the first set of coupled transmission lines) and the differential voltage across the output (measured at the output of the second set of coupled transmission lines).