Transmission Line – Understanding Current in a Transmission Line

An interconnection should be considered a transmission line if reflections are detrimental to your application. As Leon Heller says, 1/10th wavelength is a good rule-of-thumb.

However, all this depends on the nature of the signal (1/10th of what wavelength?). With sine waves, the wavelength in the cable will be about $\frac{2 X 10^8}{f}$ where f is the frequency, but will vary according to the cable type. The effect of reflections on sine waves is to cause standing waves, so the amplitude will vary along the length of the cable but will always be sinusoidal. With square waves the situation is complicated by harmonics. Although the square-wave may have a low fundamental frequency, the harmonics introduced by the edges will be significant. The effect of reflections in this case is undershoot, overshoot or ringing depending on where the mismatches are (if both $R_S$ & $R_L$ are lower than $Z_0$ for example, you will get ringing).

So do you need to match the source? No. As long as the load is perfectly matched, the wavefront will travel down the transmission line and be fully absorbed by the load. Consider a digital system with 3.3V logic for example. If you terminated both source and load you would only get 1.65V swing at the load. Terminating only at the load is termed parallel termination.

There is also a technique known as series termination in which the source is terminated but the load is left open circuit. In this case, a half-amplitude edge propagates down the line and is reflected back to the source, lifting the voltage to full amplitude as it returns until it is absorbed by the source termination. Along the line, you would see a 'stepped' signal except right at the load where you would see a clean waveform.

So when talking to one board, the other 9 are high impedance;

This isn't quite right. The input impedance of the receiver doesn't change when it is listening or not listening. So all 10 loads will be high impedance (or capacitive).

If this is the case, should I series terminate each line according to the transmission line impedance (ribbon cable, so ~100 Ohms)?

This won't do any good. 100 ohms in series with a high impedance is still a high impedance. If you are going to terminate these lines at the receiving end, you would need the termination to be in parallel with the load. But be careful before you do that and make sure your driver can actually drive a 100 ohm load.

Series terminations are more often seen at the source, since the driver tends to be low impedance, and, say, 95 ohms in series with the driver might match a 100-ohm line reasonably well.

If you have no series termination and high impedance, you get up to 2*Vin at the source on the back reflection. That was just one wire; I just scaled that up for 9 more reflections. Is my logic flawed?

Yes, your logic is flawed. Because if you split up the signal to lead to 9 (or 10) loads, only a fraction of the energy would travel down each line. If all the lines were the same length, you'd end up with a total reflection of 2*Vin (or probably a little less because some of the signal would have been reflected back at the fanout point, and returned out of phase with the other reflections).

So what should you actually do?

Depending on your design constraints, you could try

• Connecting the loads in daisy-chain configuration and provide a parallel termination only at the end.

or

• Use a fan-out buffer to drive the signal to each load separately