How to select source termination resistors by looking at signal?

If I have two devices that some distance apart on a fairly large PCB (could be 1 in, or it could be 20 in). If I arbitrary select a value for R1, lets say 33 ohms in this example and if my digital signal is distorted due to reflection, how can I determine if I should increase or decrease my resistance to get my digital signal correct ? What indicators when I look on a scope, should be key characteristics that my voltage divider ratio is too high or too low (referring to source impedance + R1 and Zo)

Assume that the characteristic impedance (ADD: of the trace) is (ADD: constant but) not controlled (ADD: / known).

simulate this circuit – Schematic created using CircuitLab

Measured at the receiver end:

• Undershoot means your series termination resistor value is too big
• Overshoot means your series termination resistor value is too small

For low-to-high edges there is only one definition of overshoot and undershoot, so that's the one used here. I like to use a similar definition for high-to-low edges, but not all agree so be careful.

The purpose of the source terminator is to damp the initial reflection from the end of the line when it returns to the source. The easiest way to pick The right one is to find the output impedance of your driver and then use the resistor to match that to the line impedance.

Here's another question that addresses this.

I think if you terminate too low your reflection coefficient is negative so the reflection will come out inverted. That older question seems to agree.

Occasionally you will also see people use high value series terminators to try and slow down an edge for emi. Not the best for ringing of course.

Any reflections that might distort your originating signal depend on the terminating impedance of the cable and not on a resistor placed in series with one of the cable wires. Also, if your originating signal is a true voltage source, the effect of reflections, at that voltage source will be zero.

I'll also add that if the originating source is not a perfect voltage source i.e. it has a finite impedance, you'll only see distortion if the originating signal is NOT a sinewave. In the case of a sinewave you might see modifications to its amplitude but no distortion.

EDIT - this demo is for Nick so he can understand what I'm saying: -

Animated picture taken from here

Red is the forward sine wave travelling down a cable from left to right. Green is the reflected sine wave travelling back up the cable to the originating source. Blue is the standing wave - the standing wave (at any fixed point on the cable) is actually a constant amplitude sine wave of the same frequency as the originating wave but at points along the cable it will have zero amplitude and at other points it will have maximum amplitude.

The blue trace looks like a modulated sine wave BUT IT ISN'T. It's a standing wave and predicts what the RMS amplitude of the combined forward and relected signals are at various points along the cable.

As for the rest of this question, without understanding the end-termination impedances of the cable, any amount of series resistance is just speculation and I'm surprised other answers haven't addressed this because it is of fundamental importance to the question.

• It's not a cable. It's a trace on a really big board. What is a "true voltage source" ? Apr 5, 2015 at 23:32
• Hm, then why do people add series termination of 33 ohm (if source impedance is 7-8 ohm) on a controlled 50 ohm trace for a high speed digital line ? Perhaps my question didn't imply digital signal (I'll edit). Apr 5, 2015 at 23:37
• Cable or traces on PCB make no difference because they act identically. A true voltage source produces a signal that cannot be distorted or reduced in amplitude. A non true voltage source is a true voltage source in series with an impedance. Apr 5, 2015 at 23:38
• Well this would be a non true source, because these are coming from ICs (microcontroller to some logic device). Apr 5, 2015 at 23:40
• Then you need to specify exactly what your situation is in your question. Apr 5, 2015 at 23:42