PCB Trace as transmission line :RISE TIME vs Propagation Delay

I have seen the answer for when to consider PCB trace as a transmission line in many places. The answer is "Typically, if the signal pulse rise time is ‘small’ compared to the time it takes for the signal pulse to propagate (e.g. ½ of the total time the signal takes to travel along the trace) then you need to consider your PCB as a high-speed circuit".

But no one explains how the SI problems arise when the rise time is smaller than 2XPropagation delay.

May I know
1) what happens to a signal when it's RISE time is less than the propagation delay.
2) How this condition will make SI issues to come.

 

[FvM]

Unterminated transmission lines are causing signal overshoot and reflected pulses, the actual appearance of these waveform distortions depends on the ratio of rise time and propagation delay.

You can check in a SPICE simulation.

A similar rule of thumb tells that conductor segments shall be analyzed as transmission lines if there length is larger than 1/10 of the highest signal frequency components wavelength.

 

[hioyo]

D

Unterminated transmission lines are causing signal overshoot and reflected pulses, the actual appearance of these waveform distortions depends on the ratio of rise time and propagation delay.

You can check in a SPICE simulation.

A similar rule of thumb tells that conductor segments shall be analyzed as transmission lines if there length is larger than 1/10 of the highest signal frequency components wavelength.

Do I need to terminate a transmission line if my rise time is larger than propagation delay

 

[FvM]

According to your rule of thumb, termination can be omitted if the rise time is larger than 2x propagation delay. I'd say the order of magnitude sounds right, you should check the actual signal quality with a simulation or measurements.

Consider that different termination schemes are possible, for CMOS point-to-point connection, I'd prefer source series termination and unterminated load.

 

[crutschow]

Below is the simulation of a 4ns risetime signal into a 2ns propagation delay,100Ω transmission line, with a 10Ω source resistance.
Note the output ringing (yellow trace).

If the source resistance is increased to be equal to the transmission line impedance of 100Ω then there is no overshoot or ringing (blue trace)