How to know if your design is high speed design?

First of all when I say high speed, it doesn’t mean high frequency, it means quick signal transition.

If the trace is longer than the ¼ distance travelled by the signal during its rise time, then it’s a high speed design. There is a lot of information in the last sentence. Let’s break it down.

The rise time of the signal is usually measured in nanoseconds.

Now you need to know the speed of the signal in the PCB. And the speed depends on the dielectric of the PCB. Let’s assume it’s 4 for simplicity.

So the speed of the signal on the PCB is about 150,000,000 meters per second.

So say in 1 nanosecond, the signal will travel 15cm. So now we can take the rise time of different technologies and compute the length the signal would travel in that time. For TTL rise time is 5ns, distance travelled 75 cm, LVDS - rise time is 0.3ns. Distance travelled 4.5 cm.

This length is called TEL - Transition Electrical Length.

If you are working LVDS and your trace is longer than ¼ of 4.5 cm that is 1.12 cm. The signal will overshoot. You need to manage the impedance to control the reflections.

So to sum it up, the design is high speed if the trace is longer than ¼ TEL.