Gareth Waymark

High Speed Digital Design

26 November, 21

Category: electronics

It occurred to me just now that I haven’t anything about my uni work or electronics in general. One of the more interesting modules I’m doing is called “Microwave & High Speed Digital Design”.

In a nutshell, at high frequencies, 100 MHz and up, wires and printed circuit board (PCB) traces start to behave differently. It is a complex world where the physical layout of the wires and the materials in the vicinity of the traces change the behaviour of the signals. If you have ever noticed a PCB trace on a motherboard or stick of RAM that snakes and wiggles around then this is to ensure that all the signals arrive together at the same time. A 2666MHz digital clock signal (typical of DDR RAM) has a period (the time between pulses) of 384 picoseconds, the time it takes light to travel just under 12 cm. Suddenly we have to take into consideration the length of every trace.

A simple change of PCB track width such as a sharp 90 deg bend can send reflections back down the line to the sending device. In some cases this can be destructive to the chip. It’s even possible to create a filter using a short length of PCB trace that goes nowhere!

It’s a fascinating subject that I’m really enjoying and would like to pursue a little further after uni.