Signal integrity

Question

I am working on signal integrity of the PCBs. I have some misunderstanding in handling high frequency signals. Resistance and inductance increases with high frequency signals, so we have to take care while routing high speed signals.

But while I am referring some documents, they always mentioning faster rise time.

Why?

Why are they mentioning rise time instead of using high frequency? Do the circuits have different effects on faster rise times?

I am looking forward to the answer.

score 7 · Answer 1 · edited Dec 09 '19 at 12:24

Faster rise time is just another way to refer to a high frequency. The higher the rise time the more "squared" a signal is, and a square signal is nothing else but the sum of many sine signals of increasing frequency (the more components of high frequency, the more "squared" the signal will appear)

As frequency rises, the impedance that comes from inductances (PCB traces for instance) also rise. Other things to take into account for high speed PCB design:

Impedance matching: you need your tracks to be impedance matched as to have maximum power delivery along the line.
Dielectric material: FR4 is the standard core material of PCBs, but for high speed designs you might want to look at other materials like RO4003, which attenuates less the signals.

Additional information: Bandwidth = 0.35/Rise_time. Bandwidth is defined as the highest "significant" frequency content of the signal (lowest frequency is always dc). A faster rise time implies a low value of rise time, which means higher Bandwidth. Higher bandwidth implies signal contains higher frequency content. — skt9, Dec 18 '19 at 05:37

score 2 · Answer 2 · answered Apr 06 '18 at 08:18

Resistance and Inductance increases with high frequency signals.

Resistance increases proportional with \$\sqrt{F}\$ (due to skin and proximity effects) but inductance stays largely the same. Inductive reactance rises with frequency - maybe you meant that?

But while I am referring some documents. They always mentioning Faster Rise time. Why?? Whether they are mentioning Rise time instead of using high frequency.

The reactance of an inductor is \$2\pi f L\$ in AC circuits but ultimately this is based on Faradays formula: -

V = \$L\dfrac{di}{dt}\$

In other words the rate of change of current produces a proportionally sized blocking voltage, V. This is why trying to push a fast edge of current through an inductor is problematic.

So, in effect fast rise times and frequency are very related.

Signal integrity

2 Answers2