Why is ground meaningless in high speed circuits whereas more important is the path of return current?

Question

I know everything about this topic but I don't know why ground is meaningless in high speed circuits. So basically in high speed circuits these are the factors we need to consider-:

Decoupling caps

Coupling/Crosstalk

Bandwidth

Ground bounce

Impedance matching

Timing

I know about routing signal traces, we should keep check of these things-:

1. Distribution of signal and return

2. Coupling

3. Common impedance

4. Trace impedance matching

5. Trace density

6. Transmission line concerns(Basically stuffs of high speed design)

Also I know that in low speed, return current goes the path of least resitance whereas in high speed circuits, return current returns to path of loweset inductance.

Similarly, I know there should not be slot in ground plane(Although I don't know why, maybe because it increases loop area). But I am not able to conclude why ground is meaningless in high speed circuits and why return current path is more important?

Answer 1

First off, I believe the main proposal of your question is ill-cited.

I would agree that some properties that are usually associated with the term "Ground" in low frequency do not hold at high frequencies (where 'high' depends on the circuit size).

• High frequency currents cause return currents in any nearby conductor, regardless of its potential, so it does not matter e.g. for trace impedance if you run over a Ground plane or power plane, or whatever-other-node plane. Usually Ground and power work best because the signals are references to them directly by the senders and receivers so the current loop will be smallest.
• The potential of the Ground node can vary locally based on the wavelength of the respective frequency
• Therefore, high frequency signals are only properly defined locally with respect to their local reference. You can't just measure them with respect to a global "Ground" point, or at least you would obtain meaningless figures. For low frequency circuits in contrast, it is often enough to know that they are ground-referenced and you will measure the same value with respect to any point on the Ground node.

Answer 2

Usually in high speed circuits (50MHz+) you need a transmission line to avoid attenuating the signal an ringing on the lines. This means the transmission line inductance resistance and capacitance needs to be balanced. This also means that the return current path will need to be balanced (depending on the geometry of transmission line that the designer selects).