Inductively Coupled Noise and Resistively Coupled Noise: A Signal and Power Integrity Tutorial

Figure 6. Measuring the inductively coupled noise
on the victim trace adjacent to the aggressor
signal with no gap and separated by a gap.
The inductively coupled noise is reduced by
about 40% on the victim trace separated by a gap.
This is a small impact.

The following is excerpted from Professor Eric Bogatin's article in the Signal Integrity Journal, The Case for Split Ground Planes. Reprinted by permission of Signal Integrity Journal.

This section continues from the discussion on Return Current at Low Frequency.

. . .

Inductively Coupled Noise

In a plane, at frequencies below about 10 kHz, return currents will not flow under the signal path, but will spread out in the return plane. Above 10 kHz, the return currents are localized under the signal paths. 

When we have two adjacent signal paths that are over a wide, continuous plane, they will show inductive cross talk at high frequency. Even with minimal overlap of the return currents, there is still loop mutual inductance between the two signal-return paths. This inductive noise is driven by the changing current, the dI/dt, in the aggressor signal-return path, which will get smaller at lower frequency.

Fundamentals of Power Integrity: Mutual Aggressors and Rail Transient Response Measurement

Fig 1. Rail droop in response to a load step is
a typical case of mutual aggressors in a PDN.

A third type of noise found in PDNs is what we call mutual aggressors, which is crosstalk coupling from one component of the PDN onto another.

An obvious example is a load step in the PDA, where something in the system being turned on pulls current from the VRM that supplies a rail. In Figure 1, you can see how the output voltage of the VRM supplying a 1 V rail droops in response to a load step before it recovers. This is still noise: it is a signal variation that we're not expecting and don't want.

We want to be able to characterize that noise, because too much droop could affect the operation of other components that are already consuming power from that device.

In order to do so, we’re going to measure the rail transient response to the load application. We need only look at two signals: the voltage and the current on the rail of interest. Figure 1 shows the voltage on C5 (the green trace) and the current on C8 (the orange trace).