The Fourier series of a periodic signal f(t) is given by
f(t) =Equation 1.1
For a square wave this reduces to
f(t) =Equation 1.2
(For the purpose of this discussion we assume a 50% duty cycle with rise and a fall times tending to “0”.)

As can be seen, this is a series of sinusoids of the odd harmonics of the fundamental frequency ω0 and can be depicted in the frequency domain as shown above in Figure 1.

These energy peaks are the elements that compliance engineers battle.

Managing EMI / RFI

Current Solutions.

Solutions today are plenty and can broadly be divided into these categories “conventional” and “non-conventional”. The pros and cons of these are listed below

Current and Conventional:

Passives: Chokes, Capacitors, Shields, Sprays, Multi-layer PCBs etc
Pros: Easy to use, very well known, point solutions
Cons: Degrade signal and compromise signal integrity, add weight, and bulk, “trial and error” approach, do not provide a system-wide benefit.

Current and non-conventional:

Active solutions: "Dithering" and legacy (1st-Gen) SS technology.
Pros: Highly effective when applicable, system wide EMI / RFI benefit, deterministic.
Cons: Requires design-in and technical and architectural provisioning to use, not so well known, current generation (1st-Gen) not applicable to numerous applications especially timing sensitive ones and mobile platforms due to excessive, jitter, high power consumption and part-to-part variations.