EMC Concept (4)

EMC Concept (4)

In EMC concept (2), it was introduced that there is a periodic signal and after taking Fourier series, we could see the waveform in the frequency domain. Now in this chapter we talk about the nonperiodic signals. It is converted by using Fourier Transform. Suppose it is the following function:

Taking Fourier Transform, we get:

the frequency domain is shown:

This is Fourier series from the periodic signal:

Compare the series and transform.

1. The series is not a continuous wave but the transform is.

2. The Co is A/2 and At. A/2 >> At

Because of the reason, the periodic signal usually causes EMI test fail.

Following figure is a sample for EMI test photo. The red arrow means we sometime could see the harmonic signals. The Green arrow we called it base, it is like the strength as nonperiodic signal.

Conculsion:

The periodic signal generates the uncontinous wave and larger energy field.

The nonperiodic signal generates the continuous wave but smaller energy field.

Note:

In real case, when the EMI engineer discovers the base is too high, he will recommend to do re-layout. This is because even we want to eliminate the peak value to under the limitation, it is impossible to under the base and still causes the test fail. If the EMI engineer finds it is harmonic signal, we give a solution on the clock system.