#038 EMI Suppression ~Signal Transmission Line Design and EMI~

When sending a clock signal to a receiver, you might concern that a long signal line affects an accurate signal sending.
The longer the transmission line, the more susceptible it is to a noise exposure as well as signal deterioration.

One of the things you have to consider in a signal transmission line is the effect of common mode noise.
A radiated emission level stemming from a common mode noise current is much higher than from a normal mode noise current.

So what should we do? (see #33)

Many people design signal transmission lines with a single end, but in a single end, the return side of the circuit is used as the GND, so the impedance of the sending and the return is not symmetrical.
This is why it is called an unbalanced circuit. On the other hand, a balanced circuit is one in which the impedance of the sending and the return is symmetrical and equivalent so that the impedance is also symmetrical even when the sending and the return are viewed from the GND (see the figure below).



A common mode current can be reduced if a sending and a return current are kept the same (normal mode) (see #028). Here, the differential transmission line can be treated as a balanced circuit.

When EMI suppression components are required on transmission lines, it is important to balance the line impedance so that the signal, performance and safety quality can be maintained while ensuring EMI performance.


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