DocID025260 Rev 1

3/14

AN4356

EMI and noise issues

14

Moreover, the data rates of new standards are even higher:

•

MIPI M-PHY up to 5.4 Gbps

•

USB 3.0 up to 5 Gbps

•

MHL up to 6 Gbps

The frequencies used, and especially their harmonics, are in the same range as the mobile

Rx frequencies, between 700 MHz and 2.7 GHz, and in the same range as the various RF

frequencies used by FM, TV, GPS etc.

These high data rates involve clock and data signals with very fast transition times in the

range of a hundred picoseconds and lower, inducing fast transitions and highly radiated

fields. The effect of this is amplified by more complex sub-circuits using connectors and flex

that can act like antennas, spreading unwanted frequencies and noise (Figure 2).

Figure 2. Radiated and conducted emissions

An important source of noise is the skew that can appear on a flexible PCB between interior

and exterior traces on a differential link. (Figure 3).

Figure 3. Effect of skew on differential link

Internal links

External links

FLE

X

t

t

t

t

t

t

t

t

D+

D-

[D+]-[D-]

[D+]+[D-]

Transmitter

Receiver

Differential signals

Skew