EMI issue on the mobile phone

AN3141

2/14

Doc ID 17004 Rev 1

1

EMI issue on the mobile phone

The baseband part of the mobile phone can be either the object or the source of EMI.

A mobile phone can generate a power up to 2 W in the GSM band. This radiation can impact

external wires, such as the hands-free kit, or USB cable. This can generate TDMA noise on

the audio part, but also errors on the USB link. In this case the mobile phone is the object of

EMI, as shown on Figure 1.

Figure 1.

Baseband is a victim of EMI

The mobile phone can be also a source of EMI, regarding the RF part.

The sensitivity of a mobile phone needs to be very good, especially concerning

communications far from the base station. According to TS125101 and TS100910

standards, sensitivity must be as low as -104 dBm in GSM mode and -107 dBm in

W-CDMA. These sensitivities can be achieved by a radio system, but it needs to be free of

external noise.

As the digital signal spectrum is wide-band, harmonics can occur at the same frequency as

the receiving frequency. Radiation at these frequencies, by a flex for example, can impact

the antenna, producing noise at the LNA input.

Figure 2 presents a case where the base station (BST) signal received by the antenna is

lower than the flex radiation. The base station signal is below the noise floor, and cannot be

used by the mobile phone. Only a BST signal higher than noise floor can be received.

Mobile phone sensitivity is thus defined by the flex radiation noise, and not by the RF system

performance.

Antenna

LCD

BB

Flex

Camera

t

t

Antenna radiation, TDMA

TDMA

demodulation

Antenna

LCD

BB

Flex

Camera

Antenna

LCD

BB

Flex

t

t

Antenna radiation, TDMA

TDMA

demodulation

Camera