ST AN3141 Application note

AN3141

Application note

LC filters for mobile phone LCD and camera links

Introduction

The mobile phone is a compact, small device, which radiates high power, and needs to have a very good sensitivity for signal reception. This functional conflict generates an EMI issue, which can be addressed by filtering.

September 2010 Doc ID 17004 Rev 1 1/14

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EMI issue on the mobile phone AN3141

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

Antenna radiation, TDMA

Antenna

LCD

Camera

Flex

TDMA

demodulation

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.

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AN3141 EMI issue on the mobile phone

Figure 2. Baseband is a source of EMI, which reduces the mobile’s sensitivity

Only flex radiation, BST signal is canceled

Antenna

Camera

Flex

LCD

Flex radiation

Flex

radiation

Coming from BST

GSM

Coming from Flex radiation

Flex signal, frequency domain

Flex signal, temporal domain

To keep the mobile phone sensitivity at the radio level, the radiated digital signal needs to be filtered.

Doc ID 17004 Rev 1 3/14

Link between baseband processor and LCD or camera AN3141

2 Link between baseband processor and LCD or

camera

One of the noisy digital links is the LCD or camera bus. The trends for LCD size and camera resolution are both increasing, thus requiring the transmission of greater and greater volumes of data. These trends result in clock frequencies which are now in the range of 1050 MHz. Harmonics are at the same frequencies as the GSM reception band. Baseband processor are also getting faster and faster, inducing very low rise and fall times, making the digital signal spectrum wide band.

In a mobile phone, these links use a Flex PCB several centimeters long, which acts as an antenna.

Consequently, combining all these parameters, digital links can drastically reduce the sensitivity of a mobile phone.

Figure 3 shows the signal waveform measured on an LCD link.

Figure 3. Signal to control the LCD

1 V/div 20 ns/div

This signal has rise and fall time in the range of 2 ns. These edges will radiate electromagnetic fields in the in the range of RX mobile phone frequencies.

To evaluate the spectrum generated by this waveform, a measurement has been done on the similar signal, with a spectrum analyzer:

● Signal amplitude: 3 V

● Signal frequency: 20 MHz

● Modulation: PRBS 2n-1

● Rise and fall time: 1.8 ns

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AN3141 Link between baseband processor and LCD or camera

We choose to perform a measurement at 940 MHz, which is near the middle of the GSM RX band.

The measurement setup is presented in Figure 4. Due to the spectrum content of a digital signal, band-pass filter is needed to avoid saturation of the spectrum analyzer input stage.

Figure 4. Harmonic measurement test setup

Spectrum analyzer

Cavity band-

pass filter

Signal generator

50 Ω output

Isolator

DUT

Measurement results is shown in Figure 5 (DUT short-circuited).

Figure 5. Spectrum at 940 MHz of a 1.8 ns rise/fall time, 20 MHz signal

The result shows that the emission of this signal is significant regarding the required sensitivity of a mobile phone.

This measurement was performed on one line, while digital links integrate several lines in parallel. In addition, measurements have been performed on 50 environment, which is not the case in the application (link is not impedance matched, drivers are low impedance and receiver high impedance). Consequently, radiated field can be very high.

Without any filtering, this signal can drastically decrease the sensitivity of a mobile phone.

Doc ID 17004 Rev 1 5/14

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