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AN3141Application noteLC filters for mobile phone LCD and camera linksIntroductionThe 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 2010Doc ID 17004 Rev 11/

EMI issue on the mobile phone AN31411 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 Figure1.

Figure nd is a victim of EMIAntenna radiation,TDMAAntennaCameratLCDFlexBBtTDMAdemodulationThe 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.

Figure2 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.

2/14Doc ID 17004 Rev 1

AN3141Figure issue on the mobile phone

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

Only flex radiation,BST signal is canceledAntennaFlexradiationCameraFlexLCDFlexradiationf+Coming from BSTGSMfComing from Flex radiationfFlexfFlex signal,frequency domainBBFlex signal,ttemporal domainTo keep the mobile phone sensitivity at the radio level, the radiated digital signal needs to be

filtered.

Doc ID 17004 Rev 13/14

Link between baseband processor and LCD or camera AN31412 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 10-50 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.

Figure3 shows the signal waveform measured on an LCD link.

Figure to control the LCD

1V/div20 ns/divThis 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

4/14Doc ID 17004 Rev 1

AN3141Link 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 Figure4. Due to the spectrum content of a digital

signal, band-pass filter is needed to avoid saturation of the spectrum analyzer input stage.

Figure ic measurement test setup

Spectrum analyzerCavity band-pass filterSignal generator50ΩoutputIsolatorDUTMeasurement results is shown in Figure5 (DUT short-circuited).

Figure um 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 15/14

Filtering the LCD or camera link AN31413 Filtering the LCD or camera link

The filter choice is made according two parameters:

●●Bandwidth, to keep the signal integrity in case of digital signals

Rejection at the frequency to attenuate, i.e. at RX frequencies of mobile phone

LCD and camera link clock frequency is in the range of 10 - 50 MHz, and the digital signal is

a wide band spectrum signal. Consequently, bandwidth of the filter needs to be in the range

of 200 MHz to keep good signal integrity.

As shown before (see Figure2), the filter must reject the mobile phone reception

frequencies: GSM to W-CDMA gives frequencies from 869 MHz to 2.17 GHz.

In this frequency range attenuation needs to be as high as possible to limit the emissions of

the digital link.

Three types of device have been developed by STMicroelectronics to ensure good signal

integrity:

●●●EMIF10-LCD02F3, 10-line version, (or EMIF07-LCD02F3, 7-line version)

EMIF08-LCD04M16,

EMIF10-LCD03F3 (or EMIF07-LCD03F3)These devices offer a bandwidth of 200 MHz, as shown in Figure6, Figure7 and ation at RX frequencies are not the same:●●●EMIFxx-LCD02F3 is an RC type filter and provides attenuation of -25 dB at 900 08-LCD04M16 is an LC filter and provides attenuation of -35 dB at least on RX

band, which is better than the RC type -LCD03F3 is an LC type filter and provides attenuation of -50 dB on RX band -

better than the RC type filter.6/14Doc ID 17004 Rev 1

AN3141Figure 6.0Filtering the LCD or camera link

Frequency response of EMIF10-LCD02F3, RC type

dB-10-20-30-40-50F (Hz)-60100k1MLine 1Line 3Line 5Line 710M100MLine 2Line 4Line 6Line 81G6GFigure ncy response of EMIF08-LCD04M16, LC type

0-5-10-15-20-25-30-35-40-45-50-55-60-65-70dBI2I3I4I5I6I7I8-------O2 #1O3 #1O4 #1O5 #1O6 #1O7 #1O8 #1F (Hz)100k1M10M100M1GDoc ID 17004 Rev 17/14

Filtering the LCD or camera link

Figure 8.0-10-20-30-40-50-60-70-80F (Hz)AN3141Frequency response of EMIF10-LCD03F3, LC type

dB-90100k1MLine 1Line 3Line 5Line 710M100MLine 2Line 4Line 6Line 81G6GAll filters have a high bandwidth so they have a very small impact on the signal in the time

domain, as shown on the Figure9.

Figure ncy response of EMIF10-LCD03F3, LC type

Without filterWith LC type,EMIF08-LCD04M161V/div.10 ns/ LC type,EMIF10-LCD03F3With RC type,EMIF10-LCD02F38/14Doc ID 17004 Rev 1

AN3141Filtering the LCD or camera link

The same measurements have been performed in the frequency domain, at 940 MHz.

Figure10, Figure11, and Figure12 show the harmonic power with RC and LC type filter.

Figure um at 940 MHz, with RC type filter, EMIF10-LCD02F3Figure um at 940 MHz, with LC type filter, EMIF08-LCD04M16

Doc ID 17004 Rev 19/14

Filtering the LCD or camera link

Figure um at 940 MHz, with LC type filter, EMIF10-LCD03F3AN3141Power at 940 MHz, without filter was -92.5 dBm (see Figure5):

●●●With EMIF10-LCD02F3, RC type filter, the power is decreased to -120 EMIF08-LCD04M16, LC type filter, the power is decreased to -126 EMIF10-LCD03F3, LC type filter, the power becomes insignificant, as it is lower

than the -130 dBm, which is the noise floor of the measurement system.

This shows that the use of a filter decreased the harmonic levels of the LCD and camera

links.

An LC type filter, such as the EMIF08-LCD04M16 or EMIF10-LCD03F3, ensures the best

filtering, because harmonic power is insignificant, while maintaining good signal integrity.

10/14Doc ID 17004 Rev 1

AN3141Filter placement

4 Filter placement

The goal of filters is to avoid flex radiation at mobile phone reception frequencies.

Consequently, filters need to be placed just before the flex connector, at the digital signal

generator side.

For implementation on the LCD link, filters should be placed on the main PCB, between

baseband processor and the flex connector. By contrast, for the camera link, filters will be

placed on the camera PCB, as the signal comes from the camera to go to the baseband

processor. In case of a bidirectional link, both sides of the flex need to be filtered.

Figure13 show filter placement for LCD and camera links.

Figure s placement on LCD and camera linksAntennaCameraLCDFlexFilters placementBBDoc ID 17004 Rev 111/14

Conclusion AN31415 Conclusion

Mobile phone LCD or camera links can radiate on the RX frequencies, which can impact

mobile phone sensitivity.

To evaluate this radiation, a spectrum measurement has been performed at RX GSM

frequencies, on a digital signal having 1.8 ns rise and fall time. The measurement identifies

a power of -92 dBm at 940 MHz, on a single line, and 50 Ω environment. This is huge

regarding the requested sensitivity on GSM band.

To reduce this noise, filtering is needed. It must respect the signal integrity, and filter RX

band frequencies.

EMIF08-LCD04M16, EMIFxx-LCD02F3 and EMIFxx-LCD03F3 have been designed to filter

parallel LCD and camera links, while fully respecting signal integrity. Time domain

measurement shows a very low impact on the signal.

Frequency domain measurements with filters show attenuation at RX band frequencies:

●●●From -92 dBm without filter to -120 dBm with EMIF10-LCD02F3, RC type filter, at

F=940 MHzFrom -92 dBm without filter to -126 dBm with EMIF08-LCD04M16, LC type filter, at

F=940 MHzFrom -92 dBm without filter to a power lower than -130 dBm with EMIF10-LCD03F3, LC

type filter, at F=940 MHzConsequently, using filters, before the flex connection, reduces the emission of the flex at

RX frequencies.

Using LC type filters:

●●Does not impact the signal integrity

Decreased the reception frequency range power to an insignificant valueConsequently, the RF system sensitivity is not affected. It remains at its own value.

12/14Doc ID 17004 Rev 1

AN3141Revision history6 Revision historyTable 16-Sep-2010

Document revision historyRevision1Initial sDoc ID 17004 Rev 113/14

AN3141

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