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.
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EMI issue on the mobile phone
1 EMI issue on the mobile phone
AN3141
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
Camera t t
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t t
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
+ f
Flex
Flex radiation
Coming from BST
GSM f
Coming from Flex radiation f f
Flex signal, frequency domain t
Flex signal, temporal domain
To keep the mobile phone sensitivity at the radio level, the radiated digital signal needs to be filtered.
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Link between baseband processor and LCD or camera
2 Link between baseband processor and LCD or camera
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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.
Figure 3
shows the signal waveform measured on an LCD link.
Figure 3.
Signal to control the LCD
1 V/div
20 ns/div
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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 bandpass 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.
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Filtering the LCD or camera link
3 Filtering the LCD or camera link
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●
●
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
Figure 2
), 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
Figure 6
,
Figure 7
and
Figure 8
.
Attenuation at RX frequencies are not the same:
●
●
EMIFxx-LCD02F3 is an RC type filter and provides attenuation of -25 dB at 900 MHz.
EMIF08-LCD04M16 is an LC filter and provides attenuation of -35 dB at least on RX band, which is better than the RC type filter.
●
EMIFxx-LCD03F3 is an LC type filter and provides attenuation of -50 dB on RX band - better than the RC type filter.
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AN3141 Filtering the LCD or camera link
Figure 6.
Frequency response of EMIF10-LCD02F3, RC type
0
dB
-10
-20
-30
-40
-50
-60
100k 10M 1M
Line 1
Line 3
Line 5
Line 7
100M
Line 2
Line 4
Line 6
Line 8
1G
F (Hz)
6G
Figure 7.
Frequency response of EMIF08-LCD04M16, LC type
-30
-35
-40
-45
-50
-55
-60
-65
-70
0
-5
-10
-15
-20
-25
dB
I2 - O2 #1
I3
-
O3 #1
I4
-
O4 #1
I5
-
O5 #1
I6 - O6 #1
I7 - O7 #1
I8 - O8 #1
100k 1M 10M 100M
F (Hz)
1G
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Filtering the LCD or camera link AN3141
Figure 8.
Frequency response of EMIF10-LCD03F3, LC type
-50
-60
-70
-80
-90
0
dB
-10
-20
-30
-40
100k
Line 1
Line 3
Line 5
Line 7
1M 10M 100M
Line 2
Line 4
Line 6
Line 8
1G
F (Hz)
6G
All filters have a high bandwidth so they have a very small impact on the signal in the time domain, as shown on the
Figure 9
.
Figure 9.
Frequency response of EMIF10-LCD03F3, LC type
1 V/div.
10 ns/div.
Without filter
With LC type,
EMIF10 -LCD03F3
With RC type,
EMIF10 -LCD02F3
With LC type,
EMIF08 -LCD04M16
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The same measurements have been performed in the frequency domain, at 940 MHz.
Figure 10
,
Figure 11
, and
Figure 12
show the harmonic power with RC and LC type filter.
Figure 10.
Spectrum at 940 MHz, with RC type filter, EMIF10-LCD02F3
Figure 11.
Spectrum at 940 MHz, with LC type filter, EMIF08-LCD04M16
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Filtering the LCD or camera link
Figure 12.
Spectrum at 940 MHz, with LC type filter, EMIF10-LCD03F3
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●
●
●
Power at 940 MHz, without filter was -92.5 dBm (see
Figure 5
):
With EMIF10-LCD02F3, RC type filter, the power is decreased to -120 dBm.
With EMIF08-LCD04M16, LC type filter, the power is decreased to -126 dBm.
With 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.
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AN3141 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.
Figure 13
show filter placement for LCD and camera links.
Figure 13.
Filters placement on LCD and camera links
Camera
Filters placement
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Conclusion
5 Conclusion
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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 MHz
●
●
From -92 dBm without filter to -126 dBm with EMIF08-LCD04M16, LC type filter, at
F=940 MHz
From -92 dBm without filter to a power lower than -130 dBm with EMIF10-LCD03F3, LC type filter, at F=940 MHz
Consequently, 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 value
Consequently, the RF system sensitivity is not affected. It remains at its own value.
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Table 1.
Date
Document revision history
Revision
16-Sep-2010 1 Initial release.
Changes
Revision history
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AN3141
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