ST AN3141 Application note
September 2010 Doc ID 17004 Rev 1 1/14
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 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
AN3141 EMI issue on the mobile phone
Doc ID 17004 Rev 1 3/14
Figure 2. Baseband is a source of EMI, which reduces the mobile’s sensitivity
To keep the mobile phone sensitivity at the radio level, the radiated digital signal needs to be
filtered.
Antenna
LCD
BB
Flex
Camera
Antenna
LCD
BB
Flex
Camera
t
f
f
GSM
f
f
Coming from BST
Coming from Flex radiation
Flex radiation
Flex signal,
temporal domain
Flex signal,
frequency domain
Flex
radiation
Only flex radiation,
BST signal is canceled
+
Flex
Link between baseband processor and LCD or camera AN3141
4/14 Doc ID 17004 Rev 1
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 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
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
1 V/div
20 ns/div
AN3141 Link between baseband processor and LCD or camera
Doc ID 17004 Rev 1 5/14
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
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.
Signal generator
50 Ω output
Cavity band-
pass filter
Isolator
Spectrum analyzer
DUT
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