This document discusses several common radio frequency interference (RFI) and desense issues encountered in mobile devices and potential solutions. Issues covered include DDR memory clock desense, transceiver noise coupling, switching regulator noise radiating and coupling to antennas, LCD and touchscreen driver noise, and interference from USB, HDMI and other ports radiating or coupling to antennas. Solutions proposed involve modifying clock frequencies, adding decoupling capacitors, improving shielding and isolation between components, modifying circuit board layouts, and adding EMI filters.

1. 1

2. DDR Desense
 DDR2 frequency of 198.4 MHz and 297.6 MHz degrade
conducted Rx sensitivity: CH246 at GSM850
and CH871 at PCS1900[1].
 As shown below, change DDR2 clock frequency to 200
MHz and 300 MHz to eliminate desense[1].
2

3. DDR Desense
 Of course, to change DDR2 clock frequency is merely to
change the desense channels.
 Nevertheless, for these high speed CLK signal, there
will be frequency selectivity response in PCB. In other
words, the sensitivity degradation level may be
mitigated as the desense channels change.
3

4. LNA Abnormal Issue
 TD-LTE B38/B39/B40 sensitivity only reaches up to -80
dBm.
 At transceiver input, the issue already appears.
Transceiver
 Chances are that the RX IQ signals are coupled by
noise[2].
Transceiver
BaseBand
Chip
RX IQ
Noise
4

5. LNA Abnormal Issue
 The iLNA integrated in transceiver has 8 gain modes,
but the iLNA state cannot switch to the highest
gain(G0) to lower the cascade noise figure while
receiving weaker RX signal, and always stays at G2[1].
 The root cause is due to incorrect connection between
transceiver and baseband chip. So, iLNA cannot switch
to the highest gain(G0).
G0 G1 G2 G3 G4 G5 G6 G7
Max Min
5

6. Desensed by LCD_PCLK
 The Rx channel frequency
= 884.7 MHz = 24.576 MHz ×36,
just 36 frequency multiple of 24.576 MHz LCD_PCLK[1].
 At transceiver input, the issue disappears.
6

7. Desensed by LCD_PCLK
 But, changed the 0 Ω to 10 Ω/100 Ω/1 kΩ, inductor, and
bead to attenuate spur on LCD_PCLK without much
improvement[1].
 Removed the 0 Ω resistor completely to disconnect
LCD_PCLK (avoid LCD_PCLK leaking), and eliminate
desense[1].
BaseBand
Chip
LCM_PCLK
0 Ohm
7

8. Desensed by LCD_PCLK
 There is no GND isolation between LCD_PCLK and
VPH_PWR and CLK couples to power trace.
 And, PA input via is close to VPH_PWR (also routed
under PA).
8

9. Desensed by LCD_PCLK
 Thus, the path is : LCM_PCLK Noise => VPH_PWR =>
PA input => RX Trace
 That’s why the issue disappears at transceiver input.
PA
VPH_PWR
LCM_PCLK Noise
Transceiver
RX Trace
 The only solution is to modify the layout.
9

10. CPU LTE B5 1.6 MHz Radiated Spur Issue
 There are 1.6 MHz noise when LTE is connected, which
degrades DRX TIS[1].
 In general, there are two suspected 1.6 MHz noise
sources : Switching noise from PMIC and ET/APT PMIC.
As marked the green circles.
BB Chip
VREG_S1/S2/S3/S4 I/Q
TransceiverPMIC
VPH_PWR
VPH_PWR
ET/APT
PMIC
PA
Vcc
10

11. CPU LTE B5 1.6 MHz Radiated Spur Issue
 Change VREG_S1/S2/S3/S4’s SMPS frequency. Test
result shows that when VREG_S2 works at 4.8 MHz, the
spur frequency interval is 4.8 MHz and if VREG_S2
works at 3.84 MHz, the spur frequency interval is
1.92/3.84 MHz[1].
 So, a coarse conclusion is that VREG_S2 is the
noise source[1].
PMIC
BB Chip
DRX ANT
 Because VREG_S2 is between Baseband chip and PMIC,
both the shielding can are the suspected radiators
due to cavity resonator mechanism.
11

12. CPU LTE B5 1.6 MHz Radiated Spur Issue
 After adding a 82 pF capacitor on VREG_S2 to filter the
in-band noise, the DRx TIS improves ~ 6 dB[1].
BB Chip
VREG_S2
PMIC
82 pF
12

13. TIS Desensed by PMIC 1.6 MHz Switch Noise
 Conducted sensitivity is ok, but GSM850/900 OTA
sensitivity is ~ 6‒7 dB degraded at channel:
1.6 MHz ×X, e.g.,: 870.4 MHz = 544 ×1.6 MHz,
872 MHz = 545 ×1.6 MHz, etc.[1].
13

14. TIS Desensed by PMIC 1.6 MHz Switch Noise
 It means the issue is NOT due to layout because
conducted sensitivity is okay[1].
 As mentioned earlier, there are two suspected 1.6 MHz
noise sources : Switching noise from PMIC and ET/APT
PMIC.
14

15. TIS Desensed by PMIC 1.6 MHz Switch Noise
 PMIC and transceiver share the same shielding can, the
1.6 MHz noise easily couples without a separate
shielding[1].
15

16. TIS Desensed by PMIC 1.6 MHz Switch Noise
 Nevertheless, because conducted sensitivity is okay, it
means that the issue is NOT due to coupling
mechanism in the common shielding can.
 While in PWM mode, the SMPS frequency is 1.6 MHz.
The dominant mode of desense is the 1.6 MHz pulse
train from ~ 700–900 MHz which are then picked up by
the antenna due to cavity resonator mechanism[1].
PMIC
ANT
1.6 MHz switching Noise
16

17. TIS Desensed by PMIC 1.6 MHz Switch Noise
 Placing PRX/DRX ANT far away from PMIC and
separated shielding helped to avoid the switching noise
coupling to ANT[1].
17

18. Desensed by BL 1.2 MHz Switch Noise
 For conductive test, some PCBs has desense issue, but
some do NOT.
Transceiver
Backlight
Driver IC
Switching Noise
Transceiver
Backlight
Driver IC
Switching Noise
 For bad boards, there is aperture between transceiver
and BL driver IC, which makes shielding effect and
grounding poor. So, the switching noise leaks to
transceiver through the aperture.
Aperture
18

19. Desensed by BL 1.2 MHz Switch Noise
 As shown below, with aperture, the noise floor arises.
Transceiver
Backlight
Driver IC
 Consequently, the transceiver and BL driver IC should
NOT share the same shielding can.
19

20. GSM HB Rx Desensed by USB 3.0 Port
 GSM HB Rx radiated test shows it was desensed on
frequency 1843.2 MHz. USB 3.0 port is close to antenna
and the USB port radiates 19.2 MHz 96th harmonics.
(19.2 MHz * 96 = 1843.2 MHz)[1].
 A hardware solution is to add a bypass capacitor(15 pF)
on USB 3.0 port to improve the desense[1].
ANT
USB 3.0 CLK Noise
USB 3.0 Connector
20

21. GSM900 Radiative Sensitivity Degrade When LCD is On
 Desense happens on full bands, but it is worse on the
low channel side. The peak value is CH1000 = 930.2
MHz[1].
 In general, LCD panel (including driver circuit)
generates wideband noise when LCD is on; MIPI DSI
CLK and backlight driver switching noise generate
numerous narrowband spurs[1].
21

22. GSM900 Radiative Sensitivity Degrade When LCD is On
 Thus, there are four ways to improve LCD On desense
issue :
 MIPI DSI CLK
 LCD Power supply
 Backlight Driver IC
 LCD FPC Grounding
22

23. GSM900 Radiative Sensitivity Degrade When LCD is On
 As shown below, for LCD power, the two 10 uF
decoupling capacitors should be close to PMIC. And
(240R + 4.7 uF + 100 pF) should be close to B-to-B
connector because this combination is filter.
PMIC VREG_DISP_5V
B-to-B Connector
PMIC
10 uF 10 uF
240 R
4.7 uF 100 pF
To LCM
23

24. GSM900 Radiative Sensitivity Degrade When LCD is On
 As shown below, this filter has at least 90 dB rejection
in LB range. Even though anti-resonance frequency and
HB range, the rejection is at least 70 dB.
4.7 uF 100 pF
To LCM
240 R
 In general, the desense issue due to power supply
occurs in LB range (700MHz ~ 900MHz).
Thus, this filter should focus on LB
noise suppression.
24

25. GSM900 Radiative Sensitivity Degrade When LCD is On
 The backlight driver IC is as shown below[3]. For PWM
pin, reserve 100 pF ; for LED 1 and LED 2, reserve (100
pF + 1uF). Do NOT put 1uF capacitor on PWM pin, or
the waveform distorts.
 These filters should be close to B-to-B connector.
1uF100 pF
100 pF
25

26. GSM900 Radiative Sensitivity Degrade When LCD is On
 For LX pin, reserve (240 R + 0.47 uF + 100 pF), which
should be close to B-to-B connector as well.
VREG_WLED_ANODE
To B-to-B connector
0.47 uF 100 pF
To LCM
240 R
26

27. GSM900 Radiative Sensitivity Degrade When LCD is On
 DSI MIPI CLK can customized up to 500 MHz depending
on LCM requirement. the single DSI bus interference
frequency is usually CLK/12, which is up to
500 MHz/12 = 41.67 MHz[1].
27

28. GSM900 Radiative Sensitivity Degrade When LCD is On
 Because MIPI DSI CLK is differential mode, which
generates common mode noise[4].
28

29. GSM900 Radiative Sensitivity Degrade When LCD is On
 Thus, the EMI filter should have high common mode
noise suppression. In other words, we should choose
the EMI filter with high Scc21, and low Sdd21(low
insertion loss).
29

30. GSM900 Radiative Sensitivity Degrade When LCD is On
 As shown below, vendor A has high Scc21 in 900 ~ 1000
MHz range than vendor B. In theory, vendor A has
higher noise suppression than vendor B.
30

31. GSM900 Radiative Sensitivity Degrade When LCD is On
 Apparently, the GSM 900 desense issue is mitigated
with high Scc21 EMI filter.
 Of course, to change the DSI CLK to avoid interference
falling to GSM Rx band is a solution as well[1].
31

32. GSM900 Radiative Sensitivity Degrade When LCD is On
 The LCM and FPC is as shown below :
32

33. GSM900 Radiative Sensitivity Degrade When LCD is On
 For better shielding effect, the FPC should have
conductive silver paste. In addition, as marked red
circles, the FPC should have metal
GND pad for better grounding.
 Otherwise, without these design,
the FPC may act like a
radiator, and radiates noise to
Antenna[5]. ANT
33

34.  Nevertheless, cut the SD card connector, and the spur
near antenna is reduced about 12 dB.
WiFi 5G radiative Sensitivity Degrade When HDMI is On
 There is WiFi 5GHz desense issue when HDMI is on.
And this is because there is 5.192 GHz (ch 40) spur near
antenna.
-87 dBm -99 dBm
34

35.  With cut in SD card connector, the resonant frequency
moves away from WiFi 5 GHz. That is why the spur near
antenna is reduced.
WiFi 5G radiative Sensitivity Degrade When HDMI is On
 Due to resonant cavity mechanism, the SD card
connector will act like a radiator. Thus, it radiates noise
to antenna and desense issue occurs.
WiFi 5Ghz Range
S11
Frequency
w/ Cut
w/o Cut
HDMI Noise
ANT
SD Card
Connector
35

36.  Finally, modify the layout and replace these 0 ohm
resistors with ferrite bead to suppress HDMI CLK noise.
And the issue is solved.
WiFi 5G radiative Sensitivity Degrade When HDMI is On
 However, to cut the SD Card connector is not a suitable
solution. After checking the layout, the HDMI CLK noise
couples to SDIO related traces due to insufficient
isolation.
SD Card
Connector
HDMI CLK trace
SDIO related trace
HDMI Noise
36

37. Reference
[1] Rx Desense Common Issue, Qualcomm
[2] DDR Desense Issue, Slideshare
[3] Dual-Channel LED Driver For Smart Phone Application, NOVATEK
[4] Noise Suppression Products/EMI Suppression Filters, Murata
[5] Antenna desense on handheld equipment Application note AN4356, STMicroelectronics
37