Introduction To Antenna Impedance Tuner And Aperture Switch

2.  Aperture Tuning
 Impedance Tuning
 Introduction

3. 3
Introduction

4. 4
Modern Mobile Antenna Design Challenges
 The challenges are as following[19] :

5. 5
What is Antenna Tuning?
 Due to the increase in new features, functionality
and industrial design requirements, the space available
for the mobile system antenna shrinks at a rapid
rate and lowers the antenna's efficiency[1,21].
 Some of this lost performance can be recovered with
antenna tuning, in which the system uses dynamic
impedance tuning techniques to optimize the antenna
performance for both the frequency of operation
and the environmental conditions[1,19].

6. 6
Industry Trends Drive Performance
 LTE-Advanced Network and Carrier Aggregation
specifications are pushing RF Front End performance
demands higher. Further demands on antenna size
or tuning selectivity.
 Tunable devices can support the increased bandwidth
demanded by LTE handsets by enabling small antennas
that are efficient across the entire LTE bands from
700MHz to 3GHz, saving battery power and enabling
slim and thin designs[1,20].

7. 7
Antenna Tuning Methods
 In general, there are two antenna tuning methods :
RF Front End
Impedance
Tuning Element
RF Front End
Aperture
Tuning Element
 Impedance Tuning :
Aperture Tuning :

8. 8
Impedance Tuning

9. 9
Impedance Tuning
 Duplexer’s characteristics, including insertion loss and
isolation, changes with non 50 Ω input or output port.
 Front-end power is lost with antenna mismatch. This
mismatch also causes the handset PA's output to drop
due to change in load-pull, further reducing the
handset's radiated power.
Insertion Loss
Frequency
ANT
Matching
PA
Duplexer
 Thus, duplexer’s insertion loss increases with antenna
mismatch, further reducing the handset's radiated
power more seriously.
non 50 Ω
50 Ω

10. Impedance Tuning
 For frequency-division-duplex (FDD) systems such as
WCDMA and FDD-LTE, the transmitter and receiver
operate simultaneously, thereby creating Tx desense
issue.
ANT
Matching
PA
Duplexer
LNA TX Leakage
TX-to-RX
Isolation
non 50 Ω
50 Ω
 Thus, duplexer’s isolation aggravates with antenna
mismatch, further enhancing the Tx desense issue[22].

11. Impedance Tuning
 Besides, antenna mismatch leads to change in load-pull,
thereby enhancing harmonics level. This may result in
Radiation Spurious Emission(RSE) issue.
RF Front End
Antenna
Matching

12. 12
Impedance Tuning
 Impedance tuning is usually achieved by tuner, which
must have low loss to avoid degrading the radiating
efficiency of the antenna[1-2].
 The tuner is composed of several tunable capacitors
and switches. Besides, it needs some external passive
components as well.

13. 13
Antenna
Performance
Impact Requirement
Antenna
Efficiency
TRP, TIS Low Cmin
High Q
Low loss
Tuning Range Band Coverage Wide range of C
value
Low Noise TIS, CA High linearity
 The requirements of tuner element are as following[3,5] :
Impedance Tuning

14. 14
Impedance Tuning
 This solution tunes the antenna to the entire system,
creating a tuned matching network which is added to
the antenna input. It optimizes power transfer from
the RF Front-End into the antenna terminals and
improves total radiated power (TRP) and total isotropic
sensitivity (TIS). The solution improves return loss and
bandwidth mainly[1-2].
RF Front End
Impedance
Tuning Element

15. 15
Impedance Tuning
 Closed-loop implementation is more complicated, but
pre-determined parameters can be adjusted
dynamically, allowing the impedance tuner to track the
optimal frequency and matching for the antenna in all
use cases[6,21].
 Open-loop implementation is easier than the closed-
loop design. But pre-determined parameters only works
at the pre-defined situation and can’t be adjusted
dynamically. So it cannot take into account changing
environmental conditions [6,8,21].
Pre-determined
Parameters
Impedance
Tuning
Tuning
Algorithm
Pre-determined
Parameters
Impedance
Tuning

16. 16
Impedance Tuning
 The overall closed-loop block diagram for impedance
tuning[6].

17. 17
Impedance Tuning
 Predetermined Parameters – This block has a pre-
set tuning table that contains the tuning parameters
based on the input from the processor and FEM.
The smartphone developer sets these parameters
for different conditions such as that at free space the
tunable capacitor set value at 1 pF and at (head+hand)
case to set value to 3 pF, etc.[6,8].
 Processor interface – This block receives input from
the processor and the Front End Module (FEM).
How impedance tuning will operate is based on the
calculation results from this block[6,8].

18. 18
Impedance Tuning
 Real-Time Calculation – This block is used when
the closed-loop design is implemented. After tuning
parameters are sent to the tunable RF device, the
antenna efficiency is sent back via feedback loop
to the real-time calculation block to determine if
additional tuning is required[6].
 SPI/RFFE Controller – This block is the main
interface block that sends tuning parameters to the
tunable RF device[6].

19. 19
Impedance Tuning
 Given any hand, head, or environment changes causing
antenna frequency shift, impedance tuning can adjust
the antenna resonance back to original condition
dynamically and rapidly with closed-loop design[7,8,21].

20. 20
Impedance Tuning
 Besides, impedance tuning can improve part-to-part
TRP variation as well[4].
Output Power Varies between
25 and 31.5 dBm
Output Power Varies between
29 and 31 dBm

21. 21
 The less Cmin is, the higher achievable efficiency will
be[5]
Not much change high end of the band
Significant improvement at the low end of the band (2-3
dB) is possible with lower Cmin
Impedance Tuning

22. 22
 CA (carrier aggregation): multiple TX and RX channels
are used simultaneously to increase uplink and
downlink speed.
 First implementations have 1TX and 2RX channels
(improve download speed): same antenna shared for
both bands
 Some combinations of bands create potential
intermodulation and / or harmonics issue[3]
Impedance Tuning

23. 23
Antenna Tuner – QFE2550
 QFE2550, the antenna tuner, is configured to improve
the matching between the antenna and the RFFE
circuits, thereby maintaining RF performance as the
operating conditions change (band, airlink
mode, Tx power level, temperature, antenna impedance,
etc.).
 Its operating frequency range is from 698 MHz to 2690
MHz, thereby supporting GSM, CDMA2000, UMTS, LTE,
TD-SCDMA, and WiFi 2.45 GHz[14].
 It supports CA, and Its improved linearity can mitigate
potential intermodulation and / or harmonics issue with
CA[15]

24. 24
Antenna Tuner – QFE2550
 Its low Cmin (0.5 pF) can achieve higher efficiency[15].
 Highly flexible – supports four sub-bands, and up to
eight tuning scenarios per band[15].
 Intended to support diversity (Rx-only) antennas, but
can be used for primary (Rx/Tx) antennas as well.
RF Front End RF Front End
Passive
Matching RF Front End QFE2550
 Conventional passive ANT matching may alter the fine-
tuned load-pull, but QFE2550 can adjust the load-pull
dynamically for optimal power delivery at high Tx levels.

25. 25
 QFE2550 functional block diagram[15] :
 Five on-chip
circuit elements provide
flexibility in matching
configuration: three
switches (S1, S2, and S3)
and two variable
capacitors (C1 and C2).
 Each switch can be open
or closed, and each
capacitor has a nominal
range and step size[15].
Antenna Tuner – QFE2550

26. 26
 The real schematic[15] :
Antenna Tuner – QFE2550

27. 27
 Although QFE2550 has merely 0.25 ~ 0.3 dB insertion
loss, there are a lot of external passive components,
which lead to additional loss.
 Remove the unused components to minimize layout
parasitics and additional loss[13].
 Use minimum number of components (active and
passive) to reduce matching circuit loss[13].
Antenna Tuner – QFE2550

28. 28
 Provide as much GND clearance as possible around all
RF components and traces, at least 7.2 mil, both co-
planar as well as vertical (layer-to-layer) to mitigate
parasitic effect [13].
Parasitic
Capacitance
Antenna Tuner – QFE2550

29. 29
 As shown below, Pin3 and Pin15 are the GND pins of
internal C1 and switch3 respectively[13].
Antenna Tuner – QFE2550

30. 30
 Thus, make all the GND pins group together and add
GND vias connecting to main GND plane directly under
all GND pins to mitigate the parasitic inductance,
especially pin3 and pin15. Otherwise, the impedance
will be unexpected while activating C1 and switch3[13].
Pin3
Pin15
Antenna Tuner – QFE2550
C1
LParasitic
LParasitic
SW3

31. 31
 Conversely, if the antenna radiator does not have a DC
path to ground, a shunt inductor to ground as the first
component from antenna feed pad to provide additional
ESD protection[13].
RF Front End QFE2550
Feed line
GND
Radiator
 If the antenna radiator has a DC path to ground, such
as IFA(Inverted-F Antenna), the additional ESD
protection is not needed.
Antenna Tuner – QFE2550

32. 32
 Besides, it features with ACL(Advanced Close Loop)
 QFE2550 supports MSM8996 and WTR3925[14].
QFE2550
WTR3925
RF Front End
FBRX
MSM8996
 It adopts FBRx-based closed loop solution, and the
coupler is integrated in RF Front-End. Thus, it achieves
lower loss due to no additional coupler[14].
 The coupler integrated in RF Front-End is used to
detect the incident and reflected RF power and give to
power detector to convert to DC respectively, then
MSM8996 is used to calculate S11 and decide how to
optimize S11[8].
Antenna Tuner – QFE2550

33. 33
 More accurate closed loop algorithm
 ACL goals:
Applicable to all tuner use-cases (Tx, Rx)
 Compensate for device variation[14]
 ACL highlights:
 No need of a call box tester(such as CMW500)
 No OTA(over-the-air) measurements needed to determine
tuner states
 Wide Smith chart coverage (multimode capability)
 Continuously measures antenna impedance and adjusts
match dynamically
 Compensates for antenna-to-antenna variation[14]
Antenna Tuner – QFE2550

34. 34
 Before proceeding with ACL characterization, place the
pigtail between the antenna and tuner; includes the
QFE2550 + external passive components[14].
RF Front End QFE2550
 A call box is not needed because this setup uses FTM
sweep.
 Characterization is only needed for one technology, and
LTE is recommended because it covers the most
bands[14].
Antenna Tuner – QFE2550

35. 35
 Applied and measured loads at the antenna port[14] :
 Measured load is the pink dot
Applied load is the red “x”
Antenna Tuner – QFE2550

36. 36
 The real example[14] :
Antenna Tuner – QFE2550

37. 37
 Connect the pigtail ground to the PCB RF ground close
to the QFE2550 and avoid connecting the pigtail ground
to any ground areas on the PCB that are part of the
antenna radiator[14].
Antenna Tuner – QFE2550

38. 38
Aperture Tuning

39. 39
 The part with most loss has the most room for
improvement. If used properly, aperture tuning has less
loss than impedance tuning and the ability to improve
antenna radiation efficiency more[8].For efficiency,
aperture tuning is 2x better results than impedance
tuning[3,5].
 Antenna Efficiency runs between 15% up to 50%
efficient. In other words, in radio portion of mobile
device, the loss of antenna itself will be 50% ~ 85%(3 dB
~ 8 dB).
Aperture Tuning

40. 40
 Typically a high-end smartphone will have to cover
multiple bands in all spectrum region, but not
simultaneously. So, band-select tuning is possible, and
aperture tuning elements are becoming a “must have”
in high-end mobile devices due to a lot of bands[3].
Aperture Tuning

41. 41
Aperture Tuning
Modify Structure
Aperture
Tuning Element
With Aperture Tuning
Change ANT performance
(Band of operation, Return
Loss, Bandwidth, Gain,
Efficiency, etc. )
 Impedance Tuning can improve S11 and system
efficiency accordingly. But it hardly improves the
antenna radiation efficiency because antenna structure
is not changed. Hence, we need aperture tuning to
change the antenna radiation principle flexibly [8].

42. 42
 Aperture tuning is usually achieved by a switch, which
must have low loss to avoid degrading the radiating
efficiency of the antenna[1-2].
 Shunt type is the most widely used aperture tuning
method due to less ohmic loss than series type,
thereby achieving higher radiation efficiency[8,16,19].
Aperture Tuning

43. 43
Antenna
Performance
Impact Requirement
Antenna
Efficiency
TRP, TIS Low Ron
High Q
Low loss
High Isolation
Low Coff
Tuning Range Band Coverage Wide range of C
value
Low Noise TIS, CA High linearity
 The requirements of aperture tuning switch are as
following[3,5] :
Aperture Tuning

44. 44
 Ohmic loss, also known as dissipative loss or heat loss,
is the most critical factor for antenna tuners regarding
performance[16].
 Ohmic loss is composed of mismatch loss and
insertion loss. But mismatch loss is not important for
an aperture switch because it will be adjusted with
antenna and loading components.
 Conversely, insertion loss can’t be compensated. Thus,
design for the least ohmic loss across bands to
maximize antenna performance.
Aperture Tuning

45. 45
 The influence of mismatch loss and insertion loss on
Smith Chart[16] :
Aperture Tuning

46. 46
 Traditionally, we change the antenna structure to
change its performance[12].
Aperture Tuning

47. 47
 Without changing the antenna structure, the
performance of the proposed antenna with aperture
tuning switch still alters while activating the switch. The
on/off performances are almost the same as the
previous (c) and (d). This proves that aperture tuning
changes the antenna performance indeed even though
there is No modification on the antenna structure[12].
Aperture Tuning

48. 48
 Any antenna can be regarded as RLC model. That’s to
say, the antenna performance varies with any
modification of the equivalent RLC model.
 Thus, both modifying antenna structure and aperture
tuning can alter the equivalent RLC model, thereby
changing the antenna performance.
Aperture Tuning

49. 49
 With aperture tuning elememt, the electrical length of
antenna ground leg is adjusted to shift its resonance to
the desired frequency band of operation. Band
switching has the advantage of being able to achieve
higher levels of performance than input tuning since
the actual radiating element is being tuned[1,3].
RF Front End
Aperture
Tuning Element
Aperture Tuning

50. 50
 As shown below, there will be leakage from common
port to port 2, port 3 ,and port 4 while activating state 1.
RF Front End
Aperture Tuning
with Switch
1 2 3 4
Leakage
Common Port
Aperture Tuning
 In other words, the state 1 performance will be a bit
unexpected due to these leakages.
 Thus, the higher isolation is, the less leakage
will be, and the state 1 performance will be more
expectant.
S11
Frequency
Ideal
Real

51. 51
Aperture Tuning - QAT3514
 The QAT3514 device is a high linearity, low series
resistance SP4T switch, addressing antenna
aperture tuning needs for multimode (2G/3G/4G) RF
front-ends from 600 MHz up to 2700 MHz
 The ultra-low on resistance enables high-Q antenna
aperture tuning, thereby achieving higher radiation
efficiency[17].
 QAT3514 supports MSM8996[16].
 Low ohmic loss : 0.28 dB[17]

52.  The real schematic[16] :
Aperture Tuning - QAT3514

53. 53
 Antenna loads are the reactances used at the aperture
port to tune the frequency response of the antenna.
Frequency
S11
 QAT3514, in this case an SP4T, enables different
antenna loadings to be selected, which produces shifts
in antenna frequency response accordingly[16].
 RFC to antenna
 For RFC, there are two configurations :
 RFC to GND
Aperture Tuning - QAT3514

54. 54
 Traditional aperture switch typical example:
 Unwanted resonances in the switch and
with the antenna are seen
in the mid and high bands
for the two switch states.
 Switch loss efficiency is the reduction
in the antenna efficiency due to
unwanted resonances[16].
RF Front End
RF4
RF3
RF2
RF1
RFC
Aperture Tuning - QAT3514

55. 55
 Common aperture switches are capacitive in their OFF
state, with value COFF, and resistive in their ON
state, with value RON[16].
 Branches are inductive, with the value Lbranch
 Thus, in OFF state, the resonance mechanism like a
notch filter is destined to exist because
of (Lbranch + COFF) whether ANT load is
inductor or capacitor.
RFC
Coff
LBranch
To ANT
ANT Load
Resonance
Mechanism
Loss
Frequency
Undesired Resonance
Aperture Tuning - QAT3514

56. 56
 Hence, when RF1 = ON, due to the inherent (Lbranch +
COFF) in RF2/RF3/RF4 branches and their ANT loads,
and (CANT load + Lbranch ) in RF1, there will be in-band
resonance.
0.9 pF 1 nH 6 nH 35 nH
RFC
0.9 pF 1 nH 6 nH 35 nH
RF1 RF2 RF3 RF4 RF1 RF2 RF3 RF4
Resonance
Mechanism
LBranch
Coff
RON
 There will also be resonance when RF2 = ON and RF3 =
ON, but their resonances are out-band and absent in in-
band range(0.6 ~ 2.8 GHz)
Aperture Tuning - QAT3514

57. 57
 To suppress the undesired resonance, the shunt switch
(SWsh) is actuated with reverse logic with respect to
the main (or series) switch for each branch.
 The SWsh will be ON and resistive(RSWsh) when the
main switch is OFF.
 Thus, due to RSWsh, the resonance mechanism is
destroyed and undesired resonance disappears[16].
RFC
SWsh
To Antenna
Coff
LBranch
RSWsh
Loss
Frequency
Inband
No undesired
Resonance
Aperture Tuning - QAT3514

58. 58
 These shunt switches are called resonance stoppers,
which are introduced by QAT3514 in the industry for the
first time. The feature aims at CA applications[16].
 Please keep in mind that this feature can only be used
with the RFC-to-antenna type of application circuit.
 Otherwise, if RFC-to-GND configuration is adopted, and
the antenna load is capacitor, (CANT load + LBranch) will
constitute resonance mechanism again.
RFC
Coff
LBranch
SWsh
RSWsh
To Antenna
CANT load
Resonance
Mechanism
Aperture Tuning - QAT3514

59. 59
 Besides, the trace from antenna to RFC should not be
too long or narrow.
 Otherwise, even though RFC-to-antenna configuration
is adopted, if Ltrace is large enough, (Ltrace + COFF ) will
constitute resonance mechanism again.
RFC
SWsh
To Antenna
Coff
LBranch
RSWsh
LTrace
Resonance
Mechanism
Aperture Tuning - QAT3514

60. 60
 The same as antenna tuner layout rule, provide as
much GND clearance as possible around all RF
components and traces, at least 7.2 mil, both co-planar
as well as vertical (layer-to-layer) to mitigate parasitic
effect [13,16].
 Otherwise, COFF will become larger due to shunt
parasitic capacitance, CP [16].
Antenna
aperture Port
Aperture Tuning - QAT3514

61. 61
 With CP, even though RFC-to-antenna configuration is
adopted, even though Ltrace is small, [ Ltrace + (COFF // CP) ]
will constitute resonance mechanism again.
RFC
SWsh
To Antenna
Coff // CP
LBranch
RSWsh
LTrace
Resonance
Mechanism
Aperture Tuning - QAT3514

62. 62
 Thus, this feature, resonance stoppers, can NOT be
used with[16] :
 RFC-to-GND configuration
 large Ltrace
 large CP
RFC
Coff
LBranch
SWsh
RSWsh
To Antenna
CANT load
Resonance
Mechanism
RFC
SWsh
To Antenna
Coff
LBranch
RSWsh
LTrace
Resonance
Mechanism
RFC
SWsh
To Antenna
Coff // CP
LBranch
RSWsh
LTrace
Resonance
Mechanism
Aperture Tuning - QAT3514

63.  QAT3514 MIPI control can easily enable more than one
RF path simultaneously. This feature is called
Hyperport[16].
 Hence, RF paths can be combined to convert the SP4T
into SP3T, SP2T, or SPST, respectively.
 If two ports are combined, the Ron of the combined
ports is reduced to 0.6 Ω, thereby achieving lower loss
further[16].
1
2
3
4
10
11
9
8
7
65
Ron = 1.2 Ω // 1.2 Ω
= 0.6 Ω
RF Port
Aperture Tuning - QAT3514

64.  SP4T, SP3T, SP2T, and SPST configurations are as
below :
1
2
3
4
10
11
9
8
7
65
1
2
3
4
10
11
9
8
7
65
1
2
3
4
10
11
9
8
7
65
1
2
3
4
10
11
9
8
7
65
SP4T SP3T SP2T Floating
SPST
 Unused ports should be left open (floating) to reduce
loading on the ANT port[2].
Aperture Tuning - QAT3514