introduction to antenna impedance tuner and aperture switch

Aperture Tuning

Impedance Tuning

Introduction

3

Introduction

4

Modern Mobile Antenna Design Challenges

The challenges are as following[19] :

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

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

Antenna Tuning Methods

In general, there are two antenna tuning methods :

RF Front EndImpedance

Tuning Element

RF Front End

Aperture

Tuning Element

Impedance Tuning :

Aperture Tuning :

8

Impedance Tuning

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

PADuplexer

Thus, duplexer’s insertion loss increases with antenna mismatch, further reducing the handset's radiated

power more seriously.

non 50 Ω

50 Ω

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

PADuplexer

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].

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 EndAntenna

Matching

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

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

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 EndImpedance

Tuning Element

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

Impedance Tuning

The overall closed-loop block diagram for impedance

tuning[6].

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

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

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

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

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

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

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


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 EndPassive

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

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].


26

The real schematic[15] :


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].


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


29

As shown below, Pin3 and Pin15 are the GND pins of

internal C1 and switch3 respectively[13].


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


C1

LParasitic

LParasitic

SW3

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.


32

Besides, it features with ACL(Advanced Close Loop)

QFE2550 supports MSM8996 and WTR3925[14].

QFE2550WTR3925

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].


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]


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].


35

Applied and measured loads at the antenna port[14] :

Measured load is the pink dot

Applied load is the red “x”


36

The real example[14] :


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].


38

Aperture Tuning

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

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

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

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

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

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

The influence of mismatch loss and insertion loss on

Smith Chart[16] :

Aperture Tuning

46

Traditionally, we change the antenna structure to

change its performance[12].

Aperture Tuning

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

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

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

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

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]

The real schematic[16] :


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


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 EndR

F4

RF

3

RF

2

RF

1

RFC


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


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)


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


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


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


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


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


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


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

6 5

Ron = 1.2 Ω // 1.2 Ω

= 0.6 Ω

RF Port


SP4T, SP3T, SP2T, and SPST configurations are as

below :

1

2

3

4

10

11

9

8

7

6 5

1

2

3

4

10

11

9

8

7

6 5

1

2

3

4

10

11

9

8

7

6 5

1

2

3

4

10

11

9

8

7

6 5

SP4T SP3T SP2T Floating

SPST

Unused ports should be left open (floating) to reduce

loading on the ANT port[2].


introduction to antenna impedance tuner and aperture switch

Engineering