Switchable Radial Stub Resonator for Isolation Improvement of SPDT Switch.
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
S
Switch
hable Radiial Stu
ub Reesonattor forr
Issolatioon Im
mprovement of SPDT Switcch
#1
*3
N. A. Shairi
S
, B. H
H. Ahmad #2, Peng Wen Wong
W
#
Cenntre for Teleccommunication
n Research & Innovation (C
CeTRI), Facullty of Electronnics and Comp
puter
Engineeering, Universsiti Teknikal Malaysia
M
Melaaka (UTeM), Hang
H
Tuah Jay
ya, 76100 Dur
urian Tunggal, Melaka,
Malaysia.
1
[email protected]
2
Correesponding autthor: [email protected]
*
Electrical & Electronnic engineerin
ng Departmennt, Universiti Teknologi
T
Petronas (UTP), Bandar Seri Iskandar,
I
31750 Trronoh, Perak, Malaysia.
3
wong_penngwen@petro
onas.com.my
per proposess switchable radial stub resonator
r
forr isolation im
mprovement of Single
Abstraact—This pap
Pole Doouble Throw (SPDT) disscrete switch
h for Time Division
D
Dup
plex (TDD) sswitching off wireless
nications. A commercializeed discrete PIIN diode is used
u
to switch
h the radial sttub resonatorr between
commun
bandstop
p to allpass response
r
wheere an analyttical modeling of the switchable resonnator is preseented and
analyzed
d. In this anaalysis, correla
ation between
n inner radia
al and angle radial
r
stub w
with the charracteristic
impedan
nce and atten
nuation pole is determined
d. Isolation im
mprovement is
i analyzed w
with two-portt network
of singlee shunt PIN diode with switchable
s
raadial stub reesonator wheere it is founnd that an additional
a
isolation
n can be obtaained with th
he switchablee radial stub
b resonator. In
I measurem
ment result, the SPDT
switch w
with switchab
ble radial stu
ub resonator has shown more than 30
3 dB of trannsmit-receivee (Tx-Rx)
isolation
n at centre frrequency of 3.5
3 GHz givin
ng better isolation compa
ared to conveentional SPD
DT switch
design. The potential applicatio
on of this S
SPDT switch
h is TDD sw
witching forr WiMAX and
a
LTE
nication system.
commun
Keyword-RF switcch, SPDT, ressonator, radiaal stub, switcchable resona
ator
I. INTRODUCTIION
ns using Timee Division Du
uplex (TDD), Single Pole D
Double Throw
w (SPDT)
In wiireless data coommunication
switch iss commonly used
u
in RF fro
ont-end system
m [1] to switch between traansmitter and receiver. As illustrated
in Fig. 1,, the SPDT sw
witch is part of
o other subcoomponents su
uch as antennaa [2]-[3], filter
er [4], amplifieer [5] and
mixer (uup-converter or
o down-converter) [6]. Unntil now, the switching eleements in the SPDT switch
h can use
either PIN
N diodes or FETs
F
where diiscrete PIN dioodes are still desirable for higher
h
power levels used in
n military,
satellite ccommunicatioon or base stattion applicatioons [7].
One oof the key parrameters in SP
PDT switch ddesign is the reequirement of high isolatiion between trransmitter
and receiiver port. How
wever, it is diifficult to get isolation of SPDT
S
switch higher than 220 dB (for ap
pplications
above 3 GHz) if usingg single low performance
p
P
PIN diodes. The
T PIN diodees are usuallyy in standard packaging
p
SOT23, SOT3223, SOD323 or
o SOD523.
such as S
Fig. 1. Application of SPDT switch in RF front-end system for wirele
ess communicatioons.
Theree are two mosst popular tech
hniques to incrrease isolation
n. First, multiples connectioon of PIN dio
odes either
in series, shunt or com
mbination of seeries-shunt [8]]. However, using
u
multiples PIN diodes,, it will increaase current
ductor with PIN
N diode [9]; bbut it has a lim
mitation of
consumpption of the cirrcuit. Second, the parallel reesonant of ind
discrete iinductance vallues.
Recenntly, there is a technique using switchaable resonato
or for isolation
n improvemeent of SPDT switch as
reported in [10] and [11]. The key advantage of thhis technique is reduction of circuit size w
with minimum
m usage of
ET) comparedd to the techniique using mu
ultiples connecction of PIN diodes.
d
As
switchingg elements (PIIN diode or FE
suggestedd in [11], to cater for wireeless broadban
and application
n, wider isolaation bandwiddth can be ob
btained by
ISSN : 0975-4024
Vol 5 No 1 Feb-Mar 2013
460
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
wideningg the width off open stub ressonator. Howeever, the gap between
b
transm
mission line aand the input port
p of the
open stubb resonator coontributes to RF coupling. Such RF cou
upling effect has
h been repoorted in [12] where
w
the
series cappacitance of thhe gap increases as the gap sspacing decreases, thus allo
owing RF couppling.
In ordder to improvee isolation of SPDT switchh with minimu
um RF coupling effect, raddial stub reso
onator is a
good canndidate. Furthermore, the raadial stub of aangle 90° or greater
g
is morre broad-bandded than a con
nventional
quarter w
wavelength oppen stub of sttraight microsstrip line of similar
s
resonaance frequenccy, in the sensse that its
dispersioon is smaller [13]. Thus, wiider isolation bbandwidth caan be achieved
d by adjustingg the angle of the radial
stub whille maintainingg the same wiidth at the inpput port of thee stub. It is fou
und that radiaal stub was stiill popular
until todaay and used for
f the design such as antennna [14], filteer [15], ampliffier [16] and phase shifter [17]. The
only appllication of raddial stub in sw
witch design is biasing circu
uit as reported in [18] and [119].
Thereffore, this papeer proposes sw
witchable raddial stub reson
nator for isolaation improvem
ment of SPDT
T discrete
switch. A
An analytical modeling
m
of th
he switchable resonator is presented
p
and analyzed for isolation imprrovement.
The SPD
DT switch withh switchable radial stub reesonator is dem
monstrated att 3.5 GHz whhere the PIN diodes
d
are
based onn commercializze PIN diode (BAP64-02) in SOD523 package
p
from NXP Semicoonductors. Thiis paper is
organizedd as follows. The circuit design
d
and anaalysis of the proposed
p
switchable radiall stub resonattor and its
applicatioon in SPDT sw
witch design are presented in section II. The analysis of isolation im
mprovement is
i focused
more in this section. Then,
T
simulattion and meassurement resu
ult of SPDT switch
s
with thhe proposed switchable
s
n section III. C
Concluding reemarks are giv
ven in section IV.
radial stuub resonator arre discussed in
II. CIRCUIIT DESIGN AND
D ANALYSIS
In thiss section, the concept and operation
o
of sswitchable rad
dial stub resonator are expplained. The correlation
c
between radial stub paarameters and attenuation poole are analyzzed with mathematical moddeling. Next, analysis
a
of
wo-port netwo
ork of single shunt PIN diode with
isolation improvemennt performance is performeed with a tw
switchable radial stub resonator.
or
A. Switcchable Radial Stub Resonato
A sim
mple structure as illustrated
d in Fig. 2 (aa) is a switchaable radial stu
ub resonator which is based on the
geometryy of radial stuub in Fig. 2 (b) [20]. It is connected in shunt to 50 Ω transmissioon lines. The proposed
switchable radial stub resonator can
n be reconfiguured between bandstop and
d allpass respoonse which iss the same
o
betw
ween bandstop
p and allpass response is coontrolled by PIN
P diode
concept aapplied in [211]-[23]. The operation
which is connected in series with the radial stub.
(a)
(b)
(
Fig. 2. (a) The proposed switchable radiall stub using PIN diode
d
and (b) geo
ometric of radial sstub [20].
(a)
(b)
Fig. 3. (a) Banddstop response du
uring PIN diode inn ON state and (b
b) allpass responsse during PIN dioode in OFF state.
ISSN : 0975-4024
Vol 5 No 1 Feb-Mar 2013
461
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
In ON state condition of the PIN diode (Fig. 3 (a)), the RF signal in 50 Ω microstrip line will be short
circuited because of low impedance at the input shunt PIN diode due to λ/4 impedance transformation of radial
stub (from high to low impedance). The radial stub is equivalent to series inductance and capacitance having a
bandstop response between Port 1 and Port 2. In OFF state condition of the PIN diode (Fig. 3 (b)), the radial stub
resonator is disconnected from 50 Ω microstrip line having allpass response between input and output port.
As shown in Fig 3 (a), the bandstop response of the radial stub resonator can be modeled using transmission
matrix.
|
(1)
where Ts is transmission matrix of radial stub resonator, T1 is transmission matrix of Microstrip Line at Port 1
and T2 is transmission matrix of Microstrip Line at Port 2.
Therefore, we have
cos
cos
sin
sin
(2)
tan
sin
cos
sin
cos
Since the impedance of the microstrip lines are 50 Ω, thus they are fully matched with the characteristic
impedance of 50 Ω and exhibit very minimum power losses. Therefore T1 and T2 can be ignored in order to
simplify the following equation where S parameter of the radial stub resonator can be obtained by converting the
transmission matrix in (2) as
|
.
(3)
where Zo is characteristic impedance of microstrip line and Ys is characteristic admittance of radial stub.
The characteristic impedance of radial stub can be obtained as [20]
(4)
where d is thickness of substrate, ri is inner radial of radial stub and is effective dielectric constant.
The effective dielectric constant of microstrip, can be calculated from [24]
where
(6)
/
is relative dielectric constant of substrate. As suggested in [20] for radial stub, W in (6) is calculated as
sin
Then, rearrange (3) with (4), we get
|
θ
(7)
π
π
.
(8)
To calculate length of radial stub resonator, we know
where
Therefore, rearrange (9) and by substituting
determined as
(9)
.
, the λ/4 length of the radial stub resonator can be
(10)
where c is speed of light and f is resonant frequency.
Considering fixed value of d and of printed circuit board (PCB) in (4) and (8), we found that angle, θ and
inner radius, ri of radial stub are significantly influence Zs and S21. Further analysis is carried out using (4) and (8)
by considering resonant frequency at 3.5 GHz and a FR4 substrate having thickness of 1.6 mm and dielectric
constant, of 4.7.
As shown in Fig. 4, angle, θ is fixed at 90° and inner radius, ri is varied in order to observe the correlation
with Zs and S21. It is found that as the ri is increased it will produce lower Zs and higher attenuation pole of S21.
ISSN : 0975-4024
Vol 5 No 1 Feb-Mar 2013
462
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
Fig. 5 shhows the correelation of anglle, θ with Zs aand attenuation
n pole of S21. In this calcullation, inner raadius, ri is
d. It is observeed that as the θ is increased
d it will produuce lower Zs and
a higher
fixed at 1 mm and anggle, θ is varied
attenuatioon pole of S21.
Thereffore, we can conclude
c
that higher attenuaation pole of S21 will be obtained if angle
le, θ and innerr radius, ri
are increaased. This result has a poten
ntial to impro ve the isolatio
on of SPDT sw
witch design.
220
65
Attenuation pole @ 3.5 GHz (dB)
200
180
Impedance ()
160
140
120
100
80
60
40
20
0
60
55
50
45
40
0
2
4
6
8
10
0
2
4
6
8
Inner Radius (m
mm)
Inner Radius (mm)
(a)
(b)
10
Figg. 4. Inner radius, ri versus (a) charrateristic impedan
nce, Zs and (b) atttenuation pole off S21.
52
Attenuation pole @ 3.5 GHz (dB)
700
600
Impedance ()
500
400
300
200
100
0
50
48
46
44
42
40
38
36
34
32
20
40
60
80
100
120
140
160
1880
200
20
2
40
60
80
100
1200
Angle ( )
Angle ()
(a)
(b)
140
160
180
200
v
(a) charateeristic impedancee, Zs and (b) attenu
uation pole of S211.
Fig. 5. Angle, θ versus
B. SPDT
T Switch with Switchable Ra
adial Stub Ressonator
A conventional sinngle shunt PIIN diode in SPDT switch
h [25] is consstructed (Fig. 6) for a perrformance
ng the SPDT switch
s
in transsmit mode, D1
D must be
comparisson with the proposed shuntt SPDT switchh. By assumin
turned O
OFF and D2 must
m
be turned
d ON . In thiss condition th
he isolation beetween Port 1 and Port 2 (or
( Tx-Rx
isolation)) is obtained solely due to ON
O state of D22 in the receivee arm.
(a)
(b)
PDT switch. (a) C
Circuit design and
d (b) isolation bettween Port 1 andd Port 3.
Fig. 6.. Conventional SP
ISSN : 0975-4024
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463
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
The O
ON state of a single shunt PIN diode is equivalent to forward resisstance (Rf) andd series inducctance (Ls)
[24]. Thuus, a simplifiedd transfer matrrix of the SPD
DT switch for isolation analy
ysis is given bby
.
(11)
Then the isolation between
b
Port 1 and Port 3 ((S31) can be ob
btained by con
nverting the trransfer matrix in (11) to
S-parameeter. Hence,
(12)
.
From (12), we know
w that isolatio
on is obtainedd solely due to
o Rf and Ls off the ON statee of shunt PIN
N diode. It
can be shhown that diffficult to achieeve more thann 20 dB of isolation betweeen Port 1 andd Port 3 if usiing single
shunt disscrete PIN Dioode in standarrd package (e.gg. SOD523). This can be seeen in Fig. 6 ((b) where the simulated
single shhunt PIN diodde (BAP64-02 from NXP S
Semiconductorrs) in SOD523 package sho
hows very low
w isolation
performaance between 2 to 5 GHz.
As prroposed in thiis paper, radiaal stub is usedd to improve isolation of the
t single shuunt PIN diodee in SPDT
switch deesign. The raddial stub resonaator is cascadeed in shunt wiith the existing
g PIN diode. T
This can be an
nalyzed by
consideriing a two-portt network con
nsist of a shunnt PIN diode and
a a switchab
ble radial stubb resonator as shown in
Fig. 7 (a)). A voltage suupply of +5 V is used to turrn ON the PIN
N diode and thee radial stub rresonator. Thee resonator
will then produce an addditional isolaation between Port 1 and Port 2.
(a)
(b)
d Port 2 with
Fig. 7. Twoo-port network off shunt PIN diodee with switchablee resonator. (a) Ciircuit diagram and (b) isolation beetween Port 1 and
differrent angle of radiaal stub.
A sim
mple transfer matrix
m
of two-port networkk of the circuiit in Fig. 7 (a)) is analyzed bby considering an ideal
PIN diodde in the switchhable resonato
or. Hence,
.
(13)
Convertinng the transferr matrix in (13
3) to S-parameeter, we get
.
(14)
High iisolation can be
b obtained iff
and Zs ≈ 0 in the rad
dial stub reson
nator where thhe total isolatiion in (14)
is a cumuulative of isolaation of shuntt PIN diode annd isolation off the radial stu
ub resonator. T
This can be ob
bserved in
Fig. 7 (bb) where diff
fferent isolatio
on performannce is simulaated with diffferent angle oof radial stub
b. In this
simulatioon, the inner radius, ri is fixed at 1.5 mm in ordeer to minimizze any RF cooupling effectt between
microstriip line and thhe input of radial stub resoonator. Besidees, it is obserrved that largger angle of radial stub
producess wider isolatioon bandwidth in spite of higgh attenuation
n pole.
A circcuit diagram and
a prototype of SPDT swittch with switcchable radial stub
s
resonatorr are depicted in Fig. 8.
As a tradde-off betweenn radial stub size and its aattenuation pole performancce, the angle oof radial,
° is
chosen aand ri is fixed at 1.5 mm. All
A the PIN diiodes are supp
plied with +5 V (ON state)) and -5 V (O
OFF state)
control vvoltage.
In trannsmit mode (RF signals frrom Port 1 too Port 2), the D1 and Reso
onator-1 are tturned OFF. Then, the
Resonatoor-1 becomes allpass respon
nse. The D2 an
and Resonatorr-2 are turned ON. Then, thee Resonator-2
2 becomes
a bandstoop response inn the receive arm producinng additional isolation (bettween Port 1 and Port 3). The same
operationn and responsse can be obtaained in the reeceive mode (RF signals from
f
Port 2 too Port 3) wheen D1 and
Resonatoor-1 are turnedd ON; and D2
2 and Resonatoor-2 are turned OFF.
ISSN : 0975-4024
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464
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
(a)
(bb)
Fig. 8. SPDT switch with switchable raddial stub resonator (a) circuit diagrram (b) circuit proototype.
IIII. SIMULATION
N AND MEASU
UREMENT RESU
ULT
Fig. 9 shows the sim
mulated resultts of insertionn loss (IL), retturn loss (RL)) and isolationn (ISO) of con
nventional
witch and SPD
DT switch with
h switchable rradial stub ressonator. The simulated
s
IL aand RL of the proposed
SPDT sw
SPDT sw
witch are com
mparable with the conventioonal SPDT sw
witch. The sim
mulated ISO of the propossed SPDT
switch shhows a signifficant improveement of isollation which is
i higher than
n 20 dB betw
ween 3.05 to 3.85
3
GHz
comparedd with the connventional SPD
DT switch.
(a)
(b)
Fig. 9. Si mulated (a) inserrtion loss, return loss
l
and (b) isolattion between sing
gle shunt SPDT switch (conventioonal) and SPDT sw
witch with
switchaable radial stub reesonator.
The pperformance comparison
c
at 3.5 GHz betw
ween the conv
ventional SPD
DT switch andd the SPDT sw
witch with
switchablle radial stub resonator are listed in Tablee I. The perfo
ormance is com
mpared betweeen insertion lo
oss, return
loss and iisolation.
TABLE I
Performan
ance comparison at
a 3.5 GHz
Convention
nal shunt SPDT
switch
Shunt SPD
DT switch with
switchablee radial stub
resonator
Insertion
Loss
Return
Loss
Tx-Rx
Isolation
0.88 dB
29 dB
11.5 dB
0.65 dB
32.7 dB
39 dB
Fig. 110 shows the simulated and
d measured reesult of SPDT switch with switchable
s
raddial stub reson
nator. It is
successfuully demonstraate an isolation higher than 20 dB at 3.5 GHz
G compareed with the connventional cirrcuit. Both
simulatioon and measurrement show a comparable iinsertion loss and isolation result but the measured retu
urn loss is
slightly llower comparred with simu
ulated ones. H
However, it is
i still higherr than 10 dB
B which is a minimum
specificattion of return loss.
ISSN : 0975-4024
Vol 5 No 1 Feb-Mar 2013
465
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
(a)
(b)
Fig. 10. Siimulation and meeasurement result of SPDT swith ((a) double switchaable radial stub re
esonator and (b) ssingle switchablee radial stub
resonator.
IV
V. CONCLUSIO
ON
A swittchable radial stub resonato
or is proposed in SPDT disccrete switch att 3.5 GHz for isolation imprrovement.
The potential applicatiion is TDD sw
witching for W
WiMAX and LTE
L commun
nication system
m. The characcteristic of
nalyzed in teerm of charaacteristic impeedance and aattenuation pole using
the switcchable radial stub was an
mathemaatical modelinng. Then, the isolation imprrovement wass analyzed wiith two-port nnetwork of sin
ngle shunt
PIN diodde with switchhable radial stu
ub resonator. T
There is a corrrelation betweeen inner radiial and angle radial
r
stub
with the characteristicc impedance and attenuatioon pole, thus provide an additional
a
isol
olation in SPD
DT switch
Finally, we succcessfully fabricated the SP
PDT switch with
w switchablee radial stub re
resonator to vaalidate the
design. F
simulatioon result whichh shows an iso
olation improvvement higheer than 20 dB between
b
3.05 to 3.85 GHz.
ACCKNOWLEDGM
MENT
The w
work describeed in this pap
per was fully supported by
y Universiti Teknikal
T
Malaaysia Melaka (UTeM).
Melaka. T
Therefore, wee would like to
o acknowledgee the contributtion of our collleagues from Fabrication Laboratory
L
and Microwave Labooratory, Facullty of Electroonics and Co
omputer Engin
neering, UTeM
eM for fabriccation and
ment of the ressearch works.
measurem
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ISSN : 0975-4024
Vol 5 No 1 Feb-Mar 2013
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N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
[16]
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[21]
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[23]
[24]
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Stubs," Microwave and Wireless Components Letters, IEEE , vol.18, no.2, pp.109-111, Feb. 2008.
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MTT-S International , vol.3, pp. 1485- 1488 vol.3, 8-13 June 2003.
Yunus, N.A.M.; Wagiran, R.; Postoyalko, V.; , "Design of a microstrip SPDT PIN diode switch," Semiconductor Electronics, 2002.
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no.3, pp. 269- 271, Mar 1985.
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Theory and Techniques, IEEE Transactions on , vol.60, no.5, pp.1258-1265, May 2012.
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Kai Chang, Inder Bahl and Vijay Nair. RF and Microwave Circuit and Component Design for Wireless Systems. Third Avenue, N.Y.:
John Wiley & Sons, Inc. 2000, Chapter 7.
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467
S
Switch
hable Radiial Stu
ub Reesonattor forr
Issolatioon Im
mprovement of SPDT Switcch
#1
*3
N. A. Shairi
S
, B. H
H. Ahmad #2, Peng Wen Wong
W
#
Cenntre for Teleccommunication
n Research & Innovation (C
CeTRI), Facullty of Electronnics and Comp
puter
Engineeering, Universsiti Teknikal Malaysia
M
Melaaka (UTeM), Hang
H
Tuah Jay
ya, 76100 Dur
urian Tunggal, Melaka,
Malaysia.
1
[email protected]
2
Correesponding autthor: [email protected]
*
Electrical & Electronnic engineerin
ng Departmennt, Universiti Teknologi
T
Petronas (UTP), Bandar Seri Iskandar,
I
31750 Trronoh, Perak, Malaysia.
3
wong_penngwen@petro
onas.com.my
per proposess switchable radial stub resonator
r
forr isolation im
mprovement of Single
Abstraact—This pap
Pole Doouble Throw (SPDT) disscrete switch
h for Time Division
D
Dup
plex (TDD) sswitching off wireless
nications. A commercializeed discrete PIIN diode is used
u
to switch
h the radial sttub resonatorr between
commun
bandstop
p to allpass response
r
wheere an analyttical modeling of the switchable resonnator is preseented and
analyzed
d. In this anaalysis, correla
ation between
n inner radia
al and angle radial
r
stub w
with the charracteristic
impedan
nce and atten
nuation pole is determined
d. Isolation im
mprovement is
i analyzed w
with two-portt network
of singlee shunt PIN diode with switchable
s
raadial stub reesonator wheere it is founnd that an additional
a
isolation
n can be obtaained with th
he switchablee radial stub
b resonator. In
I measurem
ment result, the SPDT
switch w
with switchab
ble radial stu
ub resonator has shown more than 30
3 dB of trannsmit-receivee (Tx-Rx)
isolation
n at centre frrequency of 3.5
3 GHz givin
ng better isolation compa
ared to conveentional SPD
DT switch
design. The potential applicatio
on of this S
SPDT switch
h is TDD sw
witching forr WiMAX and
a
LTE
nication system.
commun
Keyword-RF switcch, SPDT, ressonator, radiaal stub, switcchable resona
ator
I. INTRODUCTIION
ns using Timee Division Du
uplex (TDD), Single Pole D
Double Throw
w (SPDT)
In wiireless data coommunication
switch iss commonly used
u
in RF fro
ont-end system
m [1] to switch between traansmitter and receiver. As illustrated
in Fig. 1,, the SPDT sw
witch is part of
o other subcoomponents su
uch as antennaa [2]-[3], filter
er [4], amplifieer [5] and
mixer (uup-converter or
o down-converter) [6]. Unntil now, the switching eleements in the SPDT switch
h can use
either PIN
N diodes or FETs
F
where diiscrete PIN dioodes are still desirable for higher
h
power levels used in
n military,
satellite ccommunicatioon or base stattion applicatioons [7].
One oof the key parrameters in SP
PDT switch ddesign is the reequirement of high isolatiion between trransmitter
and receiiver port. How
wever, it is diifficult to get isolation of SPDT
S
switch higher than 220 dB (for ap
pplications
above 3 GHz) if usingg single low performance
p
P
PIN diodes. The
T PIN diodees are usuallyy in standard packaging
p
SOT23, SOT3223, SOD323 or
o SOD523.
such as S
Fig. 1. Application of SPDT switch in RF front-end system for wirele
ess communicatioons.
Theree are two mosst popular tech
hniques to incrrease isolation
n. First, multiples connectioon of PIN dio
odes either
in series, shunt or com
mbination of seeries-shunt [8]]. However, using
u
multiples PIN diodes,, it will increaase current
ductor with PIN
N diode [9]; bbut it has a lim
mitation of
consumpption of the cirrcuit. Second, the parallel reesonant of ind
discrete iinductance vallues.
Recenntly, there is a technique using switchaable resonato
or for isolation
n improvemeent of SPDT switch as
reported in [10] and [11]. The key advantage of thhis technique is reduction of circuit size w
with minimum
m usage of
ET) comparedd to the techniique using mu
ultiples connecction of PIN diodes.
d
As
switchingg elements (PIIN diode or FE
suggestedd in [11], to cater for wireeless broadban
and application
n, wider isolaation bandwiddth can be ob
btained by
ISSN : 0975-4024
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460
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
wideningg the width off open stub ressonator. Howeever, the gap between
b
transm
mission line aand the input port
p of the
open stubb resonator coontributes to RF coupling. Such RF cou
upling effect has
h been repoorted in [12] where
w
the
series cappacitance of thhe gap increases as the gap sspacing decreases, thus allo
owing RF couppling.
In ordder to improvee isolation of SPDT switchh with minimu
um RF coupling effect, raddial stub reso
onator is a
good canndidate. Furthermore, the raadial stub of aangle 90° or greater
g
is morre broad-bandded than a con
nventional
quarter w
wavelength oppen stub of sttraight microsstrip line of similar
s
resonaance frequenccy, in the sensse that its
dispersioon is smaller [13]. Thus, wiider isolation bbandwidth caan be achieved
d by adjustingg the angle of the radial
stub whille maintainingg the same wiidth at the inpput port of thee stub. It is fou
und that radiaal stub was stiill popular
until todaay and used for
f the design such as antennna [14], filteer [15], ampliffier [16] and phase shifter [17]. The
only appllication of raddial stub in sw
witch design is biasing circu
uit as reported in [18] and [119].
Thereffore, this papeer proposes sw
witchable raddial stub reson
nator for isolaation improvem
ment of SPDT
T discrete
switch. A
An analytical modeling
m
of th
he switchable resonator is presented
p
and analyzed for isolation imprrovement.
The SPD
DT switch withh switchable radial stub reesonator is dem
monstrated att 3.5 GHz whhere the PIN diodes
d
are
based onn commercializze PIN diode (BAP64-02) in SOD523 package
p
from NXP Semicoonductors. Thiis paper is
organizedd as follows. The circuit design
d
and anaalysis of the proposed
p
switchable radiall stub resonattor and its
applicatioon in SPDT sw
witch design are presented in section II. The analysis of isolation im
mprovement is
i focused
more in this section. Then,
T
simulattion and meassurement resu
ult of SPDT switch
s
with thhe proposed switchable
s
n section III. C
Concluding reemarks are giv
ven in section IV.
radial stuub resonator arre discussed in
II. CIRCUIIT DESIGN AND
D ANALYSIS
In thiss section, the concept and operation
o
of sswitchable rad
dial stub resonator are expplained. The correlation
c
between radial stub paarameters and attenuation poole are analyzzed with mathematical moddeling. Next, analysis
a
of
wo-port netwo
ork of single shunt PIN diode with
isolation improvemennt performance is performeed with a tw
switchable radial stub resonator.
or
A. Switcchable Radial Stub Resonato
A sim
mple structure as illustrated
d in Fig. 2 (aa) is a switchaable radial stu
ub resonator which is based on the
geometryy of radial stuub in Fig. 2 (b) [20]. It is connected in shunt to 50 Ω transmissioon lines. The proposed
switchable radial stub resonator can
n be reconfiguured between bandstop and
d allpass respoonse which iss the same
o
betw
ween bandstop
p and allpass response is coontrolled by PIN
P diode
concept aapplied in [211]-[23]. The operation
which is connected in series with the radial stub.
(a)
(b)
(
Fig. 2. (a) The proposed switchable radiall stub using PIN diode
d
and (b) geo
ometric of radial sstub [20].
(a)
(b)
Fig. 3. (a) Banddstop response du
uring PIN diode inn ON state and (b
b) allpass responsse during PIN dioode in OFF state.
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461
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
In ON state condition of the PIN diode (Fig. 3 (a)), the RF signal in 50 Ω microstrip line will be short
circuited because of low impedance at the input shunt PIN diode due to λ/4 impedance transformation of radial
stub (from high to low impedance). The radial stub is equivalent to series inductance and capacitance having a
bandstop response between Port 1 and Port 2. In OFF state condition of the PIN diode (Fig. 3 (b)), the radial stub
resonator is disconnected from 50 Ω microstrip line having allpass response between input and output port.
As shown in Fig 3 (a), the bandstop response of the radial stub resonator can be modeled using transmission
matrix.
|
(1)
where Ts is transmission matrix of radial stub resonator, T1 is transmission matrix of Microstrip Line at Port 1
and T2 is transmission matrix of Microstrip Line at Port 2.
Therefore, we have
cos
cos
sin
sin
(2)
tan
sin
cos
sin
cos
Since the impedance of the microstrip lines are 50 Ω, thus they are fully matched with the characteristic
impedance of 50 Ω and exhibit very minimum power losses. Therefore T1 and T2 can be ignored in order to
simplify the following equation where S parameter of the radial stub resonator can be obtained by converting the
transmission matrix in (2) as
|
.
(3)
where Zo is characteristic impedance of microstrip line and Ys is characteristic admittance of radial stub.
The characteristic impedance of radial stub can be obtained as [20]
(4)
where d is thickness of substrate, ri is inner radial of radial stub and is effective dielectric constant.
The effective dielectric constant of microstrip, can be calculated from [24]
where
(6)
/
is relative dielectric constant of substrate. As suggested in [20] for radial stub, W in (6) is calculated as
sin
Then, rearrange (3) with (4), we get
|
θ
(7)
π
π
.
(8)
To calculate length of radial stub resonator, we know
where
Therefore, rearrange (9) and by substituting
determined as
(9)
.
, the λ/4 length of the radial stub resonator can be
(10)
where c is speed of light and f is resonant frequency.
Considering fixed value of d and of printed circuit board (PCB) in (4) and (8), we found that angle, θ and
inner radius, ri of radial stub are significantly influence Zs and S21. Further analysis is carried out using (4) and (8)
by considering resonant frequency at 3.5 GHz and a FR4 substrate having thickness of 1.6 mm and dielectric
constant, of 4.7.
As shown in Fig. 4, angle, θ is fixed at 90° and inner radius, ri is varied in order to observe the correlation
with Zs and S21. It is found that as the ri is increased it will produce lower Zs and higher attenuation pole of S21.
ISSN : 0975-4024
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462
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
Fig. 5 shhows the correelation of anglle, θ with Zs aand attenuation
n pole of S21. In this calcullation, inner raadius, ri is
d. It is observeed that as the θ is increased
d it will produuce lower Zs and
a higher
fixed at 1 mm and anggle, θ is varied
attenuatioon pole of S21.
Thereffore, we can conclude
c
that higher attenuaation pole of S21 will be obtained if angle
le, θ and innerr radius, ri
are increaased. This result has a poten
ntial to impro ve the isolatio
on of SPDT sw
witch design.
220
65
Attenuation pole @ 3.5 GHz (dB)
200
180
Impedance ()
160
140
120
100
80
60
40
20
0
60
55
50
45
40
0
2
4
6
8
10
0
2
4
6
8
Inner Radius (m
mm)
Inner Radius (mm)
(a)
(b)
10
Figg. 4. Inner radius, ri versus (a) charrateristic impedan
nce, Zs and (b) atttenuation pole off S21.
52
Attenuation pole @ 3.5 GHz (dB)
700
600
Impedance ()
500
400
300
200
100
0
50
48
46
44
42
40
38
36
34
32
20
40
60
80
100
120
140
160
1880
200
20
2
40
60
80
100
1200
Angle ( )
Angle ()
(a)
(b)
140
160
180
200
v
(a) charateeristic impedancee, Zs and (b) attenu
uation pole of S211.
Fig. 5. Angle, θ versus
B. SPDT
T Switch with Switchable Ra
adial Stub Ressonator
A conventional sinngle shunt PIIN diode in SPDT switch
h [25] is consstructed (Fig. 6) for a perrformance
ng the SPDT switch
s
in transsmit mode, D1
D must be
comparisson with the proposed shuntt SPDT switchh. By assumin
turned O
OFF and D2 must
m
be turned
d ON . In thiss condition th
he isolation beetween Port 1 and Port 2 (or
( Tx-Rx
isolation)) is obtained solely due to ON
O state of D22 in the receivee arm.
(a)
(b)
PDT switch. (a) C
Circuit design and
d (b) isolation bettween Port 1 andd Port 3.
Fig. 6.. Conventional SP
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463
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
The O
ON state of a single shunt PIN diode is equivalent to forward resisstance (Rf) andd series inducctance (Ls)
[24]. Thuus, a simplifiedd transfer matrrix of the SPD
DT switch for isolation analy
ysis is given bby
.
(11)
Then the isolation between
b
Port 1 and Port 3 ((S31) can be ob
btained by con
nverting the trransfer matrix in (11) to
S-parameeter. Hence,
(12)
.
From (12), we know
w that isolatio
on is obtainedd solely due to
o Rf and Ls off the ON statee of shunt PIN
N diode. It
can be shhown that diffficult to achieeve more thann 20 dB of isolation betweeen Port 1 andd Port 3 if usiing single
shunt disscrete PIN Dioode in standarrd package (e.gg. SOD523). This can be seeen in Fig. 6 ((b) where the simulated
single shhunt PIN diodde (BAP64-02 from NXP S
Semiconductorrs) in SOD523 package sho
hows very low
w isolation
performaance between 2 to 5 GHz.
As prroposed in thiis paper, radiaal stub is usedd to improve isolation of the
t single shuunt PIN diodee in SPDT
switch deesign. The raddial stub resonaator is cascadeed in shunt wiith the existing
g PIN diode. T
This can be an
nalyzed by
consideriing a two-portt network con
nsist of a shunnt PIN diode and
a a switchab
ble radial stubb resonator as shown in
Fig. 7 (a)). A voltage suupply of +5 V is used to turrn ON the PIN
N diode and thee radial stub rresonator. Thee resonator
will then produce an addditional isolaation between Port 1 and Port 2.
(a)
(b)
d Port 2 with
Fig. 7. Twoo-port network off shunt PIN diodee with switchablee resonator. (a) Ciircuit diagram and (b) isolation beetween Port 1 and
differrent angle of radiaal stub.
A sim
mple transfer matrix
m
of two-port networkk of the circuiit in Fig. 7 (a)) is analyzed bby considering an ideal
PIN diodde in the switchhable resonato
or. Hence,
.
(13)
Convertinng the transferr matrix in (13
3) to S-parameeter, we get
.
(14)
High iisolation can be
b obtained iff
and Zs ≈ 0 in the rad
dial stub reson
nator where thhe total isolatiion in (14)
is a cumuulative of isolaation of shuntt PIN diode annd isolation off the radial stu
ub resonator. T
This can be ob
bserved in
Fig. 7 (bb) where diff
fferent isolatio
on performannce is simulaated with diffferent angle oof radial stub
b. In this
simulatioon, the inner radius, ri is fixed at 1.5 mm in ordeer to minimizze any RF cooupling effectt between
microstriip line and thhe input of radial stub resoonator. Besidees, it is obserrved that largger angle of radial stub
producess wider isolatioon bandwidth in spite of higgh attenuation
n pole.
A circcuit diagram and
a prototype of SPDT swittch with switcchable radial stub
s
resonatorr are depicted in Fig. 8.
As a tradde-off betweenn radial stub size and its aattenuation pole performancce, the angle oof radial,
° is
chosen aand ri is fixed at 1.5 mm. All
A the PIN diiodes are supp
plied with +5 V (ON state)) and -5 V (O
OFF state)
control vvoltage.
In trannsmit mode (RF signals frrom Port 1 too Port 2), the D1 and Reso
onator-1 are tturned OFF. Then, the
Resonatoor-1 becomes allpass respon
nse. The D2 an
and Resonatorr-2 are turned ON. Then, thee Resonator-2
2 becomes
a bandstoop response inn the receive arm producinng additional isolation (bettween Port 1 and Port 3). The same
operationn and responsse can be obtaained in the reeceive mode (RF signals from
f
Port 2 too Port 3) wheen D1 and
Resonatoor-1 are turnedd ON; and D2
2 and Resonatoor-2 are turned OFF.
ISSN : 0975-4024
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464
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
(a)
(bb)
Fig. 8. SPDT switch with switchable raddial stub resonator (a) circuit diagrram (b) circuit proototype.
IIII. SIMULATION
N AND MEASU
UREMENT RESU
ULT
Fig. 9 shows the sim
mulated resultts of insertionn loss (IL), retturn loss (RL)) and isolationn (ISO) of con
nventional
witch and SPD
DT switch with
h switchable rradial stub ressonator. The simulated
s
IL aand RL of the proposed
SPDT sw
SPDT sw
witch are com
mparable with the conventioonal SPDT sw
witch. The sim
mulated ISO of the propossed SPDT
switch shhows a signifficant improveement of isollation which is
i higher than
n 20 dB betw
ween 3.05 to 3.85
3
GHz
comparedd with the connventional SPD
DT switch.
(a)
(b)
Fig. 9. Si mulated (a) inserrtion loss, return loss
l
and (b) isolattion between sing
gle shunt SPDT switch (conventioonal) and SPDT sw
witch with
switchaable radial stub reesonator.
The pperformance comparison
c
at 3.5 GHz betw
ween the conv
ventional SPD
DT switch andd the SPDT sw
witch with
switchablle radial stub resonator are listed in Tablee I. The perfo
ormance is com
mpared betweeen insertion lo
oss, return
loss and iisolation.
TABLE I
Performan
ance comparison at
a 3.5 GHz
Convention
nal shunt SPDT
switch
Shunt SPD
DT switch with
switchablee radial stub
resonator
Insertion
Loss
Return
Loss
Tx-Rx
Isolation
0.88 dB
29 dB
11.5 dB
0.65 dB
32.7 dB
39 dB
Fig. 110 shows the simulated and
d measured reesult of SPDT switch with switchable
s
raddial stub reson
nator. It is
successfuully demonstraate an isolation higher than 20 dB at 3.5 GHz
G compareed with the connventional cirrcuit. Both
simulatioon and measurrement show a comparable iinsertion loss and isolation result but the measured retu
urn loss is
slightly llower comparred with simu
ulated ones. H
However, it is
i still higherr than 10 dB
B which is a minimum
specificattion of return loss.
ISSN : 0975-4024
Vol 5 No 1 Feb-Mar 2013
465
N. A. Shairi et al. / International Journal of Engineering and Technology (IJET)
(a)
(b)
Fig. 10. Siimulation and meeasurement result of SPDT swith ((a) double switchaable radial stub re
esonator and (b) ssingle switchablee radial stub
resonator.
IV
V. CONCLUSIO
ON
A swittchable radial stub resonato
or is proposed in SPDT disccrete switch att 3.5 GHz for isolation imprrovement.
The potential applicatiion is TDD sw
witching for W
WiMAX and LTE
L commun
nication system
m. The characcteristic of
nalyzed in teerm of charaacteristic impeedance and aattenuation pole using
the switcchable radial stub was an
mathemaatical modelinng. Then, the isolation imprrovement wass analyzed wiith two-port nnetwork of sin
ngle shunt
PIN diodde with switchhable radial stu
ub resonator. T
There is a corrrelation betweeen inner radiial and angle radial
r
stub
with the characteristicc impedance and attenuatioon pole, thus provide an additional
a
isol
olation in SPD
DT switch
Finally, we succcessfully fabricated the SP
PDT switch with
w switchablee radial stub re
resonator to vaalidate the
design. F
simulatioon result whichh shows an iso
olation improvvement higheer than 20 dB between
b
3.05 to 3.85 GHz.
ACCKNOWLEDGM
MENT
The w
work describeed in this pap
per was fully supported by
y Universiti Teknikal
T
Malaaysia Melaka (UTeM).
Melaka. T
Therefore, wee would like to
o acknowledgee the contributtion of our collleagues from Fabrication Laboratory
L
and Microwave Labooratory, Facullty of Electroonics and Co
omputer Engin
neering, UTeM
eM for fabriccation and
ment of the ressearch works.
measurem
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