Planar diplexer for microwave integrated circuits

Z.C. Hao, W. Hong, J.X. Chen, X.P. Chen and K. Wu

Abstract: A planar microwave diplexer based on the substrate integrated waveguide (SIW)technique is presented in the paper. The SIW-microstrip transition and SIW filter areexperimentally investigated, and then a C-band SIW diplexer is designed and fabricated using astandard PCB process. The relative bandwidths of up and down channels are 3% and 4.8% at5.96GHz and 5.42GHz, respectively. The measured results show good channel isolation, moderateinsert losses and small return losses in pass-bands. The diplexer takes a planar form and can beeasily integrated in microwave integrated circuits.

1 Introduction

Increasing development of microwave communicationsystems, such as wireless local-area networks (WLAN)and local multipoint distributions systems (LMDS) etc.,greatly stimulates the need for compact low-loss microwavediplexers or multiplexers, which serve as channel separators.The waveguide diplexers/multiplexers are well studied andwidely used in wireless communication systems, due to theirexcellent performance [1, 2]. However, waveguide compo-nents cannot be integrated with microwave or millimetre-wave planar integrated circuits. To overcome this drawback,substrate integrated waveguides (SIWs) are the appropriatechoice for the design of microwave and millimetre-waveintegrated circuits [3–25]. SIW is synthesised by metallic-viaarrays in substrate which can be easily integrated with othermicrowave and millimetre-wave circuits [3–8]. As the fielddistribution in an SIW is similar to that in a conventionalrectangular waveguide, then SIW components take advan-tages of high Q-factor, low insertion loss and high-powercapability etc. [9–25]. Similar structures realised in LTCCprocesses are also known as laminated waveguides andpost-wall waveguides [4–6, 21–25]. This scheme is alsofeasible for ridged waveguides in LTCC [4, 5, 9–12], wheresome wideband ridge-waveguide bandpass filters andmultiplexers with good experiment results were reported.

In this paper, based on the studies in [7, 8], we obtain theequivalent conventional rectangular waveguide for SIWwith a closed-form formula, and then equivalent circuits forSIW steps are proposed. Using these models, we designedthe SIW inductive window filter. To ensure the SIWcomponents can be measured or integrated with othercomponents in a planar form, SIW-microstrip transition isstudied. Finally, an SIW diplexer is designed, fabricated andmeasured. The measured results show that the microstrip-SIW transition can be operated in a wide frequency range,the performances of SIW inductive window filters arereasonably good, and the diplexer has good channel

isolation, moderate insertion loss and small return loss.The whole structure is fabricated on a single substrate. As aresult, not only the size, weight and cost of the componentsare reduced, but also the manufacturing repeatability andreliability are enhanced.

2 SIW-microstrip tapered transitions

To measure or integrate the SIW diplexer with othermicrowave integrated circuits conveniently, the design forSIW transitions is very important [3, 13]. The SIW-microstrip tapered transition shown in Fig. 1 with itsgeometrical parameters influences the performance ofSIW diplexer directly. An SIW-microstrip tapered transitionincludes a segment of 50Omicrostrip, a segment of taperedmicrostrip and the discontinuity between microstrip andSIW. The tapered microstrip is used to excite the waveguidemode and match the impedance with the SIW. Byoptimising the length and width of the taper, the transitioncan be operated in a wide frequency range.

To ensure the SIW supports the TE10-like modeexclusively in the operating frequency range, the SIW shouldbe designed carefully. The necessary physical parameters inthe design of the SIW include the diameter D of metallic vias,the space VSP between metallic vias, and the SIW widthWSIW. From the viewpoint of dispersion characteristics andthe leakage loss, the SIW can be treated as a conventionalrectangular waveguide if the SIW physical parameters aredetermined by using the following closed-form formula [8]:

�a ¼ arec

WSIW¼ x1 þ

x2VSPDþ x1 þ x2 � x3

x3 � x1

ð1Þ

where �a is the normalised coefficient, arec is the width of thecorresponding conventional rectangular waveguide, and x1,x2, x3 are defined as

x1 ¼ 1:0198þ 0:3465

WSIWVSP

� 1:0684

x2 ¼ �0:1183�1:2729

WSIWVSP

� 1:2010

x3 ¼ 1:0082� 0:9163

WSIWVSP

þ 0:2152

ð2Þ

The authors are with the State Key Laboratory of Millimeter Waves,Department of Radio Engineering, Southeast University, Nanjing 210096,People’s Republic of China

E-mail: zchao@emfield.org

r IEE, 2005

IEE Proceedings online no. 20050014

doi:10.1049/ip-map:20050014

Paper received 18th January 2005

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 455

Using (1) and (2), the SIW can be treated as a conventionalrectangular waveguide, and the equivalent impedance Ze ofthe SIW can be determined as

Ze ¼p2

8� harec�ffiffiffiffiffiffiffiffiffiffiffiffiffier � e0mr � m0

r� 1:0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1:0� ðl=2 � arecÞp ð3Þ

where h, er, mr are the thickness, relative permittivity andpermeability of the substrate, respectively, e0 and m0 are thepermittivity and permeability in vacuum, l is the operatingwavelength.

Using (3), the design for SIW-microstrip taper transitioncan be treated as the design for a linearly tapered microstripline (LTML) [27, 28] to match the SIW and the 50Omicrostrip. The characteristic impedance Zc of a taperedmicrostrip line can be defined as

Zc ¼ C0 �Zt

jo� 1:0ffiffiffiffiffiffiffiffi

eeffp ð4Þ

where C0 is the light velocity in vacuum, o is the operatedangle frequency, Zt is the distributed series impedance ofequivalent transmission line for the linearly tapered micro-strip line (LTML), eeff is the effective relative dielectricconstant for an LTML [28]. Using (3), (4) and solving amatrix equation from [27], the width and length of theSIW-microstrip transition can be determined and the Sparameters can be obtained.

A C-band SIW-microstrip taper transition has beendesigned and fabricated on a substrate with h¼ 0.5mmusing a standard PCB process. The substrate has a relativepermittivity of er¼ 3.0 (75%) and a loss tgd of 0.001at 10GHz. The width of the SIW is WSIW¼ 20.5mm, thediameter of the metallic vias is 0.5mm, the space betweenmetallic vias is VSP¼ 1mm, the length of taper is 25mm,the width of the taper is 6mm and the width of the 50Omicrostrip is 1mm. The summation length of two tapertransitions and a segment of SIW is 10.0 cm, and a 10.0 cm50O microstrip is measured for comparison. The measuredresults are shown in Fig. 2. It can be seen that the transitioncan be operated from 5.0GHz to 6.8GHz with return lossof less than �12dB, and the insert loss is less than 1.5dB inthe operating frequency range.

3 Design for SIW inductive window filter

The channel filter plays an important role in designing theSIW diplexer. In this design, we adopt SIW inductivewindow filters as channel filters. The configuration of anSIW inductive filter is shown in Fig. 3 with its geometricalparameters. In Fig. 3, the SIW steps play the role of

inductive irises with thickness corresponding to theconventional rectangular waveguide inductive windowfilters.

The equivalent circuits for the SIW steps can each beroughly treated as an iris which has a round corner andsome thickness, and their equivalent circuits are shown inFig. 4, where the normalised parameters are determined asfollows [29]:

Xa

Z0¼ 2arec

lg

arec

pD0

� �2;

pD0

l� 1

Xb

Z0¼ arec

8lg

pD1

arec

� �4

;pD0

l� 1

ð5Þ

Ltaper W

tape

r

D

WSI

W

VSP

h

Fig. 1 SIW-microstrip tapered transition

0

10.0cm microstrip

the transition

−5

−10

−15

−20

−25

−30

5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0frequency, GHz

mag

nitu

de, d

B

6.0 GHz, −0.6 dB

6.0 GHz, −1.0 dB

Fig. 2 Measured results for SIW-microstrip taper transition and10 cm 50O microstrip

Ltaper

Wvr

WSIW

Li (i = 1,2... n)

W tape

r

SH i (i

= 1,2.

.. n)

Fig. 3 Configuration of SIW inductive window filter with itsgeometrical parameters

− j Xb− j Xb

j Xa

Z 0Z 0

ba

Lvs

arec

d′ /2

Fig. 4 Equivalent circuits for SIW stepsa SIW step with its geometric parametesb Equivalent circuits for the SIW step

456 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005

where lg is the guided wavelength, D0 and D1 are defined as

D0 ¼

d 0ffiffiffi2p a0

Eða0Þ � a2F ða0Þ ; otherwise;

d 0ffiffiffi2p 1þ Lvs

pd 0ln4pd 0

el

� �;

Lvsd 0� 1

8>>><>>>:

D1 ¼

ffiffiffiffiffiffiffiffiffiffia2a02

3

4

rd 0

EðaÞ � a02F ðaÞ otherwise;ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4

3pLvsd 03

4

rLvsd 0� 1

8>>><>>>:

ð6Þ

In (6), the functions E(a) and F(a) are the complete ellipticintegrals of the first and second kind, respectively, and a isdetermined by

Lvsd 0¼ Eða0Þ � a2F ða0Þ

EðaÞ � a02F ðaÞ ; a ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� a02p

ð7Þ

Using (5)–(7), we first get the equivalent circuits for SIWsteps, and then, taking a canonical filter design procedure[29], an SIW filter can be designed. Practically, the lengthsof SIW cavities obtained by this model will be adjusted alittle bit to satisfy the specification, and we can use full-wavecommercial simulators, such as HFSS, CST etc., to achievethe specifications [26].

We designed a C-band SIW inductive filter using thisdesign method. It is fabricated on a substrate with thicknessof 1mm, relative permittivity of 2.65(73%) and loss tgd of0.001 at 10GHz. The geometrical parameters are listed inTable 1. After the design was finished, we simulated thewhole structure with a full-wave field solver HFSS. Thesimulated results and the measured results which includethe effects of the two SMA connectors are shown in Fig. 5.

It can be seen from Fig. 5 that a return loss of less than�14.4dB, and an insertion loss of about 1.2dB are achievedin the pass-band from 6.05GHz to 6.55GHz. A 50MHzfrequency shift between the simulated results and themeasured results should be caused by the tolerance ofrelative permittivity (73%).

4 Design and optimisation for SIW diplexer

Figure 6 depicts the configuration of the proposed SIWdiplexer with its physical parameters. The diplexer consistsof two SIW channel filters and three SIW-microstriptransitions for input or output purpose. The whole structureis fabricated on a single-layer substrate using a standardPCB process.

In the designing of the SIW diplexer, direct integration ofchannel filters into the SIW branches may result in a poorperformance. Hence, an optimum procedure for diplexerdesign is necessary. In the optimisation design of SIWdiplexers, initial values of the SIW diplexers must beprovided. The initial dimensions of the channel filterssatisfying the given specifications can be obtained from thepreceding description. As shown in Fig. 6, because thetwo filters have a relatively narrow bandwidth, the firstresonator of filter 1 is relatively loosely coupled to themain SIW, hence the input SIW steps in filter 1 introduceonly small discontinuities in the main SIW, and thesediscontinuities disturb the response of filter 2 slightly. Toovercome these discontinuities, the SIW steps of the firstresonator in filters 1 and 2 should be tuned in the design.If the filter 1 is placed approximately one-quarter guided-wavelength (or three quarters guided-wavelength forfabrication purpose of the centre frequency of filter 2) fromthe first SIW steps of filter 2, the filter 2 reflects an opencircuit to the plane of these SIW steps. Thus, the filter 2 willbe roughly decoupled. Then the initial values for distancesspace4 and space5 shown in Fig. 6 are set to be lg1/4and 3lg2/4�WSIW/2, respectively. Here, lg1 and lg2 arewaveguide wavelengths of filters 1 and 2, respectively. Inthe first step of the optimisation procedure, only thejunction parameters, i.e. lengths of the connecting SIW’sspace4 and space5 shown in Fig. 6 are optimised.In the next optimisation step, more parameters should be

Table 1: Dimensions of SIW step filter

L1, mm 12.565 L2, mm 15.451 L3, mm 16.128

L4, mm 15.451 L5, mm 12.565 Wvr, mm 4.0

SH1, mm 2.9365 SH2, mm 4.5075 SH3, mm 5.034

SH4, mm 5.034 SH5, mm 4.5075 SH6, mm 2.9365

WSIW, mm 21.5 D, mm 0.5 VSP, mm 1.0

Ltaper, mm 19.0 Wtaper, mm 9.6 W50, mm 2.76

4.5

0

−10

−20

−30

−40

−50

5.0 5.5 6.0 6.5 7.0 7.5frequency, GHz

measured resultsdesign results

6.35GHz, −1.2dB

mag

nitu

de, d

B

Fig. 5 Measured and simulated results for SIW inductive windowfilter

L taper

space4

channel filter 1

space3Wtaper

SH2i (i = 1–5)

SH1i (i = 1–6)

L2i (i = 1– 4)

L1i (i = 1–5)

WSIWspace1 space5

Wvrspace2

channel filter 2

Fig. 6 Configuration of the proposed SIW diplexer with itsgeometrical parameters

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 457

included in the optimisation. The width of SIW steps andthe length of SIW resonators in the channel filters closer tothe junction should be chosen earlier in the optimisation.Then the obtained parameters are used as the initial valuesfor the sequent optimisation steps. This will be repeateduntil all the required diplexer specifications are satisfied.Practically, we can obtain the parameters for the SIWdiplexer just by tuning space4, space5, lengths of the firstresonator and widths of the first steps in filter 1 and filter 2.

A C-band SIW diplexer is developed, and has beenmeasured without any tuning. The structure was fabricatedon a 0.5mm thick substrate with er¼ 3.0(75%) usingstandard PCB process, and the substrate has a loss tgd of0.001 at 10GHz. An SIW-microstrip transition is designedin each port with good performances in a wide frequencyrange. The geometric parameters of the proposed diplexerare given in Table 2, where WSIW is the width of SIW, L1i

(i¼ 1–5) and L2i (i¼ 1–4) are the length of SIW cavities infilters 1 and 2, respectively, SH1i (i¼ 1–6) and SH2i (i¼ 1–5)are the height of SIW steps, D is the diameter of the metallicvias, VSP is the space between metallic vias and W50 is thewidth of a 50O microstrip.

The measured results of the SIW diplexer are given inFigs. 7 and 8, where the SMA effects of the transitions arealso included. The channel 1 has a centre frequency of

5.96GHz, with bandwidth from 5.87GHz to 6.05GHz,and the channel filter 2 has a centre frequency of 5.42GHz,with bandwidth from 5.29GHz to 5.55GHz. The maximalinsertion loss of channel 2 is�2.6dB, while it is�3.2dB forchannel 1. The return loss is less than �17.5dB for channel2, and is less than �12.5dB for channel 1. The measuredchannel isolation is presented in Fig. 8, which shows thatthe channel isolation is less than �65dB for channel 2, andless than �50dB for channel 1.

5 Conclusions

A planar SIW diplexer has been presented in this paper.A C-band SIW-microstrip taper transition, a C-band SIWinductive window filter and a C-band SIW diplexer aredesigned, fabricated using standard PCB process andmeasured, respectively. As the whole structure of the SIWdiplexer is made by various metallic vias on a planarsubstrate, it can be easily fabricated and conveniently beintegrated into microwave and millimetre-wave integratedcircuits, which means the SIW components can be massproduced with small size, low weight and low cost.

6 References

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Table 2: Dimensions of SIW diplexer

Filter 1 Filter 2 SIW and SIW-

microstrip taper

transition

space4, mm 23.18 space5, mm 21.47 D, mm 0.5

L11, mm 22.22 L21, mm 18.35 VSP, mm 1.0

L12, mm 24.14 L22, mm 20.73 WSIW, mm 20.5

L13, mm 25.24 L23, mm 20.69 Ltaper, mm 25.0

L14, mm 24.81 L24, mm 18.35 Wtaper, mm 6.0

L15, mm 21.5 W50, mm 1.0

SH11, mm 3.27 SH21, mm 4.195 Wvr, mm 4.0

SH12, mm 4.615 SH22, mm 6.25

SH13, mm 5.295 SH23, mm 6.6 space1, mm 10.0

SH14, mm 5.355 SH24, mm 6.25 space2, mm 10.0

SH15, mm 4.865 SH25, mm 4.335 space3, mm 10.0

SH16, mm 3.14

5.42GHz, −2.6dB

5.96GHz, −3.2dB

mag

nitu

de, d

B

0

−10

−20

−30

−40

−50

4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.504.50frequency, GHz

S31S32S33

Fig. 7 Measured S-parameter response of the proposed SIWdiplexer

S 21−40

−50

−60

−70

−80

−90

−100

4.50 4.75 5.00 5.25 5.50 5.75 6.00 6.25 6.50frequency, GHz

mag

nitu

de, d

B

Fig. 8 Measured channel isolation of the proposed SIW diplexer

458 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005

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