76 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 20, NO. 2, FEBRUARY 2010
Development of a Compact Broadband Folded HybridCoupler on Multilayer Organic Substrate
Hai Hoang Ta, Student Member, IEEE, and Anh-Vu Pham, Senior Member, IEEE
Abstract—We present the design and fabrication of a wide band-width folded hybrid coupler on a multilayer organic substrate. Thecoupler is designed with a tandem connection of two 5-sections witha coupling of 8.34 dB. We propose a novel center coupling structurethat achieves tight coupling while being compatible with printedcircuit board fabrication processes. The folded coupler achieves abandwidth of 2 to 18 GHz with 42% reduction in size.
Index Terms—Multichip module, multilayer, wide-bandwidthhybrid coupler.
I. INTRODUCTION
B ROADBAND hybrid couplers are a staple in many hybridmicrowave and millimeter wave circuits and multichip
modules [1]–[3]. A wide bandwidth 3 dB coupler is typically de-signed by cascading multiple coupled sections, each having theproper even- and odd-mode impedances and is a quarter-wavelong at the center frequency. A major challenge with this de-sign technique is that the coupling of the center section is muchtighter than the others. In planar microstrip lines, tight cou-pling requires small gaps that cannot be fabricated with stan-dard printed circuit board processes. Since each section has dif-ferent even- and odd-mode impedances, there are discontinuesbetween adjacent sections. These discontinues create extra re-actance and decrease the directivity of a coupler.
Tandem connections and non-uniform structures were pro-posed to develop wide bandwidth 3 dB couplers [4], [5]. In [4],Uysal and Aghvami used non-uniform structures to avoid abruptchanges and bond wires to provide crossovers for tandem con-nections to realize a 2–18 GHz, 3 dB coupler. The resulted cou-pler had large dimensions of by 25.4 mm, and thebond wires had significant effect on the coupler performance.In [5], Salem et al. used the same method as in [4] to develop a2–18 GHz, 3 dB coupler. The tandem connection was realizedby using offset parallel coupled strip transmission lines. Sincenine coupled-line sections were used in this design, the couplerachieved good amplitude balance but resulted in large dimen-sions of by 44.5 mm. Only simulation results wereprovided in this publication [5].
Manuscript received August 11, 2009; revised November 12, 2009. First pub-lished January 22, 2010; current version published February 10, 2010. This workwas supported in part by the Vietnam Education Foundation, Boeing Company,and UC MICRO.
The authors are with the School of Electrical and Computer Engineering,University of California at Davis, Davis, CA 95616 USA (email: [email protected]).
Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LMWC.2009.2038516
Fig. 1. 3-D view of the multi-layer broadband hybrid coupler.
TABLE INORMALIZED EVEN MODE IMPEDANCES
In this letter, we present the design and fabrication of a com-pact wide bandwidth folded hybrid coupler on a multilayer or-ganic substrate. By using tandem connections of two five-sec-tion, 8.34 dB couplers, the wide bandwidth 3 dB coupler is con-structed and folded in a multi-layer substrate. The coupler isdesigned to have a bandwidth ratio of 9:1 from 2–18 GHz. Thefolded coupler is 6.3 mm by 14.4 mm, which represents a 42%size reduction.
II. DESIGNOF THE HYBRID COUPLER
Fig. 1 demonstrates a 3-D view of our multi-layer hybrid cou-pler that is constructed from two five-section, 8.34 dB couplersthat are tandem connected together. The even- and odd-modeimpedances of the th section of each five-section, 8.34 dB cou-pler are related by
where is the terminating impedance at the ports. From [6],we can determine the normalized even-mode impedance valuesfor a 5-section, 8.34-dB coupler with a 9:1 bandwidth ratio. Theselected values are shown in Table I.
To achieve a normalized even-mode impedance value of2.09021 for the center section (section 3), a coupled transmis-sion line requires a gap of less than , which is notcompatible with printed circuit board processes. For printedcircuit board fabrication, the minimum line width and spacingare typically 75 . In this letter, we propose novel asym-metric broadside-coupled microstrip lines with a meandered
TA AND PHAM: DEVELOPMENT OF A COMPACT BROADBAND FOLDED HYBRID COUPLER 77
Fig. 2. Center section of the folded hybrid coupler.
Fig. 3. Cross section of a multi-layer LCP board.
Fig. 4. Prototype of multi-layer compact hybrid coupler on LCP (a) – Top view;(b) – Folded sections on the bottom-metal layer.
upper-layer conductor to realize tight-coupling center section.Fig. 2 shows a diagram detailing this technique. With theproposed structure, the center section can easily achieve ahigh normalized even-mode impedance value of 2.09021 whileproviding the crossover for a streamlined tandem connection.By using a meandered upper-layer conductor, the phase dif-ference of the coupled and through ports of the center sectionis kept around 90 . After all the sections are designed to havepredetermined coupling factors, they are connected together toform the complete 8.34 dB coupler. Two 8.34 dB couplers arethen tandem connected together to form a 3 dB coupler.
The coupler is built in a multi-layer Liquid Crystal Polymer(LCP) substrate, which has a dielectric constant of 2.9 and tan-gent loss factor of 0.0025. Fig. 3 illustrates the cross section ofthe multi-layer LCP board. Coupler sections 2, 3, and 4 are con-structed on the top metal layer while sections 1 and 5 are on thebottom metal layer. The coupler sections 2 and 4 are connectedwith the sections 1 and 5, respectively by using 200 diameter
78 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 20, NO. 2, FEBRUARY 2010
TABLE IICOMPARISION OF HYBRID COUPLER DESIGNS
vias. A ground is inserted in the middle for all sections. Fig. 4shows the prototypes of the folded coupler.
III. MEASUREMENT RESULTS
The simulation results for all -parameters of the couplerwere done using Sonnet EM software [7]. Electrical perfor-mance of the folded hybrid coupler was measured on a Cas-cade Microtech RF probe station, with an Agilent E8364 2-portnetwork analyzer. Before taking measurement, the probes werecalibrated by using on-wafer Thru-Reflect-Line calibration ona Picoprobe CS-5 substrate [8]. Two ports of the coupler areterminated with a broadband 50 resistor while a two-port net-work analyzer was used for measurements. For example, whenS31 were measured, port 2 and port 4 were connected to 50terminations while port 1 and port 3 were connected to the net-work analyzer. Fig. 5 shows the measurement results.
The measurement and simulations results are well correlated.The folded hybrid coupler achieves a return loss better 20 dB upto 8 GHz, better than 15 dB up to 14 GHz and has a minimumaround 10 dB around 18 GHz. The isolation is better than 16 dBup to 17 GHz. The measured phase difference between the cou-pled and through port is around 90 from 2 to 17 GHz.The measured through power is correlated to the simulated data.The measured coupled power is a little bit deviated from thesimulation. Theoretically, the ripple level is 0.8 dB or the cou-pling factor is around [6]. The coupling factor startsto deviate from the theoretical value due to fabrication toler-ances, losses and discontinuities between adjacent sections. Forexample in this design, for each 25 misalignment of the cou-pled lines of the center section, the coupling factor of the center
section decreases 2.5% and the phase difference between thecoupled and through ports of the center section changes 2.3%.The folded coupler has a size of 6.3 mm by 14.4 mm and is42% smaller than the un-folded hybrid coupler. Table II com-pares this design with published wide bandwidth couplers onthe same operating frequency range.
IV. CONCLUSION
We present the design and development of a wide bandwidthfolded coupler in multi-layer liquid crystal polymer. The mea-surement results show good agreement with the simulation re-sults. The coupler achieves a bandwidth ratio of 9:1. The isola-tion is roughly better than 15 dB and the phase difference isfrom 2 to 17 GHz. By folding the coupler in a multi-layer sub-strate, we achieve a size reduction of 42%. To the best of ourknowledge, this coupler achieves the highest bandwidth ratioproportional to size reported in literature to date.
ACKNOWLEDGMENT
The authors would like to thank T. T. Lee, Boeing Company,for valuable discussions and support.
REFERENCES
[1] P.-S. Wu, “New miniature 15–20-GHz continuous-phase amplitudecontrol MMICs using 0.18 �� CMOS technology,” IEEE Trans.Microw. Theory Tech., vol. 54, no. 1, pp. 10–19, Jan. 2006.
[2] S. J. Kim and N. H. Myung, “A new active phase shifter using a vectorsum method,” IEEE Microw. Guided Wave Lett., vol. 10, no. 6, pp.1677–1680, Jun. 2000.
[3] C. Y. Chi and G. Rebeiz, “Design of lange-couplers and single-sidedmixers using micromachining techniques,” IEEE Trans. Microw.Theory Tech., vol. 45, no. 2, pp. 291–294, Jan. 1997.
[4] S. Uysal and A. H. Aghvami, “Synthesis and design of widebandsymmetrical nonuniform directional coupler for MIC applications,” inIEEE MTT-S Int. Dig., 1988, pp. 587–590.
[5] P. Salem, C. Wu, and M. C. E. Yahoub, “Non-Uniform taperedultra-wideband directional coupler design and modern ultra-widebandbalun integration,” in Proc. Microw. Conf. (APMC’06) Asia-Pacific,Dec. 2006, pp. 803–806.
[6] E. G. Cristal and L. Young, “Theory and tables of optimum symmet-rical TEM-mode coupled-transmission line directional couplers,” IEEETrans. Microw. Theory Tech., vol. 13, no. 5, pp. 544–558, Sep. 1965.
