Low transmission loss, simple, and broadband waveguide-to-microstrip line transducer in V-, E- and W-band

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1 LETTER IEICE Electronics Express, Vol.14, No.15, 1 10 Low transmission loss, simple, and broadband waveguide-to-microstrip line transducer in V-, E- and W-band Kohei Fujiwara a) and Takeshi Kobayashi Tokyo Metropolitan Industrial Technology Research Institute, Aomi, Koto-ku, Tokyo , Japan a) fujiwara.kohei@iri-tokyo.jp Abstract: A low transmission loss, simple mechanical structure, and broadband waveguide-to-microstrip line transducer has been developed with a polytetrafluoroethylene printed circuit board. This can fully cover the V-band, E-band, and W-band, and the transmission losses are typically 0.68 db, 0.86 db, and 0.78 db, respectively, at the center frequency of each band. The transducer in the W-band was already implemented in a W-band 2nd-order harmonic mixer. In this letter, we report the development of the millimeter-wave waveguide-to-microstrip line transducer. Keywords: W-band, UWB, waveguide-to-microstrip line transducer Classification: Microwave and millimeter-wave devices, circuits, and modules References [1] K. Fujiwara and T. Kobayashi: Low-cost W-band frequency converter with broad-band waveguide-to-microstrip transducer, Proc. Global Symposium of Millimeter Waves (2016) (DOI: /GSMM ). [2] X. Ma and R. Xu: A broadband W-band E-plan waveguide-to-microstrip probe transition, Proc. Asia-Pacific Microwave Conference (2008) (DOI: /APMC ). [3] E. W. Bryerton, et al.: A W-band low-noise amplifier with 22K noise temperature, IEEE MTT-S International Microwave Symposium Digest (2009) (DOI: /MWSYM ). [4] J. H. C. Van Heuven: A new waveguide-microstrip transition, IEEE Trans. Microw. Theory Techn. 24 (1976) 144 (DOI: /TMTT ). [5] L. J. Lavedan: Design of waveguide-to-microstrip transitions specially suited to millimetre-wave applications, Electron. Lett. 13 (1977) 604 (DOI: / el: ). [6] J. Dong, et al.: Broadband stripline to rectangular waveguide transition, IEICE Electron. Express 12 (2015) (DOI: /elex ). [7] Y. Leong and S. Weinreb: Full band waveguide-to-microstrip probe transitions, Proc. IEEE International Microwave Symposium (1999) (DOI: /MWSYM ). 1

2 [8] T. Suzuki, et al.: Band broadening of waveguide/microstrip-line transformer for millimeter-wave band, IEICE Technical Report ED MW (2012-1). [9] K. Fujiwara and T. Kobayashi: Japan Patent Application No [10] Japanese Industrial Standards: JIS H 4040, Aluminum and aluminum alloy rods, bars and wires (2006). [11] A. Technologies: De-embedding and Embedding S-Parameter Networks Using a Vector Network Analyzer, Application Note Available from [12] K. Technologies: Signal Integrity Analysis Series Part 3: The ABCs of De- Embedding, Application Note. Available from com/litweb/pdf/ en.pdf. [13] K. Technologies: De-embedding Techniques in Advanced Design System, Application Note. Available from EN.pdf. 1 Introduction Recently, several ultra-wide band (UWB) millimeter-wave applications have been deployed for instance, the IEEE ad wireless local area network (WLAN) in the V-band and an automotive radar in the E-band. In order to increase the penetration of millimeter-wave equipment into the consumer market, new products should be developed and fabricated in a small size and medium firm. The main mission of our institute is technical fostering to achieve products of small size and medium firm. Therefore, it is necessary to support the entry of an enterprise into the new millimeter-wave industry and develop components with low fabrication cost and high quality. As well known, most millimeter-wave components are generally transferred or inter-connected using a rectangular waveguide owing to lower transmission loss as compared to a coaxial cable or a microstrip line (MSL). However, surface mount devices have to be placed on a printed circuit board (PCB) when a circuit is built. A typical millimeter-wave circuit employs an alumina or quartz substrate using a metal sputtering process owing to a low loss tangent at high frequency. However, the fabrication is more expensive as compared to a general PCB process and the handling is difficult owing to the fragile and thin substrate. Thus, a low loss and broadband direct conversion of waveguide-to-msl transducer should be built using the polytetrafluoroethylene (PTFE) substrate via an etched PCB process for a low-cost millimeter-wave component. We have already developed a W-band frequency converter in a 2nd-order harmonic mixer for implementing the transducer [1]. The frequency conversion loss is approximately 20.0 db at an RF signal of 89 GHz and an IF signal of 9 GHz, and the total fabrication cost is approximately $800 USD [1]. We confirmed that the developed transducer exhibits satisfactory performance to include a millimeter-wave component. In this paper, we describe the development of a new broadband waveguide-to- MSL transducer for UWB application in the V- (50 75 GHz), E- (60 90 GHz), and W-bands ( GHz). The transducer has a high repeatability and a very simple mechanical structure. 2

3 2 Design and simulations A coaxial-msl transition is not a popular method for millimeter-wave applications because it is not cost-effective and incurs a large transmission loss; therefore, we have developed a waveguide-to-msl transducer with high performance and low cost using a general PCB process and a simple mechanical structure. 2.1 Design concept First, most millimeter-wave applications have an ultra-wide bandwidth of a few GHz. Therefore, the following are the development goals: the transmission loss should be within 1 db at the center frequency of the frequency band, the 1 db loss bandwidth should cover more than 80% of the band, and the loss flatness should be within 0:1 db. Second, the common rectangular waveguide to MSL transducers are proposed as the right-angle E-plane probe [2]. Although the structure is very simple, the type requires additional waveguide bends when an in-line arrangement is configured for an amplifier application [3]. Thus, to avoid a complex structure, we developed an in-line type transducer in the report. Third, in order to achieve a low cost and high repeatability transducer, an easyto-etch PCB pattern and a simple mechanical structure for an enclosure are required. Consequently, we chose a Finline type [4,5] instead of a probe transition type [6,7]. The function of the waveguide-to-msl transducer is to convert a TE 10 electric field in the rectangular waveguide by 90 and concentrate the field in the MSL mode by using an antipodal tapered ridge structure. The transducer PCB is sandwiched without a solder or silver epoxy glue in the E-plane at the center of a waveguide. The transducer consists of four segments, as explained in Fig. 1: the rotating part, parallel part, balun part, and 50 Ω MSL part. The top side (the MSL is printed) is indicated by a hatched pattern [1]. (a) PCB Pattern (b) Flange Side View Fig. 1. Outline of single waveguide-to-msl transducer. (a) PCB pattern. (b) Flange side view. The rotating part is often employed in a streamline shape such as a cos 2 function curve [4]. In the transducer, the part has been designed using a polygonal shape for a simple simulation model. The balun part acts as a matching transformer for the symmetrical structure to the asymmetrical MSL [4]. The substrate is a 3

4 Rogers RT/Duroid 5880 ( r ¼ 2:2, tan ¼ 0:001), and the thickness is mm. The copper layer is 17.5 µm thickness. The via radius is 0.2 mm and the pitch is smaller than a quarter length of the maximum frequency in a frequency band. The transducer PCB is sandwiched between two aluminum blocks divided by the E-axis, as shown in Fig. 1(b). Both the design and optimization are executed using an FEM engine on the Keysight EMPro and D electromagnetic simulator. The optimization parameters are as follows: the length and shape of the rotating and parallel parts; the length and clearance of the balun part; and the height of the MSL part inside the waveguide as illustrated in Fig. 1(a). 2.2 Simulation The simulation model with parameters is shown in Fig. 2. The model is a single transducer; however, when executing the simulation, two transducer models are cascaded to build a symmetrical waveguide-msl-waveguide transducer model. Both transmission and return losses are optimized by changing these parameters on the simulator. The optimized parameters are summarized in Table I for the V-, E-, and W-bands. Fig. 3 shows a calculated typical electric field in the waveguide face at 92.5 GHz inside the W-band transducer by the electro-magnetic simulator. Fig. 4 shows a calculated typical electric field along the MSL at 92.5 GHz. Consequently, the fundamental TE 10 mode at the waveguide port is brought to the MSL mode as the quasi-tem mode by the PCB pattern. (a) Bottom View (b) Top View (c) Side View of enclosure Fig. 2. Simulation model with parameters of waveguide-to-msl transducer, (a) Parameters for bottom view, (b) Parameters for top view, (c) Parameters for side view. 4

5 Table I. Summarized simulation parameters for each frequency band. Parameter Name WR-15 WR-12 WR-10 a mm mm mm b 2.00 mm 1.64 mm 1.16 mm c 3.00 mm 2.41 mm 1.88 mm d 4.90 mm 4.05 mm 3.31 mm e 5.55 mm 4.60 mm 3.70 mm f 5.90 mm 4.90 mm 4.00 mm g 6.25 mm 5.20 mm 4.15 mm h 1.26 mm 1.03 mm 0.89 mm j 0.69 mm 0.54 mm 0.46 mm k 0.24 mm 0.20 mm 0.17 mm m 0.23 mm 0.20 mm 0.20 mm n 0.64 mm 0.50 mm 0.50 mm r 0.35 mm 0.30 mm 0.30 mm s 0.35 mm 0.35 mm 0.35 mm MSL length 7.1 mm 8.1 mm 5.0 mm Height of MSL Part 0.90 mm 0.90 mm 0.80 mm Via Pitch 0.75 mm 0.75 mm 0.75 mm Thickness 20 µm Conductivity 5: S/m Fig. 3. Electric field of the W-band waveguide-to-msl transducer in the waveguide face at each part at 92.5 GHz. 2.3 Suppression of a partial transmission loss point inside a transducer In high frequency regions, a waveguide-to-msl transducer tends to generate a large transmission loss. A solution to rectify this problem is to place an electromagnetic wave absorber inside a waveguide [8]. Fig. 5 shows the calculated transmission and return losses of the W-band transducer. These are dips indicated by the dashed line at GHz, GHz, 5

6 Fig. 4. Electric field along the MSL of the W-band waveguide-to-msl transducer at 92.5 GHz. Fig. 5. Simulated transmission and return loss of waveguide-to-msl transducer in W-band. The dashed line represents the result of not improved the waveguide hight in the MSL part. The solid line represents the result of improvement. and GHz, respectively. The phenomenon occurs because of resonance in the transducer. Fig. 6 shows a typical disturbed electric field of the W-band transducer with a resonance at the frequency of GHz. The disturbance is likely induced around the balun part. In order to suppress the resonance in the development, the following measures are very effective: reducing the height of the waveguide in the MSL part, or an adjustment of the balun part [9]. The improved electrical field at GHz is shown in Fig. 7. The improved characteristics of the W-band transducer are plotted by the solid line as shown in Fig. 5. The suppression level is 18.2 db at the frequency of GHz. 3 Fabrication A photograph of the W-band transducer PCB and a split waveguide-to-msl transducer are shown in Fig. 8. Two transducers are cascaded to form a back-toback waveguide-msl-waveguide transducer because the measurement of the S- parameter must be performed using waveguide vector network analyzer (VNA) extenders. The PCB is gold-plated on the copper layer by an electroless plating. The measured conductivity of the PCB is 5: S/m. 6

7 Fig. 6. Typical disturbed electric field of the W-band waveguide-to- MSL transducer with a resonance along the microstrip line at GHz. Fig. 7. Electric field of the improved W-band waveguide-to-msl transducer along the microstrip line at GHz. The PCB is fixed by the four metal pins on the enclosure block for position alignment; subsequently, the blocks are screwed. The blocks are based on a 6061 aluminum alloy. The alloy has a good surface finish and an excellent corrosion resistance to atmospheric conditions. The conductivity is 3: S/m [10]. The flange complies with the type UG-387/U flange. The PCB length is 26 mm for the V- and E-band transducers and 18 mm for the W-band transducer, and the PCB width is 16 mm in each case. The MSL length of the E-band transducer is longer than that of the V-band transducer as summarized in Table I. 4 Evaluation The measurement of S-parameter is performed using the Agilent N5247A VNA with both the V-band extender V15VNA2-T/R and the W-band extender V10VNA2-T/R. In terms of the E-band evaluation, WR-12 to WR-10 and WR- 12 to WR-15 taper waveguides are used. Subsequently, the S-parameters of the 7

8 Fig. 8. Photograph of W-band waveguide-to-msl transducer PCB and enclosure. E-band transducer obtained with these taper waveguides are extracted by the S-parameter of these waveguides using the T-matrix method [11,12,13]. The mean and standard deviation values of the measured S 21 parameters of the transducers at the center frequency of each band are summarized in Table II. The standard deviations are 0.04 db in each case. Consequently, the transducers have repeatability. Table II. Mean and standard deviation values of fabricated transducers for each frequency band. WR-15 (f c ¼ 62:5 GHz) WR-12 (f c ¼ 75:0 GHz) WR-10 (f c ¼ 92:5 GHz) js 21 j 0.67 db 0.90 db 0.71 db 0.04 db 0.04 db 0.04 db N Figs. 9, 10, and 11 show a comparison of the typical transmission loss (S 21 parameter) and return loss (S 11 parameter) via simulation and measurement in each band. These results include a transmission loss from the back-to-back transducers because the measurement was performed using the WR-10 and WR-15 VNA extenders. The 1 db loss bandwidths achieved are 86.5% frequency range of the V-band, 82.3% frequency range of the E-band, and 87.0% frequency range of the W-band. The fluctuations of the flatness are 0:09 db for the V-band, 0:06 db for the E-band, and 0:09 db for the W-band. In order to investigate the difference between the simulation and measurement results for the above results, a simulation is performed for the W-band transducer using the conductivities of both fabricated PCB and aluminum enclosure described in the section three. The S 21 parameter by the simulation is 0.52 db at 92.5 GHz. The designed value in the Fig. 11 is 0.51 db at 92.5 GHz. Thus, the dependence 8

9 Fig. 9. Simulated and measured transmission and return losses of V-band back-to-back waveguide-to-msl transducer. Fig. 10. Simulated and measured transmission and return losses of E-band back-to-back waveguide-to-msl transducer. Fig. 11. Simulated and measured transmission and return losses of W-band back-to-back waveguide-to-msl transducer. of these conductivities is not dominant. The difference between the simulation and measurement results is probably caused by both r and tan at only 10 GHz from the supplier and not including the effect of the surface roughness of the metal layer in the electro-magnetic simulator. In the case of the E-band transducer, if the MSL length is shorter than the V-band transducer, the characteristics can be improved. Consequently, the three types of transducers achieve a low transmission loss of less than 1 db for each band at the center frequency, more than 80% of 1 db loss bandwidth in own frequency range, and good flatness. Therefore, the transducer exhibits a sufficiently good performance for implementation in a UWB millimeterwave component. 9

10 5 Conclusion Low transmission loss and broadband waveguide-to-msl transducers for the V-, E-, and W-bands can be fabricated on a PTFE substrate using the standard PCB process. The conversion losses achieved are within 1.0 db at the center frequency of all bands. The 1 db loss bandwidth covers more than 80% of each frequency band. The flatness in the bandwidth is approximately 0:1 db in each band. A dip caused by a resonance in each frequency band can be successfully suppressed by the improvement of the height in the MSL part and the optimization of the balun part. Therefore, the transducer is suitable for a UWB application and is simple to manufacture. The W-band transducer has already been implemented in the W-band frequency converter, and its performance has been verified. The transducer can contribute to cost reduction and construction process. As the next step, we are developing a UWB application for V- and E-bands using the transducer. Acknowledgments The authors would like to thank Dr. Kouji Shibata at Hachinohe Institute of Technology for fruitful discussion, Aida Print Seisakusyo Co. Ltd., Fuchu-city, Tokyo, for the PCB fabrication, and Hachiyoo Inc., Chofu-city, Tokyo, for the aluminum enclosure fabrication. 10