Millimeter-wave micromachined tunable filters - IEEE Xplore

5 downloads 0 Views 309KB Size Report
Hong-Teuk Kim, Jae-Hyoung Park, Yong-Kweon Kim, and Youngwoo Kwon. School of Electrical Engineering, Seoul National University. San 56-1 Shinlim-dong ...
WE4C-5

Millimeter-wave Micromachined Tunable Filters Hong-Teuk Kim, Jae-Hyoung Park, Yong-Kweon Kim, and Youngwoo Kwon School of Electrical Engineering, Seoul National University San 56-1 Shinlim-dong, Kwanak-ku, Seoul, 151-742, Korea e-mail :[email protected] ABSTRACT

fabricated bandpass filters opevated at Ka-band with the tuning range of as much as 1.1 GHz (4.2 YO)for 2-pole lumped elements filter, 0.8 GHz (2.5 %> for 2-pole resonators filter. The design, fabrication and measured results of micromachined tunable filters are presented.

Novel types of mm-wave tunable filters using micromachined variable capacitors are presented. In order to demonstrate this concept, two bandpass filters, one with 2-pole lumped elements and the other with 2pole resonators are designed at Ka-band and fabricated using MEMS technology. With applied bias, the center frequency shift as much as 1.1 GHz (4.2 %) at 26.6 GHz, and 0.8 GHz (2.5 YO)at 32 GHz, was measured for lumped elements and resonators filter, respectively. These results show the potentials of micromachined tunable filters for low cost, highly integrated transmitter and receiver for mm-wave multi band communication systems.

DESIGN Two types of tunable filters were designed; one using 2-pole lumped elements with 4.7 % bandwidth, 0.5 dB ripple at 26.8 GHz and the other using 2-pole resonators with 8.5 % bandwidth, 0.5 dB ripple at 30.6 GHz.

INTRODUCTION Recent developments in microelectromechanical systems (MEMS) have made possible the fabrication of microwave and mm-wave planar filters on thin dielectric membrane [1][2]. They show low loss, and are suitable for low cost, compact, high performance mm-wave one-chip integrated circuits. Recently, a hlly integrated, micromachined, micropackaged filter banks was also reported at WKu-band and showed the possibility of low cost filter banks for multi-band communication systems [3]. However, these filters are fixed-frequency filters and are, therefore, not suitable for modern multi-band communication systems. In this paper, we propose new types of tunable filters using micromachined cantilever-type variable capacitors. With the application of DC bias voltage on cantilevers, the electrostatic force between the plates increases and the distance between the two plates reduces, resulting in a capacitance increase. These micromachined capacitors were employed in Ka-band bandpass filters as a part of resonators to demonstrate the frequency tunability. The

Port2 -0

Micromachined

(a)

Fig.1. Topologies of two micromachined tunable filters. (a) 2-pole lumped elements filter, (b) 2-pole resonators filter 1235

0-7803-5135-5/99/$10.000 1999 IEEE

1999 IEEE MTT-S Digest

k (a)

Micromachided movable cantilevers

Fig. 3. Micromachined movable cantilever structure.

capacitors are connected to both ends of half wavelength resonators, shifting h12 resonance frequency. The microphotographs of two fabricated filters are shown in Fig.2. These circuits were patterned on a 560-omthick glass (Coming #7740) substrate using 2 ,um gold electroplating and subsequent releasing technique. Cantilevers are suspended 6 pn above a common GCPW x 200 ,m. To avoid top ground plate. The area is 200 DC voltage short, an I-,m SiOz dielectric layer was deposited on the GCPW top ground plate. The fabricated cantilever beam is shown in Fig.3. RF choke for DC voltage bias was realized using a h/4 radial stub and a h/4 high impedance line. Application of DC voltage between the cantilever and the GCPW top ground plate causes the upper cantilever to deflect downwards. When the cantilever go down as much as a third of the initial gap from top, it collapses onto the dielectric layer due to the pull-in instability [4]. We simulated the tuning ranges of the filter for the gap distances between two parallel plates of movable capacitor, 6 om, 5 pm, 4 ,um. The simulation results are shown in Fig.4. 2-pole lumped elements filter showed 6.4 % center frequency shift, from 26.8 GHz to 25.1 GHz, with a decrease in the gap by 2 om. 2-pole resonators filter showed 2.6 % center frequency shift, from 30.6 GHz to 29.8 GHz, with the gap change of 1 ,m.

Fig. 2. Microphotographs of two micromachined tunable filters. (a) 2-pole lumped elements filter (3.4 mm x 2.9 mm), (b) 2-pole resonators filter (4.1 m m x 3.4 mm).

Commercial software IE3D was used for the full wave analysis of GCPW micromachined filters. Fig. 1 shows the topologies of two micromachined tunable filters. In case of 2-pole lumped elements filter shown in Fig. 1 (a), interresonators coupling is mostly through magnetic field and adjusted with the distance between two spiral inductors. For frequency tuning, micromachined cantilever-type variable capacitors are connected to both ends of spiral inductors, varying LC resonance frequency. In the 2-pole resonators filter shown in Fig.1 (b), these variable

MEASUREMENT

The responses of the filters were measured using HP 85 1OC network analyzer. A Short-Open-Load-Through calibration was used with 150 p m pitch Picoprobes and

1236

0

0

10

5

-10

-10

-20

0

-m- -20

p.

-% 7

rn

N

5 -30

- 5 *

-3 0 -1 0

4 0

-1 5 22

24

.Frequency (GHz)

26 28 Frequency ( G b )

30

32

(a)

(a) 0

20

-10

10

-20

0 -

I

B

B

..-

r

v) cu

7

-30

-10

40

-20

-50 15

2 6 2 8 3 0 3 2 3 4 3 6 3 8

-30 20

25

30

35

40

45

Frequency(GHz)

Frequency (GM)

(b)

(b) Fig. 4. Simulated S2 1 of two micromachined tunable filters. (a) 2-pole lumped elements filter, (b) 2-pole resonators filter.

Fig. 5. Measured responses of two micromachined tunable filters. (a) 2-pole lumped elements filter, (b) 2-pole resonators filter.

to the conductor loss which can be reduced with the thicker metal and unaccounted radiation loss. Comparison of Fig.4 (a) with Fig.5 (a) shows that maximum measured tuning range (4.2 %) is smaller than the simulated result (6.4 %), which is attributed to the partial deflection of the cantilevers; the edge of the cantilever deflects as designed while the its neck deflects much less. Aside from the higher insertion loss, the measured results showed good agreement with simulated results.

a calibration substrate from GGB Industries. DC bias voltage was applied between the movable cantilever beams and a common GCPW top ground plate using DC probes. The measured bias dependence of the two filters is shown in Fig.5. The center frequency of the 2-pole lumped elements filter shifted from 26.6 GHz with no bias to 25.5 GHz with 65 V bias (4.2% tuning). That of 2-pole resonators filter shifted from 32 GHz with no bias to 3 1.2 GHz with 50 V bias (2.5 % tuning). The passband insertion losses did not change appreciably throughout the tuning range of the filters. The measured 4.9 dB and 3.8 dB minimum passband insertion losses for lumped elements and resonators filters, respectively, were 2 dB higher than simulated results. These losses are attributed

CONCLUSION In this paper, novel types of mm-wave tunable filters

1237

using micromachined variable capacitors are designed and fabricated using MEMS technology. Experimental results of two filters indicate 4.2 % and 2.5 % tunability at 26.6 GHz and 32 GHz, respectively. These results demonstrate the feasibility of new micromachined tunable filters for low cost, highly integrated transmitter and receiver for mm-wave multi band communication systems.

ACKNOWLEDGMENT This work is supported in part by Korea Institute of S&T Evaluation and Planing.

REFERENCES [ l ] S.V. Robertson, L.P.B. Katehi, and G.M. Rebeiz, “Micromachined Self-packaged W-Band Bandpass Filters”, IEEE Microwave Theory Tech. Symp., pp 1543-1546, June 1995. [2] P. Blondy, A.R. Brown, D. Cros, G.M. Rebeiz, “ Low Loss Micromachined Filters for Millimeterwave Telecommunication Systems”, IEEE Microwave Theory Tech. Symp., pp 1181-1 184, June 1998. [3] A.R. Brown, G.M. Rebeiz, “Micromachined micropackaged Filter Banks”, IEEE Microwave and Guided Wave Letters. Vo1.8, No. 4, pp 158-160, April 1998 [4] C. Goldsmith, J. Randall, S . Eshelman, T.H. Lin, D. Denniston, S.Chen, B. Norvell, “Characteristics of micromachined switches at microwave frequencies”, IEEE Microwave Theory Tech. Symp., pp 1141-1 144, June 1996.

1238