W-Band RF MEMS Double and Triple-Stub Impedance Tuners

RF MEMS

Abstract — Reconfigurable integrated impedance tuners have been

developed for W-Band on-wafer noise parameter and loadpull

measurement applications. The impedance tuners are based on double

and triple-stub topologies and employ 11 switched MEMS capacitors

producing 2048 (211) different impedances. Measured |ÃMAX| for the

double-stub tuner is 0.92 and 0.82 at 75 and 100 GHz from 110

measurements out of 2048 possible impedances, and 0.92 and 0.83

for the triple-stub tuner. To our knowledge, this represents the first

W-band integrated impedance tuner to date.

 

INTRODUCTION

Impedance tuners based on waveguides are used at frequencies

above 50 GHz in noise parameter and load-pull measurements of

active devices. In practice, these are large in size and expensive.

The maximum achievable reflection coefficient, |ÃMAX|, which can

be generated using waveguide tuners is very high (about 0.97)

but this is referred to the waveguide flange. For on-wafer

measurements, the loss of the connecting waveguide sections

and W-band-to-CPW probe can limit |ÃMAX| to about 0.7-0.8 at

50-110 GHz [1,2]. If a tuner can be integrated inside an RF probe,

the loss between the tuner and the DUT can be minimized. Also,

measurement automation can be increased with electrically

controllable tuners.

In this work, we present the first integrated impedance tuners

operating at W-Band (75-110 GHz). The impedance tuners are

based on double- and triple-stub topologies and switched

MEMS capacitors. The MEMS switches are used since they

provide excellent performance compared to mmwave transistors

or varactor diodes [3].

IMPEDANCE TUNER DESIGN

The impedance and electrical length of the stubs and connecting

t-line are controlled at discrete positions by digitaltype RF MEMS

capacitors (Fig. 1) as done previously in [4-6].

This method results in more wideband tuning and better

impedance coverage compared to standard reconfigurable

impedance tuners [7,8]. In the switched capacitor-based tuner,

the number of the switched capacitors (N) and their capacitance

values (CU, CD) have the most important effect on the tuning

range and bandwidth. In general, a larger N yields more wideband

operation and better impedance coverage but results in an increased

component size and loss. Other parameters that need to be

optimized are the spacing of the switched capacitors and the

lengths of the stubs. There are many variables and several

acceptable solutions. The tuners were optimized to have 11 switched

capacitors producing 2048 (211) different impedances, and Agilent

ADS.1 was used to for the optimization procedure. The

fabricated impedance tuners are shown in Fig. 2, and the

process is similar to [4-6] having the same materials and

layer thicknesses.

The switched capacitor is a series combination of a

MEMS switch and metal-air-metal (MAM) capacitors

(Fig. 3). The size of the MEMS switch is 200 ìm x 40 ìm x 0.9

ìm and the area of the fixed MAM capacitor is 280 ìm2,

respectively. The MAM capacitors are realized as a part

of the anchor area of the MEMS switch. These are

electroplated to 3 ìm, and being very stiff, they are

not actuated with the bias voltage. The CPW dimensions

are 50/50/50 ìm (G/W/G) and under the MEMS switches

45/90/45 ìm in both designs. The center conductor was

widened under the MEMS switches for a lower pull-down

voltage. Measured pull-down voltage was 28 V, and a 36 V

bipolar actuation was used for getting a firm downstate

contact and avoiding charging in the dielectric layer.

The switched MEMS capacitor total up and down-state

capacitances are CU = 23 fF and CD = 46 fF (XU=-j78 Ù

and XD=-j39 Ù at 90 GHz), which results in total capacitance

ratio of 2.0. Previous lower frequency designs employed a

capacitance ratio of 3.5-4.5 for the entire coverage of the

Smith Chart [4-6]. The quality factor of the switched MEMS

capacitor is calculated using Q = (2ðfC(RMEMS+ RMAM))-1

and results in QU = 111 and QD = 54 at 90 GHz. This means

that the switched capacitors do not contribute a lot to the

loss of the tuner circuit. Measured and simulated S-

parameters for the tuners are shown in Fig. 4. The

T-junctions were simulated with Sonnet2, and the

S-parameters for the tuners with Agilent ADS using

the equivalent circuits of the switched capacitors

(Figs. 3 and 4, and Table I).

IMPEDANCE COVERAGE

the tuners can produce 2048 (211) different impedances.
Measured (110 points for the 2-stub tuner, and 90 for
the 3- stub) and simulated (all 2048 points) impedance
coverage for the tuners are shown in Figs. 5 and 6.
Measured |ÃMAX| was 0.93, 0.87 and 0.87 at 75, 90, and
105 GHz for the double-stub tuner from 110 measurements
 and 0.92, 0.87, and 0.83 for the triple-stub tuner from 90
measurements. The tuners have reasonably good impedance
coverage with high reflection coefficients at W-Band
frequencies, and demonstrate that stub tuners can be
employed at 110 GHz and above.

Tjunctions can be improved by making them more

compact and lowering the transmission line impedance.

With these improvements, we feel that the double and

triple stub tuners can cover the entire Smith chart over

the W-band frequency range (see Fig. 7).

CONCLUSIONS

Reconfigurable W-Band double- and triple-stub

impedance tuners were presented. The tuners were

developed for onwafer noise parameter and load-pull

measurement purposes. The electrical tuning of

impedance is realized with 11 switched MEMS

capacitors, and 2048 (211) different impedances

can be generated with the tuners. The tuners are

small enough to be integrated inside W-band RF probes.

Página: www. my.ec.ucsb.edu.com

Realizado: Franco A Rivera C.

Asignatura: CRF