miércoles, 10 de febrero de 2010

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


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.

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].
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).

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).


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

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