miércoles, 10 de febrero de 2010

An X-Band to Ku-Band RF MEMS Switched Coplanar Strip Filter


Abstract—Radio frequency microelectromechanical systems (RF MEMS)
are key enabling technologies for miniature reconfigurable circuits such
as microwave filters. We present a two-pole monolithic RF MEMS
switched filter, fabricated on GaAs, that employs surface-micromachined
capacitors to present a variable capacitance to a coupled coplanar strip
filter, thereby switching the filter center frequency 37% between
10.7 GHz and 15.5 GHz with voltages of 20 and 0 V, respectively.
This 15% bandwidth filter occupies a chip area of 2.2 1.5 mm and
demonstrates less than 2-dB of loss, making it promising for numerous
applications within these critical frequency bands.


Reconfigurable microwave circuits have generated great
interest for both military and commercial applications because
these networks allow increased system functionality with
lower weight and cost than existing systems [1]. Due to
their low-loss and other attractive characteristics, radio
frequency microelectromechanical systems (RF MEMS) are
key to meeting these objectives [1], [2]. One of the most critical
elements enabled by RF MEMS is the tunable filter, of which
there have been several examples. These filters have
included both lumped designs [3]–[5] and distributed designs
[6]–[9] at frequencies ranging from L-band to millimeter-wave.
However, there have been few examples of RF MEMS-enabled
filters in the 8 to 18 GHz range where numerous radar and
communications applications reside. In this work, we present
an RF MEMS switched filter operating in this important
frequency range.


This filter design is enabled by a low-loss RF MEMS
switched capacitor. The device used in this work is optimized
for low loss and high-Q at microwave frequencies. To enable
post-processing on arbitrary substrates ranging from quartz
to InP MMICs, the device was fabricated using a l
ow-temperature surface micromachining process described
in detail previously [10]. This process uses a polymer
sacrificial layer and evaporated gold films to fabricate the
capacitor, and a low-temperature dry etch process to release
the devices. In the future, we anticipate adding thin-film
resistors using a high-resistivity layer to allow individual
biasing of the switches, enabling more complex and
higher-order filter designs.
The switched capacitor consists of a 1.2 m-thick gold
cantilever suspended 5 m over a ground pad coated
with silicon oxynitride dielectric. A cantilever device is
used to reduce sensitivity due to thermal mismatches
between the substrate and device, but has not been
characterized over a broad temperature range. The
device is switched by increasing the actuation voltage
until the plate pulls down onto the substrate at
approximately 15 V. The up-state capacitance is
approximately 150 fF while the down-state capacitance
is approximately 400 fF at 20 V. Additionally, the downstate
capacitance can be tuned between approximately 350 and
400 fF by varying the holding voltage between the
pull-off voltage of 10 V and maximum voltage of 20 V,
allowing approximately 10% tuning in the downstate.
Even though this slight tuning is possible, in this filter
application the device is used as a two-state switch for
improved stability and reproducibility in the presence of
vibration or bias voltage noise. This MEMS capacitor has
a-factor of over 100 through the entire band of operation
up to 25 GHz, and an extrapolated minimum self-resonant
frequency of over 60 GHz in the highest capacitance state.
Additionally, switching times between the down-state and
up-state were measured to be under 100 s. Finally, while
the device has been operated to over a billion cycles
without any charging or stiction-related failures, during
the lifetime test the pull-in voltage gradually shifted from
15 to 7 V due to metal fatigue at the anchor. This reliability
limitation will be addressed in future designs.


The filter was designed as a second-order capacitively-loaded
interdigital filter [11]. The resonators were grounded at one
end by a large grounding capacitor for bias isolation, their
lengths were chosen to be approximately 45 long at the center
frequency of 12.5 GHz to allow for maximum tuning range, a
nd the tap locations were chosen for optimum filter
characteristics at 12.5 GHz. The filter elements, to take full
advantage of the high performance of the switched MEMS
capacitor, were fabricated in a coupled coplanar strip (CPS)
configuration on a GaAs substrate [12]. Coplanar technology,
rather than microstrip, was:

 Chosen both for process simplicity and the ability to fabricate these
components on arbitrary substrates. This filter differs from
previously reported RF MEMS switched CPS-filters [9]
in that this design is switched by changing the loading
capacitors rather than by physically changing the length of the
line. This approach allows for a filter layout that is up to 50%
smaller at a given frequency than previous examples.
 After using standard filter design techniques [13] and
software to design a 15% bandwidth, 0.25-dB ripple filter,
the layout accuracy was verified with a commercial 2.5D
method-of-moments simulator [14] and a commercial
three-dimensional (3-D) finite element simulator [15].
An optical micrograph of the filter, which occupies only
1.5 2.2 mm of chip area, is shown in Fig. 1, which also includes
a simple sketch of the equivalent circuit as an inset.
The coplanar waveguide (CPW) feedlines are designed for
50 impedance, while the CPS resonators are 80 m wide
with a 35 m gap between the signal line and the ground
plane, producing a characteristic impedance of 58 and an
effective dielectric constant of 6.85. The resonators are
1100- m long and separated by 450 m, producing even and
odd mode impedances of approximately 62ohms and 52
ohms. The switched capacitors are 300 300 m with an initia
l air gap of 5 m. Because of the short resonators
(about 45 ohms), this filter occupies less than half the
chip area as earlier designs.
The filter was tested using coplanar Cascade probes and
an HP8510C network analyzer from 50 MHz to 25 GHz.
For testing, wirebonds were added across the CPW feed
to suppress spurious modes at the feed junctions.
The control voltage, ranging from 0 to 20 V, was introduced
to the CPW center conductor through the internal bias
tees on the network analyzer. The transmission and return
loss properties of the filter at voltages of 0 and 20 V are
shown in Fig. 2, which also shows the circuit model response.
At 0 V, the filter's center frequency is 15.5 GHz, with a
bandwidth of 2.2 GHz. The minimum passband loss is
approximately 1.4 dB in that state, with the return loss
better than 20 dB in the passband. When the switches are
pulled in at 20 V, the filter center frequency is 10.7 GHz with
1.8 GHz bandwidth. In this state, the insertion loss is less
than 2 dB, with in-band return loss better than 10 dB. This
37% center frequency switching is achieved with only two
simple RF MEMS devices.
 The rejection of the filter at two bandwidths above the
center frequency is greater than 15 dB for both of the
states, as expected for a two-pole filter. In similar filters
operating at lower frequency, we have observed a slight
decrease in stop-band rejection at three times the center
frequency, but the rejection of this particular filter remains
greater than 20 dB because of the electrically short resonators.
 While the measured and modeled responses correspond well,
there is a discrepancy between the measured and modeled
bandwidths which can be accounted for by several factors.
 First, the circuit-level simulation does not accurately account
for the coplanar-strip tees, which impact the performance
of the resonators. Second, accurately accounting for the
coupling between the coplanar strip resonators and end ground
planes requires full 3-D electromagnetic simulation. Finally,
the resonator grounds are not exactly fixed at the end of the
resonator, but instead distributed across the large area of the
grounding capacitor. All of these effects were captured by full
electromagnetic simulation and considered in the final design,
but were not incorporated into the circuit model. The filter
properties change with frequency because the physical
locations and lengths of the resonators and taps are fixed.
Therefore, as the filter is switched, the effective tap position
and coupling coefficient vary, deviating from the ideal desired
filter response. The filter was optimized for a response at
12.5 GHz, resulting in larger insertion and return losses at
lower frequencies and merging of the poles at the upper frequency.
Except for these slight departures from an ideal filter shape, the
filter response is symmetric about the center frequency. The loss
of the filter is limited by the low (30–40) of the coplanar strip line,
rather than by losses in the switched capacitors. By using a
transmission line technology with improved unloaded, the loss
of this filter can be significantly improved.

In this work, we have presented an RF MEMS enabled
switched filter with 37% tuning from a center frequency of
10.7 to 15.5 GHz. The filter has a bandwidth of approximately
15% and less than 2 dB of loss in the passband. The monolithic
design takes full advantage of the RF MEMS device by
eliminating losses associated with off-chip connections and
allowing the total filter to occupy only 3.3 mm. This filter has
lower loss, smaller size, and a larger tuning range than previous

RF MEMS filters of this type, and switches between two
critical radar bands, enabling frequency agile radars and
other systems. Future work to improve the filter passband
properties is ongoing, and is expected to make this design and
fabrication technique a strong candidate for insertion into
advanced RF system designs.

Realizado: Franco A Rivera C.
Asignatura: CRF

No hay comentarios:

Publicar un comentario