SKY33100-360LF Datasheet, PDF (4/5 Pages) Skyworks Solutions Inc. – BAW Band Pass Filter 2.4 GHz

PRELIMINARY DATA SHEET • SKY33100-360LF
Electrical signals are coupled into the piezoelectric layer via the
electrodes and due to the piezoelectricity, that layer changes
dimension, primarily thickness. The longitudinal acoustic wave
created is resonant in the layered structure and as such the fre-
quency of operation is dictated by the layer thicknesses and
acoustic properties of the layers used. The large currents that
result at the acoustic resonance produce the sizeable impedance
changes that are used in the design RF filters.
The substrate on top of which the electrode-peizoelectric-elec-
trode "sandwich" is formed is an important part of the BAW
structure. Alternating layers of high and low acoustic impedance
materials are deposited to a total thickness of about one fourth of
the acoustic wavelength of the resonant frequency. These layers,
known as the acoustic mirror or the Bragg Stack, act in a manner
analogous to an electrical quarter-wave transmission line termi-
nated in a short circuit. Construction of optimized acoustic
reflections is vital to the low loss operation of BAW filters.
A simplified electrical equivalent circuit of a single BAW resonator
and its impedance versus frequency are shown below.
BAW Ladder Network
The construction of BAW resonators is similar to that of parallel
plate capacitors. This geometry allows for very robust filters to be
constructed, with tolerance of higher DC voltages and static dis-
charges than those tolerated by typical SAW filters. BAW filter
losses and temperature coefficients of frequency can also be
lower than those of typical SAW filters so that more aggressive
near band rejection requirements can be addressed.
Evaluation Board
Frequency
BAW Resonator Equivalent Circuit and Impedance vs. Frequency
This structure produces a series resonance, f_SERIES, as well as a
parallel resonance, f_PARALLEL. The spacing between resonance
and antiresonance and the magnitude of the impedance excur-
sions is determined by layer construction.
A filter is formed by cascading series and shunt resonators in a
ladder configuration. Simplistically one places the series low
impedance and the shunt high impedance frequencies roughly
mid band resulting in a band pass shape. The number of seg-
ments used is determined by required rejection and tolerable
insertion loss. The impedance, band shape, and out of band
rejection of the filter are determined by optimizing the properties
of the various resonators.
The evaluation board for the SKY33100-360LF BAW filter allows
the part to be fully exercised and evaluated. Although the RF
ports on the evaluation board are marked "Input" and "Output",
the filter is actually fully bilateral, so either RF port can be uti-
lized as the RF input or the RF output with no degradation in
performance.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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August 10, 2007 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • 200627 Rev. D