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Oscilent
SAW Filter Division
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Oscilent
designs, manufactures, and promotes a full line of IF and
RF SAW Filter products and solutions for wireless applications
including GPS, PCS, PCN, CDMA, Wireless LAN, GMS, CATV, Bluetooth,
keyless entry, and other datacom, handset, and broadband commercial
and military/aerospace applications. Our capabilities span
both low and high volume programs with particular focus on
signal processing applications. In addition to our extensive
offering of standard SAW Filter products, Oscilent has diverse
Engineering experience spanning hundreds of custom designed
SAW Filters, Bandpass Filters, Low Loss Filters and SAW based
subsystems. Our custom/design/testing capabilities are unparalleled
in our industry.
SAW
Filter General Information
Advantages
of SAW Filter technology include compact packages, low shape
factors, superior linear phase characteristics, rejection
qualities, and the relatively stable performance over temperatures.
Many other advantages are derived from the physical structure
of SAW Filters which allow for extremely robust and reliable
designs that remain stable in the field/application. Additionally,
the inherent design and wafer processing techniques of Saw
Filters provide for a repeatable device in both low and high
volume production.
Surface
Accoustic Wave (SAW) fundamentals provide for a piezoelectric
material that converts an incoming electromagnetic signal
into an acoustic signal, and vice versa. In its most basic
form, a SAW filter consists of a polished piezoelectric substrate
with a deposit of two transducers with interdigital arrays
of thin electrodes. The electrodes making up the arrays alternate
polarities so that when an RF signal voltage is applied across
them, a surface wave is then generated. In designing a SAW
Filter, the overall frequency response characteristics are
determined by deriving two impulse responses for the two transducers
whose transforms are added together in dB. The surface of
a piezoelectric substrate is then etched with the two impulse
responses.
Selecting
an Oscilent SAW Filter
Generally,
a SAW Filter manufacturer will offer their most popular devices
as standards creating a reference for Engineers to design
from. Typically, and especially true in the RF SAW Filter
category, a standard device is available for most common applications.
However, for applications requiring parameters that are not
currently considered industry standards, Oscilent is uniquely
equipped to offer design and development services at comparatively
lower costs than our competitors. In this case, the following
information is required to effectively design a SAW Filter:
The
following link can be used to print and fax your SAW Filter
requirements to our Engineering Staff:
a.
Center Frequency (Fo)
b.
Passband Width (Bp)
Simply stated, the Passband Width will pass a signal occupying
a specific frequency band, and reject others falling outside
the band. From a SAW Filter design perspective, the first
parameter to consider is the Fractional Bandwidth (Bp/Fo)
because of the influence on the substrate material to be used
in the design. The substrate material influences many parameters,
most importantly the Temperature Stability specifications.
c.
Amplitude Ripple over Passband Width (AR)
The Amplitude Ripple is a measure (dB) of the variation, or
differential value, of attenuation in the passband of a filter,
typically a SAW Filter will be specified as having a Typical
and Maximum allowable value.
d.
Group Delay Variation over Passband Width (GDR)
From a mathematical perspective measured in time, the Group
Delay of a SAW Filter is the first differential value of time
for phase frequency of phase changing (variation) in pass
band. Otherwise, we can reference the Group Delay as the slope
of the Phase vs. Frequency Curve. In simple terms, the Group
Delay represents the time it takes for the signal to pass
through the SAW Filter.
e.
Transition Bandwidth (Bt)
Otherwise referred to as Skirts, the area between the Stop
Band and the Passband found on both sides of the Passband.
f.
Rejection (REJ.)
All ranges of the SAW Filter not including the Passband. The
Rejection can also be referred to as the Rejection Range,
or Stop Band. We can refer to this as the range in which the
Relative Attenuation is larger than the specified Rejection
side. With proper material selection and design, Rejection
of 50dB, or greater, is possible within a wide selection of
fractional bandwidths and shape factors.
g.
Insertion Loss (IL)
Advances in SAW Filter design techniques routinely allow for
a design incorporating a specification of under 10dB Insertion
Loss, however, the minimum attainable Insertion Loss is generally
influenced by the Fractional Bandwidth and the influences
of this ratio on the applicable substrate material. The Insertion
Loss value will generally increase when approaching the fractional
bandwidth limit of a substrate material. For instance, a Fractional
Bandwidth value of 8% will generally produce lower Insertion
Loss than a Fractional Bandwidth value of 30% using the same
substrate material.
h.
Package
Factors affecting the size of the package used in SAW Filter
design include parameters associated with Center Frequency,
Bandwidth, and Shape Factor, among other minor considerations.
For instance, lower Frequencies require larger substrate,
thereby increasing the size of the packages available to the
designer. Consequently, an equally important challenge of
package size reduction is always considered by Oscilent Design
Engineers in an attempt to meet the desired parameters in
the smallest package possible. In selecting a package, we
recommend stating general preferences. Without this input,
Oscilent will design using the most cost effective approach
balancing parameter requirements with cost and manufacturability.
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