SAW Filter

Description

Filter Specifications Order Form
SAW (Surface Acoustic Wave) devices SAW Filter have become well known as a rugged low cost means to offer a variety of signal processing functions covering a frequency range of 20MHz to 2.5GHz. The applications can range from as small as 0.01% in fractional bandwidth for resonators to as large as 50% for wideband filter requirements. Examples of some of the many popular applications for SAW include bandpass filters (RF and IF) for spectral shaping with image rejection, delay lines, coded waveform generation, correlators and resonators for phase noise clean up in synthesizers along with many more examples.

SAW CHARACTERISTICS
A SAW product SAW Filter consists of Aluminum patterns deposited on a piezoelectric substrate that is wire bonded to a hermetically sealed package connecting it to the "outside" world, your PCB. Typically it would consist of an input and output transducer that is composed of an interdigital structure. In many cases the package is externally interfaced on the board with input and output matching elements to transform the SAW impedance to the source and load impedance. The Aluminum transducer pattern is designed to shape a signal in the manner prescribed in the customer specification. When a signal is applied to an interdigital pattern, an electric field is generated between the hot and ground elements, which generates an acoustic field in the substrate due to the piezoelectric coupling. The strength of this coupling (K2) is dependent on material as noted below. This acoustic field then propagates along the surface of the material (hence the word Surface Acoustic Wave) to the output Aluminum pattern where it generates an electric field between the Al elements producing a time delayed spectrally shaped signal at the output.

Because the designer has control on the overlap and spacing of these interdigital patterns there is more freedom to control passband linearity, transition skirts and rejection then in typical lumped element filter design. The designer also has the option in many cases to offer single ended input and output or balanced input and output or a combination as requested. In addition to the main desired response, there are three typical spurious responses associated with SAW and are discussed separately.

FEEDTHROUGH
SAW Filter FeedThrough (or Break Thru as is commonly used overseas) is the electromagnetic signal that is applied to the input and is measured at the output at zero delay. It basically arises from electromagnetic coupling between the input and output transducers either through the air or through the substrate (due to the high permittivity) or it results from ground currents on the PCB due to layout and other components near the SAW mounting location. It has the tendency to corrupt the passband linearlity as well as reducing the amount of rejection of the filter.

TRIPLE TRANSIT
As noted above once a voltage is applied to the input transducer it generates an acoustic wave that travels to the output transducer. As it propagates under the output transducer Aluminum pattern, SAW Filter generates a voltage that is measured at the output and is the main desired response. However, this voltage at the output transducer then generates an acoustic wave back to the input transducer and in turn is regenerated back to the output transducer. Having traveled three times the distance between input to output (1 path, the main desired signal), back to input (2 trips) and finally back to output (three trips) it is called Triple Transit. This regeneration continues but is very rarely seen as it is too far down in the noise typically. The output triple transit signal is a replica of the main signal and arrives at two times the delay of the main signal. It can cause ripple in the passband of the SAW device and is typically controlled thru design choices.

BULK MODES
In addition to the input transducer launching an acoustic wave to the output that travels along the surface of the substrate,SAW Filter also launches energy into the substrate, which is termed bulk acoustic energy. This bulk energy measured at the output typically reduces the rejection of the filter and is managed typically during the manufacturing stage by sand blasting the backside of the substrate.

TECHNICAL HIGHLIGHTS
Over the 35+ years that SAW has been used, many sophisticated transducer architectures have been developed to address a variety of specific requirements. MtronPTI offers the most advanced design methods available today to meet your requirements.SAW Filter is therefore very important to completely define your specification noting the important parameters to emphasize. As a guide, we have included a data sheet with the parameters necessary to completely specify a SAW requirement. Once the specification is received a typical design procedure will proceed as follows:

SAW Filter Design-

Input the specification paramaters (this sets the chip size and hence package)

Perform a digital filter synthesis

Extract the roots

Separate the roots into two groups (input and output transducer)

Optimization of performance

Mask generation

SAW Filter Manufacturing-

Substrate + mask

Photolithography

Probe

Substrate dicing

Die assembly into package

Wirebond

Seal package

Final test

Package Tape & Reel or box with conductive foam layers

Ship

SAW Filter MATERIAL OVERVIEW

LiNbO3: excellent for Insertion Loss, wideband applications but large temperature coefficient

LiTaO3: reasonable Insertion Loss, medium bandwidth applications and reasonable stability over temperature

Quartz: low coupling coefficient, narrowband applications, and excellent stability over temperature