Surface Acoustic Wave (SAW) filters are used in many applications, including, for example, in mobile/wireless transceivers, radio frequency (RF) filters, Intermediate Frequency (IF) filters, resonator-filters for one and two-way pagers, resonator-filters for one and two-way medical alert transmitters, filters for mobile and wireless circuits, IF filters in a Base Transceiver Station (BTS), RF front-end filters for mobile/wireless circuitry, multimode frequency-agile oscillators for spread-spectrum secure communications, delay lines for low power time-diversity wireless receivers, nyquist filters for microwave digital radio, voltage controlled oscillators for first or second stage mixing in mobile transceivers, Pseudo-Noise-coded delay lines for combined code division multiple access/time division multiple access (CDMA/TDMA) access, clock recovery filters for fiber-optics communication repeater stages, synchronous, spread-spectrum communications, televisions, video recorders, and many other applications.
One problem with current SAW filters is the differential between the pass band and the stop band. SAW filters have an acoustic response, and also have electromagnetic (EM) feed through. Often, much of the stop band of the SAW filter is made up of the EM feed through in the SAW filter. The greater the differential between the pass band and the stop band of the SAW filter, the better the device will typically perform in a system or apparatus. Such EM feed through is a combination of internal and external electromagnetic feed through. Thus, a reduction of the EM feed through will usually improve the differential between the pass band and the stop band of the SAW filter.
Some SAW filters are designed to reduce or minimize the amount of internal EM coupling that is present in the SAW filter. One solution to the problem has been to attempt to reduce the amount of EM coupling, thus improving the differential between the pass band and the stop band. One way in which the internal EM coupling can be reduced is by including electrodes of opposing polarity within the SAW filter. This reduces the internal EM feed through by feeding current in opposite directions to two sets of electrodes on the acoustic substrate. Additionally, most SAW filters will have some measure of internal EM coupling that has not been properly shielded or has not been shielded at all and will be present in the output of the SAW filter. Thus, while a manufacturer may attempt to reduce internal EM coupling the manufacturer of the SAW filter has no control over the external EM coupling that may take place as this will vary from application to application.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.