SAW filter: basic concept

 Rejection SAW filters (Notch-filter, Rej-SAW).

Rejection filters are designed for rejecting narrow-band noise signals.

Application domains: cable TV systems and other high frequency signal transmission systems where pickups, repeated reflections, intermodulations and other phenomena cause a permanent frequency noise.
Main technical parameters :

- Nominal rejection frequency frej = 30... 700 MHz;
- Rejection band from 0,5 to 1,5 % of frej at the level of -30 dB;
- Rejection depth аrej = 35...40 dB at the frequency frej ;
- Design: metal and glass package of SIP/ DIP type, or a screening construction with high frequency «F»-type connectors, or others, to the customer specification.

Fundamentals of SAW Devices

Surface acoustic wave (SAW) components have been in production for many years.

Many of the advantages of SAW devices are derived from their physical structure. They are inherently very rugged and reliable. Because their operating frequencies and responses are set by photolithographic processes, they do not require complicated tuning operations nor do they become detuned in the field.

The perfomance advantages realized with SAW technology are dependent on the application and frequently include small-size, linear phase, low shape factor, excellent rejection, and temperature stability. The semiconductor wafer processing techniques used in the manufacturing of SAW components permit largevolume production of economical and reproducible devices.

Although this discussion generally refers to transversal filter operation, the basic principles apply to nearly all of the SAW device types. These alternate device types, such as low-loss filters, delay lines, resonators and resonator filters differ primarily in their physical construction.

In its simplest form, a transversal SAW filter consists of two transducers with interdigital arrays of thin metal electrodes deposited on a highly-polished piezoelectric substrate such as quarts, langasite or lithium niobate (see figure 1). The electrodes that comprise these arrays alternate polarities so that an RF signal voltage of the proper frequency applied across them causes the surface of the crystal to expand and contract. This generates the Rayleigh wave, or surface wave, as it is more commonly called.

These interdigital electrodes are generally spaced at 1/2 or 1/4 wavelength of the operating center frequency. Since the surface wave or acoustic velocity is 10 -5 slower than the speed of light, an acoustic wavelength is much smaller than its electromagnetic counterpart.

As a result of this relationship, physical limitations exist at higher frequencies when the electrodes become too narrow to fabricate with standart photolithographic techniques and at lower frequencies when the devices become impractically large. Hence, at this time, SAW devices are most typically used from 10 MHz to about 3 GHz.

The basic SAW transducer is a bidirectional radiator. That is, half of the power is directed toward the output transducer while the other half is radiated toward the end of the crystal and is lost. By reciprocity, only half of the intercepted acoustic energy at the output is reconverted to electrical energy; hence, the inherent 6 db loss associated with this structure. Numerous second-order effects, such as coupling efficiency, resistive losses and impedance mismatch, raise the incertion loss of practical filters to 15 db - 30 db.

Fortunately, a great deal of effort in recent years has resulted in the development of numerous low-loss filter structures that significantly reduce this loss to 1-7 db using the effects of multiple reflections within a transducer. (see an example, electrode configuration of longitudinally-coupled filter, Fig. 2.) It must be noted, however, that not every SAW requirement can or should be implemented as a low-loss filter due to various performance trade-offs that exist.

SAW devices meet both the performance and cost requirements of modern system design. To optimally specify and properly apply SAW bandpass filters, systems engineers should understand the technical options that they offer our engineering staff is always available to assist you as you develop your specification.