In today’s crowded RF spectrum, system designers must satisfy strict regulatory requirements without sacrificing performance. For filters, this translates into the need for high selectivity, low insertion loss, flat passbands, and uniform group delay. Filters also must be highly repeatable, small in size, and low in cost—all while potentially operating in adverse environmental conditions. Answering these needs are surface-acoustic-wave (SAW) filters, which can simplify RF designs as long as they are correctly specified. An overview of that specification process is provided in a white paper from Spectrum Microwave titled, “Specifying the Proper SAW Filter.”
That four-page document begins by explaining that SAW filters depend on the mechanical properties of piezoelectric crystals. Through transducers that are deposited on the crystal, SAW devices convert an RF signal into a mechanical displacement. In doing so, they create a surface wave across the device. They then convert it back into an RF signal.
It should be noted that the filtering characteristics of SAW devices depend on the properties of the selected crystalline material, the length of total displacement, and the transducer’s design, placement, and thickness. Thus, designers must consider a number of factors beyond center frequency and filter bandwidth. System operation will be affected by insertion loss, signal group delay, out-of-band rejection, and thermal stability. In addition, the allowable ripple must be specified for amplitude, phase, and group-delay responses.
To illustrate these points, the paper provides an example of a typical filter and discusses the issues that it may invite. For instance, the stop-band level is limited by the device’s ability to dampen undesired vibrations. If a design requires greater rejection than what one filter provides, two or more SAW filters will have to be run in cascade.
Another key filter parameter is insertion loss, as these devices are passive. Thus, no internal amplification will compensate for the energy lost in piezoelectric coupling, which converts the signal between electrical and mechanical forms of energy. The magnitude of the insertion loss is mostly a function of filter bandwidth together with the crystalline bandwidth used. Yet material affects more than insertion loss. The note explores its effect on group delay, for example. It then closes with a discussion of computer simulation and how it can aid the analysis of any and all filter tradeoffs.
Spectrum Microwave, 324 Clark St., Worcester, MA 01606; (508) 852-5400, www.spectrummicrowave.com.