Specifying a custom SAW filter
An engineer needs to give the filter manufacturer details on bandwidth, band edge attenuation and stopband rejection frequencies over all conditions to ensure that out-of-band emissions are adequately suppressed.
This article is an accumulation of the combined contributions of several design engineers working in filter development for Sawtek, Orlando, FL. Sawtek manufactures a variety of bandpass filters.
Surface acoustic wave (SAW) technology allows the construction of efficient filters for suppressing emissions outside of an authorized bandwidth. What are the advantages of SAW technology? What are the limits of the technology? What are the trade-offs that influence size, and, consequently, price?
All the design trade-offs among key parameters of surface acoustic wave (SAW) technology for filters cannot be described completely here, but the information that follows provides the essential knowledge to discuss the various design choices. Remember that the design of SAW components is not entirely straightforward because of the interdependencies among the key parameters and the vast number of SAW design approaches currently available.
Correctly specifying a SAW filter begins with the system requirements. Table 1 below allows a determination that the desired performance falls within the capabilities of SAW technology.
As with all filter technologies, it is not possible to optimize all parameters simultaneously in one filter because of their interdependence. Also, a common pitfall of the systems engineer seeking a SAW filter is a tendency to think in terms of the characteristics of other technologies, such as those of LC (inductance-capacitance) filters. Despite tempting analogies, SAW filter specification is approached most easily from the fundamental system requirements as a unique technology.
Fractional bandwidth The first and most important parameter to consider when specifying a SAW filter is the fractional bandwidth, or the passband divided by the center frequency. The fractional bandwidth directly determines the substrate material that will be used, which, in turn, establishes the temperature stability of the resulting device. In addition, it limits the design options available because resonator filter technologies can rarely be used for fractional bandwidths greater than 0.3%, and low-loss filter techniques are rarely useful at fractional bandwidths greater than 20%. Table 2 below provides a summary of the most commonly used substrate materials, the optimum fractional bandwidths for each and their corresponding temperature dependence.
The materials in Table 2 are presented in order of increasing acoustic coupling coefficients, a measure of the efficiency with which the RF signal is converted to a surface acoustic wave. As the fractional bandwidth of a SAW filter increases, the number of interdigital electrodes on the surface of the crystal decreases. This creates the need for higher coupling of materials at wider fractional bandwidths.
The most straightforward method of specifying the SAW filter frequency response is to precisely convey to the manufacturer the information bandwidth and maximum allowable band edge attenuation of your signal, along with the stopband rejection frequencies and their minimum allowable attenuation over all conditions of temperature, aging and manufacturing tolerance. In short, the manufacturer must design the actual filter with a wider passband and a narrower stopband to ensure that your system requirements are met over all these conditions. This can often have a profound effect on the difficulty of the specification, as shown in Table 3 on page 38.
Although the raw system requirements fall well within the capability limits presented in Table 1, the required filter design, which includes the appropriate bandwidth adjustments to allow for temperature and manufacturing, exhibits a shape factor and transition bandwidth that is too steep to be realized. Therefore, despite the temptation, one should not attempt to pad the system requirements with excess design margin. It is up to the SAW filter designer to provide the necessary margins to meet the system requirements over all environmental conditions.
In general, the frequency response of a SAW filter determines its physical size and, ultimately, its cost. In general, the narrower the bandwidth and the lower (steeper) the shape factor of a SAW filter, the larger the device becomes. More precisely, it is the transition bandwidth that determines the length of a transversal SAW filter. The relationship is roughly linear with the smallest transition bandwidths requiring the largest SAW die.
This relationship is not true for resonator filters, however, which are generally quite small despite their narrow bandwidths and steep skirts.
Insertion loss SAW devices typically have been associated with high insertion loss. Although bidirectional SAW filters exhibit losses in the 15dB to 30dB range, recent advances in low-loss transversal and resonator filter design techniques routinely permit the manufacture of SAW filters with less than 10dB of insertion loss.
The ability to produce high-performance SAW devices with low insertion loss, at low cost, has opened many new market opportunities that never before were able to be served by SAW manufacturers. A word of caution, however. As Table 1 indicates, not every SAW requirement can be satisfied with the various low-loss techniques. Due to fundamental physical limitations, low-loss can be achieved only at fractional bandwidths below 20% and shape factors greater than 1.5:1. In any case, the minimum attainable insertion loss is generally a function of the filter’s fractional bandwidth for a given substrate material. The insertion loss of a SAW filter generally will be greater as the upper fractional bandwidth limit is approached on a given substrate material. For instance, Table 2 identifies a fractional bandwidth range of 7% to 30% for YZ lithium niobate. Thus, a 7% bandwidth filter on this material will generally exhibit a lower insertion loss than a 30% bandwidth filter on this same material.
Impedance matching Because the basic SAW transducer produces capacitive reactance, some impedance matching of the SAW device is often required. Generally, a simple one- or two-element matching circuit at the input and/or output of the SAW filter is used to reduce insertion loss and to improve voltage standing wave ratio (VSWR). Impedance matching the SAW device, however, may tend to generate a time-spurious response known as triple transit, which causes periodic amplitude, phase and group delay ripple. As a result, some SAW filters are specifically designed to operate into a 50V source and load impedance without matching to minimize this effect. The precise effects of impedance matching on SAW filter performance are too complex to analyze without the help of a computer-aided SAW modeling program. It should be noted that low-loss SAW filters and resonator filters do not exhibit these potentially detrimental effects of impedance matching.
Once it is determined that some impedance matching is necessary, the systems engineer has the choice to place the matching components external to the SAW package, or to ask the manufacturer to internally match the filter. The externally matched option will generally be smaller and less costly, while the internally matched option provides the maximum ease of interface to the rest of the circuit.
Stopband rejection Rejection of greater than 50dB is achievable in SAW devices for a wide range of fractional bandwidths and shape factors, with correct material selection and proper design. Low-loss SAW filters generally achieve 40dB to 55dB rejection. Frequently, the full performance of the filter itself is not realized in printed circuit board environments because of RF feed-through. However, this type of crosstalk can be avoided by adjustments in the circuit board layout.
Another limitation is encountered if the matching components are mounted within the hermetic metal package where input and output inductors cannot be shielded. A planar construction of the integrated spiral matching components serves to minimize this problem; and it is rare for the rejection performance to be worse than 45dB. Many systems use modules with surface-mounted assembly (SMA) connectors and separate impedance matching compartments to achieve the full rejection performance of the SAW filters.
Variations Transversal SAW filters, both bidirectional and low-loss, inherently exhibit linear amplitude, phase and group delay across the entire usable passband and, as a result, are generally superior to any other available technologies in this regard. In addition, the SAW designer has independent control over the amplitude and group delay or phase responses, thus permitting the incorporation of amplitude and delay equalization directly into the SAW filter, if desired.
The group delay response of a SAW filter exhibits a property that merits further discussion. Although the SAW filter’s phase response is extremely linear to within a few degrees of ideal, it often contains tiny but rapid perturbations as a function of a frequency caused by time-spurious signals that are attenuated significantly more than the main response. Because group delay is the first derivative of phase with respect to frequency, these small but rapid phase variations cause group delay ripple that is often larger than one might expect, given the SAW’s excellent phase linearity. These rapid variations are often ignored by the system, depending on the modulation scheme and the periodicity of the ripple.
Note that SAW resonator filters are not transversal in nature and exhibit phase and delay responses that more closely resemble those of LC filters. As a result, the performance presented in Table 1 on page 32 can only be achieved over the central portion of the passband.
Cost factors Cost can be a major consideration in specifying a SAW filter. Occasionally, a device costs several times more than it should because the specification contains an overly difficult, often unnecessary, requirement.
Generally, the most important factors in determining price are die size and quantity. SAW devices are etched onto a metalized substrate. SAW materials are currently supplied in three-inch and four-inch round wafers. Not only is the material cost per device lower when it is smaller, but more die per wafer implies that fewer wafers need to be processed to yield a given quantity of devices, thus saving labor costs. Because SAWs are generally housed in the smallest standard package that will accommodate the die, a smaller device will require a less expensive package.
Summary SAW devices meet both the performance and cost requirements of modern system design. To optimally specify and properly apply SAW bandpass filters, system engineers should understand the technical options that they offer.