Intro: Why are SAWs and BAWs Useful?
Filters evaluate signals and remove undesirable frequencies while preserving desirable frequencies. Acoustic filters are the most common filter used in mobile devices. Modern smartphones are required to filter, transmit, and receive paths for 2G, 3G, and 4G in up to 15 bands, as well as support Bluetooth, Wi-Fi, and other wireless communications. Phones such as these could require up to 40+ filters; with the constant demands and innovation of next-gen technology, phones will require even more filters in the future.
Filters constructed from discrete components can’t meet the performance, size, and cost demands of today’s products. Thankfully, acoustic wave filters (such as SAWs and BAWs) allow engineers and developers to select their filters in a complete, monolithic-like package. Acoustic filters can operate at both high and low frequencies (up to 6GHz), are among the physically smallest filters, and have the best performance and cost points for complex filtering requirements. This article will discuss the characteristics, differences, construction, and applications for SAW and BAW filters.
| Terminology | |
|---|---|
| SAW | Surface acoustic wave |
| BAW | Bulk acoustic wave |
| Attenuation | An amplitude loss, usually measured in decibels (dB), incurred by a signal after passing through an RF filer |
| Insertion Loss | Loss of signal power resulting from the insertion of a component into the signal path |
| Isolation | Separation of one signal from another to prevent unintentional interaction between them (for example, transmit and receive interaction). |
| Q Factor | The quality factor (Q factor) is one of the main determiners of filter loss. Lower Q leads to higher loss and rounding of the filter corners. This rounding of the corners can be problematic for narrowband modulations. |
| Passband | The region through which the signal passes relatively unattenuated. |
| Ripple | The variation of insertion loss in the passband. |
| Selectivity | A measurement of the capability of the filter to pass of reject specific frequencies relative to the center frequency of the filter. Selectivity is usually stated as the loss through a filter that occurs at some specified difference from the center frequency of the filter. |
| Band pass | Allows all frequencies between two frequencies to pass while rejecting all others |
What are the uses for Acoustic Filters?
- Front end filtering
- Narrow multi-band filtering
- Eliminating specific interference sources
- Narrow or wide band pass filtering
- Low or high-pass filtering
Note: the primary technical parameters you should consider when choosing an acoustic filter are frequency, power handling, bandwidth, insertion loss, attenuation, and temperature stability.
| SAW Summarized Info | |
|---|---|
| (+) Generally cheaper than BAWs | |
| (+) Smaller than traditional cavity and ceramic filters | |
| (+) Low insertion loss/good rejection | |
| (+) Works with GSM, CDMA, 3G, and some 4G bands | |
| (-) Above 1GHz, selectivity declines, but SAWs can operate up to about 2.7GHz | |
| (-) Above 2.7GHz, SAW usage is limited due to decline in performance | |
| (-) These devices are temperature sensitive - substrate material becomes more pliable at higher temperatures and acoustic velocity is negatively affected. |
SAW In-Depth Information
What is the Construction of a SAW Filter?
- The actual filter is made from a piezoelectric substrate material (a material that generates an electric charge in response to mechanical stress) such as lithium niobite, lithium tantalite, quartz, or lanthanum gallium silicate. Each material has different electrical properties and different temperature coefficients.
- The filter substrate is capped on both sides with a metal layer formed from comb-like fingers that act as the interdigital transducer (IDT). (Fig. 1)
Fig. 1: Internal Composition of a SAW filter (Courtesy of the University of South Florida)
How Does the Signal Travel Through the Device?
- An electrical signal is given to one end of the device; the comb-like IDT at that end will convert the signal to acoustic energy and send it across the substrate as a surface-acoustic wave. The acoustic wave is then transformed back into an electronic signal at the opposite end of the component by the other IDT.
How Does It Filter out Specific Frequencies?
- The speed of the acoustic movement across the surface of the substrate is slower than the electrical speed of the IDTs on either end. The delay that occurs from the wave traveling across the substrate combines at the IDT on the receiving end, producing a finite-impulse-response (FIR) filter response.
- By adjusting the travel distance across the substrate and the dimensions of the IDT fingers, the impulse response is changed. This determines bandwidth, center frequency, type, and other factors.
Additional Considerations
- Center frequencies span from 50MHz to about 2.7GHz.
- Can handle 10-30dBm signals, not intended for high-power signals.
- The frequency temperature coefficient in standard SAWs is problematic (about -50 ppm/C), more costly temperature-compensated models are available that are as low as -15 to -25 ppm/C.
| BAW Summarized Info | |
|---|---|
| (+) Higher performance than SAWs at high frequencies (1.5GHz-6GHz) | |
| (+) Excellent Q - very low loss and very steep filter skirts are typical | |
| (+) Filter size decreases with higher frequencies, making these ideal for demanding 3G and 4G applications | |
| (+) Far less sensitive to temperature variation than SAWs | |
| (-) More expensive than SAWs |
BAW In-Depth Information
What is the Construction of a BAW Filter?
- BAWs usually use a quartz crystal as a piezoelectric substrate. Metal patches are located on the top and bottom of the quartz. (Fig. 2)
Fig. 2: Construction of a BAW filter
How Does the Signal Travel Through the Device?
- The metal patches on the top and bottom of the quartz excite acoustic waves, which then bounce back and forth between the patches and the crystal. Acoustic waves in BAWs travel vertically, versus the horizontal travel path of SAW filters.
How does It Filter Out Specific Frequencies?
- The resonant frequency is inversely proportional to film thickness - meaning both the metal and dielectric layers. For example, removing some of the top layer metal thickness can increase the resonant frequency. This is why filter size decreases with higher frequencies.
- By storing acoustic wave energy in the piezoelectric material, BAWs can achieve very high quality (Q) that translates into very selective filters with steep filter skirts.
Additional Considerations
- Additional types of BAW filters include FBAR (film bulk acoustic resonator) and BAW-SMR (solidly-mounted resonator BAW) devices, which include additional microstructures that trap acoustic waves very well and create high acoustic energy - making the Q in these types of filters the highest of any other filter this size at microwave frequencies.
Article Summary
Filters are essential in all signal-processing applications. The need for filters to be smaller and higher quality has been increased by the advancement of modern wireless technology. Instead of making size-limited filters from discrete components and dealing with the impact of parasitic capacitances in filtering, acoustic wave filters offer a better solution. These passive, monolithic-like filters have a compact IC design that are very small, low cost, have high Q factors, and are made to meet highly specific and high-performance filtering requirements. SAW filters are used for lower frequencies (up to 2.7GHz) and BAW filters are used at higher frequencies (2.7GHz-6GHz.) Thanks to acoustic filters like these, Filter design for RF applications has been changed to filter selection, making the process simpler.
