Detailed explanation of the structure, principle, and considerations of SAW and BAW filters

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Detailed explanation of the structure, principle, and considerations of SAW and BAW filters [Copy link]

Filters evaluate signals and remove unwanted frequencies while retaining desired frequencies. Acoustic filters are the most common filters used in mobile devices. A high-end smartphone must filter transmit and receive paths for up to 15 frequency bands for 2G, 3G, and 4G wireless access methods, as well as Bluetooth, Wi-Fi, and other wireless communication paths. These phones may require up to 40 filters, and future phones will require more filters as the demand and innovation of next-generation technologies continue to expand.
Filters constructed from discrete components no longer meet the performance, size, and cost requirements of today's products. Fortunately, acoustic wave filters (such as SAW and BAW) allow engineers and developers to select the appropriate filter in a complete package similar to a monolithic chip. Acoustic filters can operate at high and low frequencies (up to 6GHz), are one of the smallest filters physically, and have the best performance and cost point for complex filtering requirements. This article will discuss the characteristics, differences, structure, and applications of SAW and BAW filters.

What is the purpose of a sound filter?

Front-end filtering
Narrowband multi-band filtering
Eliminate specific interference sources
Narrow or wideband pass filtering
Low-pass or high-pass filtering
Note: The main technical parameters to consider when selecting an acoustic filter are frequency, power handling, bandwidth, insertion loss, attenuation, and temperature stability.

What is the structure of a SAW filter?

The actual filter is made of a piezoelectric substrate (a material that generates an electrical charge in response to mechanical stress) such as lithium niobate, lithium tantalate, quartz, or lanthanum gallium silicate. Each material has different electrical properties and temperature coefficients.
Both sides of the filter substrate are covered with a metal layer formed by comb-like fingers that act as interdigital transducers (IDTs). (Figure 1)

Figure 1: Internal components of a
SAW filter (Courtesy of University of South Florida)

How do signals travel through the device?
An electrical signal is presented to one end of the device; a comb-shaped IDT at this end converts the signal into acoustic energy and sends it as a surface acoustic wave onto the substrate. The acoustic wave is then converted back into an electronic signal at the other end of the component by another IDT.
How does it filter out specific frequencies?
The acoustic wave traveling through the substrate surface moves slower than the electrical speed of the IDTs on either end. The delays incurred by the wave traveling through the substrate combine at the IDTs at the receiving end to produce a finite impulse response (FIR) filter response.
The impulse response can be varied by adjusting the distance traveled through the substrate and the size of the IDT fingers. This in turn determines the bandwidth, center frequency, type, and other factors.
Other Considerations
Center frequencies range from 50MHz to about 2.7GHz.
Can handle 10-30dBm signals, but not for high power signals.
The temperature coefficient of frequency in standard SAWs is problematic (about -50ppm/°C), but more expensive temperature compensated models are available with coefficients as low as -15 to -25ppm/°C.

BAW Details
What is the structure of a BAW filter?
BAW typically uses a quartz crystal as the piezoelectric substrate. The quartz has metal patches on the top and bottom. (Figure 2)

Figure 2: Structure of a BAW filter

How does the signal travel through the device?
Metal patches on the top and bottom of the quartz excite acoustic waves that bounce back and forth between the patches and the crystal. Sound waves in a BAW travel vertically, while SAW filters take a horizontal path.
How does it filter out specific frequencies?
The resonant frequency is inversely proportional to the film thickness, and this applies to both the metal and dielectric layers. For example, reducing the thickness of the top metal increases the resonant frequency. This is why the size of the filter decreases as the frequency increases.
By storing the energy of the acoustic wave in the piezoelectric material, BAW can achieve very high quality (Q), which translates into a very competitive filter with high out-of-band attenuation.
Other considerations
Other types of BAW filters include FBAR (film bulk acoustic wave resonator) and BAW-SMR (solid-state mounted resonator BAW) devices, which include additional microstructures that capture the acoustic wave very well and produce high acoustic energy - resulting in higher Q values than any other filter of the same size at microwave frequencies.

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