Modern cellular and other wireless communication devices require extensive filter banks to provide isolation to the multitude of operating frequency bands not present in modern wireless devices.
Modern cellular and other wireless communication devices require extensive filter banks to provide isolation to the multitude of operating frequency bands not present in modern wireless devices. Achieving the diversity of filtering needs for modern wireless communication devices with lumped-element, cavity, or other discrete resonator/filter technologies would result in excessively large and bulky devices that may not be suited to handheld, or even portable, use. For many of these applications, the performance of on-chip integrated filters doesn’t exhibit adequate Q factor or other performance metrics to address these modern filtering challenges.
A solution to this challenge: the use of acoustic wave resonator (AWR) filter technologies. Namely, these filter types fit into two categories, surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices, as shown in Figure 1 and Figure 2, respectively.
Figure 1 The basic SAW filter structure highlights electrical and acoustic building blocks. Source: Qorvo
Figure 2 BAW filters mainly differ from SAW filters in terms of bulk of substrate. Source: Qorvo
Acoustic resonator devices are made on piezoelectric substrates and function as electroacoustic transducers. This means that an electrical signal induced on an acoustic resonator is converted to an acoustic signal and another properly designed acoustic resonator can be paired with the first to convert the signal back to an electrical signal. In this way, an acoustic resonator filter can be implemented in between the electroacoustic transduction stages to provide signal conditioning functions.
The relative size of acoustic waves on certain piezoelectric substrates is far less than the size of electrical waves induced in the acoustic resonator filter device. So, a much more compact and higher performance filter design can be implemented in a fraction of the footprint of a comparable electromagnetic filter type, such as lumped element inductor/capacitor/resistor filters or cavity resonator filters.
There are a variety of piezoelectric substrates that can be used for acoustic resonator devices, including lithium niobate, piezoelectric ceramics, and quartz. The substrates used for acoustic resonator devices are typically planar with metallization deposited or otherwise developed on the surface to form the electroacoustic transducers and acoustic resonator filters.
Where BAW and SAW filters mainly differ is the direction, in relation to the bulk of the substrate where the acoustic wave is channeled. In a SAW device, the acoustic wave is channeled across the surface of the substrate, which enables interaction with surface metallization and the top surface of the substrate.
With a BAW device, the acoustic wave is channeled through the bulk of the substrate. This requires metallized structures and substrates to be stacked and arranged on top of each other to achieve electroacoustic transduction and acoustic resonator filtering. A simple BAW device may consist of only a top and bottom layer, though there are more complicated BAW variations with additional layers in between.
The construction difference results in BAW devices that are more complex and generally more expensive than SAW devices. However, BAW devices can generally be fabricated to achieve much higher RF performance than SAW devices. This includes insertion loss, Q factor, and maximum frequency. For this to occur, the slab of a BAW filter substrate is typically on the order of hundreds of micrometers or less. This typically requires thin-film fabrication techniques such as deposition and micro-machining.
For instance, practical SAW devices typically reach the highest frequency of operation at 2.5 GHz to 2.7 GHz; whereas, BAW devices can be made to operate in the tens of gigahertz. This difference is largely due to the relative size of the SAW and BAW structures compared to the electromagnetic and acoustic wavelengths.
Hence, for applications that justify the additional expense, BAW filters generally present higher Q factor, better power handling, and higher frequency operation than SAW devices. SAW devices are slightly more mature technology, and there is generally a wider variety of SAW filter devices to choose from on the market compared to BAW devices. Another factor to consider is that SAW devices tend to be more temperature sensitive than BAW devices, so temperature compensation may be necessary for SAW devices in some applications.
Given the maturity of the technology, there are SAW and BAW devices available that feature temperature compensation, so this is less of a practical concern now than it has been in the past. However, with narrower guard band frequencies becoming increasingly common in wireless standards, ensuring that the filter response is temperature insensitive is a much greater concern than it has been previously.
For many RF front-end (RFFE) devices, it is possible for BAW and SAW filters to be integrated into heterogeneous integrated circuits. A common method for this is to use copper flip chip technology. This allows for much more compact RFFE modules that integrate amplifiers, filters, switches, mixers, and other RFFE components into a single packaged solution. Both BAW and SAW filters are implemented in these multi-chip modules.
This article was originally published on Planet Analog.
Jean-Jaques (JJ) DeLisle, an electrical engineering graduate (MS) from Rochester Institute of Technology, has a diverse background in analog and RF R&D, as well as technical writing/editing for design engineering publications. He writes about analog and RF for Planet Analog.