What is SAW Filter? Working and Applications

SAW – Surface Acoustic Wave filters are one of the significant components in mobile handsets and digital televisions sets for decades. SAW filters are still an integral part of the communication module in every Smartphone and other IoT devices. Find out what is SAW filter, how it works, and what are the applications of SAW filters.

Radio Frequency (RF) and wireless applications require a lot of filtering circuits to avoid potential interference from surroundings and improve received signal quality. Wide varieties of filters (low pass, band pass and high pass) are used in every wireless device ranging from 10 kHz up to 6 GHz range. SAW filters are effective solutions for many applications due to their reliability, cost-effectiveness, and smaller size.

What is SAW Filter?

SAW filters are passive components used in radio frequency applications to filter out unwanted frequency components in an application. A typical bandpass filter will have a passband (with minimum insertion loss) within its operating frequency range and a stop-band with maximum attenuation. SAW filters are developed using semiconductor technology and are used in several applications.

How does the SAW filter work?

Surface Acoustic Wave filters work based on the principle of the Inverse Piezoelectric Effect. SAW filters use semiconductor materials such as Lithium Niobate (LiNbO3) or Lithium Tantalate (LiTaO3) which exhibit piezoelectric properties.

Inverse Piezoelectric Effect

Piezoelectric Effect is a unique property of crystals and some materials when the external stress is applied across this material, it can generate a small amount of EMF or voltage. Similar way if a voltage is applied to a piezoelectric material, it will either compress or expand according to the polarity applied to the material. This phenomenon is known as Inverse Piezoelectric Effect.

SAW filter has two electrodes known as Interdigital Transducer – IDTs at each end (input and output). IDTs are comb-like patterns etched on the surface of a semiconductor plate. The size, shape, and material height of this structure define the properties of the filter.

When a voltage is applied to the IDT, it generates a continuous compression, and expansions on the chip correspond to the amplitude of the applied EMF. These vibrations are transferred to the other end of the chip like a wave. At the other end, the vibrations (waves) generated from input DUT will be transferred (via waves on the chip) into corresponding electrical pulses at output IDT.

Advantages of SAW filters

Frequency Stability – developments in SAW technology have contributed to improving the performance of filters compared to earlier generations. The current generation of SAW filters offers high-frequency stability over temperature ranges.

Reduced size – advancements in semiconductor processing and packaging allow manufacturers to significantly reduce the device size. Component size is an important factor for a device to qualify by customers for their next project.

Cost-effective solutions – SAW technology offers cost-effective solutions for a wide variety of applications. Affordable cost makes SAW components a greater choice for a customer who requires high volume production.

Faster Development – with advanced simulation techniques and the latest semiconductor process technology, the development time of SAW components is reduced to a few weeks.

Applications of SAW filters

Smartphones and tablets

Manufacturers have been using SAW filters for many years since the first generation of handsets and it is still one of the significant components in all smartphones. Since the beginning of LTE, smartphones supports many radio frequency bands (LTE bands) from 600MHz to 2700MHz. Manufacturers will include most of the common LTE bands on smartphones to make sure it is compatible with networks across the globe.

5G introduced the sub-6GHz spectrum for earlier deployments, SAW filters can cover most of the bands in the lower range. SAW filter solutions powers modern mobile devices with high-performance duplexers and multiplexers for RF communication modules.

GPS and tracking

SAW filters are used in GPS and other satellite navigation applications on smartphones and other wireless devices.

Wi-Fi

Wi-Fi modules use SAW filters as filtering solutions for multiband operations. SAW filter can provide a large bandwidth to support a higher data rate.

Wireless sensors

SAW filters can be used in wireless sensing applications. A SAW band pass filter connected to the antenna will only allow a particular frequency (within the filter passband) to pass through. Using this method, detection/sensing of a signal is possible with SAW filters.

Automotive

SAW filters are used in the automotive industry for many years. Due to its stability under high temperatures, well suited for automotive applications such as keyless access to wireless connectivity. In future autonomous vehicles, these filters will be an integral part of RF communication modules.

Base stations

Mobile base stations are using SAW filters for all common LTE bands. Especially in small cell concepts like Femto Cell / Pico Cell base stations, SAW filters are providing filtering solutions to avoid interference.

Medical

SAW filters are used in several medical applications. Sensors are significant components in many medical equipments. It can be used in wireless communication devices and near-field communication devices and tags in the medical industry.

Digital TVs

Digital television is one of the early adopters of SAW filter technology and it is still in use in many countries.

Military Radars

Due to their high stability and frequency response performance, SAW filters are the perfect solution for military RADAR applications. SAW filters are integrated into the latest generation of radars like Active Electronically Scanned Array (AESA) Radars as well.

Aerospace

Due to high selectivity and higher attenuation on the out-of-band (low distortion from other systems), SAW band pass filters are used in aerospace applications.

RFID Applications

SAW filters are used in RFID technology to improve the selectivity of desired frequency used.

Other Applications

In Software Defined Radio transceivers as intermediate frequency (IF) filters.

Intermediate frequency (IF) filters in superheterodyne radio systems.

Wireless repeaters

Future of SAW technology

Like other semiconductor technologies, SAW technology has also been advancing over the years. With current technology, high-performance filters with improved power handling and better temperature stability can be produced. SAW filters will be used in future wearable tech, Internet of Things applications, drone devices, and smart home appliances.