Regarding SAW and BAW RF filters

Dec 26,2024
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Why do we need an RF filter?

Radio frequency (RF) filters are the fundamental components of all RF/microwave systems, especially wireless communication systems with multiple channels or frequency bands. The main function of an RF filter is to attenuate signals in certain unwanted frequency bands, while having minimal impact on signals in the desired frequency band.

RF filters are crucial because in many cases, poor signals (known as interference) can cause a decrease in system functionality or even damage. In wireless communication systems, various types of RF filters are used at the receiver input to attenuate signals outside the desired frequency band.

RF filters are also used to reduce harmonics, spurious content, and out of band leakage from transmitter circuits. In many application scenarios of modern electronic products such as smartphones, these devices are equipped with several wireless communication technologies. If RF filters are not used for proper isolation, these technologies may interfere with each other, known as the coexistence design challenge (Figure 1)

Figure 1: Example of a 5G smartphone

Which applications require RF filters?

Due to the compact design limitations of most modern communication technologies, engineers use RF filters to enhance the required isolation effect and ensure that these products meet the necessary standards. These standards can be specifications developed by national and international regulatory agencies, such as the Federal Communications Commission (FCC) and the Global Electronic Communications Commission (ECC), as well as Wi Fi, 4G/5G, Bluetooth ® And wireless standards such as Zigbee. Many modern electronic products also deploy specific functions that require additional filtering, such as Global Positioning System (GPS) and other geolocation technologies, as well as Near Field Communication (NFC) technology. Considering the extremely compact size of modern wireless communication systems, highly compact filters are therefore required; Nevertheless, these filters still require a high Q factor (quality factor) and can be easily integrated into filter banks for multi band filtering applications. In many cases, different RF filters are required for each frequency band to minimize crosstalk and mitigate nonlinearity. To meet this demand, engineers often use acoustic wave filters (AWFs). Essentially, AWF is composed of electroacoustic transducers on a piezoelectric substrate, which can convert electrical energy into acoustic/mechanical energy, and vice versa. Based on this method, AWF converts high-frequency signals into acoustic signals, which are then adjusted through acoustic resonators and filtering techniques, and ultimately converted back into high-frequency signals. Compared with other electromagnetic filter technologies, its advantage lies in the fact that the acoustic wave phenomenon is about five orders of magnitude smaller than the electromagnetic filtering phenomenon. In fact, these factors result in acoustic filters being one order of magnitude smaller than traditional electromagnetic filters under similar performance.

AWF consists of two main types - BAW filters and SAW filters - which use interdigital transducers to convert electrical and acoustic signals. The BAW filter guides the signal energy through the main body of the substrate, while the SAW filter guides the signal energy to propagate along the surface of the substrate. Although this distinction may seem simple at first, in reality, different methods can lead to significant differences in performance and frequency capability.

Due to the fact that the manufacturing process mainly involves the development of surface structures, the design and manufacturing of SAW filters are usually not as complex. On the contrary, BAW filters require precise control of substrate thickness and layered structure, such as precise spacing of acoustic reflectors in the stack.

However, due to the physical limitations of surface acoustic conduction, the relative size and physical characteristics associated with BAW filters allow them to be designed for higher frequency operation and higher Q factor than SAW technology. In addition, BAW filters can be manufactured using technologies compatible with standard IC processing systems and typically exhibit higher power processing capabilities. Although some SAW filter technologies incorporate temperature compensation design features or are manufactured in other ways to minimize temperature sensitivity, the temperature drift of BAW filters is still lower than that of SAW filters.

Generally speaking, SAW filters can be manufactured in practice for frequencies of 2000MHz or 2500MHz, while BAW filters can reach 10GHz or even higher.

When will engineers use BAW and SAW?

When selecting a filter for a specific application, it is necessary to understand the requirements of the filter application and interpret the electrical specifications of the filter. Each filter application will have requirements for center frequency, bandwidth, required signal level, and suppression requirements. System engineers typically list these system requirements, and when selecting filters, engineers must ensure that the filters meet these requirements while maintaining budget, and develop a plan to incorporate the filters into the system design. For modern wireless devices, it typically involves designing filter banks consisting of many filters to meet strict requirements and comply with wireless and regulatory standards.

The following are the key electrical specifications for RF filter design:

Filter type (low-pass, high pass, band-pass, notch/pass suppression), passband frequency (Hz), suppression frequency (Hz), suppression or out of band suppression (dB), attenuation (dB), insertion loss (dB), isolation (dB), selectivity (dB), Q factor, ripple (dB), input power processing (dB), input and output impedance matching (ohms)

Most wireless communication standards emphasize the use of bandpass filters in series with several other bandpass filters to achieve multi band filtering. This has become a widely used typical scenario for SAW and BAW filters, as their compact size allows for relatively small filter banks compared to other filter technologies.

The function of these bandpass filters is to reduce the out of band signal content that the receiver is exposed to. This is an important feature because out of band frequency content can form a desensitized receiver with lower signal-to-noise ratio (SNR) or higher bit error rate (BER). In addition, filters are commonly used at the transmitter output to reduce nonlinear results such as harmonics and spikes generated by relatively high power transmitter devices.

The auxiliary application of filters enhances the Adjacent Channel Leakage Ratio (ACLR) and Adjacent Channel Power Ratio (ACPR). In addition, through the use of this filter, wireless transmitters that were originally unable to pass wireless standard testing may pass the test without the need for extensive redesign work.

There are also some cases where a wireless communication system that has been designed and deployed may not have been discovered in early assessments and may face challenges when operating in the field. In this case, if stricter filtering can be used to solve these problems, enhanced filters can be used to upgrade the radio to alleviate these issues. For example, if a radio encounters interference near a critical operating frequency band, a filter with higher Q-factor and better out of band suppression capability will result in better operation than existing filters.

Generally speaking, SAW filters are cheaper than BAW filters, and there are a large number of SAW filters available in the market for selection. SAW filters generally do not provide operating frequencies exceeding 250MHz. However, if the design requires the strongest performance or higher operating frequencies (such as higher frequencies in 3G, 4G, Wi Fi 6E, and sub-6GHz 5G), then BAW filters are a better choice (Figure 2). The design of BAW filters is usually aimed at achieving higher frequency operation and better passband attenuation, out of band suppression, power processing, and Q factor compared to SAW filters.

Is there a dedicated filter for specific applications?

SAW and BAW filters must be precisely designed to achieve the desired operation. SAW and BAW filters can be customized and mass-produced for a specific application. Fortunately, BAW and SAW filter manufacturers are very inclined to design and mass produce specialized SAW and BAW filters that meet the needs of various markets and applications such as Wi Fi and 4G LTE/5G. Specific BAW and SAW filters are also designed to address the prominent challenges of wireless standard coexistence, such as Wi Fi and low-power Bluetooth ® （LE）。

Conclusion

RF filter is a key component in modern wireless communication systems; They attenuate unwanted frequency bands to minimize interference and ensure the normal functioning of the system. Due to physical limitations of technology, RF filters are not always ideal and may result in some loss of passband frequency. Acoustic filters, especially BAW and SAW filters, have become popular choices due to their small size and high Q-factor. The design and manufacturing of SAW filters are not very complex, while BAW filters bring higher frequency operation and stronger Q-factor performance. Engineers must consider the requirements of their filtering application and the electrical specifications of the filter, such as filter type, passband, stopband, and insertion loss, in order to choose the appropriate filter for their system design.