Why RF filters?
RF filters are fundamental components in all RF/microwave systems, especially wireless communication systems with multiple channels or frequency bands. Their primary function is to attenuate signals in unwanted frequency bands while minimally affecting signals in the desired bands.
RF filters are critical because unwanted signals, or interference, can degrade or damage system performance. In wireless receivers, filters at the input attenuate signals outside the desired band. Filters are also used to reduce harmonics, spurious content, and out-of-band leakage from transmitter circuits. Many modern devices, such as smartphones, incorporate several wireless technologies; without proper filtering and isolation, these technologies can interfere with each other, creating coexistence design challenges.
Figure 1
Figure 1: 5G smartphone example
Which applications require RF filters?
Compact designs in most modern wireless products limit physical isolation, so engineers use RF filters to improve isolation and ensure compliance with regulatory and industry standards. These standards include requirements from agencies such as the FCC and ECC and wireless specifications such as Wi-Fi, 4G/5G, Bluetooth, and Zigbee. Many devices also use functions that require additional filtering, for example GPS and other positioning systems, and NFC.
Because modern wireless systems are extremely compact, they require highly compact filters that still deliver high quality factor (Q) and can be integrated into filter banks for multiband filtering. In many cases, each band needs a distinct RF filter to minimize crosstalk and mitigate nonlinear effects. To meet these demands, engineers frequently employ acoustic wave filters (AWF).
Acoustic wave filters use electroacoustic transducers on a piezoelectric substrate to convert electrical energy to acoustic/mechanical energy and vice versa. AWF convert high-frequency electrical signals into acoustic waves, apply resonator and acoustic filtering techniques, and then convert the signals back. Compared with electromagnetic filter technologies, acoustic phenomena occur at much smaller physical scales, enabling acoustic filters to be roughly an order of magnitude smaller than equivalent electromagnetic filters for similar performance.
AWF include two main types: bulk acoustic wave (BAW) and surface acoustic wave (SAW) filters. Both use interdigital transducers to convert between electrical and acoustic signals. BAW filters guide acoustic energy through the bulk of the substrate, while SAW filters guide acoustic energy along the substrate surface. Although this distinction seems simple, it leads to significant differences in performance and frequency capability.
SAW filter design and manufacturing are typically less complex because the process primarily involves surface structures. BAW filters require precise control of substrate thickness and layered structures, for example precisely spaced acoustic reflectors in a stack.
Due to physical limitations of surface electroacoustic transduction, the relative dimensions and mechanical properties associated with BAW allow higher frequency operation and higher Q compared with SAW. BAW filters can be fabricated with techniques compatible with standard IC processing and usually handle higher power. Some SAW technologies incorporate temperature compensation or other manufacturing methods to reduce temperature sensitivity, but BAW filters generally exhibit lower temperature drift.
In general, SAW filters are practical up to about 2.0–2.5 GHz, whereas BAW filters can reach 10 GHz and beyond. Thus, SAW and BAW technologies directly compete in the frequency range from roughly 100 MHz to about 2.5 GHz.
When should engineers use BAW versus SAW?
Choosing a filter requires understanding application requirements and the filter's electrical specifications. Each application specifies center frequency, bandwidth, required signal level, and suppression levels. System engineers typically list these requirements; designers then select filters that meet the specifications within cost and integration constraints and plan how to incorporate the filters into the system. Modern devices often use filter banks with many filters to meet strict radio and regulatory requirements.
Key electrical specifications for RF filter design include:
- Filter type (low-pass, high-pass, band-pass, notch)
- Passband frequencies (Hz)
- Stopband frequencies (Hz)
- Stopband attenuation or out-of-band suppression (dB)
- Attenuation (dB)
- Insertion loss (dB)
- Isolation (dB)
- Selectivity (dB)
- Q factor
- Ripple (dB)
- Input power handling (dB)
- Input and output impedance matching (ohm)
Many wireless standards emphasize using band-pass filters, often several in series, to achieve multiband filtering. This is a common use case for SAW and BAW filters because their compact size makes relatively small filter banks feasible. These band-pass filters reduce out-of-band signals reaching the receiver, which is important because out-of-band content can desensitize the receiver, leading to lower SNR or higher BER. Filters are also used at transmitter outputs to reduce nonlinear effects, such as harmonics and spurious peaks, generated by relatively high-power transmitters. This use improves adjacent channel leakage ratio (ACLR) and adjacent channel power ratio (ACPR). Proper filtering can allow a transmitter to pass industry tests without extensive redesign.
In some cases, a deployed radio encounters interference in field operation that was not apparent during early evaluations. If the issue can be resolved by stricter filtering, the radio can be upgraded with enhanced filters to mitigate the problem. For example, if interference occurs near a critical operating band, a filter with higher Q and better out-of-band suppression can outperform the existing filter.
Generally, SAW filters are less expensive than BAW filters and are widely available. SAW filters typically do not operate above about 2.5 GHz. However, if the design requires the highest performance or higher operating frequency—for example in 3G, 4G, Wi-Fi 6E, and sub-6 GHz 5G bands—BAW filters are often the better choice. BAW design targets higher frequency operation, improved passband attenuation, stronger out-of-band suppression, higher power handling, and higher Q.
Are there application-specific filters?
SAW and BAW filters must be precisely designed for the intended operation and can be customized for specific applications and mass production. Manufacturers commonly design and produce SAW and BAW filters tailored to markets and applications such as Wi-Fi and 4G/5G. Specific SAW and BAW products are also used to address coexistence challenges between standards, such as Wi-Fi and low-energy Bluetooth.
An example of a widely used acoustic filter is the Qorvo QPQ4900 BAW sub-band filter for 5G NR TDD band n79. This sub-band BAW filter is intended for macro and small cells that may experience potential interference from nearby Wi-Fi 6E networks. Another example is the Qorvo QPQ1063 GPS SAW duplexer, designed for L1/L2 GPS bands.
Conclusion
RF filters are essential in modern wireless systems. They attenuate unwanted frequency content to reduce interference and ensure proper system function. Due to physical limitations, filters are not ideal and can introduce some passband loss. Acoustic wave filters, especially BAW and SAW, are popular because of their small size and high Q. SAW filters are generally less complex to design and manufacture, while BAW filters offer higher frequency operation and superior Q performance. Engineers must consider application requirements and electrical specifications—such as filter type, passband, stopband, and insertion loss—when selecting filters for system designs.
