BAW filters: Problem-solvers for key Wi-Fi and IoT challenges
It’s no secret that the internet of things is expanding at breakneck speed. On average, today’s households have 12 clients or IoT products communicating with each other. These numbers are expected to increase dramatically, and quickly. Intel believes the number of household clients will increase to 50 in 2020, while Gartner looks at business and consumer markets and predicts 20.4 billion connected things will be used worldwide by 2020.
This rapid growth presents significant challenges for design engineers, particularly Wi-Fi front-end designers. These challenges include thermal management, interference, coexistence and RF front-end (RFFE) linearity. This article takes a look at these challenges and offers bulk acoustic wave (BAW) filters as a problem-solver, particularly for wireless access points and customer premises equipment.
Wi-Fi front-end thermal challenges
As the number of IoT connected devices increases in any given location (smart homes, business headquarters, college campuses, hospitals) and new network models (like mesh networks) are developed to service IoT demands, RF complexity within access points is increasing. In addition to more users and connected devices, RF front-ends must now support wireless radio needs as well as Wi-Fi, additional Wi-Fi bands and the ongoing challenges of how to increase functionality while decreasing the size and weight of the device.
That all adds up to an increase in the number of RF chains in routers and access points, which means thermal management challenges. Filters are one of the key RFFE components implicated by heat challenges.
RF filtering drifts to the left or the right due to changes in temperature, as shown in the following SAW versus BAW figure. These shifts can cause high insertion loss on the band edges, which could cause a low gain or POUT response from the RFFE. If the filter drifts too much (as shown in the SAW figure), then the power amplifier pushes more power output to compensate for the insertion loss. This increases current and decreases system efficiency.
Using filters with high insertion loss can decrease linearity and increase the RF chain POUT. One big advantage of BAW filters (like LowDrift BAW filters) is their stability over temperature shifts. Diplexers, bandpass filters and coexistence filters that use BAW technology with lower temperature drift help mitigate insertion loss and lead to good product thermal characteristics, making them lower cost and/or smaller in size.
Interference challenges
As more LTE bands are squeezed into the crowded global RF spectrum, the space between bands is shrinking. In some cases, the transition between the passband and stop-band is as small as 2 MHz. This makes it very tough and expensive to meet government regulation requirements using traditional filter technologies. That’s because the variation in filter response, which is dominated by temperature drift as mentioned above, can exceed the width of the transition band itself. The result is unacceptable interference, high insertion loss, or both.
High-Q BAW bandpass filters offer many advantages over traditional SAW filters, including:
- Extremely steep skirts that simultaneously exhibit low loss in the Wi-Fi band and high rejection in the band edge and adjacent LTE/TD-LTE bands
- Significant size reductions, which aid designers in creating smaller, more attractive end-user devices for residential and commercial environments
- Resolution of coexistence of Wi-Fi and LTE signals within the same device or near one another
- Unique power-handling capabilities, allowing for implementation into high-performance, high-power access points and small cell base stations
These filters address the stringent thermal challenges of multi-user multiple-input/multiple-output systems, without compromising harmonic compliance and emissions performance. This is critical to achieving reliable coverage across the full allocated spectrum.
In a nutshell, high-Q BAW filters are superior to SAW technology when it comes to band edges, because they:
- Have lower insertion loss, steeper band edges and better temperature stability then SAW technology at Wi-Fi frequencies
- Help engineers provide seamless transitioning between interfering bands
- Extend the range in Wi-Fi band channels 1 and 11 by a factor 2 to 3
Summary
Wi-Fi front-end designers should consider bulk acoustic wave filters as an important solution to the challenges inherent in the IoT’s explosive growth of the number of RF devices and resulting demands. BAW filters are inherently less sensitive to temperature change than standard SAW filters, and they generally deliver superior performance with lower insertion loss at higher frequency levels. BAW filters can also overcome interference challenges, due primarily to their extremely steep skirts that exhibit low loss in the Wi-Fi band and high rejection in the band edge and adjacent LTE/TD-LTE bands.
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