The Challenges 5G Brings to RF Filtering

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In the race to implement mainstream 5G wireless communication, the world is waiting to see if this next-generation network will achieve a hundredfold increase in user data rates. This transformative technology not only boosts performance for the latest cell phones, but also for fixed wireless access (FWA) networks and Internet of Things (IoT) smart devices. In order to reach 10 Gbps peak data rates, the increase in channel capacity must come from somewhere. A key innovation at the heart of 5G is utilizing new frequencies greater than 20 GHz in the millimeter wave (mmWave) spectrum, which offers the most dramatic increase in available bandwidth.

A well-known downside to high frequencies is the range limitation and path loss that occurs through air, objects, and buildings. The key workaround for mmWave base station systems is the use of multi-element beamforming antenna arrays in both urban and suburban environments. Since mmWave signals require much smaller antennas, they can be tightly packed together to create a single, narrowly focused beam for point-to-point communication with greater reach.


In order to overcome the range limitations of mmWave frequencies, dense arrays of antennas are used to create a tightly focused beam for point-to-point communication.


Changes in Radio Architecture for mmWave Beamforming

Of course, these new beamforming radio architectures bring a whole new set of challenges for designers. Traditional filtering solutions for radios operating in the ranges of 700 MHz and 2.6 GHz are no longer suitable for mmWave frequencies such as 28 GHz. For example, cavity filters are often used in the RF front ends of LTE macrocells. However, an inter-element spacing of less than half the wavelength (λ/2) is required to avoid the generation of grating lobes in antenna array systems. This inter-element spacing is about 21.4 cm at 700 MHz frequency versus only about 5 mm at 28 GHz. Such a requirement therefore calls for very small form factor components in the array. Not only will the whole RF front end be reduced in size, but also the number of RF paths will be increased – which means the filters right next to the antennas must be very compact.

Screen Shot 2019-09-26 at 11.35.39 AMAs the frequency increases, the antenna size must decrease, leading to significant changes in the mmWave beamforming radio architecture.


Addressing Challenges with mmWave Filtering

Based on decades of experience working with mmWave filtering solutions, Knowles Precision Devices has a product line of mmWave filters solutions that addresses these challenges. Using specialized topologies and material formulations, we’ve created off-the-shelf catalog designs available up to 42 GHz that are 20 times smaller than the current alternatives.

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Compared to current alternatives, Knowles Precision Devices offers filter solutions that are 20 times smaller to meet mmWave inter-element spacing requirements.


As manufacturers push to increase available bandwidth, temperature stability also becomes more and more of an issue. MmWave antenna arrays may be deployed in exposed environments with extreme temperatures, and heat dissipation issues may arise from packing miniature components onto densely populated boards. In order to guarantee consistent performance, our filters are rated for stable operation from -55°C to +125°C. For example, the bandpass filters below shifted only by 140 MHz when tested over that temperature range.

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The temperature response of a Knowles 18 GHz band pass filter (BPF) on CF dielectric shows little variation in performance from -55°C to +125°C.


Finally, high performance with high repeatability is key to ensuring the best spectral efficiency and rejection possible. High frequency circuits are especially sensitive to variations in performance, so precise manufacturing techniques ensure that our filter specifications – such as 3 GHz bandwidth and greater than 50 dB rejection – are properly maintained from part to part. Plus, unlike chip and wire filter solutions, our surface mount filters standardize the form factor, reduce overall assembly time, and do not require tuning – thus saving in overall labor costs and lead times.


The performance of Knowles Precision Devices 37-40 GHz BPFs shows highly repeatable performance over 100 samples, even without post-assembly tuning.