5G mmWave filters: What is the best solution?

As cellular technology evolves, mobile bandwidth continues to increase, with more and more frequency bands. Each of these bandwidths requires a filter to separate its signal from the other bands, but the filter technology currently used in mobile phones may not scale to the entire millimeter wave (mmWave) range in 5G.

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“Millimeter wave will come eventually, but not yet,” said Mike Eddy, vice president of corporate development at Resonant, a U.S. company founded in January 2012 to create innovative filter designs for the radio frequency (RF) industry. “Earth exploration satellite services are at 23.8 GHz, which is slightly below the millimeter wave bands being deployed for 5G, so some filtering is going to have to be done on them.”

“Surface acoustic wave (SAW) devices or bulk acoustic wave (BAW) devices will not scale beyond 10 GHz,” said Anthony Lord, director of business development for FormFactor's RF division.

“None of these filters work in the millimeter wave range; they all go above 6 or 8 GHz,” said Tim Cleary, senior director of marketing for FormFactor’s RF product group. “There’s no better solution in the industry, and that’s a challenge.”

SAW and BAW filters currently dominate the handheld device industry, and while they may extend beyond the 6 GHz range with further improvements, they are still a long way from the 28 to 70 GHz range that mmWave designs need to operate.

In fact, there are already some solutions for devices that are not limited by size - but these solutions are not suitable for mobile phones, and this is where development is needed.

Frequency bands proliferate, filters explode

With each new generation of mobile communication technology, more frequency bands are developed and used. The term "frequency band" can have different meanings, such as broadband bands are allocated and auctioned, and the frequency bands represented by a single channel are a subset of these broadband bands.

The number of these small frequency bands is increasing dramatically. For a channel using frequency-domain duplexing (FDD), there are two adjacent links—one for transmission and one for reception—with a guard band between them. When using time-domain duplexing (TDD), there is only one frequency band for the entire channel.

A band-pass filter is required in each of these bands or sub-bands. As the number of bands increases, the number of filters required also explodes. For example, the number of filters in smartphones today exceeds 60, and this number will increase further in the 5G era to add higher frequencies to the millimeter wave bands.

In theory, a bandpass filter can pass all signals within the frequency band and "block out" all signals outside the frequency band. It can be simply thought of as multiplying the signals within the band by 1 and the signals outside the band by 0.

However, filters in the real world are not ideal and face many challenges.

Real-world filters are full of challenges

In reality, filters do not "stop abruptly" at the edge of the band, because the edge of the band is arc-shaped and the attenuation is inclined rather than vertical.

The center frequency, upper and lower cutoff frequencies are the key properties of the filter. The cutoff value is defined as the point where the signal passing ability drops by 3dB (corresponding to the point where the signal power drops by half). The slope beyond the 3dB attenuation is usually called the downswing and needs to drop as quickly as possible.

While it might be nice to design the three frequencies (center, upper, and lower) independently, in practice, the upper and lower cutoff frequencies move together so that both the center frequency and the overall width can be designed, so the center frequency moves with it, and the width is usually a percentage of the center frequency.

Designing wider bandpass filters can be more challenging, and the width of some 5G frequency bands can be as much as 20% of the center frequency, which puts a significant burden on the filter design.

In the front end of the receiver, spurious signals need to be filtered out as early as possible to prevent them from entering the RF chain, which means filtering the signal immediately after the antenna. With massive multiple-input multiple-output (MIMO) technology allowing beam steering, a large number of antenna element arrays are used, in which case a filter is required for each element.

“The spacing between the cells is based on millimeter wave, which means the spacing is about 5 mm,” Eddy said. “It has to accommodate that spacing.” But that’s not currently possible for millimeter wave, so any filtering is done after the mixer.

Base stations have plenty of room to accommodate filter size, but cell phones impose demanding small size requirements. For the foreseeable future, the best frequency for small filters is likely to be 28 GHz, as this is the millimeter wave frequency likely to be used in handheld devices, and higher frequencies are more likely to be used for tower-tower communications, which are not as space-constrained as cell phones.

“For things like base stations, we’re going to rely on ceramic dielectric filters and metal cavity filters,” said David Vye, director of technical marketing for AWR software at Cadence. “They’re never going to meet the space requirements inside mobile devices.”

In the early days, filtering requirements for the 28 GHz (or similar) band were more relaxed. "In the first few years, we often heard that there would not be any millimeter wave filters in the phone. Because at that time, the frequency bands were not broken down, and the antenna was mainly used for filtering," said Jeb Flemming, CTO of 3D Glass.

In this case, it’s enough to use the antenna as a so-so filter, but at some point we need real filters for the antenna elements. So how exactly are these millimeter wave filters made?

Acoustic wave filters widely used in mobile phones

Most filters in today's mobile phones use acoustic wave technology, which designs piezoelectric materials to deform slightly under the influence of an electric field, and the physical deformation generates an electric field. As a result, electrical signals can be converted into mechanical vibrations, and mechanical vibrations can also be converted into electrical signals. These mechanical vibrations are equivalent to acoustic waves in the crystal.

By building an acoustic resonant structure, an input signal can be applied to one end of the resonator. This input signal consists of many different frequencies - some are for other frequency bands, and some are ambient noise. The first task of the filter is to eliminate any signal outside the passband.

Signal frequency components within the passband will cause acoustic resonances, which are then detected by the acoustic wave filter and converted back into the electrical domain at the other end of the filter. Ideally, this output will consist of all of the input signal with the unwanted frequencies removed.

These acoustic wave filters have many advantages, including clean passbands, very small size and a favorable cost structure, especially high-volume manufacturing that reduces costs.

At lower frequencies, surface acoustic wave (SAW) filters dominate, with which waves are excited on the surface of the material and coupled to an output near the same surface.

For higher frequencies, bulk acoustic wave (BAW) filters dominate, which, in contrast to SAW at low frequencies, do not excite waves on the surface of the material, but instead use a large volume of material to resonate from top to bottom, with the output electrode located underneath. This requires more complex processing, so they tend to be more expensive than SAW filters.

There are two basic versions of BAW filters, the difference being how the internal standing waves are set up.

• One version requires reflection from the bottom to the top and does this using a free-standing resonator BAW (FBAR) filter and an air cavity.
• Another version uses a series of layers that look like acoustic mirrors (similar to Bragg reflectors for light) and is called a solid mounted resonator (SMR) BAW filter.

Both SAW and BAW filters are made using MEMS processing technology, but they appear to start failing at higher frequencies, suggesting that the industry may need to find new filters for mmWave frequencies.

Three options for millimeter wave filters

Millimeter wave radio signals are not new. Radar and microwave devices already use them, for example, but these tend to be large devices that can only handle one or two frequencies. For 5G, more frequency bands must be filtered more stringently and fit into mobile phones.

While SAW and BAW are off the table, Resonant has what it calls XBAR technology, which it claims can expand the range of acoustic technologies available. The company redesigned the BAW filter from scratch, using a different piezoelectric material—lithium niobate—and putting both contacts on the top, similar to SAW.

The main difference with SAW, however, is that with XBAR, the contacts don't physically move. “With SAW, the metal bars physically move, which means they lose momentum in the metal migration process,” Eddy points out.

“When we modeled this structure, XBAR provided the energy, bandwidth and power handling capabilities needed for 5G—especially when we focused on 3 to 5 GHz,” he continued. “Now we’re looking at WiFi at 5 to 7.1 GHz, and then ultra-wideband at 7 to 9 GHz. Can this model work for millimeter wave? We think it can.”

The XBAR filter looks promising, but the point is that it represents a new approach in this frequency range. Two other well-known mmWave filter technologies are waveguide and cavity filters, but unlike SAW and BAW filters, which use acoustic waves, they use electromagnetic waves for resonance, have a wide selection of structures, and are typically used in microwave applications.

The size of these resonators is usually sized according to the frequency range, with the size or spacing being within a quarter of a wavelength. The higher the frequency, the shorter the wavelength and the smaller the filter. For 5G frequencies, the size of the resonators is shrinking—but they are still too small to fit in a phone.

“There’s a medium called a ‘waveguide cavity,’ and its height and width determine the amount of energy that can propagate through it,” Vye said. “Below that frequency, the energy doesn’t propagate, and above a certain frequency, you have modulation problems.”

The use of resonators helps reduce unwanted modes. “A waveguide cavity filter has some posts inside it,” Vye said. “It acts the same as a ceramic filter, with the property of stopping or passing energy at a certain frequency, depending on the size of the posts. The physical size between the resonators will affect the bandwidth, and the number of resonators will affect the attenuation—the more resonators the filter has, the faster the attenuation. But then you increase the length of the filter, and you also increase the material cost of the filter.”

For base stations, the technology is suitable because it can accommodate a larger size, but for mobile phones, the filter is still too large.

Microstrip filters are another option for frequencies up to 30 GHz. With this design, microstrip lines are created on a printed circuit board (PCB) to support electromagnetic resonance. However, there is still a problem. Generally speaking, PCB materials are generally considered to be of low quality.

“Variations in the thickness of the PCB, changes in the dielectric constant of the material, and changes in the line width when printing can all change the passband frequency,” Eddy said.

There are other considerations as well. “Material properties do drive performance, but there are only a handful of materials on the market,” Flemming said. “These very high-Q resonant ceramic materials are special and typically more expensive. Historically MLCCs (multilayer ceramic caps) were a reasonable material, but they start to fail around 25 GHz.”

Attractive glass craft

Because of the shorter wavelengths of millimeter-wave frequencies, it’s possible to make waveguides in silicon or other materials. “It’s almost like MEMS, because you’re creating these channels where the microwave signal can go through etched areas that are then metallized on the silicon wafer,” Vye explains.

3D Glass uses a photolithography process to make waveguides in glass rather than silicon. Amorphous glass is selectively converted into crystals by exposure to ultraviolet light. The converted crystallized glass (actually ceramic) is more suitable for etching and is easier to create through-hole features.

“Ceramic etches in acid 60 times faster than glass,” Flemming said. “We can make cavities, but it’s a timed etch because this ceramic layer has glass running through it.”

Structures such as inductors can be made in this way, and cavities with resonators can also be created in this way for millimeter-wave filtering.

“I made the resonator out of a metal wire and etched away almost all the glass,” Flemming said. “So my resonator is mostly floating in the air. Since the limiting factor for 5G millimeter wave is the material, if I can remove the material and make it float in the air and be robust, I can call it a success. This suspended stripline can reach about 40 to 50 GHz. We demonstrated 10% to 15% bandwidth, which is quite wide.”

These air-filled cavities can be extended to higher backhaul frequencies. “We are doing a lot of customer development in the 70 to 150 GHz range,” he noted. “Some people call it 5G, some people call it 6G.”

In the past, filter design involved multiple fabrications to optimize performance, but there were too many variables and the requirements were tight, but now simulation tools are available that allow the structure of the filter to be optimized before it is built.

“How you package it and how you connect to the rest of the circuit is very important,” Vye said. “People gave up on empirical testing of the design and relied on EM (electromagnetic simulation) techniques to do the design.”

Cadence was familiar with the work on 3D Glass, having previously used Microwave Office to design and simulate 3D Glass. “You have metal resonators inside a very low-loss structure that is suspended in midair by a small glass base, which is used to build very small filters, although not as small as acoustic wave filters,” Vye said.

in conclusion

The economics of the glass process are attractive. Given the need for volume, panels can be used instead of wafers. A 9' x 9' panel can hold a lot of filters, so while work today is done on 6- and 8-inch wafers, and some customers want to move to 12-inch wafers, they see a clear path to lower costs.

While there are some exciting possibilities on the horizon, none of them are ready for commercial production and no real winner has yet emerged in the field of filtration technology.

Millimeter waves in 5G phones aren’t fully realized yet, so there’s still some time. But it’s important to note that the problem the industry is facing right now is developing a solid plan and roadmap, not some interesting ideas that might work.

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