SpectrumTalk

RF Filters: An Important but Often Overlooked Issue in Spectrum Policy

Problems With the New 3G and 4G Frequency Band

by Mike Alferman*, Guest Blogger

The current big push to ‘mobilize’ the Internet is raising the awareness in the industry to the capabilities and requirements for a passive component that normally doesn’t get into the spotlight. However, this component – the RF Bandpass Filter, has been a key item in radio spectrum policy for years. With the efforts to open up new frequency bands for wireless internet use, the RF Filter may in fact start doing some driving. However, without some intervention, it could be that the commercial realization of the needed filters may not happen.

Historically, the FCC allocated frequency bands and added additional spectrum as ‘Guard Bands’ to prevent the interference between services operating on adjacent frequency bands. In addition, the in-band frequency use was coordinated based on the best practice engineering requirements of the time. As commented in the this blog, when UHF TV was first licensed in the US, (with non-critical services located on either side of the allocated bands) the adjacent channels were left unoccupied as it was determined that the discrete component LC RF and IF Filters available in the TV receiver Tuners when the band was opened were incapable of the high performance required to eliminate adjacent stations from interfering with each other. Thus, due to the poor performance of the receiver tuner’s RF filters, large areas of the allocated spectrum were left unused. This spectrally wasteful practice wasn’t significant as there was enough spectrum capacity for the needs at that time. Obviously, times have changed.

The FCC is currently responding to the need to use every possible MHz of the public’s valuable spectrum and has eliminated the practice of adding significant Guard Bands between new services. While being more efficient in spectrum use, this has created a real problem! The reality of all RF Filter design is that a certain amount of frequency is required for the transition from the Passband to the Stopband (the roll-off). It should be noted that the theoretical ‘brick wall’ filter will forever remain theoretical because in addition to the roll-off of the basic filter design, allowances must be made in the design of practical filters for the shift of the filter due to temperature as well as for manufacturing tolerances. This means that current practical commercial RF Bandpass Filters for the new frequency allocations are not capable of preventing some interference between services.

To be certain, RF Filter performance has come a long way since the days of the old UHF TV tuner! Filters then were large multi-stage discrete LC units with modest performance – and to operate had to be aligned in manufacturing. In the late 80’s, Surface Acoustic Wave (SAW) filters became common in TV tuner IF stages. The performance of the SAW Filters was significantly better than what could be achieved with LC filters with much better manufacturability - in a much smaller package. In the last 20 years SAW Filters have continued to improve. The performance of the relatively low frequency SAW IF Filters of the late 80’s is now possible with SAW RF Filters at the much higher operating frequencies. At this time, the operating frequency for commercial SAWs has increased to above 2.5 GHz, the Filter rejection ‘skirts’ have greatly improved, the Insertion Loss of an average SAW Filter has been minimized – and all achieved at the same time as a 94% decrease in the package size and cost. BAW (Bulk Acoustic Wave – also called FBAR) Filters became common in the last 5 years and have yet higher frequency capability and better roll-off than SAW filters. However the cost, packaging and development time of BAW Filters are still a few years behind that of SAW filters.

Technically, in order to better meet the requirements for the new frequency bands, the needs for the improvement of both SAW and BAW Filter performance are similar. Adding a frequency trimming step in manufacturing improves the manufacturing yields, but adds costs and increases the price of each filter. Both SAW and BAW Filters shift with operating temperature due to the materials they are made with. Decreasing this temperature shift has been the focus of development work for several years, and it’s clear that it will be a few years before the manufacturability and electrical performance of the temperature compensated filters will be at the same point as the current generation filters.

SAW and BAW filters have become the most widely used RF Bandpass filters in the world, with Billions being used each year in Cell phones. There is no filter technology that can provide better performance in a small, cost-effective solution for the standard frequency bands around the globe. However, the move to create new frequency bands for 3G and 4G services has been regional. The US has come up with several bands which will not be allocated worldwide and the spectrum auctions have resulted in a limited number of licensees that will use a particular band. This means that instead of unifying (standardizing) the frequencies to be used, competing services will be using different frequency bands. This market fragmentation creates a limited market for the RF Filters for each band. This is a problem for the commercial SAW and BAW Filter manufacturers. It should be noted that the new requirements (without Guard Bands) make the design of the filters for each band significantly different from the design for other bands – significantly increasing the design effort required and even requiring special materials and special processing. This means it is unlikely that the filter manufacturers can profitably produce the new more technically challenging RF Bandpass Filters at the low prices demanded by the equipment manufacturers. It should be noted that this uncertain profitability falls in the Passive Component industry, which currently only has single-digit profit margins and is fighting for survival.

The new materials and special processing needed to increase the technical performance of the SAW and BAW Filters for these new bands add both high costs and uncertainty of the electrical performance. The structures being developed for the new filter designs will take a few years before they reach the stable manufacturing of the current generation of filters. If you add up the factors of the relatively small market for each new frequency band with the higher costs for producing the filters and the uncertainties of the manufacturers making a profit, you begin to realize that the commercial issues may well rival the technical challenges. When combining these significant challenges, it can be seen why there is currently a strong reluctance by the SAW and BAW filter manufacturers to move forward to develop all the necessary RF Filters.

What can be done to make the development of necessary higher-performance RF Filters attractive to develop? For one, it must be realized by the community that the RF Filters for the new bands are not just a simple commodity on which the usual high-volume pricing pressure can be applied. They are a critical component which must meet the tough new requirements – or else the rest of the higher profit radio will not function properly. In addition, as suggested by MSS in the Comments to the FCC NOI, there must be a dialog with the FCC, the NTIA and the major RF filter manufacturers to assess what can be possible with commercial technology – including discussion on both sides of performance roadmaps. Also, equipment manufacturers can not just assume that because there is a need, someone will develop technology that works as they want – and it will be the same low prices as Cellular filters.

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* Mike Alferman, mikealferman2@embarqmail.com, has been active with RF for 40 years: 12+ years at EPCOS/Siemens as head of RF Applications Engineering and Product Marketing for SAW/BAW Filters and Modules. Key role in developing RF filtering solutions for cellular handsets, WiFi consumer equipment and WiMAX RF modules. 9 years at Andersen Laboratories as SAW Filter Design Engineer and SAW Applications Engineer. Key role in developing cellular Basestation IF Filters, HDTV IF Filters and Radar Chirp Filters. Held various other Communications Technician, Engineering Technician and Field Engineering positions. Significant Experience with Cellular Systems and Handsets, Land Mobile Radio and Radar/EW systems.Holder of Commercial General Radiotelephone License (formerly called First Class) and Amateur Radio Advanced Class License (WA2NAS)