Sam Benzacar, President of Anatech Electronics, Talks About Advances in RF and Microwave Filters
by Sam Benzacar, President, Anatech Electronics
MPD: How has the filter industry changed in the more than 30 years Anatech has been in the business?
SB: The crowding of the RF spectrum presents challenges in interference because services are closer together than they were even a few years ago, and the result is an increasing need for high-performance RF rejection outside the passband. These issues are due to the exponential increase in the number of commercial, military, and first responder communication systems. 5G, in particular, uses a broader range of frequencies well into the millimeter-wave region, and to achieve higher data rates, more bandwidth is required. In short, no wireless communication system can afford to make RF filters an afterthought. Not only is the need for RF filters greater, but filters must also deliver higher performance, and as a result they are more complex to design.
MPD: A bandpass filter ultimately resolved the issues of 5G base stations potentially interfering with altimeters on commercial aircraft. Have you encountered other situations like this?
SB: The situation you mention is a classic example of the importance of RF filters and how the operating environment has changed over time. To refresh readers’ memories, the issue arose when wireless carriers began to deploy base stations operating at their newly acquired C-band frequencies between 3.7 and 3.98 GHz near airport runways. Aircraft altimeters operate between 4.2 and 4.4 GHz to detect wind shear during automated landings. The airline industry and other groups complained to the FCC that this could pose an interference problem even though the wireless industry has comprehensive specifications for reducing harmonic emissions.
The carriers vehemently disagreed, citing that at least two dozen countries had already used these frequencies without problems. After many months of arguments between the two groups, it was ultimately decided that the entire US airline fleet would have to upgrade the performance of their altimeters, an expensive undertaking, which they completed in September of this year. The solution: high-performance bandpass filters. The takeaway here is that a lack of an inexpensive component caused millions of dollars of repairs. Although this is an extreme example, Anatech Electronics gets requests to modify existing systems frequently because the spectrum, especially below about 6 GHz, is increasingly congested.
MPD: Has solving these problems has become more complicated in recent years?
SB: It has. While bandpass filters with very sharp skirts can often solve a problem, in some cases, signals are so close together that trade-offs are required. For example, a designer could choose a filter with less rejection and hope it will solve the problem, but how do you remove a harmonic of a fundamental signal far from the operating frequency that is still strong enough to cause problems in the passband of other operating bands? It’s a tricky question; sometimes, the problem cannot be effectively resolved with a single filter alone, and various RF filters solutions must be looked at. Nevertheless, there are solutions for all but the most challenging problems.
For example, if you know the interfering signal is a harmonic from a nearby system, or sometimes from a distant transmitter, you can put a lowpass filter which will pass your operating frequency and reject everything above. Of course, this also might add some loss to the passband of the affected system. Sometimes a solution consists of a bandpass filter which must reject harmonics way above the center frequency. In that case a lowpass filter must be added to effectively remove those harmonics.
MPD: Please explain some of the recent advances your company is making in RF filter design and manufacturing.
SB: We have invested in both precision CNC milling machines and very sophisticated optimization and simulation software that allows us to fabricate very complex RF filters. Obviously, this results in higher costs because of the highly complex designs. Upgrading milling accuracy and more sophisticated software proved to be beneficial, especially for RF filters operating at millimeter-wave frequencies where a minute variance can degrade the performance of the filter.
For example, when designing a filter operating at 30 to 50 GHz where a quarter wavelength can be 100 mil, fabrication would only be possible with accurate milling tools. We didn’t need this level of accuracy in the past due to more lax designs which did not require as high performance as today, but it’s now become an essential ingredient in everything we make.
MPD: Filters are an inherently custom product as customer requirements vary. How do you accommodate this while keeping lead times as short as possible?
SB: About 90% of the requests we get from customers are custom solutions that range from simple modifications, such as changing connectors, to modifications of electrical characteristics. In practice, almost every filter is a custom product because even minor changes in performance eliminate it from being a so-called “off-the-shelf” device. Each must be crafted to meet a set of requirements, sometimes for a single customer, which may not be needed by anyone else, or at least not often.
A common misconception is that custom RF filters are expensive to customize so RF engineers often choose a stock filter that almost, but not entirely, meets their needs because it is perceived to be less expensive than one designed specifically for their system or may be available almost immediately.
Unfortunately, the result of such decisions can have negative consequences. For example, a less than optimal RF filter requirement, such as a non-customized RF filter that has “acceptable performance” might fail to satisfy the system performance. This adds to the cost since the filter performance was not adequate. Worse yet, the system can reach production, and then a need for a better performing RF filter is realized, after which changes become a nightmare, causing delays, and adding costs.
Fortunately, we have a deep library of RF filter designs which we’ve built over the last three decades, so the cost difference between those already designed RF filters from our library and custom RF filters are in many cases non-existent. There is a good chance that one of those already designed will come close to what a customer needs, so meeting a “unique” requirement is not difficult and often requires making only a few minor changes. In summary, the more complex the RF filter is, the more expensive it will be, and vice versa, regardless of if it is an “off the shelf” design or a custom one.