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Debunking the Myth of Custom Filters

Debunking the Myth of Custom Filters
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by Sam Benzacar, President, Anatech Electronics

As the saying goes, RF and microwave filter manufacturers never produce models with the same specifications twice. While that’s a bit hyperbolic, the fact remains that these components are inherently custom designs, each one crafted to meet a set of requirements, sometimes for a single customer, that may not be needed by anyone else, or at least not often.

For proof, consider that back in the days when filter manufacturers printed catalogs, hundreds of “models” were listed because the companies typically made each device once or twice for a specific customer, assigned a part number to it, and may not have ever made it again. It was still in the catalog, so they could indeed resurrect it, but they certainly did not have all those “models” in stock, for obvious reasons.

They are exceptions, of course, as some requirements arise frequently, primarily for applications whose requirements are well known, so it’s economically feasible for the manufacturer or distributor to have these “catalog” items available off the shelf. In addition, filters such as SAW and BAW types, used in smartphones and other products, are mass-produced in huge quantities, so they’re not hand-crafted but instead fabricated in semiconductor foundries.

There’s a big difference between semiconductor filters and their customizable counterparts, which makes the latter the only choice for applications where small size is not critical, they aren’t needed in huge numbers, RF power handling of more than a few watts is essential, and a customer’s specifications fall outside (sometimes well outside) what are available off-the-shelf.

Defense systems are an obvious example. These systems are typically built in comparatively small quantities even through the entire lifetime of a specific platform. While the subsystem may be modified multiple times during this time, this process is completed once every few years or even longer. Some of the systems require extremely high performance in critical metrics such as out-of-band rejection along with operation over broad temperature ranges, and a variety of other requirements found nowhere else.

A common misconception is that these filters are difficult and expensive to customize. So, designers 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, or both.

Unfortunately, the result of such decisions can have devastating consequences. For example, once the filter is designed into the system, what often occurs is that performance is not what was expected. Worse yet, it can reach production before the problem is traced to the filter, after which changes become a nightmare, causing delays and adding cost.

As Anatech Electronics has learned over more than 30 years in the filter business, this problem is common, in part because filters can sometimes be considered mundane components rather than the precision devices that they are. Filters of any kind are complex devices whose performance depends on many factors that require a great deal of knowledge to design and manufacture. Taking them for granted is always a bad idea. A better one is to spend a little time on the phone with the filter manufacturer who can identify issues in a required filter specification and recommend a better solution than the one originally proposed.

The result of this effort might well cost no more than the off-the-shelf filter, either. This is because filter manufacturers create libraries of hundreds or thousands of different filter designs. There is very little chance one of these designs will not come close to what a customer needs, so meeting a “unique” requirement is not difficult and often requires making only a few minor changes.

Even if those changes are more substantial, the increase in cost, if any, is mitigated by the fact that the problem mentioned earlier won’t occur, and the performance of the system may be better than anticipated. What’s more, some of these “custom” filters require only different connectors, a different type of enclosure, or some other small modification.

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