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Improve your wideband RF design

Posted: 01 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:RF designer? RF engineer? microwave device? RF product? design assembly?

By Sam Benzacar
Anatech Electronics

The devil is in the details. For system engineers who work for U.S.-based manufacturers of RF and microwave products, that seems to be the message they are painfully discovering. As products get rushed into the marketplace and put to the test by wireless-service providers and consumers, shortcomings in frequency management crop up in the form of noise, crosstalk, signal dropouts and even complete transmission interruption.

Much of the design process of subassemblies has now been farmed out to offshore manufacturers. Few local manufacturers can afford to compete when it comes to cranking out circuit card assemblies that perform simple functions, such as frequency generation.

Many onshore designers spend their time at the line-replaceable unit and system level, combining subassemblies to ultimately create the finished goods. The result is a product that looks good in theory, but fails to deliver when put in the field. Such problems arise at the component level, and in cases of interference, the problem usually stems from incorrect choices in specifying filters.

Here are five things to look out for:

  1. Don't make the filter an afterthought. Can you imagine getting your design out in the field only to discover bad reception? This happens all too often, especially in active metropolitan areas, usually because little forethought was given to filter performance. At the start of design, look at the frequencies around the target band and ask what conflicting signals could appear in your specified time frame. Preparing up front for interference will more than pay for itself.

  2. Insist on a filter with sharp cutoff and isolation. A filter screening out a signal just 3MHz away must be able to reject more than 40dB of signal strength outside. Filters today must exhibit sharp cutoff characteristics, and this requirement becomes all the more important as various frequency bands close in on one another. On duplexers, for example, the filters must be especially sharp, with good isolation and as little insertion or return loss as possible.

  3. Don't test the product in too "friendly" an environment. In eagerness to rush a product to market, some companies may conduct environmental testing under artificially ideal conditions.

  4. Retain a filter manufacturer able to change component characteristics on short notice. Even the best of designs are occasionally compromised by a frequency conflict that could not have been foreseen. In such instances, it pays to work with a local component maker that can readily modify its filters. For instance, a manufacturer may have to change the center frequency of a filter to improve the performance of its device within a particular band.
    One cost-effective way to do that without scrapping the design is to work with a filter manufacturer that can quickly adapt a standard offering. As an example: A design might have called out a standard filter at 915MHz. But once tested in the field, it might be determined that what is actually needed is a center frequency of 920MHz. A filter manufacturer should be able to quickly shift the frequency. It's difficult to obtain that kind of response from a distributor who is getting parts offshore.
    For the same reason, working with a local filter manufacturer can enable last-minute mechanical-design changes when needed. Such flexibility simplifies cabling and lowers manufacturing costs.

  5. Consider filter size with the future in mind. As circuit card real estate declines and performance increases, old algorithms aren't enough. It's important to rely on a manufacturing partner committed to developing and using the latest algorithms and techniques to optimize filter size and performance.

About the author
Sam Benzacar
is founder and president of Anatech Electronics, a manufacturer/distributor of custom and standard RF and microwave products. Comments may be sent to

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