Global Sources
EE Times-Asia
Stay in touch with EE Times Asia
EE Times-Asia > RF/Microwave

Five filter tips for better RF design

Posted: 02 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:frequency management? wideband RF design? EMF testing?

Benzacar: As RF and microwave products get rushed into the market, shortcomings in frequency management crop up.

The devil is in the details. As RF and microwave products get rushed into the market and put to the test by wireless service providers and consumers, shortcomings in frequency management crop up through noise, crosstalk, signal dropouts and even complete transmission interruption.

Many 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 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 upfront for interference will more than pay for itself.

2. Insist on a filter with sharp cut-off 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 cut-off characteristics, and this requirement becomes more important as various frequency bands close in on one another.

3. Don't test the product in very "friendly" environment. In eagerness to rush a product to market, some companies may conduct environmental testing under artificially ideal conditions. Just because a filter may breeze through EMF testing within a lab, it doesn't mean that it won't cause interference or fail to preclude spurious signals on a street corner.

4. Retain a filter manufacturer that can 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. For example, a design might have called out a standard filter at 915MHz. Once tested in the field, however, it might be determined that a center frequency of 920MHz is needed. 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 not getting parts locally.

Moreover, working with a local filter manufacturer enables 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.

- Sam Benzacar
Founder and President, Anatech Electronics Inc.

Article Comments - Five filter tips for better RF desig...
*? You can enter [0] more charecters.
*Verify code:


Visit Asia Webinars to learn about the latest in technology and get practical design tips.

Back to Top