Avoiding Costly Pitfalls when Specifying Filter Performance
By Jeff Sloane, Integrated Microwave

Passive filters offer an attractive and efficient option for system designers. However, many misconceptions exist regarding filter specification and performance. Take this brief quiz to test your filter knowledge:

1. T/F: For a given minimum bandwidth requirement, more poles always mean more insertion loss.
2. T/F: More poles always add more cost to a filter.
3. T/F: The bandwidth (as a percentage of the center frequency) determines the group delay variation.
4. T/F: Cascading two 3-pole filters gives a 6-pole response, and the sum of the losses.
5. T/F: Ceramic filtering is unavailable above 6 GHz.
6. T/F: Larger ceramic resonators cost more, but allow higher Q.
7. T/F: Lumped-element filters cost more, but yield higher Q than ceramics.
8. T/F: Space Qualified (Class S) filters are more costly, but available.

The answer to every question is FALSE.
1. The most cost-effective way to specify most filters is using the minimum bandwidth method: indicate what you have to pass, and what you want to reject. Only the spacing between the passband and the rejection points dictates the number of poles. In many cases, adding a pole will build rejection faster than it will degrade the insertion loss, giving the N+1 pole solution less insertion loss. Many engineers will “pad” the bandwidth-of-interest beyond what is really needed for “protection,” and this can add expense. By choosing a filter company that performs 100% testing on 100% of the delivered hardware, you take the guesswork out of the passband specification process, which means you don’t need to specify extra bandwidth to “cover” your needs.

2. For most passive filter solutions, much of the manufacturing cost of a filter is determined by the tuning difficulty, which is primarily dictated by the manufacturing margin needed to meet the specification. There is always a compromise between passband specifications and stopband specifications. A 5-pole filter may be almost impossible to tune because the rejections cannot be met while maintaining the minimum bandwidth, while a 6-pole filter might be quite easily tuned to meet the same specifications. Experienced filter suppliers have accurate models to predict this behavior, and charge less for easy-to-tune solutions. The old “dollars-per-pole” thinking does not produce the most cost-effective filter solution or the most accurate costing for the manufacturer.

3. Group delay variation is based on the absolute bandwidth, not the percentage bandwidth. A 5000 MHz filter with a 100 MHz 3dB passband has the same group delay variation as a 10 GHz filter with the same absolute bandwidth. That’s good news for digital modulation users who worry about front-end group delay characteristics. If you are mixing down, the front-end won’t be any worse than the IF filtering.

4. Cascading filters may sometimes reduce cost, but often creates other issues. Keep in mind that the combined losses of the multiple filters are actually greater, even theoretically. For most filter types, the realizable Q of a 6-pole filter is higher than that of a 3-pole filter, so its efficiency is higher.

5. Ceramic filter solutions to 10 GHz, and sometimes higher, are available from Integrated Microwave. The Q is lower at these frequencies and the resonators are limited to the smaller profiles – also reducing the Q. But these higher-frequency ceramic filters still provide excellent performance, especially for wider bandwidths (10-25%).

6. Making filters fit in tight spaces can drive the cost in several ways. Larger ceramic resonators do provide higher Q, but sometimes actually cost less. Integrated Microwave charges less for 5mm-based filters than for equivalent 3mm or 4mm filters. And if the improved Q makes the filter easier to tune, this can lower the cost even further.

7. Ceramic filters usually provide higher Q than lumped element options in the bands where they are available, and therefore offer lower loss. They are also far less labor-intensive, and therefore cost less.

8. There is no standard “Space” environment or challenge; filters for Space applications are designed to meet specific mission requirements. Space product pricing is usually driven by the testing/screening requirements. IMC can help you specify filters for Space applications when the mission parameters are well characterized. IMC is a proud supplier of front-end filtering/diplexing for the Mars Orbiter and Mars Lander.

Founded in 1982, Integrated Microwave is a leading designer and manufacturer of IF, RF and microwave filters, multiplexers and integrated assemblies for the aerospace, defense, test equipment and communications industries.

Each Integrated Microwave filter is individually designed to customer specifications. All requirements are completely modeled for optimum performance, size, weight and price, underscoring an ongoing commitment to strict quality standards and conformance to customer requirements with consistent on-time delivery. Integrated Microwave’s factory meets the requirements of AS-9003, and is located in San Diego, California.

Integrated Microwave
www.imcsd.com
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