It calls to mind the old Star Trek episode “The Trouble with Tribbels”.
These little devils were free and very desirable as pets, but suffered
from a non-obvious flaw: they multiplied rather exuberantly and thus became
an unacceptable nuisance.
When you design your own filter using cookbook methods, the risk of
some unacceptable result is also quite real. Although the filter
will more than likely meet the criteria predicted by the software, it is
the unexpected problems that can cause difficulty. Such performance
characteristics as spurious resonances, temperature or humidity drift and
performance, intermodulation, power handling, mechanical stability, etc.
are the easy missing items.
Up front, free design software as offered by our competitors (K&L, Lark, etc) does not include the very pertinent facts of filter life. The attenuation slope characteristics (i.e. the dB attenuation some distance from center frequency) are not symmetrical for real filters. Thus, the free software might require the use of an 8 section filter to meet some attenuation specification, while the actual filter might not achieve the “theoretical” network transformation and thus would really need a 9 section design. You would receive the filter but would not meet your system requirements. In the other direction, it is possible that the design could have been accomplished by a properly configured 7 pole design, with a consequent savings in insertion loss.
At RS Microwave, we pride ourselves on the application of network design techniques optimum for the problem at hand. A proper combination of lumped, distributed, dielectric, evanescent, stripline, microstrip, etc., elements is configured to achieve the specified attenuation commensurate with minimum insertion loss. Our concentration on modeling and simulation enables achievement of predictions to within small margins of error, and thus we can both predict compliance and actually duplicate the predictions. Our internally-developed E-M simulation allows us to predict power handling, approximate intermodulation performance and our modeling results in accurate predictions of loss, attenuation and spurious resonances.
In conclusion, if you want to design filters, don’t try to do it the
easy way… apply to us for a job. Meanwhile, do what you do best,
fly the spacecraft, Captain Kirk, and “leave the filtering to us”.