Other resources you might find helpful.

Hardware Filter Manufacturers

Wainwright Instruments - A manufacturer of RF & Microwave Filters since 1979 under the guidance of American filter specialist C. R. Wainwright – one of the pioneers in the filter business.

Publishing Companies

PlanetEE - A comprehensive technical resource, design, solution, and commerce center for the worldwide community of electronic design engineers and engineering management. Contains communities, workshops, events, etc...

Vector Network Analyzer

OMICRON LAB - Manufactures a new Vector Network Analyzer for professionals and teachers in the electronic labs/fields. The Bode 100 performs the job of multiple devices - Gain Phase Meter, Vector Network Analyzer, Impedance Meter, and Sine Wave Generator - for the price of one.