The Controversial MIL STD 220A

D.O.D. Directive 3222.3 with its clearly stated objectives (Section IV) of operational and built-in design compatibility necessitates a fresh approach to certain inadequate MIL STD´s. One of them is in particular need of revision. MIL STD 220A, with its 50?? reference, is based on assumptions contrary to the facts which are: 1) Interfacial impedances change with frequency, time, and location and can be nearly anywhere on the Smith Chart; 2) Interfacial resonances in the stopband, and eigen and interactive resonances in the passband, render the conventionally predicted performance of the filter illusory. Hence, new, more realistic criteria for filter testing are necessary: I. Compatibility: The test method must be a qualification test and must be able to permit selection of the more insensitive filter: insensitive meaning that in the broad area about the cutoff frequency the filter is relatively unaffected by interface changes. II. Predictability: The filter test method must permit a selection of a filter to be effective as stipulated minimally by the control plan prior to installation of the system and independent of changes in the system. III. Reliability: The filter must not be destroyed by insertion gain in the so-called passband. IV. Practicability: Filters meeting the specs must be economical and practical. V. Simplicity: The test method should be reasonably simple and brief. From the various solutions which have been proposed, two try to overcome fundamentally the limitations inherent in MIL STD 220A: (a) the in situ measurement (b) the worst case measurement.