Periodically, a filter which has been subjected to numerous iterations of the measurement/fitting/tuning process does not meet the required performance specifications. Performance failure could result from a variety of possibilities. A major cause of problems typically is attributable to a defective coupling iris, usually corrected by iris rework or replacement at the pre-weld level of test. The relative ease of reworking an individual iris instead of an iris which is an integral part of a cavity underscores the advantage of the cylinder and plate construction technique.

When the measured results indicate an out of specification return loss, the output of the filter fit program is analyzed to determine the iris coupling values. It is desired that the measured coupling values be within 0.5 MHz of the design values. If an iris for a failed filter does not fall within this tolerance, the troubleshooting/rework process begins with the suspect iris. Variations in iris size of only 1 mil cause a substantial coupling change. To increase the size of an iris, there is usually a sufficient silver plating buildup around the iris opening that may be removed by polishing. As much plating as deemed necessary is removed based on the analysis of the test results.

If test results indicate the iris size must be increased such that the thin layer of silver plating has to be removed to the point that the Invar is exposed, the...