MIL-STD-220C
APPENDIX A
ANALYSIS OF FILTER MEASUREMENTS
A.1 Discussion: In the measurement of the insertion loss of a filter, there are several things that can go wrong. The purpose of this appendix is to identify the errors that can cause a reading to be incorrect. Solutions will also be suggested to assist the technician in determining if the reading is correct or other extenuating circumstances have affected the result. Readings can be either continuous wave (CW),
which is a reading at a specific frequency which is the normal condition of measurement for a production acceptance test, or read from a swept frequency chart. This analysis will be directed to the measurement of insertion loss of a pin in a filter connector. They will apply to other devices as well.
A.1.1 Elements: A filter pin connector consists of as a minimum the following components:
a. A shell of a metal, aluminum, steel or brass, or a composite material, that has been appropriately plated with a conductive finish,
b. A filter element which can be a planar array which is a disk with an appropriate capacitor grade ceramic material. The ceramic planar is constructed in layers as a multilayer printed circuit board (PCB), with active and ground electrodes placed on alternate layers creating a capacitor at each location that corresponds to the contact pattern of the connector arrangement. There also can be a tubular capacitor creating one element of the filter.
c. In addition to the capacitor, there can be ferrite beads placed on each pin to provide the inductive element of the filter.
d. There will be three elements in a planar Pi filter. There will be two capacitor arrays, one on top of the other with a ferrite bead on each pin in between the two capacitors.
e. The outer periphery of the array will have a metalized surface that is connected to all of the ground electrodes inside the array. Similarly, there is metallization inside each hole and on the surface of the array allowing connection to the internal active layers and to the pin of the connector that passes through the array. The surface electrodes are sufficiently isolated from each other to prevent interconnection and shorting to each other.
f. The ground electrode will be connected to the connector internal shell surface using a circumferential spring.
g. The connector is then finished per the standard design configuration of each manufacturer. A.1.2 Details: The measurement of insertion loss is performed by using a signal that is at a frequency
measured in Hz (cycles/sec). This frequency is usually in the range of 1 MHz to 1,000 MHz. Due to the high frequency, the currents created will travel on the surface of the conductors. Hence the quality and thickness of the surface plating will affect the measurement.
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