MIL-STD-220C
APPENDIX A
The test equipment will typically have 500 terminations and the response of the filter is a measure of how closely the filter matches the 500 of the test equipment. Frequencies at which the filter impedance is close to 500 there is little or no attenuation and full transfer of energy from source through the DUT occurs. With a Pi-filter, as frequencies increase, the ratio of the impedance of the filter to the test equipment increases and thus shows increasing attenuation. This is not just resistance but capacitance and inductance as well. These elements combine to give the distinct response typical of a filter insertion loss measurement. Each behaves differently in the presence of an RF signal. The resistance does not change very much with frequency. However, the impedances associated with the capacitance and inductance does change with frequency.
The capacitance has a property called capacitance reactance which becomes smaller as the frequency increases. This is an inverse relationship. So as a signal is passed down a conductor that has a capacitor connected between the conductor and the ground reference, the capacitor reactance with
increasing frequency will become smaller and smaller with a fixed capacitance value so that the RF signal is reduced, attenuated, so that the resultant signal reaching the load is diminished.
The inductance has a property called inductive reactance that increases as the frequency is raised. This is a direct relationship to the frequency for a specific inductance. In this case an inductance in series with the RF signal will impede the signal causing losses or delays. An example of this is that a foot of 24 gauge wire with its internal self inductance will cause a signal delay of 1 nanosecond. All conductors and conductive surfaces have a self inductance.
As noted above, another area of concern is the condition of the plating on the shell of the connector, the plating on the ground spring and its connection to the connector shell, the plating of the array and its interface with the spring, and the internal ground plane in the array (number of layers and thickness of the electrode).
A.2 Test condition: In order to measure the insertion loss of a filter connector, it is necessary to attach it to a bulkhead that is part of a shielded enclosure. The bulkhead needs to be attached to shielded enclosures at each end. The shielded enclosures and bulkhead need to be noble metal plated (silver is a very good choice) and assure that there is good conductivity between the bulkhead and each chamber. The input leads need to have their shields terminated as close and with as low an inductive clamp to each chamber's ground and as close to the bulkhead as possible. Before the connector is attached, the network/spectrum analyzer needs to be normalized. This is accomplished as noted above by attaching
the bulkhead and connecting the input and output leads together. Select the normalize function of the instrument being used through the frequency range of interest.
Once normalized, the connector shell needs to be attached to the appropriate bulkhead chosen for the shell size of the connector being tested. This bulkhead needs to be designed for the shell size to be tested with assurances that there is the maximum surface available for the connector flange. With all things properly attached and all elements of the connector design being in accordance with the guide lines listed above, the performance should look like the trace on figure 3 and the traces shown in
figure A-I.
11
For Parts Inquires call Parts Hangar, Inc (727) 493-0744
© Copyright 2015 Integrated Publishing, Inc.
A Service Disabled Veteran Owned Small Business