MIL-STO-220C
4. GENERAL REQUIREMENTS
4.1 Test setup (no load). The test circuit shall be arranged as shown on figure 1 using a network or spectrum analyzer. The test setup shall be capable of indicating a repeatable insertion loss within
±1.0 dB over the required frequency range. All test equipment shall be well shielded, and shall be filtered to the extent that leakage, either conducted or radiated, shall not affect the output level and sensitivity, respectively, needed to make the required maximum insertion loss measurement, see section 5 (method of test) for details.
4.1.1 Test setup (with load): To measure the insertion loss of a filter under conditions of load, current, or voltage, the OUT shall be isolated from the network analyzer with the use of dc blocking capacitors. A pair of appropriate LC networks will be connected to the OUT to inject the dc current or dc voltage into the filter. Normalization of the instrument, with the dc load applied will be completed through the frequency range of interest prior to making any measurements.
Caution: The direct current (dc) source used in making insertion loss measurements with
full load applied shall be a floating dc source and shall not be connected to ground.
4.1.2 Shielding test. Set up the equipment for the filter out condition of insertion loss measurement (see 5.2.2). The test setup shall then be connected for the filter in condition by substituting for the component, a solid brass or copper plate at least .25 inch thick (6.35 mm) with plane faces at least 2.38 inches (60.45 mm) wide in all directions. This plate shall be placed across the coupler and center conductor of the measuring equipment assembly so that the signal source and load are completely short circuited by the faces of the plate. The insertion loss shall be at least 80 dB greater than the insertion loss to be measured.
4.2 Test equipment.
4.2.1 Output signal. Any instrument selected for measurement shall be capable of maintaining an output signal that is within ±1 percent of nominal over a 2 minute period.
4.2.1.1 Measuring a filter. Complete characterization of filters is typically achieved with sweep- frequency measurements. The most commonly measured filter characteristics are insertion loss and bandwidth. Another common measured parameter is out-of-band rejection. This is a measure of how well a filter passes signals within its bandwidth while simultaneously rejecting signals well outside that same bandwidth.
4.2.1.2 Error correction for accurate passband measurements. Variation from a constant amplitude response within the filters bandwidth results in signal distortion. Error correction is often essential for accurate measurements of filter pass bands. When a filters pass band is measured with a network analyzer without calibration, the response may vary considerably, depending on the network analyzer and test cables used.
When the same filter is evaluated after doing a response calibration (normalization), the test systems transmission-tracking frequency-response error is removed from the measured response, resulting in a much narrower amplitude-distortion window.
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