MIL-STD-461G CS103: Conducted Susceptibility, Antenna Port, Intermodulation

MIL-STD-461G CS103 is a receiver front-end susceptibility requirement applicable from 15 kHz to 10 GHz for equipment and subsystems, such as communications receivers, RF amplifiers, transceivers, radar receivers, acoustic receivers, and electronic warfare receivers as specified in the individual procurement specification. 

MIL-STD-461G CS103 mandates that the EUT shall not exhibit any intermodulation products beyond specified tolerances when subjected to the limit requirement provided in the individual procurement specification.

This test procedure is used to determine the presence of intermodulation products that may be caused by undesired signals at the EUT antenna input ports.
 

Procedures

No test procedures are provided in the main body of this standard for this requirement. Because of the large variety of receiver designs being developed, the requirements for the specific operational characteristics of a receiver must be established before meaningful test procedures can be developed. Only general testing techniques are discussed in this appendix.

Intermodulation testing can be applied to a variety of receiving subsystems such as receivers, RF amplifiers, transceivers, and transponders.

Several receiver front-end characteristics must be known for proper testing for intermodulation responses. These characteristics generally should be determined by the conducted test. The maximum signal input that the receiver can tolerate without overload needs to be known to ensure that the test levels are reasonable and that the test truly is evaluating intermodulation effects. The bandpass characteristics of the receiver are important for determining frequencies near the receiver fundamental f_o that will be excluded from the test. Requirements for this test are generally expressed in terms of a relative degree of rejection by specifying the difference in level between potentially interfering signals and the established sensitivity of the receiver under test. Therefore, the determination of the sensitivity of the receiver is a key portion of the test.

The basic concept with this test is to combine two out-of-band signals and apply them to the antenna port of the receiver while monitoring the receiver for an undesired response. One of the out-of-band signals is normally modulated with the modulation expected by the receiver. The second signal is normally continuous wave (CW). Figure A-13 shows a general setup for this test.

 

Test Setup

For applications where the receiver would not indicate interference without a receiver signal being present, a third signal can be used at the fundamental. This arrangement may also be suitable for some receivers that process a very specialized type of modulation which would never be expected on an out-of-band signal. An option is for the two out-of-band signals to be CW for this application.

The frequency of the two out-of-band signals should be set such that f_o = 2f1 - f2 where f_o is the tuned frequency of the receiver and f1 and f2 are the frequencies of the signal sources. This equation represents a third-order intermodulation product, which is the most common response observed in receivers. f1 and f2 should be swept or stepped over the desired frequency range while maintaining the relationship in the equation. It is important to verify that any responses noted during this test are due to intermodulation responses. Responses can result from a lack of rejection of one of the applied signals or from harmonics of one of the signal sources. Turning off each signal source in turn and noting whether the response remains can demonstrate the source of the response.

For receivers with front-end mixing and filtering in an antenna module, the test may need to be designed to be performed on a radiated basis. All signals would need to be radiated and assurances provided that any observed intermodulation products are due to the receiver and not caused by items in the test area. The EMITP would need to address antenna types, antenna locations, antenna polarizations, and field measurement techniques. This test would probably need to be performed in an anechoic chamber.

MIL-STD-461G APPENDIX A 223 For frequency hopping receivers, one possible approach is to choose an f_o within the hop set and set up the signals sources as described above. The performance of the receiver could then be evaluated as the receiver hops. If the frequency hopping receiver has a mode of operation using just one fixed frequency, this mode should also be tested. A common error made in performing this test procedure is attributing failures to the EUT which are actually harmonics of the signal source or intermodulation products generated in the test setup. Therefore, it is important to verify that the signals appearing at the EUT antenna port are only the intended signals through the use of a measurement receiver as shown in Figure A-13. Damaged, corroded, and faulty components can cause signal distortion resulting in misleading results. Monitoring will also identify path losses caused by filters, attenuators, couplers, and cables. Typical data for this test procedure for the EMITR are the sensitivity of the receiver, the levels of the signal sources, frequency ranges swept, and operating frequencies.

FIGURE A-13. CS103 General test setup.




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