MIL-STD-461G CS104: Conducted Susceptibility, Antenna Port, Rejection of Undesired Signals

MIL-STD-461G CS104 is a receiver front-end susceptibility requirement applicable from 30 Hz to 20 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. For Navy ships and submarines, MIL-STD-461G CS104 is applicable for all receivers. The applicable frequencies are a function of the front-end design of the unit being evaluated. 

The EUT shall not exhibit any undesired response beyond specified tolerances when subjected to the limit requirement provided in the individual procurement specification.

MIL-STD-461G CS104 is used to determine the presence of spurious responses 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.

Front-end rejection 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. These characteristics generally should be determined by the test. The maximum signal input that the receiver can tolerate without overload needs to be known to ensure that the test levels are reasonable. The bandpass characteristics of the receiver are important for determining frequencies near the receiver fundamental that will be excluded from testing. Requirements for this test are often expressed in terms of a relative degree of rejection by specifying the difference in level between a potentially interfering signal 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 procedure is to apply out-of-band signals to the antenna port of the receiver while monitoring the receiver for degradation. Figure A-14 shows a general test setup for this test. There are two common techniques used for performing this test using either one or two signal sources. For the one signal source procedure, the signal source is modulated with the modulation expected by the receiver. It is then swept over the appropriate frequency ranges while the receiver is monitored for unintended responses. With the two signal source procedure, a signal appropriately modulated for the receiver is applied at the tuned frequency of the receiver. The level of this signal is normally specified to be close to the sensitivity of the receiver. The second signal is unmodulated and is swept over the appropriate frequency ranges while the receiver is monitored for any change in its response to the intentional signal.

The two signal source procedure is more appropriate for most receivers. The one signal source procedure may be more appropriate for receivers that search for a signal to capture since they may respond differently once a signal has been captured. Some receivers may need to be evaluated using both procedures to be completely characterized.

For frequency-hopping receivers, one possible approach is to use a one signal procedure as if the EUT did not have a tuned frequency (include frequency scanning across the hop set) to evaluate the jamming/interference resistance of the receiver. If a frequency-hopping receiver has a mode of operation using just one fixed frequency, this mode should also be tested.

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 responses 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.

A common error made in performing this test procedure is attributing failures to the EUT which are actually harmonics or spurious outputs of the signal source. 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-14. 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, operating frequencies of the receivers, degree of rejection (dB), and frequencies and threshold levels associated with any responses.

 

Test Setup

FIGURE A-14. CS104 General test setup.




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