The following diagram shows testing result according to GJB-151A RE102 of certain radar chassis, we can see interference at most frequency points between 30MHz and 100MHz exceeds the limit.
In order to judge if the radiated interference is mainly caused by chassis leakage, and highlight the points, we disconnect the cable and control lines, only leave power lines for which interference control measures already taken to keep the equipment being able to work, then we test the radiated interference again, still interference at most frequency points go beyond the limits, this means there is obvious leakage from chassis or power lines.
We moved a near field magnetic field probe( connected with spectrum analyzer) along the chassis slit or holes to fix the leakage point and observe the leakage situation at different frequencies. We found comparatively high field strength at the slit near power supply, and we pasted one conductive metal belt which was connected with the chassis metal surface with well conductivity, then there was obvious decrease of the radiated field strength. The result shows there is leakage from the chassis.
Measures taken could be adding conductive strap to make the chassis slit meeting the requirement, or reducing the interval space of the stud used to assembly the chassis( along the slit). Once the chassis is well shielded, connect the shielded cable and control lines, retest RE102, we can see there is obvious improvement, shown as Figure2, but still some points go beyond the limit.
Figure2 Still some points go beyond the limit
The result shows there is leakage from cables and control lines. This is because the electromagnetic interference of the shielded unit coupling to the wires connecting I/O interference and power lines generates interference current and transferred to the outside of the shielded unit. It is the same that outside EMI interference could enter into the unit through wires connecting I/O interference and power lines and cause radiated interference to the inner equipment of the unit.
Corrective measures could be:
a) Replace the current used connector with filtering connector.
b) Reset the input and output lines of EMI filter to prevent coupling between them, so as to secure the filtering effect.
c) The shields of the cable should be seamlessly connected with the cabinet at the entrance.
d) Insert all the pins of connector to the ground potential to prevent antenna effect.
3. Final test result of RS102