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
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.
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