Page 126 - Power Electronics Handbook

EMC measurement 119

a

To
-+ recording
t equip rn e n t
I RFI source I RFI
measurement
probe

(a)
RFI - Shielded cable
To

recording
equipment

(b) RFI antenna
Figure 4.11 Bench-top RFI testing: (a) enclosure tests; (b) cable tests

effectiveness tests, as shown in Figure 4.11(a). Cable shielding testing on
the bench top, shown in Figure 4.11(b), are also very important, since
problems here are a major cause of system noise, crosstalk and errors. The
cable is used as the receiving antenna, being connected to a spectrum
analyser, the signal coupled into the shielded and earthed cable being low
for an effective shield.
Repeatability of EMC measurements is an important consideration, and
problems which cause non-repeatable readings include the following:
(i) Electric field sensitivity of the receiving antenna, which picks up stray
electric fields coupled to the test engineer and cables. Variation in
this coupling is also caused by the orientation of the various elements.
The problem can be minimised by reducing the electric field
sensitivity of the antenna.
(ii) Screen room interactions, which should be reduced, or at least
variations in the physical layout of the tests should be avoided in
order to duplicate the interactions.
(iii) Poorly maintained and calibrated test equipment.
(iv) Not using correct assembly procedures for the equipment being
tested, when it is dismantled and re-assembled, so that its
performance varies, for example due to the variation in pressure on
joining gaskets.