IEC 61000-4-6 is widely used for compliance testing of RF immunity of apparatus for the EMC and R&TTE Directives. It applies an RF stress over the frequency range 150kHz–80/230MHz to the cables connected to the equipment under test (EUT) to determine its immunity to this stress.
The principle of the test is to excite both electric and magnetic disturbance fields within the EUT by applying the stress in common mode with respect to the ground reference plane (GRP) to certain selected cables entering it (Figure 1). The stress is applied through a defined source impedance of 150 Ω, which is also taken to be the impedance of other cables connected to the EUT. Therefore we must use networks to stabilise this impedance or to decouple it, so as to ensure that unwanted variations have little effect on the test; and also make sure that the layout of the test is controlled so that variations due to stray coupling are minimised.
Summary of best practice
The drawing below is a reproduction of Fig1 of IEC 61000-4-6. The annotations should be self-explanatory. An example is shown as well.
Choice of transducers
The standard allows three methods for injecting the stress, and assumes that results from each will be equivalent, although it is now stated that the CDN method is preferred:
•CDN (including direct injection)
•EM-Clamp
•Bulk Current Injection (BCI) probe
CDN
The Coupling-Decoupling Network (CDN) is designed to couple the disturbance signal directly to the EUT cable while at the same time preventing it from passing towards the AE (associated or auxiliary equipment). It must also provide a fixed common mode source impedance towards the EUT. The discrete components allow a more compact assembly than the other transducers to be discussed.
Advantages
•Its prime advantages are near-perfect decoupling of the AE and low uncertainty of the applied stress.
•Minimal power is required and there is minimal radiation or environmental influence.
•Additionally it defines a 150Ω common-mode cable impedance, formed from the 50Ω generator impedance in series with 100Ω resulting from the injection resistors in parallel. This impedance damps cable resonance to increase the repeatability of the test and approximates to real-life, giving a very credible test.
Disadvantages
•It is invasive, that is it requires an electrical connection to the cable shield if there is one, or to each core of an unshielded cable. Accordingly different networks are required for different cables, increasing the capital investment required for general test house use.
•This disadvantage has been partially overcome by versatile CDNs in which a variety of CDN configurations can be achieved by patching links within the AE and EUT connectors. The CDN pictured here may properly be used with coaxial and shielded cables of 1 to 6 cores, and with unshielded cables of 1,2,3 and 6 cores.
•Serious errors may result if an ordinary CDN is used with fewer than its intended number of wires, such as for instance using an M3 CDN for a mains port with only live and neutral.
EM-Clamp
The EM-clamp is a clamping device that subjects the cable under test to both capacitive and inductive coupling of the RF stress. It was invented largely for this test by Bersier and Ryser at the Swiss PTT.
Advantages
•Its principal advantage is that it is entirely non-invasive. No connection need be made to the cable under test.
•Its second main advantage is that it allows adequate decoupling of the AE at high frequencies. The design is arranged so that the capacitive and inductive coupling paths reinforce one another at the EUT end, and cancel at the AE end. This gives the clamp about 10-15dB of directivity above 10MHz.
•Thirdly, it is reasonably power-efficient, although not as good as a CDN; for the same stress, about 6dB more power is needed.
Disadvantages
•Because it uses a series of ferrite sleeves to provide the inductive coupling, it is quite long, and to provide good capacitive coupling it has a relatively narrow inside diameter. This makes it bulky to use and restricts its application for short or large diameter cables.
•Below 10MHz its directivity is negligible and therefore the AE low frequency common mode impedance is not decoupled.
•It does not provide an accurate source impedance of 150Ω across the frequency range.
BCI probe
The current injection probe (or Bulk Current Injection, BCI, probe)acts as a current transformer whose secondary is the cable under test; it provides inductive injection only.
Advantages
Its main advantage, and the reason it is widely used by many labs, is that it is both convenient and non-invasive. Because it is compact and can be made with quite a wide aperture, it can be used on virtually any cable, even short runs with limited access. This makes it the transducer of choice for in-situ tests.
Disadvantages
•Balancing this practical advantage are several technical failings. There is absolutely no decoupling of the AE, since the current induced on the cable must flow both into the EUT and the AE. Therefore the AE is being tested just as much as the EUT.
•The applied stress is very dependent on cable layout and AE impedance. The current flowing into the EUT is determined by the impedance of the cable, which acts as a transmission line at high frequencies and so may have standing waves due to mismatches, and by the impedance to the reference plane of the AE. So this offers the highest uncertainty and least repeatability of all the methods.
•The probe is lossy and has a high power requirement. The higher its internal turns ratio the more power is needed, but low turns ratios affect the coupling of the probe to the cable and are effectively forbidden by the standard.
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