EM Transducers Conducted (200's)

Section 200 Content Consideration

EMC Hire provide a range of Conducted Emissions Testing Equipment. There are three subsections under the general title Electromagnetic transducers-Conducted.

The first subsection Current Probes and Clamps deals with conducted emissions as measured by current probes and clamps. Most of the current probes are hinged & will open & then clamp around the cable to be measured with a small locking clip. Fundamentally, a current clamp becomes the secondary winding of an effective current transformer whose primary is the cable or conductor to be measured. The device is reasonably immune to outside radiated fields, offering something in the order of 60dBs of isolation. As the conducted immunity testing probe is generally connected to a frequency sensitive 50 Ω measuring instrument, to be of use it needs a conversion factor which will be the ratio of the current flowing in the primary to the voltage measured across 50 Ω from the output of the current probe. This is known as the transfer impedance (Zt) and is generally stated as dBs above one Ohm. A probe having positive values of transfer impedance for example is sometimes said to be amplifying the effect of the primary current but does so at the expense of a relatively narrower spectrum of constant Zt. Similarly, probes with negative Zt. are effectively less sensitive and are used where the primary current is expected to be relatively large. The current clamp comprises two or more ferrite split rings which provide a low reluctance path for the magnetic field to concentrate. A few turns of wire around the ferrite rings are then connected to the output connector of the clamp. The type or density of ferrite material used, the size of the clamp and the number of turns are the major factors in determining the frequency response and Zt. To read the conducted immunity testing probe accurately the two faces must come together neatly with no air gap when clipped together. Any change in the air gap from the original calibration changes the magnetic reluctance & will alter the transfer impedance.

When choosing your current probe having established it is the emission from the cable you are measuring, (not injecting) the first consideration will be its internal diameter, i.e. it must envelop the cable or harness it’s going to measure. The common value of 25-30mm seems to meet most requirements but we do have probes with larger and smaller internal diameters. The second consideration will be the frequency range of operation of the probe. This generally refers to the range over which the Zt is fairly constant or flat. However, the probe will still keep working outside this range but you will have to be aware of a changing factor with frequency when calculating the primary current in your circuit. In computer controlled measurement systems this is not a problem. One other consideration is the possible saturation of the ferrites within the probe when the RF you are trying to measure is sitting on power frequencies up to 400Hz with relatively heavy currents. The primary saturation currents are given in the main specification for each conducted immunity testing probe.

Included with this group is the emission measuring Ferrite Clamps which will be working from 30MHz on up to 1GHz. These look at the conducted emission spectrum, generally on the mains cable, beyond the range in which the LISN generally makes measurements. EMC engineers can generally make a good assessment of the radiated conformity of an item of equipment by simply looking at this RF emission signature being conducted out through the cabling. Items 214-217 cover a range of “skin current” devices which measure circulatory currents set up in conducting material.

The subsection entitled Injection Probes & Calibration Jigs covers some of the immunity tasks on conducting cables under the general specification 61000-4-6. Again, frequency range, internal diameter and this time power handling are important parameters to look for in your selection. Unless you are using other immunity test methods (CDNs & clamps covered in section 500), a calibration jig will be needed to calibrate the power you are forcing into the circuit. (BCI, Bulk Current Injection). Our software has a conducted immunity programme dealing with BCI.

Finally, the subsection General devices for Emissions & Immunity contain line impedance stabilisation networks or LISNs. 10uF Feed-through capacitors; Isolation transformers and hand-held “sniffer”probes. When choosing your LISN, check the current capability with respect to your circuit. Some of the larger current carrying LISNs will need wiring in as a standard 13amp plug will be inappropriate. Be aware of the RF spectrum you are measuring i.e. up to 30MHz for normal commercial specs and 65MHz or 100MHz for some military applications. Decide of course whether you are looking at single or three phase supplies. If you are working with three phase it will generally mean having two LISNs of the same type. These are appropriately priced under different item numbers to give a discount. The EMCO 7405 (item 272) is very handy for locating problem areas over a circuit board by holding the various loops & probes in the vicinity of an emission the end-user can locate which device or tracking is problematic.