Calibration and use of artificial mains networks and absorbing clamps
Proper use of transducers for CISPR-based emissions measurements
Executive summary of project
1. Introduction
2. Summary of results and best practice recommendations
2.1. AMNs/LISNs
2.1.1. Results
2.1.2. Use
2.1.3. Calibration
2.1.4. Design of unit
2.2. Absorbing clamps
2.2.1. Results
2.2.2. Use
2.2.3. Calibration
2.2.4. Design of unit
Introduction
The report of which this document is a summary has been written to advise test engineers and others on the best practice techniques for the use and calibration of transducers for RF conducted emissions and disturbance power measurements. These measurements are inherent in many European EMC standards that are based on the CISPR emissions methods, and form an important part of testing to meet the compliance requirements of the European EMC Directive. There are two particular aspects of these tests which are of great relevance to equipment manufacturers who must make a declaration of compliance based on the test results:
- accuracy of measurement: allowing too great a tolerance on the measurement result has the effect of artificially tightening the emissions limits, which translates into a greater cost for the manufacturer to ensure that his product emits below the limit level minus the measurement uncertainty. Uncertainties of measurement should be minimised as far as possible.
- repeatability of measurement: a manufacturer should expect that, if his compliance statement is ever challenged, he can have a product re-tested at a different test facility to the same standard as the original compliance test, and achieve the same measurement result. Otherwise, allowing for uncertainties due to lack of repeatability has the same cost impact as discussed above.
These factors are directly affected by a number of parameters that relate to the test method and the test equipment. Among them are the calibration and method of use of the transducers. Of these transducers, the most important are the artificial mains network (AMN) or line impedance stabilising network (LISN) for the conducted emissions test on the mains lead up to 30MHz, and the absorbing clamp for the disturbance power test on the mains and other leads above 30MHz. The document therefore sets out best practice in the use and calibration of these items, with the intent of minimising uncertainties which can be attributed to these particular aspects.
The project of which the document is the result has drawn on a number of sources of information:
- A literature review to investigate the existing state of the art in use and calibration of these items;
- A questionnaire sent to a number of UKAS accredited EMC test laboratories, asking for their experience in using and calibrating the items;
- A programme of experimental work using a variety of clamps and AMN/LISNs to determine and compare the factors which may contribute to their uncertainties.
This programme of work was carried out by the partners in the project, Schaffner Chase EMC and the National Physical Laboratory, both of whom are accredited by UKAS to perform these types of calibration.
Summary of results and best practice recommendations
These recommendations result from our investigations as described in the main part of this guide, as well as other sources that are referenced where relevant. They are presented roughly in order of priority. The comments are intended to amplify the instructions contained in the various CISPR standards; test procedures and design aspects that are already commonplace or typical are not discussed.
AMNs/LISNs
Results
The anticipated contributions to measurement uncertainty for calibration of and testing with the standard CISPR 16-1 AMN/LISN have been systematically investigated. The uncertainties referred to below are expanded uncertainties for k = 2. The investigation has shown that
- calibration of AMN/LISNs by different organisations can be reproduced to within 2% for impedance and 0.1dB for insertion loss, provided that the correct method is used and certain straightforward precautions are taken
- the five commercial units investigated are generally within the CISPR 16-1 specification, with a few excursions outside the ±20% impedance limit at the frequency extremes
- over the frequency range 25kHz to 15MHz it is possible to achieve test results with an expanded uncertainty of ±2dB, assuming typical system contributions, with all five AMN/LISNs; the AMN/LISN itself is not a major contributor to this figure
- the uncertainty degrades slightly below 25kHz, principally because of worsening isolation from the mains supply impedance variations
- the uncertainty degrades substantially above 15MHz to more than ±3dB, due to several factors associated with the AMN/LISN's design, construction and use; if a vertical rather than horizontal ground reference plane is used, with most designs of commercial AMN/LISN this figure may be doubled
- even greater variations are possible above 15MHz if tight control is not exercised over several aspects of the test setup and layout; the amplitude of the variations depends on the coupling modes and source impedance of the equipment under test, and cannot easily be factored into the uncertainty quoted above
The investigation has made it possible to recommend improvements to best practice in testing and calibration, and certain changes to the design of commercial AMN/LISNs.
Use
positioning on top of a horizontal ground reference plane (rather than against a vertical ground reference plane) is preferred
use the shortest, most direct wide strap from the earth bonding post to the ground reference plane
do not raise the unit on its feet (if provided)
never switch in the earth inductor (if provided), either accidentally or deliberately; if possible, modify the unit so that it cannot be accidentally selected
ensure that the mains (and other) cables never drop to the ground reference plane but are spaced from it by >10cm or as per the standard requirement
prefer standard length mains cables wherever possible
Calibration
a standard design of test adaptor (as recommended in the appropriate section) should be adopted for the calibration
both the impedance and insertion loss measurements should pay careful attention to the earthing arrangements to ensure repeatability and reproducibility
the correct way to make the insertion loss calibration is to feed the AMN/LISN and measuring system in parallel through a Tee adaptor, providing effectively a zero source impedance
although impedance and insertion loss calibration results need only be reported at spot frequencies, the measurements should be swept in frequency to detect any resonances
impedance measurements in the range 9kHz up to 25kHz should be made with the mains input port terminated with open circuit, short circuit and 50W; above this frequency an open circuit termination is sufficient
non-mandatory tests can usefully be performed by a calibration laboratory to ensure proper and safe performance of the unit, including isolation between the mains input port and the EUT port, isolation between lines, and DC or AC 50 Hz resistance
Design of unit
the enclosure construction should emphasize a low impedance bond to the ground reference plane
there should be minimum spacing between the reference earth connection(s) and the bottom of the unit; with no extendable feet
the EUT connector should be mounted near the top of the front panel, upside down (for BS1363 13A types) to encourage the mains lead to exit away from the ground reference plane and to give a short connection from the earth pin to the reference earth connection(s)
a reference earth connection close to the EUT port L and N pins should be provided for calibration
no earth lead inductor should be included
provision of locally switched 50W loads is preferable to external loads
the CISPR 50mH inductor L1 should be resistively damped if it is a single solenoidal winding, and should not be coaxial with L2
the selection of component values for CISPR C2, L2 and R2 is important to ensure that the CISPR impedance specification is met at 9kHz
Absorbing clamps
Results
The expected uncertainty contributions for calibration of and testing with the CISPR 16-1 absorbing clamp have been investigated. The results are:
- repeatability of calibration by different organisations of absorbing clamp insertion loss is generally possible to within ±0.5dB, given control over the method used, and an adequate test site and setup
- the CISPR 16-1 specification allows wide variations in construction, evident in the three units investigated; nevertheless it is possible to achieve test results within ±1dB with the most commonly used clamps on the same EUT up to 300MHz, and within ±4dB up to 1GHz
- uncertainties in calibration and testing below 100MHz are dominated by reflections from the far end of the cable or wire under test, and by the proximity of large conducting objects; as long as testing is not performed within a screened room, an expanded uncertainty for testing of ±2.5dB is achievable with proper control of the test set-up
- uncertainties in calibration and testing above 300MHz are dominated by variability in wire position within the clamp, departures of the clamp impedance from specified values and a worsening of the clamp's output reflection coefficient; with attention to these sources of error, an expanded uncertainty for testing of around ±2.5dB up to 700MHz is possible, degrading to around ±4dB above this frequency
The investigation has made it possible to clarify the important parameters in the test and calibration setups and methods, and to recommend some improvements to best practice in testing and calibration.
Use
apply a secondary absorber at the end of the cable (6 or more large ferrite clip-on sleeve absorbers are acceptable in lieu of a second clamp)
the cable under test should be kept central within the clamp
other objects including personnel should be kept at least 1m away from the setup when the measurement is made
for measurements close to the limit, the clamp output should be taken immediately through a 6dB pad before connecting to the cable to the measuring instrument; this pad can be left out for initial scans
the output cable should extend away from the set-up at right angles, above or to the side rather than straight down, and should carry ferrite absorbers
the test area should not incorporate a ground plane on the floor
Calibration
apply a secondary absorber at the end of the calibration wire (10 or more large ferrite clip-on sleeve absorbers are acceptable in lieu of a second clamp)
the calibration wire should be kept central within the clamp via centralising guide(s), and tensioned to keep it taut
the calibration should be performed in an open area devoid of large metal structures, with no ground reference plane on the floor, and with no objects including personnel closer than 1.25m to the wire when a calibration measurement is made
the calibration wire should have at least 2mm2 cross section, with 4mm2 cross section preferred if the standard is changed to allow this
the clamp output should be taken immediately through a 6dB pad; the output cable should extend away from the set-up at right angles, above or to the side rather than straight down, and should carry ferrite absorbers; this output cable assembly is to be regarded as part of the calibrated equipment
Design of unit
a wire centralising guide should be provided with each clamp as a standard calibration accessory
although our investigation does not give conclusive evidence, it appears that the current transformer should be as close as possible to the end of the clamp's body, and a non-metallic housing should give markedly lower VRC variation and sensitivity to wire position and operator hand capacitance
the BNC connection to the output cable could be brought out on the side of the clamp to encourage the measurement cable to exit perpendicular to the cable under test
