Emissions Tests

Emissions standards

The EMC Directive lists many standards applicable to different product types. There are therefore a wide range of 'product specific' standards plus a set of 'generic' standards to be used only when no product specific standard matches the product to be tested. 
In addition, there are other standards to cover military, automotive, aerospace and the US markets. A summary of the common standards is available here.

Conducted

Interference generated by the EUT (Equipment under test) may be connected via cables to victim products, eg  a serial lead may transfer interference in via the RS232 port or a mains cable, via the mains distribution network to almost any other mains powered product in the vicinity.

An EMC analyser can measure that interference, but to connect the analyser to the interference signal, a transducer or interface is required. For mains interference measurements, a LISN (Line Impedance Stabilisation Network) is used. See LISN for more information on what this is.

For other types of cables, CDNs (Coupling-Decoupling Networks) are used. In each case the interference voltage is coupled off the cable and connected to the analyser. Both the LISN and CDN (or ISN) are impedance stabilisation devices which ensure that measurements are repeatable.

An alternative device that can be used in place of a LISN is the voltage probe… not as accurate and prone to higher noise level, it is an approved technique in situations where LISNs canot be used, eg with EUTs that draw high current (>100A). 

Because the measuring device (analyser) is directly connected via copper to the source of the interference, these measurements are generally accurate and have good integrity.

Radiated 

Radiated is not so straightforward. The 'standard' technique as required by CISPR16 is to use an OATS (Open Area Test Site). This is exactly what the name suggests. Other sites such as test cells and anechoic chambers are secondary sites and must be calibrated/correlated against an OATS.
OATS are specified because they are free of reflections. RF will reflect off anything metallic and/or conductive. Reflected signals interfere with the signal that has travelled direct from the EUT to the antenna, and can cause cancelation effects, significantly reducing the apparent signal strength. The key characteristic of OATS is the lack of these reflections, thus ensuring consistent and reliable results. There are other issues related to the reflection off the ground, see here for details and here for a real example of a measurement corrected for test site reflections. The other main issue with radiated testing is the ambient (background) noise. This can easily mask the signal from the EUT making detection and measurement difficult. See here for a full study of these problems.
Laplace is unique in being able to offer cost effective and practical solutions to both problems, (reflections and ambient).
The equipment required for radiated emission measurements will need to include an EMC analyser, plus an antenna(s) or test cell. The best choice depends on budget, size of product (small products up to 60cm cube can be placed in a test cell, larger products will have to use either an OATS or an anechoic chamber). OATS are the lowest cost option, but you need a suitable space and be aware that ambient will be a disturbing factor. OATS will invariably need calibrating and the ERS provides this facility.

Harmonics & Flicker

Modern electronic equipment, and electronic lighting controls and ballasts, are very unfriendly to the mains supply system. This is because they draw their power in 'chunks' instead of drawing it as a continuous flow. This means that the current consumption is pulsed with a maximum usually occuring near the peak of the voltage waveform. The overall effect of this is that current is taken at higher frequencies than the nominal 50 or 60Hz. These higher frequencies are the harmonic frequencies. The mains supply system is simply not designed to supply current at these higher frequencies. They cause inefficiencies and reliability problems.

So the Harmonics standard (IEC61000-3-2) imposes limits on the levels of harmonics that a product can create. This requires measurement of the current consumption of a product up to the 40th harmonic (2KHz).

 The Flicker standard (IEC61000-3-3) addresses the problem where certain products may intermittently take power from the mains in such a way as to cause the voltage to dip, thus causing any lighting in the vicinity to flicker. Flicker occuring at a frequency in the range 5Hz - 15Hz is well known to be unpleasant and may cause epileptic fits in some who are susceptible. So this standard measures the current variation over time (up to 2 hours) and uses statistical techniques to derive a measure of flicker severity. Note that the standard states that this measurement is not required 'if the product is unlikely to cause flicker' (which applies to most products). BUT note that this standard also has a requirement to measure 'in-rush' current...... and this does apply to all products that are connected to the mains.

The AC2000A from Laplace is a combined Harmonics and Flicker analyser, compliant with the latest version of the standards, simple to use and very cost effective. It includes 'inrush current' measurement and is a very useful power analyser in it's own right.