RF Immunity Testing - Effect, Standard and Methodology

Introduction to RF Immunity Testing 

The radio frequency (RF) spectrum is becoming more and more congested as more electronic device is expected to work or co exist together with radio transmitters of many kinds. Many locations nowadays especially at urban areas have many fixed transmitters that there contributes to the field strengths in that location.

A product manufacturer is unlikely to know or have control over the actual location of use of his products, and products should be expected to work correctly in any environment that they are reasonably likely to encounter.

It is a requirement of the EMC Directive that any product placed on the market must have adequate RF immunity. However, in order to differentiate itself from its competitors, the manufacturer should be applying this standard to improve the quality of his products irrespective of whether this directive is enforced or not.

The purpose of RF immunity testing is to subject a product to a controlled RF stress that represents the likely level of stress that it might encountered in its operating environment, over a frequency range which is mostly dictated by practical aspects and experience of real-world problems. The test methods are divided into application of stress by conducted coupling, and by radiated field coupling.

Conducted RF Immunity Test

Cable testing is an important method for checking RF susceptibility, and IEC 61000-4-6 specifies the test methods. The frequency used to test is 150kHz to 80MHz. Any method of cable RF injection testing should require that the common mode impedance at the end of the cable remote from the EUT is defined.

Each type of cable should have a common mode decoupling network at its far end, to ensure this impedance with respect to the ground reference plane (GRP) and to isolate any ancillary equipment from the effects of the RF current on the cable. There are a few methods to couple the RF into the equipment under tests. They are:

a) Coupling/Decoupling network(CDN)

b) EM Clamp -It consists of a tube of split ferrite rings of two different grades which can be clamped over the cable to be tested, and it is therefore non invasive and can be used on any cable type. 

c) Current Injection Probe - The current probe is a clip-on current transformer which can be applied to any cable. It is shielded, and so applies only inductive coupling, without capacitive coupling of the test signal.

The test should be well decoupled to prevent any disturbances from affecting other equipment. These can be coupled out of the set-up either via the mains supply or by direct coupling to the leads.

Although the CDN will reduce both the noise on its AE port and variations in the cable impedance, it does not do this perfectly, and a permanently installed RF filter at the mains supply to the test environment is advisable. Other cables should either be kept local to the test environment, or filtered to the ground reference plane if they leave it.

Ambient radiated signals should also be attenuated and it is usual to perform the measurements inside a screened room, with the floor of the room forming the ground reference plane. 

Radiated RF Immunity Test

The radiated RF immunity standard used is IEC 61000-4-3. This requires a radiated RF field generated by an antenna in a shielded anechoic enclosure using a pre-calibrated field, swept from 80MHz to 1000MHz with a step size not exceeding 1% of fundamental and dwell time sufficient to allow the equipment under test to respond.

The equipment under test is placed on the 0.8m high wooden table (for table top devices) with its front face in the same plane as the uniform field area that was previously calibrated.

Both the antenna position and the uniform area are fixed with respect to the chamber. The standard requires at least 1m of connected cable length to be exposed to the field, and recommends the use of ferrite chokes to decouple longer cables. The cable layout cannot be generally specified, but at least some of the length should be in the same plane as one of the polarisations of the antenna.

The equipment under test is rotated on the table so that each of its four sides, and the top and bottom if it may be used in any orientation, face the antenna in turn, and are coplanar with the uniform area. For each orientation, two sweeps are performed across the frequency range, one in each antenna polarisation.

If the frequency is swept from 80 to 1000MHz in 1% steps with the conventional minimum dwell time of 3 seconds per step, each sweep should take about 15 minutes, and the whole test should take over two hours.

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