Conducted RF immunity is a test method that subjects the equipment under test (EUT) to a source of disturbance comprising electric (E) and magnetic (H) fields, simulating those coming from intentional RF transmitters. These disturbing E and H fields are approximated by the E and H near-fields resulting from the voltages and currents caused by the test setup described in standards such as IEC 61000-4-6.
To ensure test repeatability and not cause either over or under testing, standards such as IEC 61000-4-6 have specific requirements for how the test is conducted. A small portion of the standard describes the requirements for generating the fields using a RF signal generator and power amplifier. If you’re performing commercial conducted radiated immunity tests strictly by the book, you will want to utilize a radio-frequency (RF) signal generator and power amplifier combination that fully complies with IEC 61000-4-6.
For the RF signal generator, this means:
A generator capable of covering the frequency band of interest able to be amplitude modulated by a 1 kHz sine wave with a modulation depth of 80 %. The generator will have manual control (frequency, amplitude, modulation index), or in the case of RF synthesizers, programmable with frequency-dependent step sizes and dwell times.
Note: For testing according to IEC 61000-4-6, the frequency band of interest mentioned above is 150 kHz to 80 MHz.
If the RF signal generator on its own is not able to generate the required severity level required by the standard, then an additional piece of equipment called a broadband power amplifier (PA) is necessary to amplify the output signal from the RF generator to obtain the required test level.
The focus of the remainder of this article is on the output power characteristics of the PA required to generate a test level of 10V.
Annex E of the standard provides further guidance on selection of the PA required to generate a 10V test level. This is an informative annex, not normative, meaning it is there for clarifying purposes only, not part of the basic requirement of the standard.
Annex E states something to the effect:
The available output power of the PA is determined by taking into account the attenuator T2 (6 dB), the amplitude modulation depth (80 %), and the minimum coupling factor of the coupling-decoupling-network (CDN) or clamp used.
Let’s cover the last item in the above requirement – the minimum coupling factor.
Minimum Coupling Factor
The minimum coupling factor is given as the ratio given by the open-circuit voltage obtained at the equipment under test (EUT) port of the coupling and decoupling device divided by the open-circuit voltage obtained at the output of the test generator.
Note: Examples of coupling and decoupling devices are:
- direct injection networks (with decoupling);
- clamp injection devices (EM clamps).
The minimum coupling factor (± 1.5 dB) is measured by using the output level setting circuit (see IEC 61000-4-6 for details). The coupling factor is the ratio between the output voltage, obtained when using a coupling and decoupling device in series with a 150 Ω to 50 Ω adapter and the output voltage when using two 150 Ω to 50 Ω adapters in series.
Required Power Output of PA for a 10V Test Level
Annex E states that a CDN has a minimum coupling factor of 0 dB and the required output power of the PA is just 7 W.
A current clamp winding with a ratio of 5:1 has a minimum coupling factor of -14 dB and the required output power of the PA is more substantial at 176 W.
An EM clamp has a minimum coupling factor of -6 dB, and the required output power is only 28 W.
As can be observed from the above, the choice of power amplifier depends a lot on what device is used for coupling and decoupling. An EM clamp is a popular choice for many test facilities and only requires a PA rated for a maximum of 28 W, however, CDN’s are the preferred coupling and decoupling devices for reasons of test reproducibility and protection of the auxiliary equipment. If you have a product with a lot of inputs and outputs, you’ll need a lot of different types of CDNs to cover all possible equipment configurations, but you won’t require a PA that needs to output too much power.
To play it safe, picking up a PA with double or triple the necessary output power will ensure you always have enough power and won’t always be running the PA at the top of its capability.
This was a quick run-through IEC 61000-4-6 for power amplifier specifications. For more information on this important subject, please see the following.
References and Further Reading
- Electromagnetic compatibility (EMC) – Part 4-6: Testing and measurement techniques – Immunity to conducted disturbances, induced by radio-frequency fields (IEC 61000-4-6:2013).