Amplifiers
Power Amplifier Linearity Requirements per IEC 61000-4-3
T

he 2020 edition of IEC 61000‑4‑3 contains several significant technical changes.

According to the forward of the standard, the specific changes are:

  1. testing using multiple test signals has been described;
  2. additional information on EUT and cable layout has been added;
  3. the upper frequency limitation has been removed to take account of new services;
  4. the characterization of the field as well as the checking of power amplifier linearity of the immunity chain are specified.

The focus of this article is the checking of the power amplifier linearity as pointed out in item d above.

Pro Tip: When IEC standards are published with revisions, the forward usually contains a short summary of the significant technical changes from the previous edition.
If you are responsible for monitoring and communicating updates to compliance standards as many compliance engineering professionals are, this information provides an excellent, short, easy-to-read summary, suitable for communication to key stakeholders located within your organization. First-line supervisors, engineering managers, engineering directors, and executives who do not have the want, need, or desire to read a more elaborate technical breakdown of the changes, will most certainly appreciate reading a clearer, concise, and most importantly brief, executive summary. The information contained in the forward of each standard is such a summary. Take advantage of its presence in the standard.
Power Amplifier Linearity
Power amplifier linearity is mentioned in many places with the IEC 61000-4-3 standard. IEC 61000‑4‑3 first mentions linearity as part of the level setting setup under clause 6.3.1 Characteristics of the UFA (Uniform Field Area). In this section of the standard, it states:
The actual test field strength ET can be different from the level setting field strength EL provided that the linearity of the system can be demonstrated (see 6.3.2 or 6.3.3 and Annex D).
Further into the standard linearity is again mentioned:
It is required to ensure that the amplifiers can reproduce the modulation within the linearity requirements during testing (see 6.3.2 or 6.3.3 and Annex D).
Clause 6.3.2 is the constant field strength level setting method, and 6.3.3 is the constant power level setting method. Note 3 of both of these two clauses state that Step 5 (Clause 6.3.2) or Step 7 (Clause 6.3.3) describe how to check if the amplifier used is sufficiently linear and that if more information is required to refer to Annex D. Annex D (informative) covers additional and very helpful information concerning amplifier compression and non-linearity.

Based on how many times and places the word ”linearity” appears in IEC 61000-4-3, it is evident that the technical committee responsible for revision of the standard (SC 77B High frequency phenomena of IEC/TC 77 Electromagnetic compatibility) was more than likely very concerned about addressing the issue.

Amplifier sine waves
Amplifier Compression and Non-linearity (Annex D)
Annex D goes into the details and objectives of limiting amplifier distortion, the possible problems caused by harmonics and saturation, how to limit the harmonic content in the field, the effect of linearity characteristic on the immunity test, and the evaluation method of the linearity characteristic.
Pro-Tip: If ‘you’re involved in any way with IEC 61000-4-3 testing, then it would be a good idea for you to read Annex D.
According to the standard, Annex D is provided to assist test laboratories in understanding and limiting amplifier distortion, however, anyone involved in developing new electrical/electronic products and running them through radiated immunity testing should be interested in the potential issues caused by running an amplifier in saturation and harmonics that can be produced. Why should product developers and test laboratories be concerned about power amplifier linearity? The answer is “ownership” As product developers, we own getting the product out the door and into production as soon as possible. We ‘can’t always rely on the test facility to get everything right. People make mistakes. Sometimes there is a lack of training, lack of understanding, incorrect interpretation of the requirements, shortcuts taken, etc. If a product is failing a radiated immunity test, ‘it’s ok to ask the test facility to double-check amplifier linearity and that it ‘isn’t an issue. ‘It’s ok to ask them to back up their position with test data.

As noted in the standard:

Harmonics may cause an EUT failure where the EUT is robust at the intended fundamental frequency but not robust at the harmonic frequency. The false failure would be recorded incorrectly and may lead to an incorrect redesign.
Product developers want to avoid making an unnecessary design change.

Under testing may also occur due to amplifier non‑linearity and harmonics.

The harmonics may contribute significantly to the measured values taken during UFA measurement. The field strength at the intended frequency is incorrectly measured, as the broadband field probe will measure the fundamental and its harmonics.
Linearity Measurement
Annex D provides a four-step procedure, example linearity curve and example gain deviation plots to determine linearity. The gain error over the level range of the measured amplifier output cannot exceed ±1 dB linearity. If this criterion cannot be met, Annex D provides guidance and a couple of methods that can be used to adjust forward power applied during the actual EUT test to achieve compliance. See Annex D for more details.
Summary
This was a quick run-through IEC 61000-4-3 power amplifier linearity requirements. For more information on this important subject, please see the following.
References and Further Reading
  1. Electromagnetic compatibility (EMC) – Part 4-3: Testing and measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test (IEC 61000-4-3:2020).
Tips for Selecting
Amplifiers
  1. Determine the frequency range of operation needed, sometimes more than one amplifier is required.
  2. Determine if you need a Pulse or CW type of amplifier. Example: HIRF EMC applications require high power pulse amplifiers.
  3. Determine the minimum acceptable linear or saturated power needed from the amplifier. Harmonics should be considered based on the frequency range. Example: As you go up in frequency antenna gain improves so a lower power amplifier may be acceptable but the higher gain of the antenna may affect the Harmonic Level.
  4. Assess the system losses between the amplifier and the antenna/DUT. Example: If the test setup has 6dB of losses then the Amplifier power needs to be 6dBm higher.
  1. Some modulations if required for the test application, would require a higher power amplifier. Example: When performing an 80% AM modulation test the amplifier needs to have 5.1dBm of margin to accommodate the peak.
  2. Antennas, cables, DUTs, and rooms have cumulative VSWR, it is best to allocate for some power margin. Example: working into a 2:1 requires 12% more forward power.
  3. Consider the application, is this a single test or will it be used repetitively?
  4. Consider your desired RF connection types and locations to be optimal for your application.
  5. Consider if automation will be used so the appropriate remote capability is included.
These tips are presented by
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