What Every Electronics Engineer Needs to Know About: RF Amplifiers

At some point in your EMC career you may very well be tasked with specifying broadband RF amplifiers for RF immunity testing and need to quickly get up-to-speed on the key factors involved in the buying decision before actually making the purchase of an expensive amplifier. There is nothing worse than finding out after you receive your brand-new amplifier and begin testing that you discover the amplifier selected doesn’t fully accomplish what you expected it would and you’re forced to come up with some costly and sometimes embarrassing work-arounds (like having to move the antenna closer to the EUT than is allowed in order to obtain the required field strength).

The RF amplifier is the workhorse of the EMC test laboratory. Not only does it have to be reliable and operate perfectly for many hours in any given day, it must also deliver the proper field strengths, over the correct frequency ranges, with minimum spurious harmonic emissions and with low distortion, sometimes into complete open or short conditions or into other inefficient loads (i.e. antennas). If the amplifier selected is not robust or does not fully meet your specifications, your testing process is not as efficient as it could be, and this could lead to increased test time, increased test cost, over-testing or under-testing of the equipment under test (EUT).

Before getting into some of the more technical aspects of RF amplifiers for EMC testing, let’s first discuss delivery of the amplifier and the environment in which the amplifier will be installed. Think about where it will be installed. Are there adequate sources of clean power readily available? If not, have arrangements been made with the facilities department to locate the proper power source nearby? If the amplifier requires water cooling, how will that be installed? If air-cooled, is there enough air flow space available around the amplifier to ensure that heat does not recirculate back into the amplifier? Is the air surrounding the amplifier relatively dust-free and is it kept at least 72°F (22.2°C) and below? Less dusty and cooler environments are better for the amplifier. How much noise will the fans of the amplifier generate and will this negatively impact any other work located in the area near it? If the amplifier weighs a lot, how will it get from the delivery truck to its permanently installed location without getting damaged or causing damage? Will it arrive in a wooden crate and if so, how will it safely get removed from the crate? Consider having professionals install the amplifier in its permanent location and think about keeping the original packing crate in case the amplifier must be returned to the manufacturer for repair. Where will the original packing be stored after un-crating the amplifier? It should be easy to get to. Will the amplifier be located in a high traffic area and if so, how will you keep personnel from inadvertently bumping into it? Or even better, how do you plan to locate the amplifier out of the way of foot traffic in the first place?  Have you considered RF inputs and outputs to the amplifier? What is the maximum input drive level to the amplifier before damage occurs and how will inadvertent over-powering of the input be handled? How will the RF output of the amplifier be delivered most efficiently to the antenna for radiated tests or to injection probes for conducted tests? What types of RF connectors and cabling are required based on configuration and power rating of the amplifier? Are these connectors and cables currently available in-house or do they need to be purchased as well? When determining your output power handling requirements, be sure to factor in all of the line and connector losses present in the system.  Finally, have you considered installation of a door interlock to the EMC chamber so that if the chamber door is inadvertently opened while the amplifier is outputting RF energy that the RF output of the amplifier is disabled? This is a personnel safety factor and should not be over-looked.

Once all of the housekeeping requirements are thought through, consider the specifications the amplifier must meet based on anticipated testing needs.  What immunity standards do you need to perform? What are the test distances required (1m, 3m, etc.)? What are the required field strengths (i.e. 3V/m, 10V/m, 200V/m, etc.), frequency ranges, voltage standing wave ratio protection, linearity and harmonics requirements required by the chosen standards? What is the status of the standards? Is there an anticipated revision in the works that will require higher field strengths or testing to higher frequencies?  Will margin testing to higher field strengths and/or higher (or lower) frequencies be required during product development in order to fully understand where the product fails? Based on results of this investigation, try to select an amplifier that will meet current as well as these anticipated needs. Also, what antenna or antennas will be used? Does your firm already own them or do they also need to be selected along with the amplifier? Selecting an antenna along with the amplifier could be an advantage since you can choose the most efficient antenna/amplifier pair (antennas are covered separately in an upcoming article).  Unfortunately, you’re often provided with an antenna that someone else purchased long before you got involved, and because of budget or other constraints, you just have to use that particular antenna.

The most important characteristic to take into consideration when selecting a RF amplifier is the output power required to create a certain field strength at a certain frequency. This characteristic varies with frequency. It may take only 10W to generate 3V/m at one frequency, and 100W to generate the same field strength at some other frequency (the reasons are beyond the scope of this article).

The next characteristic to consider is the linearity of the amplifier over the entire frequency range of interest. Linearity, the degree of proportionality between input and output, ensures the amplifier is capable of outputting a fixed increase in output power given the same increase in input power (there is a dB for dB increase in input power versus output power). At some point, called the 1-dB compression point, all amplifiers are said to go into “compression.” The maximum amplifier RF output will be reached regardless of the RF input and the amplifier RF output will become saturated. A byproduct of running the amplifier into compression is the creation of harmonic and spurious emissions at frequencies other than the intended frequency. You may not even be aware these emissions are present and must carefully check for them using a frequency selective device such as a spectrum analyzer. The typical RF field probe used in compliance testing only provides a readout of the highest field strength present, not its frequency. This issue is important because you could be over-testing your EUT at some frequency you weren’t intending to test at and not even know it. These issues are best avoided by selecting a RF amplifier with a large enough maximum output power that is well above the 1-dB compression point.

The final characteristic to consider when selecting a RF amplifier for EMC testing purposes is a rather complex one. This characteristic is called Voltage Standing Wave Ratio or VSWR and is caused by too much impedance mismatch between source and load. It mainly occurs due to the imperfect impedance mismatch of the load to the amplifier’s RF output and in the case of radiated immunity testing the load is the antenna. If the antenna’s input impedance were a perfect 50Ω across the entire frequency band, and assuming no other losses, this impedance would match the 50Ω output impedance of the amplifier, and all of the forward transmitted power from the amplifier would be absorbed by the antenna and radiated. However, since the antenna is not a perfect load, some or all of the forward transmitted power gets reflected back into the amplifier. This reflected power can cause serious damage to the amplifier through arcing, breakdown of components, or excessive heating. Modern RF amplifiers utilize fold-back circuitry to detect mismatch and decrease the output power of the amplifier until the magnitude of the reflected power is within safe levels. Be sure to consider protection against VSWR when selecting an RF amplifier.

Final Thoughts

I hope this article gave you some ideas for selecting an RF amplifier that you may not have considered had you not read it. Good luck in choosing your first RF amplifier for EMC testing purposes. Do not be afraid to lean heavily on the manufacturers for help in the selection process. At least you now have a little more knowledge at your disposal before talking to amplifier manufacturers. Also, consider carefully documenting your selection of a RF amplifier. It is quite common to encounter a situation where the EUT is failing a RF immunity test and all fingers point to an imaginary issue with the test setup. Having documentation available to prove the test setup was sound will quickly get the team back on track and back into solving the real issue, a problem with the EUT.

References

  1. Denisowski, An Introduction to EMC Amplifiers White Paper, Rohde & Schwarz, 2016
  2. Smith, Do’s and Don’ts in the application of high-power RF amplifiers in EMC test systems, AR RF/Microwave Instrumentation, 2009

Don MacArthur is the Principal EMC Consultant at MacArthur Compliance Services, LLC. He can be reached at don.macarthur@mcs-emc.com.

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