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Questions to Ask When Looking for the Correct RF/Microwave Power Amplifier for EMC

High-power RF/microwave amplifiers are commonly used in EMC (Electromagnetic Compatibility) testing to meet radiated and conducted immunity requirements. These products can be a high overall cost, and selecting the correct models that meet your needs is imperative. There are costs associated with bandwidth power rating and the overall design of the amplifier. Looking at and comparing one data sheet over another can be very difficult to understand how each brand may market and list parameters. Making an informed decision requires understanding the amplifier’s specifications, capabilities, and compatibility with your specific needs. This article aims to guide you through the essential questions to ask when purchasing a high-power RF amplifier.

Amplifiers are used for many different applications, and some parameters are not crucial for EMC, while others are critical.

What is the frequency range and bandwidth?

You will need to cover a particular frequency range for your testing. However, having only one amplifier to cover the whole range may be nice. In some cases, this is practical; in others, it is not, requiring the range to be broken down into subranges. For conducted immunity testing, having a single amplifier cover the range is common. 10kHz – 400MHz required for DO-160 aircraft testing, for example, is one of the broader ranges for this type of testing. For most radiated immunity testing, breaking the bands into ranges is best, as power requirements will differ for different antennas and ranges. For example, 1GHz is a common breakpoint for antennas and amplifiers. Below 1GHz, more power is required to reach many fields, and above 1GHz, using a higher gain antenna may reduce the power required significantly. And breaking the system up into sub-bands results in possible cost savings. But keep in mind that more hardware switching requires more test time.

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What is the power output?

The output power of the amplifier has some sub-specifications related to power that better define the performance that can be expected. Power can be listed as CW power or saturated power. This is the amplifier’s maximum output, and the signal is saturated at this power level. In saturation, the signal becomes distorted at the top and bottom portions of the signal. The saturated signal output is not ideal since the signal is no longer as intended. There is also a side effect where high harmonics of the test signal are now present because of the distorted signal and will have unwanted effects on testing. So the amplifier should be used up to the region before saturation. On specification sheets, this is listed as the P1dB.  P1dB is the power point where the signal is compressed by one 1dB over an ideal linear line. This power level is what should be used for selecting your amplifier.

Also, it should be noted with broad-band amplifiers, the power output may not be flat across the whole band. This is not an issue in EMC testing, as levels are calibrated and controlled by the input signal from the signal generator. It is good to refer to the manufacturer’s typical power curve over frequency, and even better if you can see data from recent production units. Comparing these curves can give you a better understanding of the amplifier’s performance and what to expect. Ask the manufacturer for this information. In some cases, you can find an amplifier with marginally more power in the frequency range where it suits the test setup, such as when the level increases for conducted immunity BCI testing or when the radiating antenna’s gain is lower at the beginning of the frequency band. This may help you to select a smaller amplifier or not to get an oversized, more costly unit.

There are some other measurements that are related to linearity and power output, such as 3rd order intercept and inter harmonic distortion. These measurements are not very important for EMC testing.

What is the gain and gain flatness?

The gain of the amplifier is related to the power rating. This is the power gain from the input signal to the output signal. For example, a 100-watt amplifier will have a gain of about 50 dB. If the amplifier has a smaller gain, then it would take more input drive to reach the full output power. In many cases for EMC testing, amplifiers should only need about 0dBm of drive to reach rated power. Gain flatness is also not critical for EMC testing since the system is monitored and controlled at the input and output.

What is the technology used in the amplifier?

Amplifiers can use different technologies to produce power. Different technologies can have some advantages over others. Here is a listing of what is commonly available.

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Solid-state – is one of the best choices for EMC testing. They can produce high power and be very broadband. They become more limited in power as frequency increases into the many GHz. Though over time, Solid-state advances keep on pushing this boundary. Solid state also can be separated further into how the technology is used and powered. These refer to the Class of the amplifier. Class A amplifiers are biased fully on for peak linear power output, making these highly inefficient for power consumption. However, this offers an ideal EMC amplifier because P1dB can be better than other classes, and the amplifier can be built to be robust to handle high mismatches/VSWR, which is very common in EMC testing. Another common amplifier Class used for EMC testing is a Class AB amplifier. This amplifier is a little more efficient than the class A amplifier but, in many cases, can not handle high mismatches or VSWR. High VSWR may not be an issue when testing higher frequencies greater than 3-4 GHz. Antennas in this range are much better and may not be as much of a concern as it is in lower frequencies below 2GHz.

Traveling Wave Tube Amplifier (TWTA) – this technology is commonly used for high power and at higher frequencies. As mentioned above, Solid state, over time, has replaced many TWTA bands. TWTAs have a disadvantage as 1 TWT normally makes up the amplifier and has a life span that can sometimes be very short, only ~5 years though they can last much longer. If something happens to the TWT, it is a very expensive repair/replacement. And lead times can be very long as well. TWTAs cannot handle any high VSWR, or they can be damaged. They do not have good linearity/P1dB. In most cases, P1dB specification is not listed. But given all of these drawbacks, it is still a technology used for testing in microwave bands and when extremely high power is needed.

Tetrode Tube amplifiers – have all about disappeared from new amplifiers. These are smaller tubes combined in parallel to produce high-power output and used in amplifiers for low frequency for the 10kHz – 220MHz range. These amplifiers worked well but degraded output over time, requiring the Tubes to be replaced after so many years of use. These Tubes are harder to find, and the quality is not as high as they once were, making these amplifiers lose a spot in the market.

What is the harmonic rating?

All amplifiers will produce harmonics. This is a by-product of the nonlinearities of the amplifier. These are normally measured and listed for the P1dB rated level in the amplifiers’ linear range. Harmonics should be 20dBc (dB down from carrier). The carrier is the fundamental frequency or the test frequency being amplified. It does depend on the amplifier’s design, but in most cases, it is the 3rd harmonic that is the prominent harmonic. Once this harmonic goes beyond the top end of the amplifier’s frequency range, this starts to dissipate as the amplifiers combiners and components filter the harmonic out as this is outside of their functioning bands.

What cooling mechanism is employed?

High-power amplifiers generate significant heat, so an efficient cooling system is essential to maintain optimal performance and reliability. Look for amplifiers that employ effective cooling mechanisms, such as forced air or liquid cooling, to prevent overheating. Because amplifiers generate a lot of heat, forced air cooling can be very loud. Placing the amplifiers away from the test area may be a suggested setup for a quieter environment and a happier EMC technician.

What protections and safety features are included?

EMC testing can involve high-power signals and loads that are not ideal 50ohms. Power is reflected to the amplifier whenever a transducer/antenna/load is not matched. An amplifier must be robust to dissipate this extra energy or protect itself from damage. Ideally, an amplifier that will maintain forward power under any mismatch is suitable for EMC testing. In some cases where the amplifier is very high power or is not a robust technology such as TWTAs, it needs to protect itself by monitoring the reflected power. There are two possibilities for protection or reaction to high reflected power. One is to shut down the output power and requires the error to be cleared. Two is to fold back, reducing gain and limiting the reflected power to a set threshold. The method of folding back is preferable since the amplifier is still operational and does not require testing to be stopped to clear the error. In many cases, testing can be achieved. One should always be safe, and once you notice a high VSWR/reflected power setup, it is best to verify that all components/cables/loads are not damaged and working as intended. Other protection features would be temperature and voltage monitoring. Most high-quality amplifiers do have these safeguards in place. All these protections are critical in having an amplifier that will meet the demanding requirements for EMC testing.

 Are remote control and monitoring options available?

Remote control and monitoring capabilities can enhance convenience and operational efficiency.

Ask if the amplifier supports remote control via software, computer interfaces, or other means. Though the amplifier is a complex instrument, in many setups, the amplifier does not provide feedback. All testing is performed with other devices in the setup, Signal generator, RF Field probe, and power meter for power measurement. To the system, the amplifier is a simple gain stage. Software control may not be required.

What is the technical support and warranty offered?

Consider the availability of technical support and the warranty provided by the amplifier manufacturer. It is important to have access to reliable technical support and a warranty that covers any potential issues or defects. Getting answers when needed is vital to running an efficient EMC laboratory.

Does the amplifier comply with your EMC test standards?

This is an open question that the end user must address. Power (P1dB), frequency range, and Harmonics content are key factors. But this answer also factors into how the amplifier will be used. Pushing the amplifier past the P1dB power may be fine for harmonic content and meeting your linearity requirements. But this needs to be verified in your setup. Making the best use of your available power will save you time and money.

Purchasing a high-power RF amplifier for EMC testing requires careful consideration of various factors to ensure it aligns with your application’s needs. You can comprehensively understand the amplifier’s capabilities, performance, and compatibility by asking the essential questions outlined in this article. This knowledge will empower you to make an informed decision and select the most suitable high-power RF amplifier for your specific requirements.

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