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Selecting the Proper RF Amplifier for the Required Field Strength

Introduction

Ask any experienced EMC test engineer or technician what one of the most frequently asked questions they get is and they’ll probably tell you something along the lines of the following:

“With the current broadband RF amplifier and antenna combination we have, what’s the highest field strength obtainable?”

The requestor is usually trying to find equipment that generates some enormously high field strength level to test their product, probably already knowing beforehand that the existing amplifier/antenna combo is not suitable.

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The main reason the antenna/amplifier combo chosen is probably not adequate is because we usually only purchase what we need at the time. More power costs more money when it comes to broadband power amplifiers, so we typically only specify/buy to meet our current needs rather than what we may need in the future. This is a common cost-limiting approach practiced in most EMC test facilities.


What is a Broadband Amplifier?

Broadband amplifiers are necessary for generating the field strengths required for most EMC radiated immunity tests. In its simplest form, an amplifier is an active device that takes an input signal and produces an output signal that is a copy of the input signal, but with increased amplitude.


Factors to Consider in Selecting a Broadband Amplifier

There are several factors to take into consideration when specifying an RF power amplifier for EMC compliance testing. These factors include linearity, ruggedness, availability, and power gain. See Reference 1 if interested in learning more about these other factors. This article focuses on the last item in this list – power gain and the rules of thumb that can be used to quickly answer the question – “What’s the highest field strength possible given our current setup?”


What is Meant by Power Gain?

Gain is the amplification factor or the extent to which an amplifier boosts the strength of an input signal (usually from a signal generator). Amplification factors are specified in terms of power, usually expressed in watts (W).

Gain is simply the ratio of output power (Pout) over the input power (Pin), usually expressed logarithmically in decibels (dB).

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The amount of output power produced by an amplifier is primarily a function of the drive (amplifier input) power. For a given input level, a corresponding output level is produced. In some cases, amplifier gain may also be adjustable over a given range.

An RF amplifier used for EMC compliance testing must be able to deliver the necessary power (Watts) to generate the correct electric field (E-field) strength in volts-per-meter (V/m) at the location of the equipment under test (EUT). The amplifier must provide this power over the frequency range of the test, at the antenna-to-EUT distance specified in the standard(s), driving an imperfect load (an antenna), and with cable and other losses present in the test setup. Although the actual power level required is highly dependent on these factors, we can use a rule of thumb to get us close to an answer.


Rule of Thumb for Specifying Power

EMC engineers and technicians love it whenever we can utilize a rule of thumb. Although not as accurate as capturing every nuance of a measurement or design calculation, rules of thumb can be useful in pointing us in the right direction (and validating our intuition). Rules of thumb provide excellent rough first-order estimates, especially when answering questions (like the question described above), where the questioner probably already knows the answer – we don’t have enough power to get the E-field strength we want.

The rule of thumb often associated with specifying power is identified as effective radiated power (ERP) and is commonly known as the Friis field equation.

The simple equation for ERP is shown below:

Where:

E = Desired E-field strength (V/m)
r = Distance from source of the transmitted energy in meters

Rearranging the above equation to give results in E-field strength results in:

This equation holds only if r is in the far-field – not typical of most EMC test scenarios.

Note: There is also a rule of thumb to determine if a signal is in the near or far-field. If the frequency of a signal is less than its wavelength (λ) divided by 2π then it’s considered near-field. If the frequency of a signal is greater than its wavelength (λ) divided by 2π it’s considered far-field.

More refined versions of the above equations are:

or

If you would rather find the number of watts required to generate a given field strength, use these equations:

Where:

GainNumeric or GaindBi are provided by the antenna manufacturer for your given antenna. Since these numbers vary with frequency, checking the number of watts you’ll need to generate a given field strength must be done over the entire frequency range of the test.

Notice also that by reducing the measurement distance (r), that a higher field strength can be obtained for a given amount of power. This “trick” is often utilized but at the expense of performing a test in the near-field resulting in a less repeatable test.


Specifying Power Gain More Accurately

The problem with using the rule of thumb power equation is that it’s not entirely accurate when all the other factors affecting power output are considered. For instance, the gain of the antenna is not accurately included. It varies with frequency. This means the power for a given field strength also varies with frequency. Also, because of the mis-match of the 50Ω output of the amplifier with the varying non-50Ω input to the antenna, something called voltage-standing-wave-ratio (VSWR) can result in less “net” power delivered to the antenna than what the amplifier is outputting. Damage can occur if too much voltage is reflected back into the amplifier. The best amplifiers are designed to sink this reflected power without adversely affecting performance or reliability.

Another rule of thumb that takes these unfavorable effects into account is to increase the size of the power amplifier by a factor of between 2 to 3. This would accommodate the anticipated system losses of between 3 to 5 dB.

If performing a test with modulation, such as found in IEC 61000-4-3, don’t forget to include the additional power required to linearly support the 5.1 dB increase in signal level from the 80% modulation depth that is applied during the actual test.

If using the system in a non-anechoic screened room, the system should be further over-rated by at least 6 dB.


Summary

We can use several rules of thumb to help us determine if a certain amplifier/antenna combination will provide enough power to generate a desired E-field strength. Although rules of thumb will not provide us with precise measurements, they can be useful tools to help point us in the right direction. When specifying an amplifier, we should not only consider current usage cases but what we are likely to need in the future.

References and Further Reading

  1. In Compliance Magazine. (2018, January 5). What Every Electronics Engineer Needs to Know About: RF Amplifiers.
  2. Montrose, M.I & Nakauchi E.M., Testing for EMC Compliance, Approaches and Techniques, IEEE Press/Wiley-Interscience, 2004
  3. Rohde & Schwarz White Paper, An Introduction to EMC Amplifiers by Paul Denisowski, #1.2016-1
  4. Amplifier Research, Orange Book of Knowledge 50th Anniversary Edition, 8th
  5. Williams, T., EMC for Product Designers Fifth Edition, Newnes, 2017.

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