sk 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.
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.
Gain is simply the ratio of output power (Pout) over the input power (Pin), usually expressed logarithmically in decibels (dB).
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.
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.
- E = Desired E-field strength (V/m)
- r = Distance from source of the transmitted energy in meters
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.
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.
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.
- In Compliance Magazine. (2018, January 5). “What Every Electronics Engineer Needs to Know About: RF Amplifiers.” Retrieved from https://incompliancemag.com/article/rf-amplifiers
- Montrose, M.I & Nakauchi E.M., Testing for EMC Compliance, Approaches and Techniques, IEEE Press/Wiley-Interscience, 2004
- Rohde & Schwarz White Paper, An Introduction to EMC Amplifiers, Paul Denisowski, #1.2016-1.
- Amplifier Research, Orange Book of Knowledge 50th Anniversary Edition, 8th Edition.
- Williams, T., EMC for Product Designers, Fifth Edition, Newnes, 2017.
- Determine the Frequency range of operation needed, sometimes more than one amplifier is required.
- Determine if you need a Pulse or CW type of Amplifier. Example: HIRF EMC applications require High Power Pulse Amplifiers.
- Determine the minimum power needed from the amplifier. Example: As you go up in frequency, antenna gain improves, so a lower power amplifier may be acceptable.
- 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.
- 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.
- Antennas, Cables, DUTs & Rooms have cumulative VSWR. It is best to allocate for some power margin.
- Consider the application, is this a single test or will it be used repetitively.
- Consider your desired RF connection types and locations optimal for your application.
- Also consider if automation will be used so the appropriate remote capability is included.