Amplifiers FAQ

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FAQ for EMC Testing

When determining which amplifier to purchase, the top considerations should be output power, linearity, frequency range, and VSWR tolerance.

Not only does the amplifier need to generate enough power for your test setup to generate the required fields or injection levels, but linearity requirements are also often present within test standards. The amplifier should be used within its stated operating frequency range and be VSWR tolerant.

For an EMC application, and due to the lack of true 50 Ω loads present in EMC test environments, Class A amplifiers are superior due to the ability to withstand the amount of reverse power coming back into the amplifier’s output. AB amplifiers may be used, but this class’s inherent mismatch tolerance will prevent the amplifier from being useable with many common EMC transducers.

Since you want to test at such a wide frequency range, you will typically need two amplifiers and two antennas. Depending on the antennas selected, the power required for an amplifier to generate a given field should be somewhere in the antenna’s datasheet. If this data is not given, then you will need to base your power requirement on the antenna’s gain. Cable losses may be significant and need to be taken into consideration at higher frequencies. Chamber effects (reflections) can be unpredictable and therefore should also be considered by adding margin onto your power requirements.

If the amplifier’s output power is sufficient and you are using it within the stated operating frequency range, you can use the same amplifier. Be sure to read the test standard to ensure that the amplifier selected meets any requirements stated within the standard, such as linearity. Different antennas, loads, etc., may be required.

Typically, amplifiers are supplied with appropriate connectors based on the power output. We recommend reading our Guide to RF Coaxial Connectors and Cables, to learn about connectors and how they may affect amplifier performance. Care should be taken to use the same power rated connectors on cables, bulkheads, antennas/loads, etc. 

In most EMC tests, software control of an amplifier is not required. However, it is a great feature when utilizing equipment that is installed in another room or location further away. It is also helpful to know what the amplifier is doing in the event of issues during the EMC test.

Quality, low-loss cables can only go so far in reducing loss at high frequencies as there will always be some inherent loss. The best approach is to minimize cable length. To do this, AR routinely moves higher-frequency field generating equipment into the chamber. In these instances, AR will often mount antennas and antenna masts directly to an equipment rack or rolling system platforms. This equipment must be housed in shielded racks as the fields generated in these systems can often exceed the amplifiers’ (and other equipment’s) verified compliance levels. Another alternative is AR’s AA-series field generating systems in the 18 – 40 GHz range.

FAQ for High Power Amplifiers Used in Telecommunication Testing.

Radio Frequency (RF) testing often requires drive levels that exceed a signal generator’s maximum linear output level, typically less than 1 W. RF high power amplifiers compensate for the low output power of signal generators.

Primary Wireless/Telcom testing applications for RF high power amplifiers include high temperature operating life (HTOL) and production burn-in testing. Characterization testing, such as digital pre-distortion and intermodulation, are other testing applications.

One of the most important specifications to consider is linear performance at the required test level, this can be estimated from P1dB. Harmonics are also important as they can affect predistortion measurements. Finally, it is necessary to include your expected modulation bandwidth when determining your frequency range.

First, determine your min/max frequency ± modulation bandwidth/2. Then assure the peak power does not exceed the amplifiers P1dB – 3dB.

Example: ETM 3.1 LTE test waveform peak power = 12.2 dB

Average power = 40 dBm
40 + 12 = 52 dBm peak power
In this example, P1dB over the desired frequency range should be > 55 dBm

More detailed linear performance curves (ACLR/EVM/SEM) must be measured with swept power under user-defined modulation.

An RF test system must provide an undistorted modulated carrier of the device under test (DUT). Class A amplifiers provide the best linear performance for this as they are inherently less sensitive to load mismatch.

RF Distribution Systems (RFDS) supply multiple channels of RF to each DUT. The power level of each channel must be accurately controlled and compensate for any system loss. DUT failure and return loss must not be allowed to affect the other channels. Active monitoring of forward and reverse power is recommended.

Further clarification and details can be provided by reaching AR at 800-933-8181 or sending an email to [email protected]