Amplifier Selection Tips for EMC Applications
It is important to keep the following in mind when selecting an amplifier for EMC testing:
- Technology - In general EMC amplifiers are based on various technologies and functionality.
Solid state amplifiers’ design involves the biasing of RF transistors (primarily MOSFETs). Alternatively, TWT (Traveling Wave Tube) amplifiers are a specialized vacuum tube that is used in electronics to amplify radio frequency (RF) signals in the microwave range.
The choice of technology used is dependent on many factors. For high frequency applications (above 1 GHz) utilizing high power (above 100 W or so), a TWT would be the only choice currently available. High power, low frequency applications will utilize solid state technology. Additionally, some amplifiers will use solid state technology above 1 GHz, but power output will be lower than that of a TWT. It is all dependent on what your application requires.
In addition to choosing the appropriate technology, it is important to consider how the signal will be amplified. A CW amplifier continuously amplifies any signal introduced into its input, as long as it is within the operational range of the amplifier. This includes both continuous wave signals and modulated signals. Pulse amplifiers, however, will only amplify a signal introduced at the input when an additional pulsed signal is applied to a pulse input, resulting in a gating-type function for pulsed applications.
- Class of Operation – Amplifiers are classified as Class A or Class AB. Class A solid state amplifiers are generally the best choice for EMC RI and CI testing. They are the most reliable, durable, and rugged compared to other amplifier classes or types. EMC applications and test environments introduce the lack of true 50 Ω loads, so Class A amplifiers are superior due to the ability to withstand the amount of reverse power introduced into the amplifier’s output from load mismatch. Class AB amplifiers may be used, but this class’s inherent mismatch tolerance will prevent the amplifier from being useable with many common EMC transducers.
- Good Linearity and Low Harmonic Distortion - Amps with good linearity and low harmonic distortion result in clean, reproducible, and measurable signals. Class A amplifiers outperform other types in this area.
- Modulation (AM, FM, PM) Performance – Modulation of the CW signal is required by most RI and CI test standards. When selecting an amplifier, be sure to ask the amplifier manufacturer to provide data showing how modulation is reproduced and under what conditions. Comparing the reproducibility of the modulated signals should be a key point in the decision-making process.
- Multiple signal/tone applications - If planning to use multiple signals/tones or complex waveforms to speed up testing and better represent real-world threats, a single broadband amplifier is the most appropriate choice versus a multiband amplifier or multiple amplifiers. A broadband amplifier can amplify all the signals simultaneously whereas a multiband amplifier may require multiple inputs and outputs.
- Rated Output Power - Rated Output Power is a manufacturer-defined specification. Beware of typical, average, and maximum power level descriptions, which can be misleading. Instead, try to compare the same data across different amplifiers. If similar power data isn’t available from the specification sheet, ask the manufacturer directly for it, with an explanation of how the power measurements were derived. Just because an amplifier specification sheet states that it is rated for 400 W does not mean that the amplifier will produce 400 W across the entire frequency band.
- 1 and 3 dB Compression Points (P1dB and P3dB) - The X dB compression point (PXdB) is the output power level at which the gain decreases X dB from its constant value. Once an amplifier reaches this point, it goes into compression and becomes a non-linear device, producing distortion, harmonics and intermodulation products. The typical compression points are P1dB and P3dB. This is an important rating for applications with strict linearity requirements, such as IEC 61000-4-3 and R&D.