Design of Solid-state, Wideband, High-power Microwave Amplifiers for Radiated Immunity Testing

This article illustrates how the requirements for amplifiers used in immunity testing are dictated by various EMC standards. A case is made for using solid-state devices rather than tubes. An overview of new transistor technologies is included, showing ranges of power and bandwidth typically obtainable from single devices. Key amplifier design issues, including load VSWR, thermal constraints and power combining techniques, are discussed. It is noted that a shift in amplifier design philosophy is necessitated by new device technologies, with nonlinear analysis and heat flow analysis emerging as essential design tools.

From: Vol. 53|No. 7 | July 2010 | Page 90

by N.D. du Toit, R. Thomas, J.H. Smith, F.N. Schirk and J.A. Mogel, AR RF/Microwave Instrumentation Souderton, PA

EMC standards are created to verify the reliability of electronic equipment. The standards need not only dictate the test levels and requirements, but must specify test equipment, configuration and requirements. There exist countless possible RF threats, usually in the form of transmitters, most of which are narrow band. Because it is difficult to predict exactly what sources of interference to which a particular device may be exposed, most EMC test standards specify a broad spectrum approach, including the likely frequencies of all perceived threats:

Commercial IEC: 150 kHz to 6 GHz
Military: MIL-STD-461: 10 kHz to 18 or 40 GHz
Avionics: DO-160: 10 kHz to 18 GHz
Telecom: Bell Core GR-1089: 10 kHz to 10 GHz
Automotive: 1 MHz to 18 GHz

The standards specify the field intensity to which a product must be subjected, based on real world field measurements, past failures or even potential future threats. Based on the product's application and the level of product quality, a safety margin is often applied to the level of the perceived threat to ensure the test field exceeds that which the product may encounter in use. The required EMC amplifier power is determined based on these specifications and the gain of the antenna or transducer used to generate the specified field as well as the area to be illuminated.

Commercial IEC: 30 V/m → 1 kW at 80 MHz
Military: MIL-STD-461: 200 V/m → 10 kW at 10 kHz
Avionics: DO-160: 7.2 kV/m pulsed → kWs - MWs
Telecom: Bell Core GR-1089: 8.7 V/m → low 100s of W
Automotive: 600 V/m pulsed → 1 kW

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