Wideband High Dynamic Range Limiting Amplifier

Many modern EW systems require low noise receivers capable of withstanding wide input power variations over a multioctave bandwidth. These receivers are necessary to protect sensitive components from RF overdrive or to remove AM modulation from incoming signals. Further, multichannel system designs and proximity to the receiver antenna generate requirements for low power and small package size. Applications include IFM and direction finding front ends, DRFM, and jammer systems. These systems must operate over a wide thermal range and require a flat frequency response with low harmonic content under all operating conditions. ADI’s limiting amplifiers are ideal for many of these applications due to industry-leading package size, electrical/RF performance, and ease of integration into higher level assemblies. A microwave limiting amplifier is a high gain, multistage amplifier that limits output power by successively compressing internal gain stages as input power increases. Gain stages compress from the output stage toward the input, with the design optimized to avoid overdriving individual gain stage under all operating conditions. Challenges associated with wideband limiting amplifier design include effective power limiting, thermal compensation, and frequency equalization over a multioctave bandwidth. In addition, system requirements for low noise, low power, and a small package size add to the complexity of the design. This article reviews design considerations and techniques for a 2 GHz to 18 GHz limiting amplifier with requirements for 45 ±1.5 dB gain, an operational temperature range –40°C to +85°C, fewer than 1.5 W dc, and a 40 dB limiting dynamic range. Limiting dynamic range is defined as the input power range over which RF output power is fixed. ADI offers a wideband 2 GHz to 18 GHz limiting amplifier product, the HMC7891, that meets these requirements. This amplifier includes internal voltage regulation in a hermetically sealed, connectorized package.