A Wideband General-Purpose PIN Diode Attenuator

Introduction

PIN diode based AGC attenuators are commonly used in many broadband system applications such as cable or fiberoptic TV, wireless CDMA, etc. A popular attenuator design utilized over the instantaneous frequency range from 10 MHz to beyond 2 GHz is the PI network. The benefit of this design is its broadband constant impedance, wide dynamic range and good compatibility with AGC signals.

The PIN diode is used as a current controlled resistance component in the PI network. PIN diodes are low-cost, low-distortion elements available in commonly used small-size plastic packages. This application note describes the design of a highperformance, PIN based 4 diode PI attenuator utilizing the low-cost SMP1307-011 diode in a plastic SOD-323 package. Performance is characterized from 10 MHz–3 GHz.

PI Attenuator Fundamentals

For matched broadband applications, especially those covering low RF frequencies (to 5 MHz) through frequencies greater than 1 GHz, PIN diode designs are commonly employed. The circuit configurations most popular are the TEE, bridged TEE and the PI. All these designs use PIN diodes as current controlled RF resistors whose resistance values are set by a DC control, established by an AGC loop.

This application note describes the design and performance of a PI attenuator that uses 4 PIN diodes. The benefit of the 4 diode circuit is its symmetry that allows for a simpler bias network and a reduction of distortion due to cancellation of harmonic signals in the back-to-back configuration of the series diodes.

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•Application Note: A Wideband General-Purpose PIN Diode Attenuator