Cut Part Count and Increase Dynamic Range in the Hybrid Coupled Attenuator
Topologies commonly used for implementing constant impedance attenuators include: Pi (π), Bridged-T, Resistive Line and Hybrid Coupled. The attenuation can be made variable by replacing the resistive elements with either FETs or PIN diodes. PIN diodes behave like current-controlled resistors above ten times the cut-off frequency (fc) given by where, τ is the minority carrier lifetime. The diode junction resistance, Rj , can be varied by the application of different values of bias current. Generally, narrowband configurations like the Hybrid Coupled Attenuator (HCA) have substantially less phase change over attenuation compared to the wideband types such as Pi and Bridged-T. ttenuators with minimal phase variation are usually required in phase-sensitive digital modulation formats.
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