This application note describes several low-cost coupling methods for RF power detectors that replace directional couplers. These methods allow for the integration of RF power detectors on-chip, which reduces cost and size. These methods also allow for the integration of RF power detectors and RF power amplifiers in a single module, which further reduces cost and size.
The document introduces a technique to minimize size and cost in wireless applications, such as cellular telephones, by replacing expensive directional couplers with lower-cost components. It details two specific coupling solutions: one utilizing a 0.4pF capacitor and a 50 Ω series resistor, and another adding a shunt inductor to improve sensitivity.
A 0.4pF capacitor and a 50 W series resistor replace the directional coupler and its external components.
The minimum output power is limited by a PCTL value of 18mV because using less than this makes the output unstable.
The 0.4pF series capacitor must have a tolerance of –0.05pF or less to ensure proper signal coupling.