The RF detector alone does not drive the need for calibration. In a typical RF power measurement system, there will also be part-to-part uncertainty in components such as the directional coupler, the attenuator and the ADC. Indeed, with so much initial uncertainty in the system, there would be little value in choosing an RF detector that has absolute accuracy since the other signal chain elements would wipe out this accuracy. So calibration is required to remove system uncertainties such as part-to-part variations in the ADC, the RF detector, the attenuator and the directional coupler. In this example, an external RF power meter has been added. This meter connects to the antenna connector and must be able to somehow report its measurement results to the transmitter’s microcontroller.  So the microcontroller is receiving both the ADC codes and information from the power meter concerning the exact power at the antenna connector.