Temperature Compensated Linear Power Detector - App Note

In many RF applications where an output power level from a source varies across frequency bands and time, it is critical that these variations are either kept to their minimum or allowed to occur in a controlled manner. For example, the power output from a portable two way radio needs to be strictly regulated across frequency bands to meet the specified output power (to minimize output power variation across frequency band). Another example would be in the cellular base station transmitter employing TDMA scheme, such as GSM, where output power level has to be adjusted dynamically corresponding to different time slots (controlled variation of output power across time). In all these applications, a method to sample or to detect the output power has to be incorporated into the ALC loop to control the output power of the RF source. Figure 1 shows a very simplified ALC loop. A small portion of the output power from the RF PA is being coupled out from the directional coupler for detection. The detector produces a DC level which is proportional to the incident power from the RF PA. The DC level from the detector can then be used to adjust the variable gain amplifier or the attenuation of a variable attenuator to hold the output power from the loop at constant level across frequency.