Improving RF Power Detectors
Modern wireless transmitters generally require strict control of transmitted radio frequency (RF) power. In wireless cellular networks, strict power control allows the size of cells to be precisely set to enhance coverage. Precise power control also avoids the need for excessive thermal dimensioning of the RF power amplifier (PA), which is required when there is uncertainty about the actual transmitted power. For example, if a 50 W (47 dBm) power amplifier has a transmit power uncertainty of just 1 dB, the PA must be dimensioned so that it can safely (i.e., without overheating) transmit 63 W (48 dBm).
Power measurement and control is also used in receivers, usually at intermediate frequencies (IFs). Here the objective is to measure and control the gain of the received signal so that IF amplifiers and analog-to-digital converters (ADCs) are not overdriven. While precision in the measurement of the received signal (commonly referred to as the received signal strength indicator or RSSI) is useful for maximizing the signal-to-noise ratio, it is less important than on the transmit side; the goal is to merely keep the received signal under a certain limit.
RMS RF power detectors can measure RF power independently of signal peak-to-average ratio or crest factor. This capability is critical when the peak-to-average ratio of a measured signal is changing. This is common in wireless cellular networks due to the ever changing number of calls being carried by a cellular base station. The changing peak-to-average ratio results both from the transmission of multiple carriers at varying power levels and from the variations in code-domain power in a single code division multiple access (CDMA) carrier.