To address the challenge of improving power-added efficiency (PAE) for high-powered amplifiers, Analog Devices developed the ADL5511 TruPwr rms and envelope detector. The chip combines two RF functions, rms (root mean square) and and an envelope detector (an electronic circuit that takes a RF signal as input and provides an output that represents the envelope of the signal).
Next-generation WiMAX systems and LTE mobile data and voice systems are adopting orthogonal frequency-division multiplexed (OFDM) modulation, which supports a large number of subcarriers. However, the high peak-to-average ratio of these modulated signals causes power amplifiers (PAs) to be significantly backed off from their maximum transmit power, making the PA less efficient.
To improve PA efficiency, designers are increasingly turning to envelope detectors. In drain modulation architectures, an envelope detector modulates the PA power supply from the incoming RF signal. As a result, it gives only the PA the power needed to properly transmit the RF signal. Traditional architectures leave the PA biased at a much higher power level to ensure the PA can properly transmit peak power. However, they’re not capable of reducing PA bias when transmitting lower RF power.
Envelope Output
The ADL5511, which operates within the 1MHz to 4GHz frequency range, offers a different approach to improving PAE. According to ADI, the lower-cost, smaller-footprint device provides a higher-accuracy, easy-to-use means of extracting the envelope of a modulated signal.
The envelope output is presented as a voltage that’s proportional to the envelope of the input signal. The rms output is a linear (in V/V) output voltage that’s independent from the peak-to-average ratio of the input signal. The extracted envelope helps enhance PA linearization and efficiency, while the rms output can be used for true power measurement.