Realtime waveforms to test PA envelope tracking

October 03, 2013 // By Graham Prophet
Envelope tracking (ET) in smartphone and other power-amplifiers (PAs) enhances amplifier efficiency as it allows the amplifier supply voltage to track the envelope of the RF signal. Rohde & Schwarz has an option to test power amplifier ET performance; its SMW200A vector signal generator can now deliver RF and envelope signals in a single instrument.

SMW K540 option adds envelope tracking; this option enables quick and easy testing of envelope tracking power amplifiers for smartphones, base stations and tactical radios. Because of the specific power distribution of RF signals with modern communications standards such as LTE and WCDMA, power amplifiers generally exhibit poor efficiency since spare capacity has to be reserved for high crest factors. With envelope tracking, the amplifier supply voltage is controlled in such a way that it tracks the envelope of the RF signal. As a result, the amplifier always operates in a range close to its instantaneous maximum output power, considerably boosting amplifier efficiency. Using shaping functions, the amplifier can be optimized for linearity or efficiency.

Rohde & Schwarz meets the test requirements of chipset manufacturers by offering a solution that generates fully synchronous RF and envelope waveforms in a single instrument. Users can delay the RF waveform and the envelope waveform relative to each other by ±500 nsec with 1 psec resolution. The SMW200A delivers an extremely low-noise envelope signal at the analogue baseband outputs. The signal generator calculates the envelope waveform from the baseband signals in realtime, so that all communications standards supported by the R&S SMW200A, as well as custom waveforms, can be used. A large selection of flexible shaping functions allows users to optimise the envelope in realtime.

The envelope tracking parameters can be configured on the generator's graphical user interface. Automatic envelope voltage adaptation is available to speed up setup and testing time since the main parameters such as VCC voltage range, PAin range, DC modulator gain, DC offset and power offset only need to be entered once. Automatic recalculation of the envelope based on the power amplifier input level makes it possible to perform power sweeps over the amplifier's entire input range.

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