While at DesignCon in January, I stopped by the National Instruments booth and was amazed when I saw its PXI RF modular instrumentation setup looking at an RF power amplifier (PA) with an envelope tracking and DPD design. This modular system, along with NI LabView Modulation Toolkit and NI RF Software Measurement Suites, enables an RF signal generator to output custom or standard modulation formats as LTE, GPS, GSM, WCDMA, MIMO, and many more.
I remember the days when it took a rack of equipment to adequately test such an ET and DPD performance on a PA. Now the compact modules fit into a system the size of most bench-top power supplies or the old RF power meters I used at General Microwave in the 80s (Figure 1).
Envelope tracking is a clever way to help correct the peak-to-average power ratio (PAPR), which causes inefficiency in the PA. Digital pre-distortion (DPD) is an additional way to increase the efficiency of those power-hungry PAs.
Both of these enhancements to increasing the PA efficiency were demonstrated at DesignCon in a live demo that analyzed the efficiency of the design and showed the percent improvement of each technique by itself and then the combined overall efficiency of the duo (Figures 2 and 3).
The most difficult ET test challenge is ensuring instrument synchronization between the RF signal generator and the arbitrary waveform generator (AWG). Since maximizing the power-added efficiency (PAE) of the PA occurs when we choose an optimal Vcc value based on the input power, poor synchronization between these instruments will result in having a Vcc value that is either too high or too low for any given output power. The RF signal generator and AWG must not only be synchronized, but the synchronization should also be repeatable. NI equipment demonstrated this capability.
There are considerable challenges to designers in the design and testing of PA efficiency enhancement techniques. I suggest viewing the NI whitepaper, which discusses ET fundamentals and robust test solutions and challenges.