Future Work

There are several issues regarding the presented envelope tracking supply modulator in this thesis. For future design, the system architecture may have to be modified for improved performance.

First, the measured power efficiency of the ETHSM drops by approximately 3% to 5% compared to the simulation results. The investigation reveals that the measured average push-pull output current of the linear amplifier deviates from the ideal zero current, which causes additional power loss. The inaccurate current sensing contributes to the above issue.

Another issue lies in the tracking error of the supply modulator output. Since the actual switching frequency of the switching amplifier becomes lower than the targeted one due to additional propagation delay in the power stage, causing larger inductor current ripple and thus the non-linearity at the output. Although larger inductance can suppress the inductor current ripple, the slew rate of inductor current decreases, which degrades tracking ability of the envelope-tracking input envelope signal.

The final issue is considered from the systematic point of view. The envelope shaping function and digital predistortion defined in the Keysight N7614C software can be optimized further to improve the performance of the envelope-tracking power amplifier. Due to lack of experience and background knowledge, the envelope shaping and digital predistortion is determined by trial and error for prototype design.

To sum up, the future work regarding design of the envelope tracking supply modulator includes modification of the system architecture (particularly the current-sensing network), slew rate enhancement and optimized design for LTE signal generation.

References

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