Flexible bandwidth reduction technique for envelope tracking using low-pass finite impulse response filters

In this Letter, low-pass finite impulse response filters are proposed to flexibly reduce the envelope bandwidth (BW) requirements for envelope tracking (ET) supply modulators. A second-order power ET (PET) approach is used as a comparative study, and a `design zone' for shaping function coefficients is identified for the first time. Proposed 20 and 10 MHz low-pass filters (LPFs) are used to reduce the effective envelope BW in response to a 20 MHz long-term evolution signal applied to a 10 W power amplifier (PA). Experimental results verify that in comparison with PET, the proposed LPF method has the benefit of allowing arbitrary BW reduction and is compatible with the use of customised shaping functions. It is also shown that significant memory effects can be introduced when the effective envelope BW is reduced to less than the RF BW. To investigate the linearisability of the ET PA, the linearity before and after digital pre-distortion is also measured