Spur-Reduced and Efficiency-Enhanced Pulse-Modulated Polar Transmitters with Output Direct Absorptive Filter Connection

This article presents spur reduction and efficiency enhancement techniques for pulse-modulated polar transmitters (PMPTs). The technique allows low-power out-of-band odd spurs in PMPTs to be suppressed deeply before going into power amplifiers (PAs). By configuring the PAs as a balanced Doherty amplifier for efficiency improvement with quad-phase aliasing-free digital pulsewidth modulation (PWM), the several even spurs can be separated directly into the isolation port of the output quadrature coupler. The residual even spurs can be easily suppressed using direct connection of an absorptive bandpass filter (BPF) behind each PA. Using the proposed architecture, the prototype transmitter achieved 49.4% drain efficiency at an output channel power of 28 dBm using 20-MHz bandwidth long-term evolution (LTE) 16-quadrature amplitude modulation (QAM) test signals at 836.5 MHz. After output signal restoration by the proposed absorptive filters, an output power level of 27.1 dBm and 40% drain efficiency were achieved while keeping the spur level close to -60 dBc without using any surface acoustic wave (SAW) filter.