Active Noise Filtering for X -Band GaN Transmitters with Bitstream Modulations

Bitstream transmitters based on bandpass or envelope delta-sigma modulation promise high power efficiency for broadband communications with nonconstant-envelope modulations, but at the price of elevated quantization noise. An active noise filtering technique is proposed in this paper to address the quantization noise issue of such transmitters. The essential concept of active noise cancellation is to utilize multiple power amplifiers (PAs) with identical or similar transfer characteristics fed with the same input, but with different time delays. The difference in time delay determines whether the outputs of the PAs are combined in phase or out of phase at a certain frequency, which forms the passband or the stopband of the active filter. The active noise filtering technique can potentially replace the external high-Q passive bandpass filter typically needed for such transmitters. A two-channel bitstream transmitter with active noise cancellation operating at the X -band is fabricated with 0.25- \mu \text{m} discrete GaN HEMT devices. It has been demonstrated experimentally that the proposed active noise filtering architecture effectively suppresses quantization noise as expected and delivers a 34.5-dBm output power with 37.6% drain efficiency and 30% power added efficiency for a single-channel WCDMA signal with 100-MHz bandwidth and 5.3-dB peak-to-average ratio. Adjacent channel leakage power ratio (ACLR) at 100-MHz offsets is measured to be 36.6/35.4 dBc without using digital predistortion.