High ACLR 1-bit direct radio frequency converter using symmetric waveform

We have developed a prototype of 1-bit direct radio frequency converter (DRFC) which is a kind of RF transmitter using bandpass delta-sigma modulation (BP-DSM). This prototype has functionalities to synchronously transmit two signals up to 6 Gb/s and to reduce intersymbol interference of eye pattern. In a trial, we transmitted a pair of 1 carrier with 5 MHz bandwidth of LTE at 300 MHz using BP-DSM to aim the MIMO, with an ACLR of 47 dB. This paper investigated the relationship between the ACLR of BP-DSM and analog waveform of 1-bit pulse train using the prototype. To clarify waveform influence, we introduced waveform decomposition into symmetric and asymmetric components to the time axis. We calculated numerical simulations using several types of waveforms varying the transition time. We confirmed that the asymmetric component causes spectrum leakage at adjacent channels. We finally concluded that the optimal waveform for BP-DSM is symmetrical to the time axis and that the ACLR in the prototype will improve to 60 dB by simulation.