7.5 GHz-Band Digital Beamforming Using 1-bit Direct Digital RF Transmitter with 10GbE Optical Module

1-bit Digital RF transmitter using band-pass delta-sigma modulation which generates RF signal from 1-bit data stream can miniaturize the RF transmitter for the digital beamforming (DBF) antenna system. Some research works have been reported that demonstration of DBF transmitter using Digital RF transmitter, however, the operational frequency band is limited by half of the sampling rate (about below 6 GHz). In this study, we demonstrate 7.5 GHz band DBF by Direct Digital RF transmitter using 10GbE optical module (SFP+). By using SFP+, easy, low-cost implementation and low hardware complexity are realized. In ordinary cases, RF frequency which the transmitter can handle is limited below 5 GHz because of the operational frequency of SFP +. On the other hand, by using Direct Digital RF transmitter which used the folded image component of 1-bit delta-sigma modulated wave, the proposed DBF transmitter can handle RF signal above 5 GHz. A four-element DBF system that consists of the proposed transmitter is fabricated. In order to evaluate the DBF characteristics, radiation patterns and modulated signal quality are measured. From measurement results, the main robe can be handled in the desired direction and the beam pattern is in good agreement with EM simulation. From the QPSK signal transmission measurement, EVM
$\geq 2.0$
% and ACLR ≥-37.8 dBc are obtained.