Direct quadrature modulator MMIC for future terahertz communications at 300 GHz

We present a direct quadrature modulator MMIC based on InP HBTs for generating QPSK signals at 300 GHz. The modulator consists of two single balanced mixers and passive baluns and couplers. In this work, two different transmission line structures - thin-film microstrip lines and inverted microstrip lines - were simultaneously utilized in the single chip for passive couplers and active mixers, respectively, because the former provide less loss, resulting in better balance between ports of the couplers and the latter are suitable for short and compact interconnections between transistors. Passive couplers showed maximum imbalance in amplitude and phase of less than 1 dB and 4 degrees, respectively. A half Gilbert-cell based unit mixer including the passive balun for the RF output exhibits approximately 15 dB-conversion loss over a 40-GHz frequency span with a single-end IF input and -5-dBm LO pumping at 300 GHz, when single bias of -4 V and total current of around 10 mA are applied. The complete modulator shows >20-dB LO-RF isolation. According to the static constellation of the direct quadrature modulator, imbalance between I- and Q-components is expected to be approximately ±0.6-dB∠4-degrees at maximum.