Design Of 60GHz Direct Conversion Modulators

Direct conversion modulation can simplify transmitter design, and can be applied to 60GHz low cost, compact,high speed digital communication system. But in millimeter-wave band, LO-RF isolation of conventional fundamental balanced mixers, which are basic building blocks of modulators, is relatively low. Larger LO leakage is one of the main factors leading to modulation precision degradation. Even-order subharmonic mixing technology can be used to solve LO leakage. And more, it can simplify the design of millimeter-wave LO sources.Firstly, theories of even-order subhormonic mixer are introduced in detail. Quantitative analysis of relationship between the balance of anti-parallell diodes and LO leakage is given in detail. Two kinds of mixer circuits are designed in this paper. Some important performances of the mixers are simulated and analyzed in ADS software. 60 GHz I/Q modulators are designed including mixers, in-phase power divider, and quadrature power combiner. ADS Harmonic Balance simulation results show that the carrier rejection can be greater than 38 dBc and the residual band rejection can be greater than 21 dBc.Secondly, the performances of the designed modulator are simulated in digital channels using ADS envelope method, including EVM, ACPR, and constellation.Finally, waveguide-to-single ridge waveguide-to-microstrip transition is designed using HFSS FEM method. The return loss of the transition is more than 15 dB and the insertion loss is lower than 0.5 dB over the whole band.