fashion. The nonlinear C᎐V characteristics of the HBT produce phase errors when controlled by the conventional complementary control signal, leading to a symmetrical S-shaped constellation. The ''corrected'' constellation is achieved by a simple calibration procedure on both single-stage modulators, resulting in an almost perfect biphase amplitude constellation at 38 GHz. This result illustrates that it would be possible to realize a complete balanced vector modulator successfully at millimeter-wave frequencies using HBT technology.
Conclusions
The design and performance of single-stage and balanced biphase amplitude modulators operating at 38 GHz have been presented in this paper. The direct modulators employ cold HBTs as variable resistance reflection terminations, and it is found that they suffer from too high a series resistance in the ON stage in comparison to earlier work employing cold pHEMTs. It has been shown, however, that the balancing topology can remove the amplitude and phase errors caused by the parasitics of the cold HBTs, which ruin the single-stage performance at higher frequencies. Furthermore, by replacing the four normal Lange couplers with miniaturized microstrip couplers, the overall size of the balanced MMIC chip is reduced to 1 mm 2 . It is found that this technique is very robust, and the resulting compact layout can greatly reduce the cost of expensive transmitter chip area in millimeter-wave digital communications applications.