Gigabit QPSK modulator

We describe a QPSK (quaternary phase shift keyed) modulator operating at gigabit data transmission rates at a carrier frequency of 13 GHz. The modulator circuit can also be operated as a QPSK demodulator. A modulator-de-modulator pair operating in tandem showed clean eye diagrams of the recovered data trains on each port up to l.5 Gbit/s with corresponding error rates of less than 10¿11. The circuit can be readily scaled to higher frequencies with a proportional increase in the information rate.