Effects of Harmonics from Frequency-Multiplicative Carrier Generation in a Superheterodyne 300 GHz Transmit Frontend

In this paper, the limitations of transmit signal quality, arising from intermodulation of unwanted harmonics caused by the electronic carrier generation based on frequency multiplication, in a superheterodyne 300 GHz transmit analog frontend are analyzed. Using an electronic microwave frequency synthesizer in conjunction with cascaded frequency multipliers delivering an overall multiplication factor of 24, a 210 to 228 GHz carrier is generated. The fourth harmonic generated by the on-chip integrated frequency multiplier-by-three is measured to be present in the RF frequency domain with significant power levels, creating interferers for the modulated signals. An analysis of the spurious tone is performed as a function of the local oscillator power and frequency as well as IF input power to obtain signal-to-interferer power levels as the relevant frequencies lie within the transmitter RF frequency band of operation. Based on continuous wave measurements, system-level simulations are carried out, predicting the signal qulaity limitations for a 1.6 GBd 16-QAM modulated signal, recreating a measurement scenario using a modem-based real-time wireless link.