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.