This paper presents the design of an upconverter system based on Commercial Off-The-Shelf (COTS) components for the 17.3-21.2 GHz space-to-Earth Fixed Satellite Services (FSS) frequency band, with a variable Intermediate Frequency (IF) up to 4 GHz. The proposed system combines Upper SideBand (USB) and Lower Side-Band (LSB) frequency conversion modes and Local Oscillator (LO) leakage nulling techniques to enhance Spurious-Free Dynamic Range (SFDR). LO leakage nulling was used to show that it is possible to improve a minimum 31 dBc SFDR in the FSS bandwidth to 48 dBc. The upconverter was connected to a 10 W power amplifier, limited in the 17.3-20.2 GHz band, and its performance was compared with the International Telecommunication Union (ITU) recommendation on spurious emissions. The system is compliant for all the RF bandwidth within IF band 2.5-3.5 GHz and in the RF sub-band 17.8-20.2 GHz for IF band 3.5-4.0 GHz. The proposed system was then evaluated at sample RF and IF frequencies in terms of Error Vector Magnitude (EVM).