Up and down frequency conversions in the range of 0.5–39.5 GHz are performed by using a Semiconductor Optical Amplifier–Mach-Zehnder interferometer (SOA-MZI) in a differential configuration as a sampling mixer. The authors show that increasing the sampling frequency by a ratio of 2.5 (from 7.8 to 19.5 GHz) improves the efficiency and the quality of the optical transmission of QPSK and OFDM signals due to a better signal level and a lower aliased noise power when the sampling rate is higher. At the high sampling rate, the obtained EVM is sufficiently low for allowing data bit rates higher than 1 Gbit/s and 245 Mbit/s, respectively, for QPSK and OFDM modulated data.