An all-optical sampling mixer based on a Semiconductor Optical Amplifier Mach-Zehnder Interferometer is performed. The frequency properties of a sampled signal are exploited to obtain both up and down frequency conversion of QPSK data. The optical pulse source, generating 10-ps width pulses at the repetition rate of 7.8 GHz as a sampling signal, allows operation in the 0.5-39.5 GHz range. The conversion gain of the all-optical sampling mixer is measured over all the frequency range. It varies over this range from 20.4 to 0 dB for frequency down-conversion and from 15.5 to -13.4 dB for frequency up-conversion. QPSK data at symbol rates between 8 MSymb/s to 128 MSymb/s are frequency converted. The Error Vector Magnitude is used to evaluate the quality of the frequency conversion. Exploitable EVMs in the range 14 to 27.4% are reached for up-conversion operation to the higher frequency for all the symbol rates. For down-conversion the EVM varies from 6.7 to 29.2% at the symbol rate of 64 MSymb/s for all the frequency range.