A passive integrated unbalanced Mach Zehnder interferometer (UMZ) used in a frequency modulated fiber-optic link converts the microwave subcarrier frequencies of digital signals. As expected in theory, Bit Error Rate (BER) measurements for BPSK modulation format show better results using UMZ than for classical optical microwave mixing method using an active external optical modulator. Using the UMZ, conversions of microwave subcarrier of digital signals are reached for data rate of 500 Mbit/s. Measurements with QPSK modulation format show low BER and no IQ imbalance in the constellation diagram at 100 Mbit/s.