We present a frequency-tunable photonic millimeter- wave synthesizer based upon optical heterodyning. The photonic synthesizer consists of a wavelength-tunable two-mode laser setup, a high-power pin photodetector and a millimeter-wave (W- band) limiting amplifier. The key component of the synthesizer is the high-power waveguide pin photodiode featuring a WR10 waveguide output port. The photodiode exhibits a flat frequency response in the W-band and provides millimeter-wave output power levels up to -4.88 dBm. The developed photonic synthesizer can be continuously tuned over the entire W-band from 69-112 GHz and within that frequency range it offers an extremely flat response with power fluctuations below 3 dB. The synthesizer's total frequency-tunability is 50 GHz and its output power can be linearly adjusted from about -35 dBm to 0 dBm.