We characterize a 50-GHz comb generator with a sampling oscilloscope. With careful control of the input power, input harmonics, and comb generator temperature, we measure the output spectrum with a standard uncertainty of 0.1 dB and 0.5°. We correct the measurements for time-base distortion, impedance mismatch, an inline attenuator, and the complex frequency response of the oscilloscope's sampler. We also report on the stability of the comb-generator spectrum and the effects of the recorded time windows, drive levels, and temperature. These results provide general guidelines for practioners of high-speed measurements and a benchmark for future inter-laboratory comparisons of harmonic phase-reference calibrations. © 2008 IEEE.