Three systems have been developed to measure the frequency response of high-speed photo-receivers to 50 GHz. I will describe the systems and their calibration, and show measurements made on the three systems which are in good agreement. (1) A fiber and diffraction grating pair compresses the pulses from a mode-locked 1060 nm YAG laser to 2.5 pS FWHM. Peak optical power is adjustable to more than one Watt. We measure a photoreceiver's impulse response and the autocorrelation function of the optical impulse. From these we calculate the photoreceiver's frequency response via Fourier transform. We can observe the broadening of a photoreceiver's impulse response with increasing peak optical power. (2) Light from a pair of diode-pumped 1320nm Nd:YAG ring lasers is combined. The photoreceiver responds to the optical power envelope which varies at the difference frequency. We adjust the difference frequency by changing the lasers' temperature, and record the photoreceiver's frequency response. (3) The frequency response of a LiNbO3 waveguide optical modulator is measured using a two-tone technique. It is then used in both fundamental and second-harmonic generation modes as a calibrated source to measure a photoreceiver's frequency response.