Time-stretch photonic analog-to-digital converter (ADC) technology is used to make an optical front end that compresses radio-frequency (RF) bandwidth before input to a digital oscilloscope. To operate a time-stretch ADC in a continuous-time mode for bandwidth compression, the optical signal on which the RF is modulated must be segmented and demultiplexed. We demonstrate both spectral and temporal methods for overlapping the channels. Using the temporal method, we obtain a compression ratio of 3 with four channels. Mating this optical front end with a state-of-the-art four-channel digital oscilloscope with an input bandwidth of 16 GHz and a sampling rate of 50 GS/s gives a digitizer with 150 GS/s and an input bandwidth of 48 GHz. We digitize RF signals up to 45 GHz and obtain effective number of bits (ENOB) ~ 2.8 with single channels and ~ 2.5 with multiple channels, both measured over the 48-GHz instantaneous bandwidth of our system.