[en] In a proposed SSC linac injector the Low-Energy Booster lattice painting scheme requires a 50-MHz microbunch structure with transverse and longitudinal normalized rms emittances of less than 0.45 π-mm-mrad and 1.7 x 10^-5 π·eV·s, respectively, at 600 MeV. A 50-MHz RFQ design does not meet the longitudinal emittance requirements; a 150-MHz RFQ can do so, but requires a chopping scheme that produces a clean 50-MHz beam structure without degrading emittance. We present an rf deflector-chopper conceptual design that converts a 150-MHz RFQ beam into a 50-MHz microbunch beam structure while matching transversely and longitudinally to a following 450-MHz DTL. Multiparticle simulation with 3-D space charge shows negligible emittance growth. Output beam emittances are factors of 3 below requirements; further acceleration to 600 MeV should produce only small additional growth. The chopper is 54 cm long, comprising an rf deflector, two rebunchers, five small permanent magnet quadrupoles, and a beam dump. Similar schemes could use a 200- or 250-MHz RFQ for even smaller longitudinal emittance. 6 refs., 5 figs., 2 tabs