1. An apparatus for drilling a borehole and determining a parameter of interest of a formation surrounding the borehole, said apparatus comprising: (a) a longitudinal member for rotating a drill bit and adapted to be conveyed in the borehole; (b) a nuclear magnetic resonance (NMR) sensor assembly including at least one member slidably coupled to and spaced apart from said longitudinal member defining a flow path for drilling fluid therebetween, said NMR sensor assembly producing a pulsed RF field for obtaining measurements indicative of the parameter of interest of the formation, said RF field characterized by a plurality of parameters; and (c) a downhole processor for varying at least one parameter of the pulsed RF field.