Broadband receiver functions derived from teleseismic P-waveforms recorded on Regional Seismic Test Network (RSTN) stations in Eastern North America contain detailed information about the crust-mantle boundary. This paper examines receiver functions from three stations (RSCP, RSNY, and RSON) to illustrate the variability in response of the continental ''Moho'' and to demonstrate the utility of broadband data in studying these variations. Of particular interest is the clear evidence for frequency-dependence of the amplitudes of converted phases arriving from the crust-mantle boundary. The shear velocity structures for these stations obtained by inverting the radial component of the receiver functions in the time domain indicate that this frequency-dependence is a measure of the ''sharpness'' of the crust-mantle boundary region.