The Tektronix 3A1S is a vertical plug-in for the 561S. It is specified to have an input impedance of 47pF in parallel with 1 megohm. Input sensitivity in a 561S is from 10mV/div to 10V/div. Risetime is specified to be 14ns, giving a -3dB frequency of 25MHz. The input signal passes through a compensated attenuator switch and then into a 7586 cathode-follower biased with 2mA plate current.