The Tektronix Type 661 Oscilloscope is intended for use with two associated plug-in units in a self-contained fractional nanosecond sampling system. An external connector allows use of a digital unit for readout of time and voltage information from the oscilloscope display. The Type 661 is an indicator unit with power supplies for the vertical and horizontal plug-in units. It contains vertical and horizontal crt deflection-plate drivers, a sweep magnifier, and an external horizontal input. Auxiliary circuits offer fronf-panel amplitude and time references, output signals from both the vertical and horizontal systems, and a delayed fast-rise pulse occurring soon after the beginning of each sweep. The delayed pulse can be used to check the vertical sampling unit risetime. Input Characteristics The vertical system of the Type 661 can be driven by any of the Tektronix ‘4' Series sampling units. The equivalent-bandpass, risetime, vertical deflection factor, and input impedance is a function of the individual plug-in unit in use. The horizontal system of the Type 661 can be driven by any of the Tektronix ‘5' Series timing units or by an external input terminal at 25 k input resistance. Individual timing unit sweep rates determine the Type 661 equivalent sweeptime per centimeter. External Horizontal Input—External horizontal input sensitivities of 0.05 to 5 volts/cm in seven calibrated steps, eifher ac or dc coupled. Sampling and timing plug-in units must be in place for the external signals to provide horizontal scanning. If the vertical unit is the Type 4S1 Dual-Trace Sampling Unit, external horizontal signals cannot be applied to the horizontal amplifier when the MODE switch is at A VERT. B HORIZ.