4S1

The Tektronix Type 4S1 is a dual-trace sampling unit for the 661. Each channel has a trigger pickoff, a delay line, and a sampler. The sampling pulses for each channel are generated by a step recovery diode are are fed to a sampling bridge made with of four GaAs diodes. These diodes are fragile and exact replacements are hard to find. People have improvised replacements using modern integrated Schottky diodes such as those in HP/Agilent's HSMS-28x series. The 4S1 has a dual-trace mode, where the A waveform and the B waveform are both displayed in the time domain at the same time with a single trigger source and a single equivalent-time sweep rate. Another mode is called "A vert/B horiz". This mode is used in conjunction with an X-Y mode switch on the panel of the 661.