The Tektronix TDS 684B oscilloscope was capable of impressive memory speeds for its day, and [Tom Verbure] was keen to investigate how it was achieved.
This “analog” scope appears to actually use an analog shift register to provide its memory function for the signal being measured.
This is achieved using a chip, believed to be the now-obsolete ADG286D from National Semiconductor, which is a CMOS analog switch with integrated decoders.
This allows the scope to charge 10 parallel capacitors in parallel but read them out serially, achieving very high memory speeds with relatively low-speed ADCs and a modest processor.
It’s an elegant solution for the time, and [Tom] provides lots more insight and a great teardown of this vintage scope in his blog post.
