The 281 periodically produces a pulse that is launched down a cable. The returning reflection is allowed to pass into the oscilloscope for measurement. The pulse is specified to be 460 mV, with a rise time of 750 ps or less, with a 5 μs pulse width.
The 281 uses a 1N3130 tunnel diode as a multivibrator. The multivibrator drives a common-base output transistor.