In a time domain reflectometer a first step generator may include two branches of elements which are current sensitive and will produce a gap voltage if the current exceeds a specific threshold. Each branch may include either Josephson junction devices or tunnel diodes. When a trigger signal plus a bias current exceed a threshold value a first one of the elements in a first branch produces a gap voltage and diverts current to the second branch. The current causes all of the elements in the second branch to produce gap voltages and the current is diverted back to the first branch to switch the states of the remaining elements of the first branch. Once all of the elements in both branches produce gap voltages, the total signal produced by these elements is transmitted to a third output branch. The circuit produces a fast step signal and lends itself to expansibility whereby the amplitude of the step signal can be varied by varying the number of elements in the branches of the circuit.