Self Matched Line

Self Matched Line & Reverse Terminated Lines

Often it is necessary to produce a pulse into an undefined or arbitrary load. The conventional pulse forming circuits will only deliver rectangular pusles into matched loads. The "Self Matched" configuration overcomes this problem.
To our knowledge this was first published by M. Ishii and H. Yamada rev. Sci. Instrum. 56 (11) November 1985 although it may have been common knowledge elsewhere.

Consider the following circuit.

If both switches are closed simultaneously then any reflections from the load are absorbed by the terminating resistor at the left. The result is a square pulse into any resistive load with an amplitude that depends upon the load resistance. If the load is reactive there still will not be any reflections after two round trip times of the cable although there may still be some exponential components.
By activating the switches at different times it is possible to vary the pulse length. It is necessary however, that the switch S₁ be closed by the time the edge from S₂ arrives at S₁ This configuration is used in our Photocathode pulser.

By rearranging the switches we can obtain

and then combining the switches into one and moving the grounding point:

This configuration although simple in principal does have the drawback that in co-axial geometry the outer of the cable has to be floated by the charge voltage. Also the two ends of the cable and the terminating resistor have to be in a very low inductance configuration to obtain a good risetime. The switch has to discharge not only the cable capacitance but also the stray cpacitance of the cable outer to ground. However, it can produce very good results if built carefully.

Reverse terminated line

This configuration is useful for charging (or discharging) a capacitive load and is used in our simple "Q" switch pusler. The switch sends a step wave down the line of half the amplitude of the charge voltage. This arrives at the capacitive load which , after several RC=Z₀ times will reflect the pulse delivering twice the step voltage into the capacitor. When this step arrives back at the switch it is absorbed in the terminating resistor. The result is that the voltage on the load stays there a long time.