In the design of electronic equipment and systems, delay lines are frequently employed. Up until the present, the most reliable, accurate and accepted method of measuring delay lines made use of a pulse generator and a sampling oscilloscope. Such a method was satisfactory for the following reasons. The delay lines in use typically provided a delay greater than 10 nanoseconds. The pulse generators needed to obtain the delay measurements were required to provide output pulses having 1.0 nanosecond rise times. Such generators are relatively inexpensive and widely available. Most users of the delay lines already had a sampling oscilloscope and the required parts to construct a test fixture. The latter fixture was relatively simple and parisitic reactive effects were substantially negligible.
More recently, the quest for increased speed in electronic systems has led to the need for delay lines within the 1 to 5 nanosecond range, and even a delay of 0.50 nanoseconds is possible. The above mentioned pulse generator/oscilloscope technique is not applicable to measurements of such short lines. A number of problems arise when use of the last mentioned technique is contemplated. Pulse generators are required with pulse rise times much less than 1.0 nanosecond and these are very expensive. The sampling oscilloscope and associated probes must have extremely wide bandwidths. The parasitic effects which heretofore were negligible, became important to the extent that the layout of the test fixture is now a critical factor. Moreover, the trace width of the sampling oscilloscope introduces considerable error in the measurements. Variations in the impedance of the delay line may interfere with the ability to read the oscilloscope correctly. In addition, the process of making such measurements is tedious and time consuming.
What is desired is a delay line tester useful in the nanosecond and subnanosecond range which obviates the foregoing measurement problems. The digital tester of the present invention fills such a need and does so with an inexpensive stand-alone unit providing measurements with greatly increased accuracy and rapidity over previous measuring methods.