In Fig 1, an ECL exclusive-NOR gate (F100107) and a length of cable form a simple ring oscillator. The delay from the cable and the gate determine the ring oscillator’s frequency. A 100m cable yields approximately a 600-MHz oscillation frequency. The gate’s second output connects to a frequency counter.

The gate’s delay is the sum of its high-to-low and low-to-high transition times, t_PHL and t_PLH. Using the period of the ring oscillator, T=1/f_OSC, the length of the cable (L) is L=(T/2-(t_PHL+t_PLH))×v, where v is the propagation velocity of the cable. In the case of coaxial cable, v=1/CZ_O, where C is the capacitance per unit length and Z_O is the cable’s characteristic impedance.

To test the sensitivity and accuracy of this technique, we spliced short increments of cable into the ring. The setup could accurately measure small changes in length to better than 4% of the length of the added increment (which is accurate to 0.025% of the cable’s total length). Be sure to use the proper pull-down resistors for ECL; also, determine if the cable you are measuring actually exhibits its manufacturer’s claimed nominal impedance. (DI #1420)