Mechanical variable delays may appear simple, but their basic design has traditionally had inherent limitations that rendered them useful only for infrequent changes in delay made either manually with a screwdriver, micrometer, or by a slow-speed motor.

Several mechanical delay-line configurations have been patented, and while each varies delay differently, the principle is the same: The electrical length of a conductor or cable is varied mechanically by moving the conductor axially with respect to a contact. As a result, a signal applied to one end of the cable or conductor is delayed by a specific amount depending on the position of the contact on the cable or conductor (i.e., changing the length of conductor the signal must pass through before emerging from the other end).

The problems with mechanical delay lines arise from friction between the axially moving conductor and the contact. Friction between the sliding contact of the shorting elements against the coaxially extending conductors causes wear, which increases with the velocity at which the movement takes place. This friction also affects the delay's ability to precisely position the movable coaxial cable to achieve the desired delay with absolute accuracy. The result was, until Dr. Siegfried Knorr's invention, mechanical delay lines whose delay values could not be varied rapidly, accurately, or repeatedly over many operations without failure.

Dr. Knorr essentially removed these limitations by dramatically reducing friction (and thus wear) at two critical interfaces. The first is an interface ring soldered to the outer conductor of the semi-rigid cable whose electrical length is varied. A spring separates the interface ring from the cavity walls, forming an air gap, which prevents the outside shield and interface ring from touching the cavity walls. Contact is instead made through the spring, which provides gentle but sufficient tension to ensure a good connection. The second interface also uses a spring to form an air gap that keeps the center pin of the cable from touching the cavity walls. Both springs are plated in a proprietary process to ensure maximum operating life, and other surfaces are either gold or silver plated as well. The reduction in friction also overcomes "overshoot" (incorrect positioning) that results when the cable is moved rapidly to adjust the delay.

Dr. Knorr's invention made the mechanical delay usable for the first time for producing different delay values hundreds of thousands of times at high speed without failure. By coupling the movable elements with a precisely controllable, high-resolution stepper motor, the conductors can be accurately and quickly positioned with respect to the contact surfaces with accuracy of 0.1 percent ±0.25 ps. The stepper motor allows the delay to move from 0 to 625 ps in less than 500 ms. It was initially controlled automatically by commands over GPIB or locally by manually entering values into a keypad. In the new PDL-100A models, these commands can be communicated also via RS-232C serial communications, or over TCP/IP on Ethernet LAN.