A Precise, Wideband, Continuously Variable Delay Line

Abstract

There has long been a requirement for a device with which precise measurement of extremely short time intervals could be made. In addition, there is an ever-increasing need for a passive device through which signals may be transmitted and delayed without distortion of waveshape. This paper describes a linear, passive, jitter-free, continuously-variable delay line capable of making such measurements with high precision and good resolution. Such a line, for example, will be useful in color television systems for the precise phase matching of the three color component signals. It enables the measurement of radar ranges below 1000 feet where conventional, vacuum-tube delay circuits fail to perform. While variable delay lines have been built before, they have generally produced severe waveshape distortion, thus rendering themselves useless as signal transmission devices. The delay line to be described transmits pulse signals with a high degree of fidelity. Lines having maximum delays of 0.1 to 0.5 microseconds and impedances up to 1350 ohms have been built, and lines having delays of I microsecond or more are practical. A typical delay line that will be described has a total delay time of 0.2 microseconds, an impedance of 1350 ohms, and a rise time of O.O15 microseconds. The attenuation through this line is less than I db, and its resolution enables settings in the order of 0.01% of total delay, equivalent to 0.02 millimicroseconds. The present design has a theoretical resolution capability of better than I part in 8,000. Due to the nature of its structure, it has exhibited negligible overshoot or phase distortion, and does not require equalization. Resistive terminations Pave generally proved satisfactory. Physically, the variable delay line is a relatively small, compact unit resembling a multi-turn, helical potentiometer, and may be mounted in equipments or on a panel with an indicating dial.