The Development of a Narrow-Band Time-Domain Reflectometer

The capability of accurately determining the value of impedance discontinuities in transmission line systems is obtained by replacing the dc pulse with a gated RF source in a conventional time-domain reflectometer (TDR) system. Such a system allows complex impedance measurements to be made upon elements in a system that are physically inaccessible. Due to the sensitivity available in TDR systems, low orders of magnitude of return losses are easily detectable and measurable. In systems where multiple impedance discontinuities exist, each individual discontinuity may be isolated and hence analyzed separately. The modifications to a conventional TDR system are described in this paper. A method of using the modified narrow-band TDR system to measure complex impedance is presented. This method is complementary to that used by the Hewlett-Packard model 1580A narrow-band TDR and offers the avantages of both phase information and lower modulation frequency.