Abstract

1. Penetration Depth Measurements using Time Domain Reflectometry José Medina¹, Dereje Worku², Shalton Evans³ ¹Universidad Del Turabo School of Engineering, Gurabo, Puerto Rico ²North Carolina A&T State University College of Engineering, North Carolina ³The College of the Bahamas School of Natural Sciences, Environmental Studies and Technology, Bahamas • Abstract • Time Domain Reflectometry (TDR) can be a very powerful technique to study material properties. Surprisingly, there has not been much study of material properties using this method. The present work shows that commercially available TDR can be used to measure the inductance of a coil and the surface impedance of a superconducting sample placed inside the coil. Inductances less than 100 nH and the critical temperature of a YBCO sample were measured. Other types of instruments such as: Network Analyzer and the Tunnel Diode Oscillator (TDO) where used to compare the TDR values. The method can further be used for studies at lower temperatures and in magnetic field. • Introduction • What is Time Domain Reflectometry? • TDR measures the reflected wave from a voltage signal traveling through a transmission line. Using a step generator and an oscilloscope, a pulse of energy is launched into the transmission line under investigation. This pulse measure the characteristic impedance along the line. If impedances discontinuity exists, each discontinuity will create an echo that is monitored by the oscilloscope at that particular point of the line. The next figure is an example of a basic RL circuit measurement in the TDR. • Procedure: • Create a 2mm diameter copper coil with five turns. • Measure the inductance of the coil using the Network Analyzer. • Analyze the properties of the coil with and without the YBCO sample using the TDR during the cooling and warming processes. • Analyze the properties of the coil with the YBCO using the TDO during the same processes. • Compare the measured values. Diagrams Experimental setup TDO measurement for the coil with YBCO sample Frequency change during the warming process 2mm diameter copper coil with five turns Network Analyzer Yttrium Barium Copper Oxide structure Cooling Probe TDR measurements The evolution of the electric pulse with temperature TDR diagram & picture Results The inductance versus temperature graph of the coil containing YBCO confirms that this sample has a critical temperature in the range of 90-92K. Using the TDO and measuring the warming process in terms of frequency versus the resistance, a big change in frequency was measured at the same temperature range measured in the TDR, as the resistance of the thermometer goes down. Inductance changes with Temperature Empty coil Conclusion In this experiment a new way of measuring material properties is presented. One of which is the phase transition of conductors. We determine the transition temperature Tc, from the inductance measurements. This method can further be used for studies at lower temperatures. Once again we can say that the TDR provides amore intuitive and direct look at the device under test. Coil with YBCO sample inside References: Van Degrift Craig T. Tunnel Diode Oscillator for 0.001 ppm measurements at low Temperature (1974). Re. Sci. Instru., Vol. 46, No. 5 May 1975 Dolocan Volcu. Determination of material properties by using the reflection pulse method (1994). Re. Sci. Instru. 65 (11), November, 1994 Hewlett Packard application note 1304-2. Time Domain Reflectometry Theory. Tektronix application note. TDR Impedance Measurements: A Foundation for Signal Integrity. Acknowledgements: Thanks to Dr. Stanley Tozer, Dr. Catalin Martin, Dr. Eric Palm, and all the personnel at the NHMFL. Thanks to José Sanchez, to all the CIRL/REU staff, and The National Science Foundation.