Method and apparatus for calibrating a frequency domain reflectometer

A method for calibrating a frequency domain reflectometer system utilizing a multi-port junction coupled to an RF frequency source and a transmission line, wherein the distance to faults of which are to be measured by generating a complex reflection coefficient from the analog-to-digital conversion of the power detected by the power detecting circuitry are detected at selected outputs of the multi-port junction, and coupling the reflection coefficient to an Inverse Fourier Transform to obtain distance to the faults comprising the steps of:

• measuring the complex scattering parameters of the multi-port junction circuit;
• creating a system model using the transfer function of the frequency domain reflectometer system, including the measured scattering parameters of the multi-port junction circuit and parameters of standard component models of the other components of the reflectometer system, including calibration loads and analog-to-digital converters to create a model of what the analog-to-digital conversion of the power outputs, the model including gain and offset referring respectively to the power detecting circuitry and RF signal source;
• varying the gain and offset in the model until the error between the gain and offset of the model and the measured gain and offset is minimized;
• using the model gain and offset resulting in minimum error in the generation of the complex reflection coefficient to provide a compensated complex reflection coefficient;
• using the compensated complex reflection coefficient to measure distance to faults;
• coupling the compensated complex reflection coefficient to an Inverse Fourier Transform;
• and,
• displaying distance to faults from the output of the Inverse Fourier Transform.

A method for calibrating a frequency domain reflectometer connected to a device and having an RF frequency source, comprising the steps of:

• taking selected outputs from a multi-port junction and measuring the power thereat with power measuring circuits;
• generating a complex reflection coefficient as a function of frequency from the measured power based on a gain coefficient and an offset coefficient;
• presenting the multi-port junction with a number of calibration circuits;
• measuring the effect of the calibration circuits on the reflectometer;
• comparing the results from the measuring step with modeled characteristics of the reflectometer, including modeled characteristics of the calibration circuits;
• generating a fresh set of gain and offset coefficients for use in the generation of the complex reflection coefficient, thus to calibrate the complex reflection coefficient using the fresh set of gain and offset coefficients to provide a compensated complex reflection coefficient;
• and,
• displaying the distance to a reflection based on the compensated complex reflection coefficient.

A method for a frequency domain reflectometer system utilizing a multi-port junction coupled to an RF frequency source and a device to estimate a complex reflection coefficient from the analog-to-digital conversion of the power detected by power detecting circuitry at selected outputs of the multi-port junction comprising the steps of:

• measuring the complex scattering parameters of the multi-port junction;
• creating a system model using the transfer function of the frequency domain reflectometer system, including the measured scattering parameters of the multi-port junction and parameters of standard component models of the other components of the system including calibration loads and analog-to-digital converters to create a model of the analog-to-digital conversion of the power outputs, the model including gain and offset, referring respectively to the power detecting circuitry and RF signal source;
• calculating the gain and offset in the model that minimizes the error between the gain and offset of the model and the measured gain and offset;
• using the calculated gain and offset that minimizes the error in the estimation of the complex reflection coefficient to provide a compensated complex reflection coefficient;
• and,
• displaying the result of using the compensated complex reflection coefficient.

A method for determining distances to faults in a transmission line using a frequency domain reflectometer in a multi-fault environment comprising the steps of:

• providing a multi-port junction having detection circuitry at selected ports and predetermined scattering parameters;
• generating a compensated complex reflection coefficient from selected output ports of the multi-port junction by using a calibrated load and an RF source at input ports of the multi-port junction, including changing the un-calibrated offset and gain terms in the complex reflection coefficient for each frequency and each multi-port junction port to calibrated offset and gain terms across frequency and power, thus to characterize both variability in RF source power and variability in detection circuitry characteristics;
• coupling a transmission line under test to the multi-port junction;
• coupling an RF test signal to the multi-port junction;
• generating for the output of the multi-port junction one complex reflection coefficient for each frequency for the transmission line under test using the calibrated offset and gain terms;
• performing a modified Inverse Fourier Transform on the compensated complex reflection coefficient to obtain one complex reflection coefficient for each distance, the modified Inverse Fourier Transform using a model that takes into account phase shift per unit length of transmission line and attenuation per unit length of transmission line;
• and,
• identifying and displaying from each complex reflection coefficient for each distance the distances to candidate faults.

A method for determining distances to faults in a transmission line using a frequency domain reflectometer in a multi-fault environment comprising the steps of:

• providing a multi-port junction having detection circuitry at selected ports and predetermined scattering parameters;
• generating a compensated complex reflection coefficient from selected output ports of the multi-port junction by using a calibrated load and an RF source at input ports of the multi-port junction, including changing the un-calibrated offset and gain terms in the complex reflection coefficient for each frequency and each multi-port junction port to calibrated offset and gain terms across frequency and power, thus to characterize both variability in RF source power and variability in detection circuitry characteristics;
• coupling a transmission line under test to the multi-port junction;
• coupling an RF test signal to the multi-port junction;
• generating from the output of the multi-port junction one compensated complex reflection coefficient for each frequency for the transmission line under test using the calibrated offset and gain terms;
• performing an Inverse Fourier Transform on the compensated complex reflection coefficient to obtain one complex reflection coefficient for each distance;
• and,
• identifying and displaying from each complex reflection coefficient for each distance the distances to candidate faults.