Correction of transmission line induced phase and amplitude errors in reflectivity measurements

Abstract

Various examples of methods and systems are disclosed for correction of phase and amplitude errors that occur in transmission lines connecting transmitter/receiver devices to measurement fixtures. In one example, a method is described that includes using time domain processing to determine a phase shift from the measurement fixture that can occur between calibration measurements and measurements of the specimen under test. In another example, a method is described that includes frequency-domain processing of the signals to obtain both phase and amplitude corrections. Including these phase and amplitude corrections in the calibration procedure can reduce or minimize the errors induced in the measurements when the transmission line(s) experience either temperature changes or physical deflections, among other things.

Therefore, at least the following is claimed: 
     
         1 . A method for correction of transmission line induced errors, the method comprising:
 collecting frequency dependent reflection data of a specimen under test (SUT) via a transmission line coupled to a measurement fixture;   transforming the frequency dependent reflection data of the SUT to corresponding time domain reflection data of the SUT;   identifying a time location of a reflection from the measurement fixture based on the time domain reflection data of the SUT;   determining a SUT-reference time delay between the time location of the reflection from the measurement fixture in the time domain reflection data of the SUT and a time location of a reflection from the measurement fixture in time domain reflection data of a reflection reference; and   applying a phase shift correction to the frequency dependent reflection data of the SUT to generate corrected SUT reflection data, the phase shift correction based upon the SUT-reference time delay.   
     
     
         2 . The method of  claim 1 , further comprising determining a calibrated SUT response based at least in part upon the corrected SUT reflection data. 
     
     
         3 . The method of  claim 1 , wherein the reflection reference is free space without a calibration standard or SUT. 
     
     
         4 . The method of  claim 1 , wherein the reflection reference is a calibration standard. 
     
     
         5 . The method of  claim 1 , further comprising:
 collecting frequency dependent reflection data of the reflection reference via the transmission line coupled to the measurement fixture;   transforming the frequency dependent reflection data of the reflection reference to the time domain reflection data of the reflection reference;   identifying the time location of the reflection from the measurement fixture based on the time domain reflection data of the reflection reference.   
     
     
         6 . The method of  claim 1 , further comprising:
 determining a calibration time delay between a time location of a reflection from the measurement fixture in time domain reflection data of a calibration standard and the time location of the reflection from the measurement fixture in the time domain reflection data of the reflection reference; and   applying a phase shift correction to the frequency dependent reflection data of the calibration standard to generate corrected calibration standard reflection data, the phase shift correction based upon the calibration time delay.   
     
     
         7 . The method of  claim 6 , further comprising:
 collecting frequency dependent reflection data of the calibration standard via the transmission line coupled to the measurement fixture;   transforming the frequency dependent reflection data of the calibration standard to the time domain reflection data of the calibration standard;   identifying the time location of the reflection from the measurement fixture based on the time domain reflection data of the reference calibration standard.   
     
     
         8 . The method of  claim 6 , further comprising determining a calibrated SUT response based at least in part upon the corrected SUT reflection data and the corrected calibration standard reflection data. 
     
     
         9 . The method of  claim 8 , wherein the calibrated SUT response is further based upon isolation reflection data collected via the transmission line coupled to the measurement fixture. 
     
     
         10 . The method of  claim 9 , further comprising:
 collecting frequency dependent reflection data of free space without a calibration standard or SUT via the transmission line coupled to the measurement fixture;   transforming the frequency dependent reflection data of free space to the isolation reflection data.   
     
     
         11 . The method of  claim 1 , wherein the frequency dependent reflection data is collected at a plurality of excitation frequencies within a predefined range of frequencies. 
     
     
         12 . The method of  claim 11 , wherein a network analyzer sequentially provides the plurality of excitation frequencies and collects the frequency dependent reflection data at each of the plurality of excitation frequencies via the transmission line coupled to the measurement fixture. 
     
     
         13 . The method of  claim 1 , wherein the SUT-reference time delay is determined by iterative fitting of the time domain reflection data of the SUT and the time domain reflection data of the reflection reference. 
     
     
         14 . The method of  claim 1 , wherein the transmission line comprises a coaxial cable, a stripline, a waveguide, a microstrip, or a coplanar line. 
     
     
         15 . The method of  claim 1 , further comprising:
 collecting second frequency dependent reflection data of the SUT via a second transmission line coupled to a second measurement fixture;   transforming the second frequency dependent reflection data of the SUT to corresponding second time domain reflection data of the SUT;   identifying a time location of a reflection from the second measurement fixture based on the second time domain reflection data of the SUT;   determining a second SUT-reference time delay between the time location of the reflection from the second measurement fixture in the second time domain reflection data of the SUT and a time location of a reflection from the second measurement fixture in second time domain transmission data of the reflection reference; and   applying a phase shift correction to frequency dependent transmission data of the SUT to generate corrected SUT transmission data, the phase shift correction based upon the SUT-reference time delay and the second SUT-reference time delay, the frequency dependent transmission data of the SUT collected via the second transmission line coupled to the second measurement fixture.   
     
     
         16 . The method of  claim 1 , wherein the phase shift correction comprises an amplitude correction. 
     
     
         17 . The method of  claim 1 , further comprising:
 collecting frequency dependent reflection data of the reflection reference via the transmission line coupled to the measurement fixture; and   transforming the frequency dependent reflection data of the reflection reference to the time domain reflection data of the reflection reference.