Correction of transmission line induced phase and amplitude errors in reflectivity measurements
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.
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;
isolating reflection peak data of the measurement fixture in the time domain reflection data of the SUT;
transforming the isolated reflection peak data of the measurement fixture to corresponding frequency domain reflection data of the measurement fixture;
determining SUT-reference ratios between the frequency dependent reflection data of the SUT and the frequency domain reflection data of the measurement fixture; 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 ratios.
2. 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 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 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.
3. The method of claim 2 , 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; and
identifying the time location of the reflection from the measurement fixture based on the time domain reflection data of the reflection reference.
4. The method of claim 3 , wherein the reflection reference is free space without a calibration standard or SUT.
5. The method of claim 3 , wherein the frequency dependent reflection data of the reflection reference is collected at a plurality of excitation frequencies within a predefined range of frequencies.
6. The method of claim 2 , further comprising:
collecting the 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; and
identifying the time location of the reflection from the measurement fixture based on the time domain reflection data of the calibration standard.
7. The method of claim 6 , wherein the frequency dependent reflection data of the calibration standard is collected at a plurality of excitation frequencies within a predefined range of frequencies.
8. The method of claim 2 , 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; and
transforming the frequency dependent reflection data of free space to the isolation reflection data.
11. The method of claim 10 , wherein the frequency dependent reflection data of free space is collected at a plurality of excitation frequencies within a predefined range of frequencies.
12. The method of claim 1 , wherein the frequency dependent reflection data of the SUT is collected at a plurality of excitation frequencies within a predefined range of frequencies.
13. The method of claim 12 , wherein a network analyzer sequentially provides the plurality of excitation frequencies and collects the frequency dependent reflection data of the SUT at each of the plurality of excitation frequencies via the transmission line coupled to the measurement fixture.
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 , wherein the phase shift correction comprises an amplitude correction.
16. The method of claim 1 , further comprising determining a calibrated SUT response based at least in part upon the corrected SUT reflection data.
17. The method of claim 1 , further comprising:
collecting frequency dependent reflection data of a reflection reference via the transmission line coupled to the measurement fixture; and
transforming the frequency dependent reflection data of the reflection reference to time domain reflection data of the reflection reference.
18. The method of claim 17 , wherein the reflection reference is a calibration standard.
19. The method of claim 17 , wherein the reflection reference is free space without a calibration standard or SUT.
20. The method of claim 17 , wherein the frequency dependent reflection data of the reflection reference is collected at a plurality of excitation frequencies within a predefined range of frequencies.