An arbitrary two-port device with unknown parameters can be used as an UNKNOWN THRU in 2-port SOLR (Short-Open-Load-Reciprocal) calibration. An UNKNOWN THRU should satisfy next requirements:
•The UNKNOWN THRU must be Reciprocal (S21 = S12), which holds for most passive linear network.
•The combined transmission loss of the UNKNOWN THRU and calibration path is not recommended to exceed 40 dB.
•The approximate electrical delay of the UNKNOWN THRU should be specified manually or set to zero in order the analyzer to detect it automatically.
Requirements for automatic detection of electrical delay
It is usually not necessary to enter the electrical delay manually because the analyzer can automatically detect the electrical delay of the UNKNOWN THRU during the calibration procedure. In this case, the UNKNOWN THRU value should be set to zero.
For the Analyzer to correctly automatically detect the UNKNOWN THRU delay, the following condition must be met:
where — frequency span of calibration,
— number of points,
— delay of a UNKNOWN THRU,
In other words, the number of points must be enough to correctly auto-detect the electrical delay of the UNKNOWN THRU:
Example. An example of calculating the number of points enough for the correct automatic determination of UNKNOWN THRU delay by the Analyzer.
Let the UNKNOWN THRU is a coaxial cable having approximate length . The approximate delay of the cable will be , providing the velocity factor of the cable equals .
Let us the Span is 8 GHz.
For the analyzer to correctly automatically detect the UNKNOWN THRU delay, the number of points must be:
Manual set of Unknown Thru delay
It is possible to manually enter either the delay or physical length of the UNKNOWN THRU. The accuracy of the UNKNOWN THRU length must be known within of 1/2 of the wavelength in the Thru media at the maximum calibration frequency. Accordingly, the accuracy of the UNKNOWN THRU delay must be known within of
,
where — stop frequency of calibration.
When length specified the value of the Thru media permittivity must be also specified.
If the Thru media type is waveguide, the cutoff frequency must be specified.
