The previously discussed SOLT and offset short calibration techniques all require a known “thru (or through)” as part of the full 2-port (or 1p–2p) calibration. The “thru” is really a defined transmission line having known length, known loss, and assumed perfect match (under most conditions). There are certain cases when this is not possible when there is an undefined or unknown thru
• Coaxial cal when the two ports are different connector types
• On-wafer when the “thru” is a meandering transmission line of imperfect match
• A calibration that must take place through a test set (coax or waveguide) with unknown (and highly frequency dependent) loss and match
Illustration of SOLR or “Unknown Thru” Calibration
For these cases, and others when the “thru” cannot be very well-known, there is the reciprocal option (also known as the unknown thru). In this case, the same reflect standards are used, but no assumption is made about the “thru” except that it be reciprocal (i.e., S21=S12; no assumption made about S11 and S22). In practice, there are some limits to this.
The technique borrows from the LRL family and uses some of the redundancy available with the fully-defined families to reduce knowledge needed about something (the thru in this case). The resulting cal will generally not be quite as accurate as the regular thru version if the thru met the conditions described above. It will, however, be better than using the regular thru version when the thru has unknown loss or match.
