5 CALIBRATION AND CALIBRATION KIT
TRL and LRL calibrations have a limited bandwidth with lower to upper frequency
ratio up to 1:8. The band limits depend on the LINE length in TRL calibration or on the
difference between the lengths of the two LINEs in LRL calibration.
In theory TRM and LRM calibrations do not have limitations by frequency, however
their use at higher frequencies is limited by the quality of the MATCHes. It is
recommended to use the TRM and LRM calibrations up to 1 GHz.
Impedance of LINEs and MATCHes
All the LINEs and MATCHes used for TRL calibration must have Z0 impedance values
as precise as possible. TRL calibration transfers the impedance of standards into the
calibrated system. Precise airlines with an accurate Z0 impedance of 50 Ω are used as
LINEs in coaxial paths.
A zero-length THRU is used as the first standard in TRL calibration. In LRL calibration
a LINE, which is called REFERENCE LINE, is used instead of a zero-length THRU.
The shortest LINE is used as the REFERENCE LINE. Its length must to be known, so
that the calibration plane positions could be calculated exactly. However, LRL
calibration is also possible when the REFERENCE LINE length is not known. In this
case, its length is assumed to be equal to zero, the calibration plane being in the middle
of the LINE, and not at the ports’ edges.
TRL LINE is an airline used in TRL calibration, or the second longest LINE used in
LRL calibration. The length of TRL LINE should be known just approximately. The
LINE length is used to determine the calibration bandwidth. Let ΔL be the difference
between the two LINEs in LRL calibration. In TRL calibration this difference will be
equal to the LINE length, as a zero-length THRU is used as a REFERENCE LINE.
Then the phase difference between the TRL LINE and REFERENCE LINE or THRU
should be no less than 20° at the lower frequency and no more than 160° at the upper
frequency of the calibration.
01 LLL -=D ,
L0 – REFERENCE LINE length,
L1 – TRL LINE length,
– wave velocity in LINE (for airline it is с =2.9979·108 м/с).
So, the useful frequency range for TRL/LRL calibration is 1:8. Besides, TRL/LRL
calibration does not work at low frequencies, as it would require a very long LINE. Two