Calibration Considerations
Due to the small physical size of modern semiconductor devices it is often necessary
to use port extension or thin coaxial probes (figure2) to place the reference plane
at or close to the DUT. Whether using a standard SMA cal kit or a coaxial probe and
chip resistor type, the reference plane for each port must be accurately defined.
The maths performed on the s-parameters makes calibration errors difficult to spot.
It is always worth checking the system by measuring some known balanced structures
e.g. Load, O/C and S/C. These can be fabricated in a similar manner to the calibration
standards shown below. In the case of the unbalanced 'Thru' standard, this is equivalent
to a 'Short' in a balanced system. Breaking the 'Thru' in the middle would give
an 'Open' in the balanced system. Breaking the 'Thru' and then reconnecting the inners
with a 50ohm chip resistor would give a balanced 50ohm 'Load'.
Tip:- For the ‘Load’ use two 100ohm chip resistors in parallel to minimise stray
inductance. If you can manage 0402’s great otherwise 0603’s will be OK below 3Ghz.
When using coaxial probe calibration standards the choice of cal-kit on the analyser
is important. In the case of the SMA-type there will be an electrical delay associated
with the cal. This is because the reference plane defined by the standards is displaced
from the actual mating faces of the connectors. The coaxial probe standards have
a reference plane exactly at the end of the coax, therefore the electrical delay
is not required. If an existing SMA cal-kit definition is used in the analyser then
the associated electrical delay must be removed in software. Alternatively a user-kit
can be defined with the electrical delay set to zero. The fringing capacitance terms
C0,C1,C2,C3 can be left unchanged for frequencies below 3GHz. At higher frequencies
these terms become important and may require modification.
To correct for electrical delay in software the following equations are used :
Where
And
To validate the method the system was calibrated using the coaxial probe standards.
Then some balanced standards were fabricated for Open, Short and Load. When measured
the Short and Load were within ±0.25ohm ±0.25j of 0ohm and 50ohm respectively, the
Open was in excess of 2000ohm and within ±500j.
