Concepts and Features
R&S
®
ZNC
75
User Manual 1173.9557.02 ─ 13
Low-frequency extension with TRM
TRL calibration becomes inaccurate if the electrical length difference between Line and
Through standard corresponds to a phase shift below 20°. In practice this means that
TRL is only practicable above a threshold frequency f
TRM
which depends on the lengths
of the longest line and through standards. The threshold frequency is given by:
"f
TRM
= c
0
/[18*(l
long
– l
thr
)]"
where l
long
denotes the electrical length of the longest of the used Line standards, l
thr
the
length of the Through. The analyzer assumes l
thr
<< l
long
and calculates f
TRM
= c
0
/
(18*l
long
). At frequencies below f
TRM
, TRL calibration is automatically replaced by TRM,
provided that the necessary calibration data has been acquired. For a line with l
long
=
16.666 cm, the threshold frequency is f
TRM
= 100 MHz.
Accuracy conditions for the Line(s)
The length error of the Line, converted into a transmission phase error, must be below
the minimum difference to the singularity points 0 deg or 180 deg multiplied by two. Sup-
pose that an approximately known Line standard causes a transmission phase 30 deg
at the start frequency and of 160 deg at the stop frequency of the sweep. Its length error
must cause a phase difference below (180 deg – 160 deg)*2 = 40 deg.
3.5.1.9
TNA Calibration
A TNA (Through – Network – Attenuation) calibration requires two-port standards only.
Again, the Through standard must be ideally matched and lossless. The Symmetric Net-
work must have the same properties as the Reflect standard used for a TRL calibration,
i.e. the magnitude of its reflection coefficient can be unknown but must be nonzero; its
phase must be roughly known (
±
90 deg). The magnitude and phase of the reflection
coefficient must be the same at both test ports. The Attenuation standard must be well
matched on both sides and cause an attenuation different from 0 dB; the exact value of
the transmission coefficient is not important.
As with TRL, TNA calibration is especially useful for planar DUTs. If TNA is not practi-
cable, TRL may be an alternative.
3.5.2 Calibration Standards and Calibration Kits
A calibration kit is a set of physical calibration standards for a particular connector type.
The magnitude and phase response of the calibration standards (i.e. their S-parameters)
must be known or predictable within a given frequency range.
The standards are grouped into several types (Open, Through, Match,...) corresponding
to the different input quantities for the analyzer's error models. The standard type also
determines the equivalent circuit model used to describe its properties. The circuit model
depends on several parameters that are stored in the cal kit file associated with the cal-
ibration kit.
As an alternative to using circuit models, it is possible to describe the standards by means
of S-parameter tables stored in a file.
Calibration
