VNA concepts and features
R&S
®
ZNL/ZNLE
255
User Manual 1178.5966.02 ─ 19
Accuracy for low frequencies
TRL calibration becomes inaccurate if the electrical length difference between Line and
Through standard corresponds to a phase shift below 20°. In practice, TRL is only
practicable above a threshold frequency c
0
/[18*(l
long
– l
thr
)], where l
long
denotes the lon-
gest electrical length of the used Line standards, and l
thr
the electrical length of the
Through. The analyzer assumes l
thr
<< l
long
and uses c
0
/(18*l
long
) as the frequency
threshold.
E.g., for a line with l
long
= 16.666 cm, the threshold frequency is 100 MHz.
At frequencies below this threshold frequency, the correction term calculation makes
use of the Match calibration data, if they have been acquired for each port.
Accuracy conditions for the Lines
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.
Suppose 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.
9.5.1.6
Complementary isolation measurement
For each port pair in a manual transmission normalization or TOSM calibration, the
Through measurement can be complemented by an isolation measurement. This mea-
surement accounts for possible crosstalk between the related test ports (e.g. on a test
fixture).
If isolation is measured, the corrected transmission coefficient of the DUT is calculated
as:
(Transmission coefficient DUT – Isolation) / (Transmission coefficient Through – Isola-
tion)
There is no dedicated physical standard for isolation measurement; it is recommended
to terminate the test ports suitably (e.g. with 50 Ω loads).
9.5.2
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-parame-
ters) must be known or predictable within a given frequency range.
The standards are grouped into several types (Open, Through, Match,...) correspond-
ing to the different input quantities for the analyzer's error models. The standard type
Calibration
