Female open Male open Full one-port Calibration SMA - SMA Cal Kit
Testing SMA-K Calibrator
I. Description:
Product name:SMA-K Calibrator
| Model | SMA-K Calibrator |
| frequency range | DC-6GHz |
| Dummy load | ＜-32dB(Return Loss) |
| Material | Gold-plated brass |
| Connecting Interface | SMA |
| Torque value | 0.9 N/m |
The idea is to create correction data for the cheap SMA cal kit by
measuring the cal standards after the VNA was calibrated with the
Maury kit, and thereby transfer at least some of its precision to
the cheap kit.
As with most cal kits, the use of correction data is absolutely
necessary since the open and short have an offset length, and in
this case probably also appreciable losses due to the PTFE
dielectric. Moreover, the open is always appreciably non-ideal due to its
fringing capacity.
Let's start with the measurements. First, the VNA was configured with adapters to have a male 3.5 mm
connector at port 1, and a female 3.5 mm connector at port 2. Then a full one-port calibration was performed for each port, using
my Maury cal kit.
After some sanity checks of the calibration, the S11 of the six SMA
cal standards were measured from 300 kHz to 4 GHz. The results were stored as Touchstone (.s1p) files, each with 1001
data points. Here are plots of the results, directly from the analyzer:
Female open
Female short
Female match
Male open
Male short
Male match
The open and short standards are somewhat lossy, but this is easily
addressed by the common error correction models. The match standards are are a bigger problem;surely they are not shoddy and are certainly adequate as a general
purpose termination, but are far from what one would expect from a
calibration standard.
The matches usually will set the limit on the accuracy of a cal kit
even after correction with the coefficients method, since open and
short are usually very precise after correction, but there is no
general parametric model to adequately correct a match.
It is interesting to note that between 1 GHz and 2 GHz the return
loss of the female match worsens by about 20 dB and remains below
only −35 dB, while the male match is much better and remains well
below −45 dB almost up to 4 GHz. This despite the fact that the female standards are rated to a much
higher frequency!
Another quality feature of a cal kit is the relative phase response
of open and short over frequency. The error correction algorithm applied by the VNA is most stable
(i.e., the error terms determined during calibration are least
sensitive to variations in the measured reflection factors of the
cal standards) if the responses of the open, short and match are spaced as far apart as possible on the Smith
chart at every frequency in the band of interest. This is the case if, on the Smith chart, open, short and match are
located on a straight line at every frequency, with the match at
the center.
In the extreme case, if open and short happen to rotate at very
different rates around the outer edge of the Smith chart as
frequency is increased, they will finally meet. At that frequency, they look alike (except perhaps for small
differences in loss), and calibration fails. Thus the phase response of open and short should ideally be 180°
apart. In the following two graphs the difference in the phase response of
open and short (red trace) for the female kit as well as for the
male kit is plotted over frequency. For the female kit, the phase difference remains below 17°, and for
the male kit below 6° in magnitude up to 4 GHz. This is actually a very good result.
