Validity of a Calibration
This section helps you determine if your calibration is valid and how
the analyzer displays correction level information for your measurement.
See other Calibration Topics
Frequency
Response of Calibration Standards
In order for the response of a calibration standard to show
as a dot on the smith
chart display format, it must have no phase delay with respect
to frequency. The only standards that exhibit such "perfect"
response are the following:
There are two reasons why other types of calibration standards
show phase delay after calibration:
The reference plane of the standard is electrically
offset from the mating plane of the test port. Such devices
exhibit the properties of a small length of transmission line,
including a certain amount of phase shift. The standard is an open termination, which
by definition exhibits a certain amount of fringe capacitance
and therefore phase shift. Open terminations which are offset
from the mating plane will exhibit a phase shift due to the
offset in addition to the phase shift caused by the fringe
capacitance.
The most important point to remember is that all standards are
measured in order to remove systematic
errors from subsequent device measurements. As a result, if
calibration standards with delay and fringe capacitance are measured
as a device after a calibration, they will NOT appear to be "perfect".
This is an indication that your analyzer is
calibrated accurately and working properly. |
Validating a Calibration
At the completion of a calibration or selection of a stored Cal Set,
validation can accomplish the following:
Improve Measurement Accuracy – Once
a measurement calibration has been performed, its performance should be
checked before making device measurements. There are several sources of
error that can invalidate a calibration: bad cables, dirty or worn calibration
standards that no longer behave like the modeled standards, and operator
error.
Verify Accuracy of Interpolation – You
should validate the calibration if you are testing a device and the measurements
are uncertain because of interpolation. For
more information see Interpolation Accuracy.
Verify Accuracy of Cal Standards – To
check accuracy, a device with a known magnitude and phase response should
be measured.
Quick
Check
For this test, all you need are a few calibration standards. The device
used should not be one of the calibration standards; a measurement of
one of these standards is merely a measure of repeatability.
The following reflection and transmission Quick Check tests can be applied
to all test ports.
To verify reflection measurements, perform the following steps:
Connect either an OPEN or SHORT standard to port 1.
The magnitude of S11 should be close to 0 dB (within a few tenths
of a dB).
Connect a load calibration standard to port 1. The
magnitude of S11 should be less than the specified calibrated directivity
of the analyzer (typically less than -30 dB).
To verify transmission measurements:
Connect a THRU cable (or known device representative
of your measurement) from port 1 to port 2. Verify the loss characteristics
are equivalent to the known performance of the cable or device.
To verify S21 isolation, connect two loads: one on
port 1 and one on port 2. Measure the magnitude of S21 and verify
that it is less than the specified isolation (typically less than
-80 dB).
Note: To get a more accurate
range of expected values for these measurements, consult the analyzer's
specifications.
ECal Confidence
Check
ECal Confidence Check is a method to check the accuracy of a calibration
performed with mechanical standards or an ECal module. The confidence
check allows you to measure an impedance state in the ECal module (called
the confidence state), and compare it with factory measured data stored
in the module.
In order for this test to be valid, the test ports of the ECal module
must connect directly to the calibration reference plane (without adapters).
Note: In the N469x series of 2-port ECal modules,
from the module minimum frequency up to approximately 2 GHz, the confidence
state has a very high amount of transmission loss. In this frequency range,
calibrated measurements of transmission S-parameters for the confidence
state may vary much more than expected from the Keysight-characterized
data in the measurement memory trace. When comparing the measurement
trace and memory trace you, ignore the data for frequencies up to 2 GHz.
How to Perform ECal Confidence
Check:
Connect ECal module to the analyzer with the
USB cable. See Connect ECal
Module to the VNA. Note:
Terminate any unused ECAL ports with a 50 ohm load. See
below. Allow the module to warm up for 15 minutes
or until the module indicates READY.
Note:
N755xA series ECal modules do not require a warm-up period.
Do one of the following to start ECal Confidence
Check |
Using
Hardkey/SoftTab/Softkey |
Press Cal
> Cal Sets & Cal Kits
> ECal > ECal Confidence Check.... |
|
On
the following ECal Confidence
Check dialog box:
Click Read
Module Data. The following occurs:
ECal module
is set to "confidence state". Analyzer reads
and displays stored data. Analyzer measures
and displays "confidence state".
To view a different parameter, select Change Measurement and select
the check box for the desired parameter. The default
is the active channel parameter. Select the viewing option in the Trace View
Options block. Compare the stored and measured data for each
measurement parameter.
Notes:
After exiting ECal Confidence Check, the ECal
module remains in the same impedance state and the factory
(or user-characterized) data is still stored in the memory
trace. Therefore, you can save both the data and memory trace
as a *.csv files and import them to a spreadsheet. Learn
how. If the two traces show excessive difference,
there may be a loose or dirty connection at the test ports
or damage to the test cables. Carefully inspect the cables
and connections. Then clean and gage each connector, and re-calibrate
if needed. The User Characterization setting selects
the user-characterization data instead of the factory characterization
data (available when a User-Characterization is stored in
the ECal module). |
Determining
Effects of Not Terminating Unused ECal Ports
The following procedure can be used to determine the calibration errors
when unused ECal ports are not terminated.
Connect the ECal module to one VNA test port through
an adapter (eliminates the possibility of cable movement errors being
included) and leave the unconnected port(s) open.
Perform a 1-port cal (use 100 Hz IFBW) then save the
calset.
Connect a load(s) to the unconnected port(s) of the
ECal module.
Perform a 1-port cal then save the calset.
Compare the calset in step 2 with the calset in step
4 using Calset Viewer to evaluate
the effect of leaving the unused ECal ports open.
ECal
Confidence Check dialog box help |
Compares the accuracy of corrected (calibrated) data with stored
data in the ECal module. For the check to be valid, the module
test ports must connect directly to the calibration reference
plane (without an adapter). Learn
more about ECal Confidence Check.
Measurement
Change
Measurement Opens the Measure dialog box.
Use ECal Module
Read Module
Data
Copies
stored data from the ECal module to Memory. Changes
state of ECal module to confidence state. Measures
and displays confidence state and Memory trace. Displays
the factory and user characterizations data stored in the
ECal module. Learn
more.
Scale
Opens the Scale dialog box.
Show Prompts
Check to show a reminder for the connection (default).
Trace View Options
Data and
Memory Trace Displays
current measurement data and Memory trace.
Data /
Memory Performs
an operation where the current measurement data is divided by
the data in memory.
Data +
Memory Performs an operation where the current measurement
data is added to the data in memory. |
Verification
Kit
Measuring known devices, other than calibration standards, is a straightforward
way of verifying that the network analyzer system is operating properly.
Verification kits use accurately known verification standards with well-defined
magnitude and phase response. These kits include precision airlines, mismatch
airlines, and precision fixed attenuators. Traceable measurement data
is shipped with each kit on disk and verification kits may be re-certified
by Keysight.
See Analyzer Accessories
for a list of Keysight verification kits.