VectorStar™ MS4640A Series Microwave Vector Network Analyzers : Calibration Overview : About Calibration
 
About Calibration
The most important central concept to making good VNA S-parameter measurements is the calibration of the instrument. The background on calibration mathematics and theory will only be lightly covered in this section; more information is available in Anritsu Application Notes and in the reference literature. While the VNA is a highly linear receiver and has sufficient spectral purity in its sources to make good measurements, there are a number of imperfections that limit measurements done without calibrations:
Match
Because the VNA is such a broadband instrument, the raw match can be good but not excellent. Even a 20 dB match, which is physically very good, can lead to errors of greater than 1 dB. By correcting for this raw match, the potential error can be greatly reduced.
Directivity
A key component of a VNA is a directional coupler that allows separation of the signal incident on the DUT from the reflected back from the DUT. While the couplers used in the VNA are of very high quality, there is an amount of coupled signal even when a perfect termination is connected. This is related to directivity and can impact measurements of very small reflection coefficients.
Frequency Response
While the internal frequency response of the VNA could be calibrated at the factory, any cables connected externally will have some frequency response that must be calibrated out for high quality measurements.
The calibration is a method of correcting for these and other defects. There are an enormous number of possible calibration algorithms and many of them are implemented within the MS4640A Series VNAs. The choice between them is largely determined by the media used, the calibration standards available, and the desired accuracy/effort trade-off. Some of the choices to be made are:
• Calibration Type
Which ports are being corrected and to what level are they being corrected?
• Calibration Algorithm
How is the correction being accomplished?
 
Calibration Types
 
MS4640A Series VNA Calibration Types 
VNA Mode
Type
Parameters Calibrated
Uses
2-Port VNA Mode
Full 2 Port
S11, S12, S21, and S22
Most complete calibration
Full 1 Port
S11 or S22
or
S11 and S22
Reflection calibration only
1 Path 2 Port
S11 and S21
or
S22 and S12
1 port reflection plus simple transmission (faster, lower transmission accuracy unless DUT very lossy)
Frequency Response
Any one parameter (or pairs of symmetric parameters such as S12 and S21)
Normalization only. Fast, lower accuracy
4-Port VNA Mode
When in 4-Port mode, additional calibration types of
4-Port and 3-Port become available in the CALIBRATION menus with an associated expansion of S-Parameters, User-Defined, and Mixed-Mode options in the RESPONSE menus.
Calibration Algorithms
The use of acronyms for the various calibration algorithms is often inconsistent. The following table presents calibration algorithm acronyms as used in Anritsu documentation.
Calibration Algorithms 
Calibration Algorithm
Description
Advantages
Disadvantages
SOLT (Short-Open-Load-Through)
Common coaxially
Simple, redundant standards; not band-limited
Requires very well-defined standards, poor on-wafer, lower accuracy at high frequencies
SSLT (Short-Short-Load-Through, also called Offset Short), shorts with different offset lengths
Common in waveguide
Same as SOLT
Same as SOLT and band-limited
SSST (Short-Short-Short-Through, also called Triple Offset Short), all shorts with different offset lengths
Common in waveguide or high frequency coax
Same as SOLT but better accuracy at high frequencies
Requires very well-defined stds, poor on-wafer, band-limited
SOLR/SSLR/SSSR, like above but with “Reciprocal” instead of “Through”
Like the above but when a good through is not available
Does not require well-defined through
Some accuracy degradation but slightly less definition, other disadvantages of parent cal
LRL (Line-Reflect-Line, also called TRL)
High performance coax, waveguide or on-wafer
Highest accuracy, minimal standard definition
Requires very good transmission lines, less redundancy so more care is required, band-limited
ALRM, advanced line-reflect-match, simplifies to LRM (Line-Reflect-Match) or TRM (Through-Reflect-Match)
Relatively high performance
High accuracy, only one line length so easier to fixture/on-wafer, not band-limited usually
Requires load definition. Reflect standard setup may require care depending on load model used
 
The following table relates the Calibration Types to the Calibration Algorithms.
Calibration Types and Calibration Algorithms
VNA Mode
Type
SOLT
SSLT
SSST
SOLR/SSLR/SSSR
LRL
ALRM
2-Port Mode
Full 2 Port
YES
YES
YES
YES
YES
YES
Full 1 Port
YES
YES
YES
Can be selected for these types, but the reciprocal nature is not used and will function like the base calibration
–
–
1 Path 2 Port
YES
YES
YES
–
–
Frequency Response
YES
YES
YES
–
–
4-Port Mode
The calibration types and modes change when the VNA is in multiport (4-Port) mode. See Multiport Measurements for a discussion of multiport calibration methods and types.