S-Parameter Measurement Test System Calibration
- Updated2024-11-11
- 6 minute(s) read
S-Parameter Measurement Test System Calibration
You can use S-Parameter Measurement Library API or the calibration wizard in the InstrumentStudio S-parameter measurement panel to perform system-level calibration of your S-parameter measurement test system.
Calibration Requirements
- Before performing system-level calibration, you must complete self-calibration of your vector signal transceiver (VST). Refer to your VST user manual for more information about self-calibration.
- NI recommends using the Maury Microwave 8050CK51 Characterized Device (CD) Fixed Load SOLT Kit with Adapters.
Calibration Types
The type of calibration you perform is dependent on your hardware configuration. All types of calibration compensate for cabling losses, reflection at the VST RF ports and adapters, and the reflection and directivity of the couplers.
Refer to the following table to determine the calibration type that is appropriate for your hardware configuration.
Hardware Configuration | Calibration Type |
---|---|
1 Coupler Per Port, 1 VST | SOLT |
1 Coupler Per Port, 2 VST | SOLT |
2 Couplers Per Port, 1 VST | QSOLT or SOLR |
2 Couplers Per Port, 1 VST (solid state switch) | QSOLT, SOLR, or TRL |
S11, 1 Coupler | SOL |
S11, 2 Couplers | SOL |
Short-Open-Load-Through (SOLT) Calibration
SOLT requires short, open, and load calibration standards in the characteristic system (and DUT) impedance. The exact standard values, as determined by their mechanical dimensions, are loaded into the network analyzer prior to the calibration. The location where you attach the calibration standards (the network analyzer port, the end of a cable, or inside a test fixture) is where the measurement begins and ends. This location is known as the reference plane or measurement plane.
In addition, you must make an insertable through connection. For instance, use a male-to-female cable connection, or another connection that does not require external adapters or devices, to complete the through connection during SOLT calibration. Inserting any component during calibration and not using it in the post calibration measurement results in measurement error. If you cannot make a through connection, the component is referred to as non-insertable. You can use one of several methods to handle non-insertable devices. The simplest method is to use a set of phase-equal adapters (included in most calibration kits) along with shorts, opens, and loads of each gender. Use one adapter to complete the through connection during calibration and swap it with an appropriate adapter for the DUT connection during post calibration measurements.
Finally, there is full two-port SOLT calibration, in which you place shorts, opens, and loads on both ports, as directed by the calibration routine. The full two-port calibration concludes with the through connection.
Quick-Short-Open-Load Through (QSOLT)
Calibration
QSOLT calibration is a fast and efficient calibration method that removes systematic errors in the test setup. Although QSOLT calibration is a quick calibration method, it does not compromise on uncertainty. QSOLT calibration is a variation of the traditional SOLT calibration, which requires less measurements to correct errors. With QSOLT calibration, the short, open, and load are only measured on one port. The error coefficients are transferred to the other port with the through measurement. Therefore, a well-characterized through is important for good calibration.
Short-Open-Load-Reciprocal (SOLR)
Calibration
SOLR calibration is a combination between the short-open-load-through (SOLT) and the through-reflect-line (TRL) family of calibrations. SOLR uses the SOLT short, open, and load standards and requires the same characterization knowledge. The difference between SOLR and SOLT calibration is that SOLR does not require an electrical model of the through standard. However, SOLR calibration does require that the through standard be reciprocal, such that S21 equals S12. As with TRL calibrations, the most accurate implementation of this calibration method requires a four-receiver hardware configuration, where SOLT calibration could be performed with a three- or four-receiver design.
The TRL calibration assumption of the ideal switch, which is inaccurate, is overcome using extra measurements made during the through portion of the calibration. These measurements, and the reciprocity of the through standard, allow the through standard to be characterized and corrected for during the calibration. You need to know the approximate electrical phase of the through to allow a root choice during the characterization and correction of the through standard.
The limited knowledge requirement on the through standard makes SOLR calibration ideally suited for situations where a non-ideal through standard is desired, such as with on-wafer probing where the probes are positioned 90 degrees from each other (orthogonally) during the desired measurement. If this measurement configuration is necessary, there are two options for making calibrated measurements. One option is to move the measurement probes away from their desired calibration measurement positions using a straight through standard, which introduces errors in the final measurements. The errors are caused by the necessary cable movements when repositioning the ports for the final measurements. You also can calibrate using a through standard with a 90-degree bend. A bend makes the through behave non-ideal and therefore creates errors or degradation in those calibration models relying on the ideal characteristics. SOLR calibration does not have this limitation since it does not rely on an ideal through model and thus does not introduce this degradation during the measurement. The use of the SOLR algorithm with a 90-degree through standard simplifies both the calibration and the measurement, which provides the best calibration for these less ideal measurement configurations.
Short-Open-Load (SOL)
Calibration
SOL calibration is a simple method that calibrates the system by measuring the short, open, and load standards. SOL calibration is useful when you try to remove directivity, source match, and frequency response errors. Use the SOL calibration method when you only need to calibrate one port as S11.
Through-Reflect-Line (TRL) Calibration
TRL is one type in a family of two-port calibrations that better support non-coaxial environments such as on-wafer measurements and test fixturing. In this family, the name of the calibration is an acronym that identifies the following necessary calibration standards.
- Through-reflect-line (TRL)
- Line-reflect-match (LRM)
- Line-reflect-line (LRL)
- Through-reflect-match (TRM)
These calibrations derive error terms for the same 12-term error model as SOLT calibrations. Additional information about the port match of the hardware is required to correct perfect-match assumptions used to derive these calibration models. NI recommends you retrieve this information during the measurement of the calibration standards. Four receivers are required during calibration to provide correlated measurements of signals entering and exiting both ports, while only one port is sourcing a signal.
Fewer calibration standards, typically three, are required for this family of calibrations, compared to SOLT calibrations. In non-coaxial environments, standards are often harder to obtain but still easier to fabricate than performing a calibration and measurement with coaxial connections. The standards must be fabricated with care as the accuracy of the corrected measurements is dependent upon the quality and repeatability of the calibration standards. These calibrations generally use combinations of through, line, reflection, and match standards. The following table defines the standards that are used in these calibrations.
Standard | Description |
---|---|
Through | Ideally, a zero length through line where the measurement ports are directly connected. An alternative would be a line with an electrical length, but this line must be electrically distinct from any other line standard. The intrinsic impedance of the through and line standards set the impedance of the calibration. |
Reflection | A highly reflective device (most often a short or open). The absolute reflection coefficient is derived from the additional standard measurement information, but knowledge of the phase must be within one-fourth wavelength to ensure proper identification of the standard. It is important that you use the equivalent reflection standards to calibrate both ports. |
Line | A nonzero length through line with the same intrinsic impedance as the through standard. The difference between the phase shifts of the line and through standards require a minimum of 10 degrees to 170 degrees or a preferred 20 degrees to 160 degrees. It is not necessary to know the propagation constant because you can derive it from the extra measurement information. The required phase relationship between the through and line standards creates restrictions on the frequency span that can be measured with a given though and line combination. If you need larger frequency spans, use multiple through and line pairs. |
Match | If a match standard is used in place of a line standard, you can use its impedance to set the impedance of the calibration. The match standard should be identical for each port and should generally use characterized data or assume data to be a perfect match. |
TRL is a calibration form that makes use of fewer standards, requiring less knowledge about the standards. TRL calibration is highly suited for non-coaxial environments where traditional standards may be hard to fabricate and fully characterize. It requires high-quality, repeatable standards to ensure accurate calibrations. Frequency span limitations exist that are enforced by the requirement to identify standards by their phase relationships during measurements, but this limitation can be overcome with additional standards as required. Since much of the standard characterization is extracted during the measurements of the standards themselves, the general accuracy of the calibration is better than SOLT, which includes the use of mechanical standards electrical models.
In This Section
Related Information
- Connecting the Hardware for S-Parameter Measurements
To perform S-parameter measurements with the S-Parameter Measurement Library, you must configure your hardware as described in the following sections.
- S-Parameter Measurement Library LabVIEW VI Reference — Calibration VIs
- PXIe-5831 Self Calibration
- PXIe-5841 Self Calibration
- PXIe-5842 Self Calibration
- Calibrating the System with the S-Parameter Measurement Panel
Use the Calibration wizard in the S-parameter measurements panel in InstrumentStudio to guide you through a system-level calibration.