TRL (Thru, Reflect, Line) represents a family of calibration techniques that measure two transmission standards and one reflection standard to determine the 2-port 12-term error coefficients. For example, TRM (Thru, Reflect, Match), LRL (Line, Reflect, Line), LRM (Line, Reflect, Match) are all included in this family.
The traditional SOLT calibration measures one transmission standard (T) and three reflection standards (SOL) to determine the same error coefficients.
TRL calibration is extremely accurate, in most cases more accurate than an SOLT cal. However, very few calibration kits contain TRL standards. TRL Cal is most often performed when you require a high level of accuracy and do not have calibration standards in the same connector type as your DUT. This is usually the case when using test fixtures, or making on-wafer measurements with probes. Therefore, in some cases you must construct and characterize standards in the same media type as your DUT configuration. It is easier to manufacture and characterize three TRL standards than the four SOLT standards.
Another advantage of TRL calibration is that the TRL standards need not be defined as completely and accurately as the SOLT standards. While SOLT standards are completely characterized and stored as the standard definition, TRL standards are modeled, and not completely characterized. However, TRL cal accuracy is directly proportional to the quality and repeatability of the TRL standards. Physical discontinuities, such as bends in the transmission lines and beads in coaxial structures, will degrade the TRL calibration. The connectors must be clean and allow repeatable connections.
To learn more about Cal Standard requirements, see Cal Standards Used in TRL.
Perform a TRL Calibration in the FieldFox using the following procedure:
Preset NA Mode
Set up a measurement and the desired stimulus settings.
Press Calibration
Select the DUT connectors and Cal Kit for each port. The Cal Kit MUST include TRL standards.
Select TRL as the Cal Method.
Follow the prompts to complete the calibration.
Keysight Technologies offers two cal kits that include the required standards to perform a TRL calibration: 85050C (APC 7mm) and 85052C (3.5mm). Both kits include the traditional Short, Open, and Load standards. (The Thru standard, not actually supplied, assumes a zero-length Thru). In addition, the kits include an airline which is used as the LINE standard. To use the airline, the kits include an airline body, center conductor, and insertion / extraction tools. The APC 7 kit includes an adapter to connect the airline to the APC connector.
These standards must be defined in your TRL cal kit:
Note: All THRU calibration methods are supported in a TRL Cal EXCEPT Unknown Thru.
The THRU standard can be either a zero-length or non-zero length. However, a zero-length THRU is more accurate because it has zero loss and no reflections, by definition.
The THRU standard cannot be the same electrical length as the LINE standard.
If the insertion phase and electrical length are well-defined, the THRU standard may be used to set the reference plane.
Characteristic impedance of the THRU and LINE standards defines the reference impedance of the calibration.
If a THRU standard with the correct connectors is NOT available, an adapter removal cal can be performed.
The REFLECT standard can be anything with a high reflection, as long as it is the same when connected to both ports.
The actual magnitude of the reflection need not be known.
The phase of the reflection standard must be known within 1/4 wavelength.
If the magnitude and phase of the reflection standard are well-defined, the standard may be used to set the reference plane.
The LINE and THRU standards establish the reference impedance for the measurement after the calibration is completed. TRL calibration is limited by the following restrictions of the LINE standard:
Must be of the same impedance and propagation constant as the THRU standard.
The electrical length need only be specified within 1/4 wavelength.
Cannot be the same length as the THRU standard.
A TRL cal with broad frequency coverage requires multiple LINE standards. For example, a span from 2 GHz to 26 GHz requires two line standards.
Must be an appropriate electrical length for the frequency range: at each frequency, the phase difference between the THRU and the LINE should be greater than 20 degrees and less than 160 degrees. This means in practice that a single LINE standard is only usable over an 8:1 frequency range (Frequency Span / Start Frequency). Therefore, for broad frequency coverage, multiple lines are required.
At low frequencies, the LINE standard can become too long for practical use. The optimal length of the LINE standard is 1/4 wavelength at the geometric mean of the frequency span (square root of f1 x f2).
Note: The TRL LINE standard must have a delay that is greater than 0 (zero) ps. Otherwise, calibration correction calculations will contain unpredictable results.
If the LINE standard of appropriate length or loss cannot be fabricated, a MATCH standard may be used instead of the LINE.
The MATCH standard is a low-reflection termination connected to both Port 1 and Port 2.
The MATCH standard may be defined as an infinite length transmission line OR as a 1-port low reflect termination, such as a load.
When defined as an infinite length transmission line, both test ports must be terminated by a MATCH standard at the same time. When defined as a 1-port load standard, the loads are measured separately. The loads are assumed to have the same characteristics.
The impedance of the MATCH standard becomes the reference impedance for the measurement. For best results, use the same load on both ports. The load may be defined using the data-based definition, the arbitrary impedance definition, or the fixed load definition.
For more information, read Specifying Calibration Standards and Kits for Keysight Vector Network Analyzers (Application Note 1287-11)
Last modified:
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3-Jun-2013 |
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