General Information

Measurement Errors

S-parameter measurements are influenced by various measurement errors, which can be broken down into two categories:

• systematic errors, and
• random errors.

Random errors comprise such errors as noise fluctuations and thermal drift in electronic components, changes in the mechanical dimensions of cables and connectors subject to temperature drift, repeatability of connections and cable bends. Random errors are unpredictable and hence cannot be estimated and eliminated in calibration. Random errors can be reduced by correct setting of the source power, IF bandwidth narrowing, sweep averaging, maintaining a constant environment temperature, observance of the Analyzer warm-up time, careful connector handling, and avoidance of cable bending after calibration.

Random errors and related methods of correction are not mentioned further in this section.

Systematic errors are errors caused by imperfections in the components of the measurement system. Such errors occur repeatedly and their characteristics do not change with time. Systematic errors can be determined and then reduced by performing mathematical correction of the measurement results.

The process of measurement of precision devices with predefined parameters with the purpose of determining systematic errors is called calibration, and such precision devices are called calibration standards. The most commonly used calibration standards are SHORT, OPEN, and LOAD.

The process of mathematical compensation (numerical reduction) for measurement systematic errors is called error correction.

Systematic Errors

The systematic measurement errors of vector network analyzers are subdivided into the following categories according to their source:

• Directivity;
• Source match;
• Load match;
• Isolation;
• Reflection/transmission tracking.

The measurement results before error correction are called uncorrected.

The residual values of the measurement results after error correction are called effective.

Directivity Error

A directivity error (Ed) is caused by incomplete separation of the incident signal from the reflected signal by the directional coupler in the source port. In this case part of the incident signal energy enters the receiver of the reflected signal. Directivity errors do not depend on the characteristics of the DUT and usually have a greater effect in reflection measurements.

Source Match Error

A source match error (Es) is caused by mismatch between the source port and the input of the DUT. In this case part of the signal reflected by the DUT reflects at the source port and re-enters the input of the DUT. The error occurs effects both reflection measurement and transmission measurement. Source match errors depend on the difference between the input impedance of the DUT and test port impedance when it functions as a signal source.

Source match errors have strong effect in measurements of a DUT with poor input matching.

Load Match Error

A load match error (El) is caused by mismatch between the receiver port and the output of the DUT. In this case part of the signal transmitted through the DUT reflects at the receiver port and returns to the output of the DUT. The error occurs in transmission measurements and in reflection measurements (for a 2-port DUT). Load match errors depend on the difference between output impedance of the DUT and test port impedance when used as a signal receiver.

In transmission measurements, the load match error has considerable influence if the output of the DUT is poorly matched. In reflection measurements, the load match error has considerable influence in case of poor output match and low attenuation between the output and input of the DUT.

Isolation Error

Isolation error (Ex) is caused by a leakage of the signal from the source port to the receiver port bypassing the DUT.

The Analyzer has very good isolation, which allows us to ignore this error for most measurements. Isolation error measurement is an optional step in all types of calibration.

Reflection Tracking Error

A reflection tracking error (Er) is caused by differences in frequency response between the test receiver and the reference receiver of the source port in reflection measurement.

Transmission Tracking Error

A transmission tracking error (Et) is caused by differences in frequency response between the test receiver of the receiver port and the reference receiver of the source port in transmission measurement.

Error Modeling

Error modeling and the methodology of signal flow graphs are applied to vector network analyzers for analysis of systematic errors.

One-Port Error Model

In reflection measurement, only one port of the Analyzer is used. The signal flow graph of errors for Port 1 is represented in Figure 41. For Port 2 the signal flow graph of the errors will be similar.

Figure 41: One-port error model

Where:

• S_11a – reflection coefficient true value;
• S_11m – reflection coefficient measured value.

The measurement result at Port 1 is affected by the following three systematic error terms:

• E_d1 – directivity;
• E_s1 – source match;
• E_r1 – reflection tracking.

For normalization the stimulus value is taken equal to 1. All the values used in the model are complex.

After determining all the three error terms E_d1, E_s1, E_r1for each measurement frequency by means of a full 1-port calibration, it is possible to calculate (mathematically subtract the errors from the measured value S_11m) the true value of the reflection coefficient S_11a.

There are simplified methods, which eliminate the effects of only one or two of the three systematic errors.

Two-Port Error Model

For two-port measurements, two signal flow graphs are considered. One of the graphs describes the case where Port 1 is the stimulus source, the other graph describes the case where Port 2 is the stimulus source.

The signal flow graphs of errors effect in a two-port system are represented in Figure 42:

Figure 42 Two-port error model

Where:

• S_11a, S_21a, S_12a, S_22a – true values of the DUT parameters;
• S_11m, S_21m, S_12m, S_22m – measured DUT parameter values.

For normalization the stimulus value is taken equal to 1. All the values used in the model are complex. The measurement result in a two-port system is affected by twelve systematic error terms.

Table 14Systematic error terms

After determination of all twelve error termsfor each measurement frequency by means of a 2-port calibration, it is possible to calculate the true value of the S-parameters: S_11a, S_21a, S_12a, S_22a.

There are simplified methods, which eliminate the effect of only one or several of the twelve systematic error terms.

Analyzer Test Port Definition

The test ports of the Analyzer are defined by means of calibration. The test port is a connector accepting a calibration standard in the process of calibration.

A type-N connector on the front panel of the Analyzer will be the test port if calibration standards are connected directly to it.

Sometimes it is necessary to connect coaxial cables and/or adapters to the connector(s) on the front panel to interface with a DUT of a different connector type. In such cases, calibration standards are connected to the connector of the cable or adapter.

Figure 43 represents two cases of test port definition for 2-port measurements. The use of cables and/or adapters does not affect the measurement results if they are integrated into the process of calibration.

Figure 43 Test port defining

In some cases, the term calibration plane is used. Calibration plane is an imaginary plane located at the ends of the connectors, which accept calibration standards during calibration.

Calibration Steps

The process of calibration comprises the following steps:

• Selection of a calibration kit matching the connector type of the test port. The calibration kit includes such standards as SHORT, OPEN, and LOAD with matched impedance. Magnitude and phase responses i.e. S-parameters of the standards are well known. The characteristics of the standards are represented in the form of an equivalent circuit model, as described below;
• Selection of a calibration method (see section 5.1.6) is based on the required accuracy of measurements. The calibration method determines what error terms of the model (or all of them) will be compensated;
• Measurement of the standards within a specified frequency range. The number of the measurements depends on the type of calibration;
• The Analyzer compares the measured parameters of the standards against their predefined values. The difference is used for calculation of the calibration coefficients (systematic errors);
• The table of calibration coefficients is saved into the memory of the Analyzer and used for error correction of the measured results of any DUT.

Calibration is always made for a specific channel, as it depends on the channel stimulus settings, particularly on the frequency span. This means that a table of calibration coefficients is being stored each for an individual channel.

Calibration Methods

The Analyzer supports several methods of one-port and two-port calibration. The calibration methods vary by quantity and type of the standards being used, by type of error correction, and accuracy. The table below presents an overview of calibration methods.

Table 15Calibration methods

Normalization

Normalization is the simplest method of calibration as it involves measurement of only one calibration standard for each S-parameter.

• 1-port (reflection) S-parameters (S₁₁, S₂₂) are calibrated by means of a SHORT or an OPEN standard, estimating the reflection tracking error term Er.
• 2-port (transmission) S-parameters (S₂₁, S₁₂) are calibrated by means of a THRU standard, estimating the transmission tracking error term Et.

This method is called normalization because the measured S-parameter at each frequency point is divided (normalized) by the corresponding S-parameter of the calibration standard.

Normalization eliminates frequency-dependent attenuation and phase offset in the measurement circuit, but does not compensate for errors of directivity, mismatch or isolation. This constrains the accuracy of the method. |Item|Description| |---|---| | Note | Normalization can also be referred to as response open, response short or response thru calibrationdepending on the standard being used: an OPEN, SHORT or THRU respectively. |

Directivity Calibration (Optional)

The Analyzer offers an optional directivity (Ed) calibration feature, which can be used in combination with reflection normalization by means of measurement of a LOAD standard. Auxiliary directivity correction increases the accuracy of normalization.

Isolation Calibration (Optional)

The Analyzer offers optional isolation (Ex) calibration to be combined with the following three methods of calibration:

• transmission normalization,
• one-path two-port calibration,
• full two-port calibration.

This calibration is performed by isolation measurement using LOAD standards connected to both test ports of the Analyzer. Isolation calibration can be omitted in most tests, as the signal leakage between the test ports of the Analyzer is negligible. |Item|Description| |---|---| | Note | For isolation calibration, it is recommended to set a narrow IF bandwidth and firmly fix the cables. |

Full One-Port Calibration

Full one-port calibration involves connection of the following three standards to one test port:

• SHORT,
• OPEN,
• LOAD.

Measurement of the three standards allows for acquisition of all the three error terms (Ed, Es, and Er) of a one-port model. Full 1-port calibration is a highly accurate method for 1-port reflection measurements.

One-Path Two-Port Calibration

A one-path two-port calibration combines full one-port calibration with transmission normalization. This method allows for a more accurate estimation of transmission tracking error (Et) than using transmission normalization.

One-path two-port calibration involves connection of the three standards to the source port of the Analyzer (as for one-port calibration) and a THRU standard connection between the calibrated source port and the other receiver port.

One-path two-port calibration allows for correction of Ed, Es, and Er error terms of the source port and a transmission tracking error term (Et). This method does not derive source match error term (El) of a 2-port error model.

One-path two-port calibration is used for measurements of the parameters of a DUT in one direction, e.g. S₁₁ and S₂₁.

Full Two-Port Calibration

A full two-port calibration involves seven connections of standards. This calibration combines two full 1-port calibrations for each port, and one THRU connection, which provides transmission measurements with each test port as a source. If optional isolation calibration is required, connect LOAD standards to the both test ports of the Analyzer and perform isolation measurements for each source port.

Full 2-port calibration allows for correction of all the twelve error terms of a 2-port error model: E_d1, E_d2, E_s1, E_s2, E_r1, E_r2, E_t1, E_t2, E_l1, E_l2, E_x1, E_x2 (correction of E_x1, E_x2 can be omitted).

Full 2-port calibration is a highly accurate method of calibration for 2-port DUT measurements.

Sliding Load Calibration

In full one-port and full two-port calibrations it is possible to employ a SLIDING LOAD calibration standard instead of a fixed one. The use of the SLIDING LOAD standard allows for significant increase in calibration accuracy at high frequencies compared to the FIXED LOAD standard.

The sliding load calibration involves a series of measurements in different positions of the sliding element to compensate for reflection from the dissipation component.

To activate the sliding load calibration algorithm, the selected calibration kit should contain a calibration standard of sliding load type, and it should be assigned to the "load" class of the corresponding port. Calibration standard editing and class assignment are further described in detail in section 5.2.14.

The sliding load calibration is not suitable for low frequencies. To eliminate this limitation, use a FIXED LOAD standard in the lower part of the frequency range. For combined calibration with SLIDING and FIXED LOADS, use the procedure of standard subclasses assigning. This procedure is described in detail in section 5.3.4.

Unknown Thru Calibration (except Planar 304/1)

UNKNOWN THRU calibration standard is used only in full two-port calibration, which is also known as SOLT (Short, Open, Load, Thru) calibration.

This calibration method involves connecting the test ports to each other, referred to as the THRU. If the connectors’ gender or type prevent direct connection, a DEFINED THRU is used. But it is not always possible to know the exact parameters of the THRU, in this case UNKNOWN THRU calibration can be used.

An arbitrary two-port device with unknown parameters can be used as an UNKNOWN THRU. An UNKNOWN THRU should satisfy only two requirements.

The first requirement applies to the transmission coefficient of the THRU. It should satisfy the reciprocity condition (S₂₁ = S₁₂), which holds for almost any passive network. Furthermore, it is not recommended to use a THRU with the loss higher than 20 dB as it can reduce the calibration accuracy.

The second requirement is knowledge of the approximate electrical length of the UNKNOWN THRU within an accuracy of 1/4 of the wavelength at the maximum calibration frequency. This requirement, however, can be omitted if the following frequency step size condition is met:

where τ ₀ – delay of a two-port device.

In this case the Analyzer program will automatically determine the electrical length (delay) of the two-port device.

In other words, you can perform calibration without specifying the delay of the UNKNOWN THRU if the frequency increment is sufficiently small. For example, with an UNKNOWN THRU having and delay coefficient , the delay will be . In this case the maximum frequency increment for automatic estimation of the UNKNOWN THRU delay should be set to ; equivalently the number of points within a sweep span of 8 GHz should be no less than 16. To ensure reliable operation, set the frequency increment, or equivalently the number of points, to provide at least double margin.

To use unknown thru calibration as part of full two-port calibration, the calibration kit definition should include an UNKNOWN THRU standard, assigned to the THRU class, for the two ports. The procedure of calibration standards editing and their assignment to classes is further described in detail in section 5.2.14.

An UNKNOWN THRU is defined automatically if you set the delay to zero in the calibration kit editing menu. Otherwise the user-defined delay value will be used. This value should be set to within 1/4 wavelength of the true delay at the maximum calibration frequency.

TRL Calibration (except Planar 304/1)

TRL (Thru-Reflect-Line) calibration is the most accurate calibration method described herein, as it uses airlines as calibration standards. The TRL calibration requires the use of the following calibration standards:

• THRU or REFERENCE LINE,
• REFLECT (SHORT or OPEN),
• Second LINE or two MATCHes.

TRL is a general name for a calibration family, which comprises such calibrations as LRL, TRM, or LRM named depending on the calibration standards used.

If a zero-length THRU is used as the first standard, the method is called TRL calibration. If a non-zero length LINE is used as the first standard, the calibration method is called LRL (Line-Reflect-Line). To denote the first standard of the TRL and LRL calibration, assign TRL-Thru class, which includes THRU and LINEs. A LINE of TRL-Thru class is also called Reference Line.

An OPEN or SHORT is usually used as a second standard in TRL calibration. To denote the second standard of the TRL calibration, assign TRL-Reflect class.

A second LINE is used as the third standard in TRL calibration. At low frequencies, at which MATCHes work well, two MATCHes can be used, as they are an equivalent of a matched line of infinite length. In the latter case, the calibration method is called TRM (Thru-Reflect-Match) or LRM (Line-Reflect-Match) respectively. To denote the third standard of the TRL calibration, assign TRL-line/match class, which includes LINEs and MATCHes.

Frequency Range

TRL and LRL calibrations have a limited bandwidth, suitable for lower to upper frequency ratios up to 1:8. The band limits depend on the LINE length in TRL calibration or on the difference between the lengths of the two LINEs in LRL calibration.

In theory TRM and LRM calibrations do not have limitations in frequency, however their practical use at higher frequencies is limited by the quality of the MATCHes. It is recommended to use the TRM and LRM calibrations up to 1 GHz.

Impedance of LINEs and MATCHes

All the LINEs and MATCHes used for TRL calibration must have Z0 impedance values as precise as possible. TRL calibration transfers the impedance of standards into the calibrated system. Precise airlines with an accurate Z0 impedance of 50 Ω are used as LINEs in coaxial paths.

REFERENCE LINE

A zero-length THRU is used as the first standard in TRL calibration. In LRL calibration a LINE, which is called REFERENCE LINE, is used instead of a zero-length THRU. The shortest LINE is used as the REFERENCE LINE. Its length must to be known, so that the calibration plane positions could be calculated exactly. However, LRL calibration is also possible when the REFERENCE LINE length is not known. In this case, its length is assumed to be equal to zero, the calibration plane being in the middle of the LINE, and not at the ports’ edges.

TRL LINE

TRL LINE is an airline used in TRL calibration, or the second longest LINE used in LRL calibration. The length of TRL LINE should be known just approximately. The LINE length is used to determine the calibration bandwidth. Let ΔL be the difference between the two LINEs in LRL calibration. In TRL calibration this difference will be equal to the LINE length, as a zero-length THRU is used as a REFERENCE LINE. Then the phase difference between the TRL LINE and REFERENCE LINE or THRU should be no less than 20° at the lower frequency and no more than 160° at the upper frequency of the calibration.

ν – wave velocity in LINE (for airline it is с =2.9979·10⁸ м/с).

L₀ – REFERENCE LINE length, L₁ – TRL LINE length,

So, the useful frequency range for TRL/LRL calibration is 1:8. Besides, TRL/LRL calibration does not work at low frequencies, as it would require a very long LINE. Two or more TRL LINEs are used to extend the calibration frequency. For example, in case of using two TRL LINEs the frequency range can be increased up to 1:64.

TRL MATCH

Unlike TRL/LRL calibration, TRM/LRM calibration uses MATCHes, which are the equivalent to the infinitely long LINE, instead of a TRL LINE. Theoretically TRM/LRM calibration has no frequency limitations. However, the use of TRM/LRM calibration at higher frequencies is limited by the quality of the MATCHes. As a rule, the TRM/LRM calibration is used at lower frequencies, as it is good starting from zero frequency.

TRL REFLECT

There are no strict requirements to the TRL REFLECT standard. You should know only approximate parameters of the TRL REFLECT standard. The REFLECT standard should have high reflection coefficient, close to 1. The phase of the standard must be known within ±90°. Normally, any OPEN or SHORT meets this requirement. The next requirement is that the reflection coefficient must be the same for all the ports. If one standard is used for all the ports by turns, then this requirement is automatically fulfilled. If the ports have different genders or types of connectors, use special standards with the identical electrical specifications, which are available in pairs.

TRL Calibration Frequency Extension

To extend the frequency of TRL calibration a method of dividing into several non-overlapping bands is applied. For each frequency band a separate TRL LINE of different length is used. The phase difference between each TRL LINE and the REFERENCE LINE must be from 20° to 160°, as indicated above. A MATCH standard is used in the lowest frequency band.

The Analyzer software allows using up to 8 LINES for calibration frequency extension. To achieve this, there are two steps of handling the calibration kits:

• defining frequency limits to calibration standards (see 5.3.2);

• assigning classes to calibration standards, where up to 8 calibration standards can be assigned to one class (see section 5.3.4).

Perform the above mentioned dividing of the calibration band into sub-bands and assign a separate TRL LINE to each of them in the calibration kit editing menu before calibration.

Multiline TRL Calibration (except Planar 304/1)

Regular TRL calibration, described in the previous section uses several LINEs of different lengths for frequency extension. It is provided by the method of dividing the frequency band into separate sub-bands.

Multiline TRL calibration also uses several LINEs. But it does not divide the frequency band into several sub-bands. Instead, all the LINEs are used simultaneously over the whole calibration bandwidth. The redundancy of the LINEs measurements allows for both extending the frequency range and increasing the calibration accuracy. The number of LINEs should be no less than three. The more LINEs you use, the higher the accuracy you will achieve.

To employ multiple LINEs in the calibration procedure, use the same method of standards subclasses assignment as in the regular TRL calibration (see section 5.2.8.1). Defining frequency limits to calibration standards is not necessary for Multiline TRL calibration method. The procedure of switching between the normal and Multiline TRL calibrations see in section 5.2.7.1.

The following table shows the differences between the regular and Multiline TRL calibrations when entering the data into the calibration standards editing menu.

Waveguide Calibration

The Analyzer supports the following calibration methods in a waveguide environment:

• Reflection or Transmission Normalization
• Full One-Port Calibration
• One-Path Two-Port Calibration
• Full Two-Port Calibration
• TRL Calibration

The Analyzer further supports use of a sliding load standard in the abovementioned calibrations, except TRL.

General use and features:

• System Z0 should be set to 1 ohm before calibration. Offset Z0 and terminal impedance in the calibration standard definition also should be set to 1 ohm.
• Waveguide calibration uses two offset short standards instead of a combination of short and open standards. Typically 1/8λ₀ and 3/8λ₀ offset sort standards are used, where λ₀ – wave length in waveguide at the mean frequency.

In waveguide calibration, one of two offset short standards must be assigned to the open class (see section 5.3.4 Calibration Standard Class Assignment). Consequently the GUI will contain an Open button with the label of this short standard.

Calibration Standards and Calibration Kits

Calibration standards are precision physical devices used for determination of errors in a measurement system.

A calibration kit is a set of calibration standards with a specific connector type and specific impedance.

The Analyzer provides definitions of calibration kits produced by different manufacturers. The user can add the definitions of own calibration kits or modify the predefined kits. Calibration kits editing procedure is described in the section 5.2.14.

To ensure the required calibration accuracy, select the calibration kit being used in the program menu. The procedure of calibration kit selection is described in section 5.2.1.

Definitions and Classes of Calibration Standards

Each calibration standard has a definition and belongs to one or several classes.

Calibration standard definition is a mathematical description of its parameters.

Calibration standard class is an application of the standard in a specific calibration method associated with a specific test port number. For example, "LOAD of Port 1" in full two-port calibration.

Types of Calibration Standards

Calibration standard type is a category of physical devices used to define the parameters of the standard. The Analyzer supports the following types of the calibration standards:

• OPEN,
• SHORT,
• FIXED LOAD,
• SLIDING LOAD,
• THRU/LINE,
• UNKNOWN TRHU (except Planar 304/1),
• Standard defined by data (S-parameters).

|Item|Description| | Note | The type of a calibration standard should not be confused with its class. Calibration standard type is a part of the standard definition used for the calculation of its parameters. |

Gender of Calibration Standard

Gender of a calibration standard is typically denoted on the calibration standard label. The label and the gender of calibration standard respectively, are not accounted by the software and are used for user information only. Nevertheless, it is recommended to follow some rules for calibration standard gender designation. A calibration standard can be labeled either with:

• the gender of a calibration standard itself, as –M– for male and –F– for female type of standard; or
• the gender of the analyzer port, which the calibration standard is mated to, as (m) for male and (f) for female port types;

For example, same standard can be labeled as Short –F– or Short (m).

The Analyzer software uses the first type of designation: the gender of a calibration standard itself denoted as –M– for male and –F– for female type of standards.

Methods of Calibration Standard Defining

The Analyzer provides two methods of defining a calibration standard:

• calibration standard model (See section 5.1.7.5),
• table of S-parameters (See section 5.1.7.6).

The calibration standards defined by the table of S-parameters are called Data-Based standards.

Besides, each calibration standard is characterized by lower and upper values of the operating frequency. In the process of calibration, the measurements of the calibration standards outside the specified frequency range are not used.

Calibration Standard Model

A model of a calibration standard presented as an equivalent circuit is used for determining S-parameters of the standard. The model is employed for standards of OPEN, SHORT, FIXED LOAD, THRU/LINE types.

A One-port model is used for the standards OPEN, SHORT, and FIXED LOAD (See 5.1.6.4). The Two-port model is used for the standard THRU/LINE (See Figure 45). The description of the numeric parameters of an equivalent circuit model of a calibration standard is shown in Table 16.

Table 16: Parameters of the calibration standard equivalent circuit model

Data-Based Calibration Standards

The calibration standards defined by data are set using the table of S-parameters. Each line of the table contains frequency and S-parameters of the calibration standard. For one-port standards the table contains the value of only one parameter – S₁₁, and for two-port standards the table contains the values of all the four parameters – S₁₁, S₂₁, S₁₂, S₂₂.

The table of S-parameters can be filled in manually by the user or downloaded from a file of Touchstone format. Files with .s1p extension are used for one-port standards, and files with .s2p extension are used for two-port standards.

Scope of Calibration Standard Definition

Different methods of calibration apply either full or partial definitions of the calibration standard kits.

The full two-port calibration, full one-port calibration, one-path two-port calibration, and normalization use fully defined calibration standards, i.e. the standards with known S-parameters. The S-parameters of OPEN, SHORT, LOAD, and THRU/LINE must be defined by the model or by data.

|Item|Description| | Note | The UNKNOWN THRU and SLIDING LOAD standards are exceptional in the above calibrations. The Sparameters of these standards are defined in the process of calibration. UNKNOWN THRU is used only in full two-port calibration. | | --- | --- |

TRL calibration and its modifications (TRM, LRL, LRM) apply partial definition of the standards:

• TRL THRU standards must have the required value of Z0 (S₁₁=S₂₂=0) and known length (delay),
• TRL LINE/MATCH standard must have the same value of Z0 as the first standard,
• TRL REFLECT standard must have the phase known as accurately as ±90°.

Classes of Calibration Standards

Along with defining a calibration standard by a calibration model or data, the standard should also be assigned a specific class. One calibration standard may belong to several classes. The class assignment is performed for each particular calibration kit. The procedure of class assignment to the calibration standards is described in section 5.3.4.

Class assignment to a calibration standard is required for specifying such properties as the calibration method, the role of a standard in the calibration, and the number of the port(s). The Analyzer supports the following classes of the calibration standards (See Table 19).

Table 19Classes of the calibration standards

For example, if you assign the class "OPEN of Port 1" to the OPEN -F- calibration standard, it will indicate that this standard is used for calibrating the first port using the following calibration methods: full two-port, full one-port, one-path two-port, and normalization.

Subclasses of Calibration Standards

Subclasses are used for assignment of one class to several calibration standards. The procedure of subclass assignment is mainly employed for calibration within a wide frequency range by several calibration standards, each of which does not cover the full frequency range. Each class of standards can contain up to 8 subclasses.

For example, suppose in your calibration kit the LOAD standard is defined as from 0 GHz to 2 GHz, and the sliding LOAD standard is defined as from 1.5 GHz to 12 GHz. To perform calibration within the full frequency range the fixed LOAD should be assigned the subclass 1, and the sliding LOAD should assigned the subclass 2 of the “load" class.

If the standards have an overlapping frequency range (as in the example above, from 1.5 GHz to 2 GHz), the last measured standard will be used.