Advanced Calibration Techniques For Vector Network Analyzers
Advanced Calibration Techniquesfor Vector Network AnalyzersPresented lentTechnologies,Inc.2006 AgilentTechnologies,Inc.2006Welcome to “Advanced Calibration Techniques for Vector Network Analyzers.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-1
Objectives Provide insight into some of the latest calibration techniques thatimprove accuracy and make calibration easier Look at performance improvements using the advanced techniquescompared to more traditional methodsUnknown thru1-PORTCALIBRATIONPLANESAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 2The objective of this seminar is to provide insight into some of the latestcalibration techniques that improve accuracy and make calibration easier.We will also look at performance improvements using the advancedtechniques compared to more traditional methods.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-2
Agenda Overview of Calibration Advanced Calibration Techniques– “Unknown Thru” Calibration– Data-Based Calibration-Standard Definitions– Expanded (Weighted Least Squares) Calibration Fixture And Probe Techniques– Automatic Port Extensions– Embedding/De-embedding– Measuring Fixtures/Probes Electronic CalibrationAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 3The first calibration technique we will cover is the “unknown thru” calibration.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-3
What is a Vector Network Analyzer?S21TransmissionS11S22Vector network analyzers (VNAs) DUT Are stimulus-response test systems ReflectionS12 Characterize forward and reverse reflection and transmissionresponses (S-parameters) of RF and microwave components Quantify linear magnitude and phaseMagnitude Are very fast for swept measurements Provide the highest levelof measurement accuracyRF SourcePhaseLOR1ATest port 1Advanced VNA Calibration Agilent Technologies, Inc. 2006R2BTest port 2Page M6- 4A vector network analyzer (VNA) is a precision measuring tool that tests theelectrical performance of high frequency components, in the radio frequency(RF), microwave, and millimeter-wave frequency bands (we will use thegeneric term RF to apply to all of these frequencies). A VNA is a stimulusresponse test system, composed of an RF source and multiple measurementreceivers. It is specifically designed to measure the forward and reversereflection and transmission responses, or S-parameters, of RF components.S-parameters have both a magnitude and a phase component, and theycharacterize the linear performance of the DUT. While VNAs can also beused for characterizing some non-linear behavior like amplifier gaincompression or intermodulation distortion, S-parameters are the primarymeasurement. The network analyzer hardware is optimized for speed,yielding swept measurements that are must faster than those obtained fromthe use of an individual source and an individual receiver like a spectrumanalyzer. Through calibration, VNAs provide the highest level of accuracy formeasuring RF components.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-4
The Need for CalibrationHow do we get accuracy?– With vector-error-corrected calibration– Not the same as the yearly instrument calibrationWhy do we have to calibrate?– It is impossible to make perfect hardware– It would be extremely difficult and expensive to make hardwaregood enough to entirely eliminate the need for error correctionWhat does calibration do for us?– Removes the largest contributor to measurementuncertainty: systematic errors– Provides best picture of true performance of DUTSystematic errorAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 5VNAs provide high measurement accuracy by calibrating the test system using amathematical technique called vector error correction (this type of calibration isdifferent than the yearly calibration done in a cal lab to ensure the instrument isfunctioning properly and meeting its published specifications for things like outputpower and receiver noise floor). Vector error correction accounts for measurementerrors in the network analyzer itself, plus all of the test cables, adapters, fixtures,and/or probes that are between the analyzer and the DUT.Why is calibration so important to network analysis? The reason is that it isimpossible to make perfect test hardware, and too difficult and/or too expensive tomake the network analyzer hardware so good that the need for error correction isentirely eliminated. Vector error correction is a cost effective way to improve theperformance of test systems comprised of good but not perfect hardware. The rightbalance between hardware performance, cost, and system performance (includingerror correction) must be achieved. If the RF performance of the hardware is poor,then vector error correction will not be able to overcome all the deficiencies, and theoverall system performance will suffer compared to that obtained from a systemusing better hardware.Calibrating a VNA-based test system removes the largest contributor tomeasurement uncertainty, which are systematic errors. Systematic errors arerepeatable, non-random errors that can be measured and removed mathematically.A vector-error-corrected VNA system provides the best picture of the trueperformance of the device under test (DUT). A network analyzer is really only asgood as its calibration, so Agilent spends a great deal of effort to provide the mostcomplete and highest-quality choices for calibration.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-5
ErrorsWhat is Vector-Error Correction?Vector-error correction MeasuredActual Is a process for characterizing systematic error terms Measures known electrical standards Removes effects of error terms from subsequent measurementsElectrical standards Can be mechanical or electronic Are often an open, short, load, and thru,but can be arbitrary impedances as wellAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 6Vector-error correction is the process of characterizing systematic errorterms by measuring known electrical calibration standards. Any deviationfrom the expected results is primarily due to systematic errors in the testsystem. Once these errors are quantified, their effects can bemathematically removed from subsequent measurements. The error termscan be expressed as vectors since they have a magnitude and phasecomponent. Since any test system is affected by more than one cause ofmeasurement error, the calibration process has to measure enoughstandards to sort out the magnitude and phase of the various errors.The electrical standards used during the calibration process can be passive,mechanical devices, like the well-known short, open, load, and thru (SOLT)standards found in Agilent (and other) commercial calibration kits, or theycan be arbitrary known impedances that are electronically switched, as isdone with Agilent’s ECal electronic calibration modules.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-6
Systematic Measurement ityDUTFrequency responseO reflection tracking (A/R)O transmission tracking lds12errortermsfortwo-portdevicesyields 12 error terms for two-port devicesAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 7Shown here are the major systematic errors associated with networkanalyzer measurements. The errors relating to signal leakage are directivityand crosstalk. Directivity limits dynamic range for reflection measurements,and crosstalk limits dynamic range for transmission measurements. Theerrors related to signal reflections are source and load mismatch. Sourcemismatch errors result from interactions between the test system’s sourcematch and the input match of the DUT. Load mismatch errors result frominteractions between the test system’s load match and the output match ofthe DUT. The final class of errors are related to frequency response of thereceivers, and are called reflection and transmission tracking. The term“tracking” is used because S-parameter measurements are ratioedmeasurements between a test and a reference receiver. Therefore, thefrequency response errors are due to imperfect tracking between the test andreference receivers.The full two-port error model includes all six of these terms for the forwarddirection and the same six (with different data) in the reverse direction, for atotal of twelve error terms. This is why two-port calibration is often referred toas twelve-term error correction. These same basic error terms are also usedin error models for test systems with more than two test ports, where the totalnumber of error terms is larger than 12.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-7
Performing the Calibration: SOLT Two most common types of calibration: SOLT and TRL– Both types remove all the systematic error terms– Type and definition of calibration standards are different SOLT– Basic form uses short, open, load, and known-thru standards– Advanced forms use multiple shorts and loads, unknownthru, arbitrary impedances (ECal)– Uses the 12-term error model Advantages:– Easy to perform– Applicable to a variety of environments(coaxial, fixture, waveguide )– Provides a broadband calibrationAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 8There are two basic types of calibration used to correct for the systematicerror terms that we discussed on the previous slide. The two types are SOLT(short, open, load, thru) and TRL (thru, reflect, line). The differences in thecalibrations are related to the types of calibration standards they use and howthe standards are defined. They each have their advantages, depending onfrequency range and application. As its name implies, SOLT calibration isbased on shorts, opens, loads, and thrus as calibration standards. Inadvanced forms, it can use multiple shorts and loads to cover a broaderfrequency range, and it can use undefined or unknown thrus, which we willcover in a later section. Electronic calibration is a form of SOLT calibration,where the shorts, opens and loads are replaced by known arbitraryimpedances. We will discuss ECal in a later section as well.SOLT cal is very easy to perform, and is used in a broad variety ofenvironments. It is the most widely used choice for coaxial measurements,since there are many commercial coaxial calibration kits available to matchmost connector types. It can also be used with fixtures and probes. SOLTinherently provides a broadband calibration, essentially from DC to the upperfrequency limit of the connector type being used.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-8
Performing the Calibration: TRL Basic form: thru, reflect, line standards Advanced forms: TRM, LRM, LRL, LRRL Uses a 10-term error model Advantages– Uses standards that are easy to fabricate and have simplerdefinitions than SOLT Only need transmission lines and high-reflect standards Required to know impedance and approximate electrical length of linestandards Reflect standards can be any high-reflection standards like shorts or opens Load not required; capacitance and inductance terms not required– Potential for most accurate calibration (depends on quality oftransmission lines)– Commonly used for in-fixture and on-wafer environmentsAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 9TRL calibration was developed for making accurate measurements of noncoaxial devices at microwave and millimeter-wave frequencies. It iscommonly used for in-fixture and on-wafer environments. The basic formuses a zero-length thru, a longer thru (the “line”), and high-reflect standardslike opens or shorts. There are many variations of TRL that substitutedifferent standards (like lines for thrus, or loads for lines), but they all use thesame error model and its associated assumptions. One of the biggestadvantages of TRL is that the standards are generally easy to fabricate andthey have simpler definitions than the standards used with SOLT. This meansthat for many non-coaxial applications, TRL can give superior accuracy. ForTRL, it is only required to know the impedance and approximate electricallength of the line standards, and the reflect standards can be any highreflection devices like shorts or opens. TRL does not require a load standard,which is desirable because it is difficult to make accurate high frequency,non-coaxial load standards. It is also not required to define the capacitanceand inductance of the reflection standards.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-9
Component Measurement ChallengesNon-insertable coaxial devices– Same sex connectors (e.g., SMA females)– Mixed connectors (e.g., SMA and Type-N)Devices without coaxial connectors– Surface-mount devices– Devices on wafer– Devices with waveguide portsMechanically difficult situations– Physically long devices– Fixed test-port positions– Non-in-line connectorsMultiport ( 4 port) devicesAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 10Now that we know the basics of measuring the electrical performance of RFcomponents, we can look at some of the measurement challenges that must beovercome to get accurate, repeatable measurements. The first challenge is forcoaxial devices that are “non-insertable”. This means that the connectors on theDUT are such that the associated test-port cables of the test system cannot beconnected directly together, without using some sort of RF adapter. During the thruportion of the calibration, the electrical characteristics of the adapter must bemeasured and removed from the calibration data. Common examples of noninsertable devices are those with the same connector sex on all ports (e.g., SMAfemale connectors), or those with mixed types of connectors (e.g., SMA on one portand Type-N on another port).In today’s world of electronics, many RF devices don’t have coaxial connectors. Forexample, RF components found in mobile (cellular) handsets and wireless LANcircuitry are all implemented in surface-mount technology. Other examples of noncoaxial devices are those measured while still a part of the semiconductor wafer, orthose with waveguide ports that are commonly found on very-high-power and/orvery-high-frequency components.Other situations that present measurement challenges are those that aremechanically difficult, such as physically long devices, fixed test-port positions, ordevices with non in-line connectors.Finally, devices with more than four RF ports are a challenge since most moderncommercial network analyzers come with up to only four integrated test ports. Testports can be increased with external test sets, and calibration methodologies mustbe expanded to include these additional ports.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-10
Unknown Thru CalibrationThe “Unknown Thru” technique is Used when a “flush” (zero-length or mate-able) thru cannot be usedor when using a flush thru would cause measurement impairment A refinement of SOLT calibration Also called short-open-load-reciprocal-thru (SOLR)Unknown Thru technique eliminates need for Matched or characterized thru adapters Moving or bending test cablesWorks great for many component measurement challenges Non-insertable devices Mechanically difficult situations Multiport devicesAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 11The unknown thru calibration technique is used when a “flush” (zero-length ormate-able) thru cannot be used or when using a flush thru would causemeasurement impairment from cable movement. It is a refinement of SOLTcalibration, and is also called short-open-load-reciprocal-thru (SOLR)calibration. The unknown thru technique eliminates the need for matched orcharacterized thru adapters, and largely eliminates the need to move or bendtest cables. It works great for many component-measurement challengessuch as non-insertable devices, mechanically difficult situations, and formultiport devices.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-11
Non-Insertable ECal ModulesECal resolves many, but not all non-insertablesituationsAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 12For non-insertable devices, Agilent’s ECal modules can often be used. Someexamples of same-sex or mixed-connector modules are shown here.However, some measurement applications cannot be solved using ECalalone, so it is desirable to have a general-purpose technique like theunknown-thru calibration which can be used in a variety of applications.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-12
Compromises of Traditional Non-Insertable MethodsCalibrationMeasurement Swap equal adapters– Need phase matched adapters of different sexes (e.g., f-f, m-f)– Errors introduced from loss and mismatch differences of adapters Use characterized thruKnown S-parameters– Two-step process (characterize thru, then use it during calibration)– Need a non-insertable cal to measure S-parameters of characterizedthru2-port cal 12-port cal 2 Perform adapter removal cal– Accurate but many steps in calibration (need to do two 2-portcalibrations) Add adapters after cal, then, during measurement – Use port extensions – doesn’t remove adapter mismatch effects– De-embed adapters (S-parameters known) – similar to characterizedthruAdvanced VNA Calibration Agilent Technologies, Inc. 2006Page M6- 13Traditional methods for measuring non-insertable devices all havecompromises compared to the unknown thru technique. The swap-equaladapter method works when phase-matched adapters are available. Manymechanical calibration kits for a specific connector type have these adapters,but they are difficult to obtain for mixed-connector-type situations. Alsomeasurement error can result from the loss and mismatch differencesbetween the adapters. Using a characterized thru is another method, but it isa two-step process as the user must first characterize the thru to obtain its Sparameters. This step often needs its own non-insertable calibration. Anadapter-removal cal is a very accurate technique that is applicable to manynon-insertable situations, but it is a lengthy process since it requires two twoport calibrations. Adding adapters after an insertable cal is often done, withmixed results. Using port extensions is a first-order correction, but it does notremove mismatch effects due to the addition of the adapter. The adapter canbe de-embedded, but this also requires extra steps taken before themeasurement to obtain the S-parameters of the adapter. All of thesecompromises are avoided with the unknown thru calibration.Modern Measurement Techniques for Testing AdvancedMilitary Communications and Radars, 2nd Edition Agilent Technologies, Inc. 2006M6-13
Comparing Unknown Thru and Adapter Removal1.85 f-f adapter comparison0.00-0.03-0.05Magnitude dB-0.08-0.10-0.13-0.1
(short, open, load, thru) and TRL (thru, reflect, line). The differences in the calibrations are related to the types of calibration standards they use and how the standards are defined. They each have their advantages, depending on frequency range and application. As its name implies, SOLT calibration is
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