VNAOperatingGuide fortheMSA - wetterlin.org

1 VNA Operating Guidefor the MSA2/1/10 The MSA as VNAI ntroductionThis Guide contains individual pages to describe the use of the MSA in Trans-mission Mode [functioning as either a Scalar Network Analyzer (SNA) or aVector Network Analyzer (VNA)], or in Reflection Mode. The pages can beaccessed by clicking on the blue headings below, or by using the Bookmarkstab. This Guide assumes familiarity with the operating procedures set forth inthe SpectrumAnalyzer Operating Guide for the first several pages include material that is relevant to a wide range ofmeasurements:Fixtures. Basic fixtures for interfacing the DUT to the Fixtures. Specifics on shunt Fixtures. Specifics on series General description of reflection Calibration procedures for Transmission Calibration procedures for Reflection ModeOSL Standards.

2 Construction and characterization of OSL cal Transmission. Basic procedures for measurement in Reflection. Basic procedures for measurement in Reflection Extension. Eliminating unwanted signal delays Analysis of filter transmission Determination of crystal parameters for use in creating Meter. Component measurements similar to an LCR Modeling devices as RLC combinations, usingTransmission Mode Modeling devices as RLC combinations, usingReflection Mode Measuring characteristics of coax cable or other S21--Generating S21 data fromS11 measurements for twoterminal DUTs The following pages cover what are called Functions, available under theFunctions menu:Return Loss Reference. Return Loss and related Chart 1. Basic description of the Smith ChartSmith Chart 2. Circular graphs on the Smith ChartAt the end is reference information:The following pages contain additional measurement procedures:Coax Z0--Determining characteristic impedance of transmission Lines--Creating Reference Lines for display or dB6-10 dBShuntFixture6-10 dB6-10 dBBridgeDUTDUT6-10 dB6-10 dBSeriesFixtureDUT is shunted to ground fromthe through-connection.

3 For impedances ohmto1 is in series with the signal path. Forimpedances from3 ohms to 100 kohms. ASeries Fixture with R0 of ohms is thestandard for measuring transmission mode, the DUT may have aseparate ground for reflection mode only. But agood bridge can be used in SNA or VNAT ransmission is shunted to ground fromthe bridgeconnection. For impedances from1 ohmto1 kohm. The bridge has the best accuracy ofany fixture for impedances close to 50 x t u re sThere are three basic types of fixtures for interfacing the MSA to thedevice under test (DUT) in SNA and VNA modes. Attenuators may beexternal or may be built into the fixture. Buffer amplifiers may be usedin place of attenuators to maintain higher dynamic main purpose of the attenuators is to present the DUT with a pre-cise impedance, which is referred to as the R0 of the fixture.

4 Home-made attenuators are far better for this purpose than commercialattenuators. R0 may be a value other than 50 ohms if the attenuatorsare designed to transformthe Modes except SpectrumAnalyzerShunt Fixture SpecificsThe shunt fixture can be constructed with two attenuators attached to acoax tee, with the DUT attached to the third arm. It is most important thatthe DUT see 50 ohms looking into each attenuator; this can be most reliablyaccomplished with homemade attenuators with precision resistors. A simplepi attenuator with values of 100-68-100 ohms makes a precision dBattenuator; makes a dB attenuator. Both have excellentreturn loss in normal use as the return loss and attenuation values are based on the assumption of a50-ohmsource and load. In the shunt fixture, the input attenuator sees aload consisting of the DUT in parallel with the output attenuator, so a zero-ohmDUT creates a zero-ohmload.

5 This can make the return loss fromtheoutside of a dB attenuator as lowas 15 can obtain higher precision by building the attenuators on a fixtureboard, and using non-standard attenuator design. The following schematicshows a shunt fixture with modified dB attenuators, and with the attenua-tor resistors nearest the DUT merged into design can significantly improve accuracy for lowimpedance DUTs ifthe source or load has return loss worse than about 25 dB, because itreduces reflections off the outside of the that for Reflection Mode measurements when OSL calibration is used(rather than the simpler Reference calibration), the issue described above isnot important, as the calibration will compensate for the impedances seenby the Fixture SpecificsThe section discussing shunt fixtures describes resistor values for home-made attenuators, and discusses the fact that simply attaching standardattenuators to a fixture does not produce optimal results.

6 For example, in thecase of the series fixture, if the DUT has a large impedance, each attenuatorsees a large load, rather than the 50 ohms that its design assumes. Thiscauses the return loss at the outside of the attenuators to be very low, caus-ing large design in the schematic belowsolves this problemand achieves aworst-case return loss of 20 dB (with an infinite DUT).The official fixture for testing crystals is a series fixture with an R0 ohms; the schematic is shown Ck components are for compensation, and aren t normally that the lowvalues of R3 and R4 also make the return loss fromtheoutside of the attenuators relatively insensitive to the DUT impedance, sothis fixture also solves the problemdiscussed above, and could be used forpurposes other than crystal are normally used only in reflection mode.

7 They are designed sothat their output is proportional to the signal reflected b y the DUT. Thereflection coefficient is then that signal divided by the output level with anopen DUT (since an open creates a 100%reflection). In dB, that division isdone by quality can be judged by their directivity, which is the reflection coeffi-cient (in dB) with a 50-ohmDUT (which should have zero reflection), and bywhether the open and short reflections have equal magnitude but oppositephase. A bridge with good directivity and well matched open/short can beused with just Reference calibration. In fact, it can evenbe used in Transmis-sion Mode, by doing a scan of the bridge with the Open attached, and compar-ing that level to a scan with the DUT attached. The difference is Active Bridge is constructed with two op amps, and has extremely highprecision to 30 MHz and very good precision beyond 100 MHz, and thus is suit-able for use with Reference calibration.

8 Its active components allowit to mea-sure reflections accurately with minimal signal passive bridges involve baluns with ferrite cores. The basic bridge design,with optional modest attenuators at the input and output, typically result in lossof 14-20 dB with passive bridges. That is, the maximumoutput (with an open orshort DUT) is 14-20 dB belowthe input Parallel Line Bridge is a passive bridge good from1 MHz to beyond 1 directivity is excellent over a broad range, but its open/short match is notideal. It therefore can be used with Reference calibration for measuring returnlosses of 20 dB and higher, where directivity is the primary consideration. WithOSL calibration, its performance is Balun-Plus-Beads Bridge is another passive bridge. It has a broader fre-quency range, fromabout 100 kHz (the practical lower limit for the MSA in VNAmode anyway) to 1 GHz.

9 It is relatively simple to bridges can also be made fromtransformers, splitters/combiners anddirectional dB6-10 dBShuntFixture6-10 dB6-10 dBSeriesFixtureCalibration is done with the DUT is done with a directthrough-connection in place of the n s m i s s i o nM o d eC a l i b rat i o nCalibration is performed to establish the strength and phase of thesignal transmitted in the absence of the DUT, to use as a referencewhen measuring the DUT Set the frequency rangeand Final Filter Pathas Open Operating Cal--> For the Series Fixture, attach a direct connectionin place of theDUT. If you need to compensate for the length of the connection,enter the delay caused by the connection, in For the Shunt Fixture, have nothing attached as a Click PerformBand Cal. The MSA will conduct one sweep to gath-er calibration Data.

10 During the calibration sweep, a minimumWaittimeof 110 ms is used, regardless of your Wait time setting. Howev-er, if your setting is greater than 110 ms, that longer time will If you want to save the Band Cal as a Base Cal file, click SaveBase Click Done. You may nowattach a DUT and click or SNA Transmission ModeRe f l e c t i o nM o d eC a l i b rat i o nVNA Reflection ModeCalibration is performed to establish the strength and phase of the signalreflected with one or more known standards . For Reference calibration,only one such standard is needed. For Full OSL, three are Set the frequency rangeand Final Filter Pathas Open Operating Cal-->PerformCal. Click PerformBand Calto open thereflection mode calibration Select whether you want to use Reference calibration(one standard; gen-erally not used above 150 MHz) or Full OSL(three standards; highestaccuracy.)