Using Test Fixtures for MeasurementsWith Scotty’s Modular Spectrum Analyzer (MSA)

Sam Wetterlin12/4/09

Corrected 3/9/2011

IntroductionMeasurement of reflection and impedance, determination of component values, or analysis of crystal parameters with the MSA requires that the device under test (DUT) be installed in some sort of test fixture. We discuss here some fixtures that can be used. We will primarily discuss use of the fixtures for measuring reflection and impedance, but we will also show how those fixtures can be used for other measurements with the MSA, such as Component Meter and Crystal Analysis.

The MSA has several operating modes, including Reflection Mode. The primary purpose of Reflection Mode is to measure the reflection or impedance of a DUT, and to display those values or other values that are calculated from those values. Reflection and impedance are closely related concepts. If one is known, the other can be calculated. To measure reflection or impedance on the MSA, we need a test fixture that, in response to a signal from the Tracking Generator, will send a signal to the MSA input from which we can calculate reflection and impedance of the DUT that is attached to the test fixture. Strictly speaking what the MSA is directly measuring is the transmission characteristics of the fixture with the DUT attached. The MSA then calculates the reflection and impedance of the DUT from the response of the fixture.

Some test fixtures, known as reflection bridges, attempt to provide an output whose level directly represents the reflection from the DUT. We can then compare that output to the reflection resulting when no DUT is attached, to find the reflection coefficient (relying on the fact that an open circuit provides 100% reflection). As we shall see, there are also fancier calculations we can do to eliminate the effects of many fixture defects.

Other test fixtures provide an output that does not directly represent the DUT reflection or impedance, but is well behaved enough that we can mathematically transform it into reflection and impedance. These fixtures generally consist of an attenuator, followed by the DUT, followed by another attenuator. Buffer amplifiers may replace the attenuators, the point being to provide a solid 50-ohm impedance to the DUT on both sides. The DUT may be installed in the fixture in series between the attenuators, or it can be shunted to ground from the common terminal of the attenuators. Hence, these fixtures are referred to as the Series and the Shunt fixtures.

The MSA has a Component Meter function to measure resistors, capacitors and inductors. That feature is available in VNA Transmission Mode, and is even available in SNA Transmission Mode, because it relies only on magnitude measurement, and does not

need phase information. Component Meter makes use of either the Series or Shunt fixtures; a reflection bridge is not suitable for that purpose.

Finally, the MSA has a feature to measure Crystal Parameters, which requires that the crystal be mounted in a Series Fixture.

Let’s begin with the Series and Shunt Fixtures, because they are the simplest.

The Series Fixture

Figure 1—The Series Fixture

In the Series Fixture of Figure 1, the signal output will decline as the DUT impedance increases, and its phase will depend on the reactance of the DUT. There is therefore a simple mathematical relationship between the fixture output level and the DUT impedance. For low impedances, the output level will not be much affected by changes in the DUT impedance, as the output level is already near the maximum. This imposes a practical minimum impedance that can be measured in the Series Fixture.

The attenuators can be used for impedance transformation, with the DUT seeing a non-50-ohm impedance at each attenuator. This can be used to alter the useful range of measurements. A 200-ohm version, for example, improves accuracy for impedances in the thousands of Kohms, but lowers accuracy for very low impedances.

The Shunt Fixture

Figure 2—The Shunt Fixture

The Shunt Fixture of Figure 2 is similar to the Series Fixture, but now the output level will increase as the DUT impedance increases. This means we have to use a different formula to calculate impedance. For high impedances, the output level will not be much affected by changes in the DUT impedance, as the output level is already near the maximum. This imposes a practical maximum impedance that can be measured in the Shunt Fixture.

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As with the Series Fixture, the attenuators can have a non-50-ohm impedance on the DUT side, to alter the useful range of measurement.

The Reflection BridgeThe Reflection Bridge can take many forms, but the concept is illustrated by Figure 3.

Figure 3—Reflection BridgeThe source and load must be 50 ohms

If the DUT is 50 ohms, then the voltages on the left and right of the transformer primary are identical. The bridge is then “in balance” and there is zero output. When the DUT rises above 50 ohms, the voltage across the transformer primary increases, with the right side having the higher voltage. The increase in voltage occurs relatively quickly at first, but the further the DUT gets from 50 ohms, the slower the voltage changes as we increase impedance. A similar situation occurs when the DUT falls below 50 ohms, but in this case the voltage on the right of the transformer becomes lower, reversing the polarity of the output. The rapid change of voltage near 50 ohms makes the bridge ideal for measuring impedances near 50 ohms. However, there is only so much voltage to go around, so the rapid change of voltage near 50 means the rate of change must become very slow when you get far from 50 ohms. The Reflection Bridge is the best fixture for measurements near 50 ohms, but it is not as good for very low impedances as the Shunt Fixture, and not as good for high impedances as the Series Fixture.

In general usage, the term “bridge” is sometimes used to describe any fixture that is used to measure reflection coefficients. Strictly speaking though, the essence of a true bridge is the existence of a null point where the bridge is in balance with no output, which causes the bridge to be very sensitive to changes in the DUT impedance in the area of the null point.

Reflection bridges do not have to use a 50-ohm reference impedance, but we will assume here that they do. If your bridge is not 50 ohms, there is a way to tell the MSA software the actual reference impedance.

Which Fixture to Use

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A good rule of thumb is that to measure very low impedances (<5 ohms), use the Shunt Fixture; to measure impedances near 50 ohms (say, 40-60 ohms) with greatest precision, use the Reflection Bridge. To measure very high impedances (>500 ohms), use the Series Fixture. For the in-between values, any of them will do.

For use in measuring return loss so as to be able to maximize RF power transfer from one device to another, such as from an amplifier to an antenna, extreme precision is usually not required, and any of these fixtures is suitable. While the bridge will be best able to distinguish 50 ohms from 50.5 ohms, that impedance difference has a negligible effect on power transfer. While the Series Fixture will be the best for measuring an impedance of 5K ohms, as a practical matter you are going to construct some impedance-matching circuit to try to get the impedance in the vicinity of 50 ohms, and then you will measure again to see how you did. A ballpark estimate of the original 5K impedance is probably adequate, and your ultimate measurement is likely to be in the neighborhood of 50 ohms, where any fixture will be suitable.

There is one special consideration for the Series Fixture: the DUT must be “floating”, with neither end grounded. This makes the Series Fixture unsuitable for a permanently grounded DUT, such as an antenna. But it is perfect for components with two leads.

The shunt fixture has one special advantage, in that its major source of error is correctible. Its major “flaw” is that there is some distance between the actual DUT and the line connecting the two attenuators. That distance may be just the length of an SMA connector, but when using Reference calibration it can distort measurements at higher frequencies. If told the time delay of that connection (typically about 0.115 ns for an SMA connector with a short PCB trace), the MSA can compensate for that delay and extend the frequency range of measurements with Reference calibration, whose limit becomes somewhere between 60 and 200 MHz, depending on how extreme the impedance being measured.

As discussed later, if a DUT is attached to a fixture through a coax cable, OSL calibration is useful to remove the effects of the cable on the measurement. The Reflection Bridge is in theory the best able to cope with very long cables, because its Open and Short calibration measurements will remain well spaced. However, I have not tested this theory, and all these fixtures proved capable of handling 8 foot cables with OSL calibration at the end of the cable.

Reference CalibrationEach of these fixtures must be calibrated in some way. The simplest calibration is Reference calibration, where you are essentially measuring the maximum output from the fixture, to which other measurements will be compared. In the bridge, if the maximum output (which occurs with an Open DUT) is 2V and the DUT generates 1V, the DUT’s reflection coefficient is 0.5. In dB terms, we call the 2V maximum 0dB, and the DUT output of 1V represents an S11 value of -6 dB, or return loss of 6 dB. The Bridge gets maximum output with either an Open or Short DUT (the magnitudes are the same, but one is the negative of the other—i.e. 180 degrees out of phase). So the Bridge can be

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calibrated with either the Open or the Short, as long as the MSA knows which it is. The Series Fixture gets maximum output with a Short DUT, so its Reference calibration is done with a Short. The Shunt Fixture gets maximum output with an Open DUT, so its Reference calibration is done with an Open.

Calibration with the MSA is generally done at exactly the frequency points to be included in the scan. If you change the frequency range, or number of steps, or shift from linear to log sweep, you need to recalibrate to get best results. Calibration at exactly the frequency being scanned is called Band calibration. It is also possible to use something called Base calibration, which is a calibration over a broad frequency range (e.g. log sweep from 100 KHz to 1GHz). Whatever the actual sweep points, the Base calibration data will be interpolated to those points. The results are not as precise as with Band calibration, especially for phase. Nevertheless, Base calibration can save a lot of trouble when you are frequently changing the sweep parameters.

OSL CalibrationDepending on the precision of the fixture, and the impedance being measured, Reference calibration can provide nice results to 100 MHz and beyond. Imperfections and parasitics in the fixture, however, will cause its measurements to deteriorate at some point. A more elaborate calibration, called OSL calibration, can eliminate the effects of many such imperfections. OSL calibration is based on the amazing fact that every signal transformation caused by a circuit with linear components, can be represented by a very simple formula that contains three parameters. Those parameters depend on the nature of the circuit. If you know those parameters, you know the behavior of the circuit. To determine those three unknown parameters, you have to make measurements with three known DUTs. The DUTs that work best for this purpose are an open circuit (the Open), a short circuit (the Short) and 50 ohms (the Load). The reason these work well is that they provide widely diverse output levels.

OSL calibration can be done with any of these fixtures. An interesting point is that the transformation required to convert the output of a perfect Series or Shunt Fixture into impedance or reflection can be incorporated into the OSL equation. Therefore, the OSL calibration will not only compensate for small imperfections; it will also accomplish the entire transformation. There is no need to first calculate what reflection is represented by the fixture output, and then to apply OSL. We just use OSL calibration, without the MSA doing anything special to take into account the type of fixture involved.

OSL calibration is quite effective, but it is also three times the hassle of Reference calibration. Especially for low frequency work (up to 60 MHz, or in many cases 150 MHz), there is often no practical benefit to OSL calibration. Even for higher frequencies, Reference calibration may be sufficient to get a good idea of the response of the DUT, and to refine the frequency range of interest, after which OSL calibration can be used to take the final data.

There is one situation where OSL calibration is an absolute necessity, even at lower frequencies. If the DUT cannot be attached directly to the fixture, but is connected via a

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coax cable, the cable will significantly affect the measurement. With Reference calibration, the MSA can accurately determine the impedance of the entire assembly—i.e. the cable plus DUT. The cable has to be considered part of the test fixture or part of the DUT. Reference calibration will consider it part of the DUT. If you want to know the impedance or reflection of the DUT by itself, you need to treat the cable as part of the DUT and do OSL calibration by connecting the calibration standards to the end of the cable where the DUT will be directly attached.

Calibration standards for OSL calibration are covered in detail elsewhere. For a fixture with a coax connector for the DUT, the Short is usually a connector with the center pin shorted to the connector body on the back of the connector. The Open is a connector of the same type, with the center pin cut off flush at the back side of the connector, so as to make the pin length of the Open the same as that of the Short. The Load is a similar connector with two precision 100-ohm resistors soldered on the back, pointing in opposite directions. The SMD resistors used for the Load typically have some series inductance (perhaps 1.3 nH) and parallel capacitance (perhaps 0.25 pF). If we could get a 50-ohm resistor (we can’t—the closest is 49.9 ohms), these parasitic elements would alter the actual impedance of the Load at higher frequencies. By putting two 100-ohm resistors in parallel, we double the capacitance and cut the effective inductance in half. As luck would have it, the resulting values cause the capacitance and inductance to have virtually no net effect up to 1 GHz, making a nearly perfect Load. (Simulate it and see.) So the two-resistor Load is the way to go.

Some Practical FixturesThe label “fixture” may suggest something fancy. That is not necessarily so, as is shown by the Series Fixture in Photo 1.

Photo 1—A Series Fixture with Alligator ClipsThe brass tube spacer reduces capacitive leakage between the clips

This fixture consists of two SMA connectors each soldered to a piece of 3/8” brass tubing. Pieces of Teflon (from a coax cable) provide standoffs for the alligator clips. The brass strip is the Short, used for calibration. Attenuators are attached to each connector when using the fixture. Obviously a fixture like this has leakage between the clips, and

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inductance within the clips, so its frequency range with Reference calibration may be limited to a few tens of MHz. However, with OSL calibration, the fixture provides very good results (generally 2% or better accuracy) in measuring resistances in the following ranges:

10 MHz 1 ohm to 100K ohms 60 MHz 2 ohms to 20K ohms150 MHz 5 ohms to 10K ohms500 MHz 30 ohms to 5K ohms

Figure 4 shows a measurement of a 1K ohm resistor with the Series Fixture, using OSL calibration.

Figure 4-Measurement of a 1K resistorThe trash above 700 MHz is the result of a low-pass filter in the test setup.

That’s pretty good for a fixture made with alligator clips, operating at hundreds of MHz. To see the limitation of this fixture, Figure 5 shows a 100K resistor.

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Figure 5—100K resistor in Series Fixture, using OSL

These results are still excellent below 10MHz. The output level of the fixture with such a large resistor is very low, and above 10 MHz it starts to become swamped by the direct capacitive leakage between the alligator clips, making it harder and harder for OSL to sort the “real” output from the leakage.

The results just shown used OSL calibration. Below 10 MHz, the Series fixture of Photo 1 would probably do a good job over a wide impedance range using just Reference calibration. A more compact probe structure, with connections much shorter than the clips, would improve that frequency range to at least 30 MHz. A Series Fixture built on a small PCB with the attenuation and pads for soldering the DUT directly would likely do well to 150 MHz with just Reference calibration. An improved Series Fixture is shown later for Component Meter.

It is very easy to build a very simple Shunt Fixture with good performance. Simply attach attenuators to 2 arms of a coax “tee”, with the DUT attached to the third arm. This requires a connectorized DUT, or perhaps a DUT soldered on the back side of an SMA connector.

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Photo 2—Shunt Fixture made with Coax Tee and AttenuatorsDUT is mounted at the top

For use with Reference calibration, performance at higher frequencies is improved if the attenuators are as close to the DUT connection as possible, as illustrated in Photo 3.

Photo 3—Shunt Fixture with On-Board AttenuatorsA DUT is attached at top center

The PCB version of the Shunt fixture in Photo 3 measured resistances very well as low as 0.25 ohm at 50 MHz with just Reference calibration. For low impedances its performance was slightly less with DUTs attached to the connector, as compared to soldering them directly on the board. On the high impedance end, even using the connector produced good results with 500 ohms at 100 MHz with Reference calibration. In a 200-ohm version (where the attenuators presented 200 ohms on each side of the DUT), good results were obtained with Reference calibration measuring a 1K resistor at 100 MHz. With OSL calibration, either of these shunt fixtures is very good for the full range of the MSA.

We have shown actual Series and Shunt Fixtures, but no Reflection Bridges. Photo 4 shows a bridge made with two op amps, called the Active Bridge:

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Photo 4—The Active BridgeInput and output are at photo bottom, DUT at the top

The Active Bridge is covered in detail elsewhere. It provides excellent performance to 150 MHz, and is operational beyond 300 MHz. A major advantage is that it provides an excellent 50-ohm interface without the loss of signal level caused by attenuators. For this reason, it is also handy as a buffer amplifier; one way to do this is to leave the DUT port open and treat the input/output ports as the ports of an amplifier.

A variety of passive bridges can be made following the concept of Figure 3, with the main variation being how the transformer is implemented. At low frequency a regular transformer can be used. At higher frequencies, a balun transformer is better. The baluns are sometimes implemented by winding coax cable around ferrites. It is not difficult to build such a bridge, but it is a challenge to make one that works from the 100 KHz range to 1 GHz.

Component MeterAs mentioned above, the MSA has a function called Component Meter, in which it acts like a RLC meter for measurement of resistors, capacitors and inductors. In VNA Transmission mode, the MSA can produce graphs of resistance, capacitance or inductance over frequency, even for devices that are not pure resistors, capacitors or inductors. But if you have a device that you know is a capacitor and just want to find its capacitance value (say you want to find the value of an unknown SMD capacitor), such a graph is overkill and the meter approach is more convenient. Such measurements are made at relatively low frequencies, where the performance of the Series and Shunt fixtures with Reference calibration is extremely good. The Component Meter function is available in Transmission mode, and in SA/TG mode for an MSA that does not have the phase modules installed. In Transmission mode, Reference calibration is the only available calibration, and it goes by the name of Line Calibration.

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The Series Fixture with alligator clips shown in Photo 1 is suitable for Component Meter. But somewhat improved results are obtained if the attenuators (which would be attached to the SMA connectors in Photo 1) are put directly on a PCB, with some sort of mechanism to attach the components being tested. Photo 5 shows such a fixture.

Photo 5—Improved Series FixtureShown with a crystal mounted as DUT

The sockets are inserts intended for header plugs. Obviously, this is a prototype, and a more durable type of socket would be desirable. With this fixture I measured some capacitors, as shown in Figure 6.

Nominal Actual MSA Error2 pF 1.59 1.57 -1.3%18 pF 18.14 18.3 pF 0.9%100 pF 100.71 pF 100.6 pF -0.1%500 pF 501.8 pF 503.8 pF 0.4%1.3 nF 1297.4 pF 1310 pF 1.0%10 nF 10.004 nF 10.05 nF 0.5%254 nF 258 nF 250 nF -3.0%

Figure 6—Capacitor measurements with Series Fixture

The “Actual” values are based on tests by Larry Phillips and Jeroen Bastemeijer on some very fancy equipment, and are accurate to 0.1% or better. The MSA measurements are within 1% or better within most of the range. Note that the Series fixture is not ideal for low impedances, so its accuracy suffers for capacitors over 0.1 uF. (Component Meter operates with simple Reference Calibration. Increased accuracy can be obtained in Reflection mode using OSL calibration, measuring S11, and graphing R, L or C values.)

Similar tests with a Shunt Fixture, which is more suited to low impedances, extended the upper end of the range above 1 uF, but became inaccurate below 10 pF.

The measurement range of either fixture can likely be extended by using the Active Bridge as a buffer amplifier at the input or output of the fixture. This will provide an

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almost perfect 50-ohm interface to the fixture without loss of signal level (in fact, a small amount of gain is possible).

A Probe-Style FixturePhoto 6 shows another assembled Series fixture, and the bare PCB on which it was constructed, that is suitable for both SMD and leaded components.

Photo 6—SMD “Probe”The 49.9 ohm resistor is here the DUT

The fixture in Photo 6 has short silver bars (standard jewelry-supply stuff), angled to allow it to contact SMD components of various sizes. It also contains component sockets (mounted through holes in the bars and the PCB) that accept leads of various sizes. RG-174 cable is mounted on the right side with its shield wrapped in #29 wire and soldered to the ground plane; the center connectors attach to attenuators at the pads on either side of “SER”. The right half of the board is wrapped with a thin strip of brass, soldered on the bottom, which provides a good place to grab the probe without interfering with measurements. The cables are 10” long and attach to the MSA through additional small attenuators, or, better yet, through buffer amplifiers. If accuracy of several percent is acceptable, they can be attached directly to the MSA. (Note: for use with OSL calibration, minimal attenuation is required.)

Crystal MeasurementFinally, the Series Fixture is perfect for measurement of crystal parameters. Photo 4 shows a crystal mounted in the fixture. The first step is to perform a Transmission mode scan that shows the series and parallel resonant frequencies, as shown in Figure 7.

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Figure 7—Scan of Crystal Series and Parallel Resonant FrequenciesThe crystal is mounted in the Series Fixture of Photo 4

Series resonance on the left, parallel on the right

With this scan in place, you open AnalysisCrystal, and click the Analyze button. The MSA will display the series frequency, parallel frequency, parallel capacitance and the motional inductance, capacitance and resistance. Alternatively, you can enter the parallel frequency manually in the dialog, which allows you to focus the original scan more closely around the series resonant frequency, making it more feasible to use a small step size to determine the series frequency more precisely.

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