Function - VNA Transmission

Created Dec 16, 2009
Update Dec 23, 2015 
Delete Special Testing

    The Function of Vector Network Analyzer is available only to the MSA Build Level 3, VNA.  The VNA Function is separated into two Modes, VNA-Transmission and VNA-Reflection.  It should be noted that there is no difference in the MSA Hardware between these two Modes.  The MSA measures the Magnitude and Phase as raw transmission data, and the software will process the data as either Transmitted or Reflected.  This page will describe the Function of VNA in theTransmission Mode.  A separate page will describe the Function-VNA Reflection.  Descriptions for all Functions of the VNA can be accessed from the Main Page.
VNA Operating Guide for the MSA, 12/12/09. Bench-top guide to VNA operation.
Illustration of Plane Extension for the MSA, 10/21/09. Use of Plane Extension
By Sam Wetterlin, PDF's

The Vector Network Analyzer, Overview
Function: The MSA as a VNA, Transmission or Reflection Mode
Basic Operation In Transmission Mode
Performing Calibration in Transmission Mode

The Vector Network Analyzer, Overview
    A Spectrum Analyzer will measure the absolute Magnitude of a signal (RF power, in dBm).  A Vector Network Analyzer will measure not only a signal's Magnitude, but also its Phase.  The Phase is somewhat meaningless unless the signal can be referenced to another signal at the same frequency.  Therefore, all VNA's have a Tracking Generator used as a Reference Signal.  A commercial VNA uses its Tracking Generator output as the Reference Signal.  The MSA/VNA uses a "product" frequency (10.7 MHz) for its Reference Signal.
    The purpose of any VNA is to obtain the electrical characteristics of a "Device".  A signal will change in Magnitude and Phase when passing through any device.  By measuring these changes, and making some mathmatical comparisons of these changes, the characteristics of a Device can be calculated.

    A device can be a passive or active component, a piece of coaxial cable, a Reflection Bridge, or even an antenna, to name a few.  In many cases a  Device can be a combination of multiple items, such as a Bridge (with connector adapters), a test bed, and a Device Under Test inserted in that test bed.  This requires multiple measurments, to factor out characteristics of all known supporting "items" to determine the parameters of the final unknown Device, the Device Under Test (DUT).
  in Transmission Mode
    The MSA/VNA can have several different Functions.  No mater what Function the VNA is being used for, the VNA is simply measuring and comparing four quantities.  The output signal of a DUT, consisting of Magnitude and Phase, is compared to the Magnitude and Phase of the Device's input signal.  This is called a Transitional Comparison.  All VNA's are "Transitional Comparitors", although we now use the term Transmission Measurement, since any Device has the ability to transmit a signal.  (For old people like me, the term "Transmission" has replaced terms such as "Forward", "Transition", and "Transfer" measurements).  The resulting comparisons are expressed as a ratio of output to input, also known as the S-Parameter, S21.
  in Reflection Mode
    When the MSA is in the VNA Reflection Mode, it is still taking a Transmission Measurement.  However, some type of external "Reflection Bridge" is used in combination with the DUT.  The Bridge transmits a "reflected" signal created by the DUT.  The MSA will measure this "reflection" as the Bridge's "transmission", thereby maintaining a Transmission Measurement.  Since the information is not a true DUT transmission, the MSA software will convert the S21 parameter into a S12 parameter.  The MSA as a VNA, Reflection Mode is described on its own, separate Web page.

The Graph Window Display
    Enter the VNA Transmission Mode from the Spectrum Analyzer Mode, using the menu item Mode / VNA Transmission.  Sweeping will automatically begin with default values left over from the SA Mode.  The following is a screen print of the Graph Window while sweeping the Tracking Generator from 0 to 1000 MHz.  The TG output is connected to the input of the MSA through a 4 inch test coax and two 10 dB attenuators.  Two signal parameters are displayed in the Graph Window.
    The indicator, "Cal = None" means that no reference calibration is used in this sweep.  Therefore, the traced information is "absolute", meaning the Magnitude is measured as power input to the MSA, -32 dBm (even though its right scale is in dB).  The Phase is a comparison of the input signal and Reference frequency, in degrees (left scale).
    Notice that there are several phase "sawtooths" displayed, one for each rotation of 360 degrees.  This is because the signal experiences a time delay traveling from the TG output to the MSA input (as well as internal cabling). That time delay translates into phase delay, and the higher the frequency, the more degrees are represented by a given amount of time. While the sawtooth pattern makes it appear that something abrupt is happening periodically, this graph actually represents a steady increase in phase delay, and every time it reaches -180 degrees the graph wraps around to +180 degrees.  The number of rotations in the above graph will differ for each individual VNA, due to variation in the internal and external lengths of cable.  This is a good self test, indicating that everything in the MSA is functioning normally.

The Sweep Parameters Window
    Most of the Controls that were described in the Spectrum Analyzer Mode are repeated in this window.  These are the differences for VNA Modes:
    The selection of Signal Generator or Tracking Generator is deleted.  The Tracking Generator is automatically selected during VNA operation.
    Note: The "Select Final Filter Path:"  drop-down box must have the correct Path selected for VNA operation.  All VNA calibrations are performed in a single Path.  Usually, this is Path 1.
    "PDM Inversion (deg) box.  This box displays the current calibration for the PDM.  It can be changed in this box for special testing.
    "Plane Extension" box.  This adds or subtracts (-) time into the calculations.  Also referred to as "Reference Plane Extension".  It is the same as physically adding cable length within the Reference Source circuit.  This is a very good way to factor out time delay effects of transitional components surrounding the DUT, such as barrel connectors, test jig, etc.  Just for reference, the Verification unit requires a plane extension of "3.2" ns to factor out its internal delays.  A new value (in nano seconds) may be entered and the "Recalc" button clicked.  The Graph will immediately retrace with the extension value.
    "Video Filter BW" and "Wait (ms)" boxes. As with Spectrum Analyzer mode, we can set the video filter to wide, middle or narrow. The narrower the setting, the longer the settling time when changing from point to point, and the more Wait time we need to specify. When phase is involved, precise measurements are likely to require extra wait time, and settings of 15-50 ms are typical. 100-150 ms of Wait time may be required for the most precise measurements. When the PDM measures raw phase within a certain “inaccurate zone”, the MSA inverts its phase reference, remeasures, and adjusts for the phase shift caused by the inversion. This inversion causes a large, abrupt shift in the raw phase measurement, which requires an extra-long settling time. The stronger the video filter (larger capacitor), the more Wait time is required. The MSA automatically imposes the extra delay, but it is important that you set the Video Filter in the sweep parameters window to match the actual hardware setting, so the software knows the state of the video filter. (The software setting does not actually set the video filter at the present time; it just informs the software of the setting.)

The Y Axis Windows, Axis Y1 (left) and Axis Y2 (right)
    The data to be graphed on each axis can be selected from the Y-axis parameters dialog, which opens when the users double-clicks in the area of the corresponding axis grid-line labels. This dialog is in the same format as for Spectrum Analyzer mode, but contains a different selection of graphs, as shown in the following:

--S21 Magnitude (db). These is conventional, processed Magnitude using a reference calibration.
--S21 Phase Angle. These is conventional, processed Phase using a reference calibration.
--Raw Power (dBm) and Raw Phase Angle. These are the "absolute" measurements without adjustment for a reference calibration. It may be desired to display these in unusual situations.
--Insertion Loss. This is simply the negative of the S21 dB value. For a bandpass filter, instead of graphing a “hill”, this will graph a “valley” with maximum transmission (lowest loss) at the bottom.
--S21 Group Delay. This is the negative of the change in phase over the change in frequency. It is sometimes a useful value. Due to being calculated from rates of change, it is very susceptible to noise, which causes the graph to be very erratic. This graph will ultimately be replaced by an item under the Analysis menu which will produce a smoother graph.
--None. This causes nothing to be graphed on that axis, the axis to be blank, and no value for that axis to be displayed in the Marker area. (As opposed to using Trace Style to turn the trace off, which leaves the axis labels in place and allows the axis values to be displayed in the Marker area.)
The “Histogram” option for Trace Style is not available for either axis in Transmission mode.

Basic Operation In Transmission Mode
    A typical Transmission mode measurement is made by setting the proper sweep settings, performing a Band calibration (or relying on a Base calibration), inserting a DUT, and hitting Restart.  Of coursc, this is an over-simplification, so some explanations are in order.
    The MSA/VNA (or any other VNA) will process Magnitude and Phase information using DUT measurments that are "absolute", or "raw".  A "raw" Magnitude measurement is straightforward, input power is measured in dBm.  A "raw" Phase measurement is the relative phase relationship of two same frequency inputs to a "Phase Detector".  The first Graph shown on this page is repeated here.  There is no DUT.  The VNA is measuring "itself".
    If a DUT were inserted into the external signal path, it would be obvious that the MSA/VNA would affect the DUT measurement.  Therefore, the internal effects of the MSA must be "factored out" to produce a relevant S21 measurement.  The factoring process begins with a "Calibration".

Performing Calibration in Transmission Mode
    You may have seen terms used by me or by others, relating to the Calibraiton of a VNA: "Line Calibration", "Reference Calibration", Reference Line Calibration", "DUT Calibration", "Test Bed Calibration", or "Cal".  They all mean the same thing.  It is simply the process to remove all factors, external to the DUT, that contribute to a DUT measurement.  I will try to use the term "Calibration", and this should not be confused with the Initial Calibration of the MSA.  The Initial Calibration of the MSA was a one-time process and "Calibrations" will be peformed quite often, usually before each critical measurement of a DUT.
    To Calibrate, it is first necessary to measure the strength and phase of the Reference Signal transmitted without the DUT in place. Typically, this involves running the TG signal through an attenuator, then possibly a short coax cable, then another attenuator, and then to the MSA input. Or the two attenuators might be directly connected without the cable. A 4 inch coax cable is used in the example Graph.
    The calibration routine is invoked under the menu Operating Cal
/ Perform Cal. It measures and stores the "absolute" Magnitude and Phase of the signal reaching the MSA input. The stored data, now called Calibration Data, will be used during the subsequent DUT measurement.  With the calibration performed and the above scan repeated, the comparison of output and input results in the value of zero.  The effect would be to flatten both the magnitude and phase lines at zero. The following picture shows such a scan after a calibration.
Note the “Cal=Band”.  The Magnitude trace is at the very top of the graph.  It is now showing 0 dB, no longer -32 dBm.  The Phase is no longer shown as sawtooths, it is a straight line at 0 degrees.  I suggest making this repeat sweep after a calibration for verification.  If the results are anything other than zero, a problem exists.  In most cases, it is usually a faulty cable or connection.  The VNA is now ready to measure a DUT.
    The following is a detailed procedure for "Calibration."
Calibration is performed by using the Operating Cal / Perform Cal menu. This opens the following window:
    For typical DUT measurements, the DUT is usually attached between two attenuators, to present the MSA with a predictable input and output impedance, since neither the TG output nor the MSA input is a steady 50 ohms.  To prepare for calibration, we need to establish a signal connection without a DUT in place.  The DUT is removed and replaced with the shortest possible interconnection.  This connection is referred to as a Through or Line connection. (More on that below.) When you click Perform Band Cal, the MSA will perform one complete sweep and save the absolute magnitude and phase as Band calibration data. “Band” refers to the fact that the calibration exactly matches the current frequency band. If the sweep settings are changed during a subsequent measurement, the Band calibration becomes outdated and will not be used.
    During the calibration sweep, the MSA will automatically use a Wait time of at least 110 ms for each step in calibration. If the Wait time in the Sweep Parameters window is set to a greater amount, that larger value will be used.

Saving Base Calibration
    The current Band calibration can be saved as a Base calibration. “Base” refers to the calibration being a rough baseline against which to adjust current measurements. It is normally performed over a broad frequency range, such as a log sweep from 0.1 MHz to 1000 MHz. The stored Base Cal can be used as a reference for future sweeps no matter what range they cover. When used, its values are interpolated to the selected sweep frequencies. It is not as accurate as a current Band calibration, but is much more convenient when the user is frequently changing sweep settings.
    When the Band calibration is saved as a Base calibration, that Base data is actually saved to a file, so it is available in future sessions. A one-month old Base Calibration in the Verification Unit showed that the MSA had shifted less than 4 degrees and had a ripple of less than .5 degrees.

Clearing Base or Band Calibration
    Once the Base Calibration is saved, it will remain usable for all future sessions.  However, the Base Line Cal is accurate only when the VNA has the same hardware test setup as when the Base Line Cal was performed. Also, the MSA must be in the same final filter Path. For example, if you increase or decrease the amount of external attenuation, or change the length of the test cables.  If any changes are made the old Base Cal will be useless. Only you will know that you made any changes. If so you should clear the Base calibration by using the Clear Base Cal button.

Likewise, if you change the final filter Path, you should either click Clear Band Cal, or simply perform a new Band calibration and save it as the new Base Cal.

Delay of Through Connection

    During calibration, the DUT is replaced by a Through or Line connector. If the attenuators on each side both end with male connectors, and the DUT has female connectors on both sides, then the two attenuators cannot simply be plugged together; an adapter is required. The adapter will create some phase delay. The Transmission mode calibration dialog shown above allows the user to indicate the time delay of the Through connection, so the resulting phase change can be removed from the calibration data. [More on this to come.]

A note on terminology: strictly speaking, a "Through" connection is a direct connection without delay. An example would be a connector on one side of the DUT would mate directly with the connector on the other side of the DUT. When that is not possible, and delay is involved, the connection is referred to as a "Line" connection. However, we use the terms Through and Line interchangeably.

Performing a Measurement in Transmission Mode
    After a calibration, the DUT is inserted in the signal chain between two attenuators. The purpose of the attenuators is to assure that the DUT is presented with a constant impedance on both sides (normally 50 ohms); failure to do so affects the accuracy of measurements. The DUT's transmitted signal is then measured at various frequencies, and compared the stored Calibration Data.  The comparison is a ratio of Magnitude (in dB) and a difference of Phase (in degrees).  The comparison values are processed by the computer and displayed on a Graph as two traces, Mag and Phase.
VNA Transmission Mode measurements are generally expressed in the form of the S-Parameter, S21. This assumes the DUT is a two-port device, an input port (1) and an output port (2). Think of “S21” as meaning “signal 2 compared to signal 1”, or that the signal that is transmitted at port 2 (the output) is the result of a signal arriving at port 1 (the input). S21 is usually expressed in two parts: the magnitude in dB and the phase in degrees.  In most VNA's these can only be displayed as comparisons.  The MSA/VNA has the capability to use "No Reference" and the displayed traces will represent the Device's transmitted "absolute" information, in dBm and degrees.

Test Fixtures
    The insertion of a DUT between the Tracking Generator output attenuator and the MSA input attenuator may require more than a few simple coaxial connections.  The prescribed attenuators can be free-standing coaxial attenuators, or they can be resistor pads built onto a PCB with some mechanism to attach the DUT between them, since many DUT's are not connectorized.  We refer to the mechanism connecting these DUT's to the MSA as a test fixture.
    A test fixture will allow the VNA to measure the characteristics of various types of DUTs, including resistors, inductors, capacitors, crystals, filters and amplifiers. These are considered "Two Port" devices.  It can even measure characteristics of a device with only one connection, a "One Port" device such as an antenna.  A test fixture will allow the DUT to be characterized as a serial element or as a shunt element.
    Most two port devices are tested as serial elements, inserted between the two attenuators.  However, some two port devices are better tested as shunt elements.  One port is inserted between the attenuators and the other port is shunted to ground.
  For example, a capacitor can be so connected, one terminal in common with the two attenuators and its other terminal grounded.  An antenna has only a single port, and must be connected in a “shunt” configuration.
There is an important distinction between fixtures in which the DUT is connected in series between the attenuators, and those in which it is connected in shunt from the line connecting the attenuators to ground. The former are called Series Fixtures, and the latter are called Shunt Fixtures. The distinction is important when analyzing resistors, inductors, capacitors, crystals and simple RLC filters. This distinction applies only when using DUTs with one or two terminals. For devices like amplifiers, with an input, an output and a ground, the distinction between Series and Shunt makes no sense.

Types of Test Fixtures
    The DUT is normally attached between two attenuators, to present it with a predictable impedance, since neither the TG output nor the MSA input is a steady 50 ohms. For devices like amplifiers, with an input, output and ground, the connection method is obvious. But for two-terminal devices there are two possible connections possible.
First, the DUT may be attached in a Series Fixture, as shown below:

The Series Test Fixture

Second, the DUT may be attached in a Shunt Fixture, as shown below:
The Shunt Test Fixture

The effect of the DUT on the transmitted signal is obviously different in the two configurations. For example, a large resistor will significantly attenuate the signal in a Series Fixture, and have very little effect in the Shunt Fixture. For some analyses performed by the MSA, it is necessary to tell the software which fixture is being used. When there is a choice, the Series Fixture generally does a better job with high impedance DUTs, and the Shunt Fixture does a better job with low impedance DUTs.

The attenuators in the fixtures are intended to present the DUT with a fixed impedance on each side, typically 50 ohms. However, by using impedance-matching attenuator designs, the attenuators can be 50 ohms on the outside and, say, 12.5 ohms on the inside. In some analyses performed by the MSA, it is necessary to tell the MSA software what impedance is being used. The impedances presented to the DUT by the attenuators, which must be equal, are referred to as R0. Note that from the point of view of the DUT, the impedances of the two attenuators are in parallel, so it actually sees a net impedance of R0/2. Nevertheless, R0 is specified as the impedance presented by each attenuator separately.

Delay Compensation in Shunt Fixture

Picture of a Shunt Fixture with attenuators directly on the PCB

Shunt Fixture (with DUT attached)

     Note that there is a direct on-board connection between the two attenuators, and the DUT connection attaches to that line. The DUT in this case is actually soldered to the back side of the male connector attached to the fixture, and is located some distance from the attenuator-attenuator connection. At higher frequencies, the travel delay caused by this distance can distort measurements. Therefore, the MSA software provides the operator to specify the one-way time delay of the DUT connection. This is typically about 0.125 ns per inch. For the above fixture, it is 0.115 ns.  The actual amount of delay can be compensated by the use of Plane Extension.

Plane Extension
    The phase values displayed in Transmission mode represent the phase difference between the transmission with the DUT in place, and the transmission with the calibration Through connection. In some cases, there may be some extra length associated with attaching the DUT. For example, it may be necessary to attach an adapter to the DUT. Or the DUT may be mounted on a circuit board, and the entire board may have to be attached to the test fixture. In such cases, we can "tell" the software to remove the phase delay that is caused by the extra length. This is referred to as Plane Extension, because it can be viewed as moving the plane of calibration.  It is also know as moving the Reference Plane.  It is implemented by entering a value for Plane Extension in the Sweep Parameters window. That value is the number of ns of delay that we wish to add or subtract from the Reference Plane. The effect of changing the value can be seen immediately by clicking the Recalc button located under the Plane Extension box. The phase delay caused by a fixed time delay increases linearly with frequency, so increasing the plane extension value has the effect of raising the phase display more at higher frequencies. This will cause a graph covering, say, 1 MHz to 10 MHz, to rotate counter-clockwise; if the graph initially had a downward tilt, increasing plane extension tends to remove that tilt.

The precise use of plane extension, and the methods of determining what values to enter, will be expanded at a later date. Plane extension is not used with some Functions of the MSA.

end of page