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.
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
Special
Testing
in
the VNA, Transmission Mode
Phase
Detector Module Control
Phase Detector Module Sync Test
VNA Phase Error Test
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.
Special Testing in the VNA, Transmission Mode This
section
will describe special testing of the MSA while in the VNA
Mode. "Special Tests" window.
Click the menu item, Setup, and select Special
Tests. The "Special Tests Window" will open and display several
items. Only those items that are relevant to the VNA Mode will be
described.
Special Test: Phase Detector Module Control "Change
PDM"
button. This will
control the phase inversion of the Phase Detector Module (PDM).
During normal VNA Mode sweeping, the software is constantly reading the
phase data. If the phase data is ever out of linear range of the
PDM, the software will automatically invert the Reference Signal inside
the PDM. This 180 degree phase inversion guarantees phase
data accuracy.
In some special testing, it is helpful to disable
this automatic feature and assure that the PDM is fixed in one of it's
inversion states, either "Normal" or "Inverted".
Follow
these steps for proper
operation. The MSA must be in the "Halted" condition:
Open the Special
Tests Window, and click the "Change PDM" button.
The
PDM will be commanded to "Normal", immediately. The button will
change it's name to "PDM in Normal". Click "Restart" to resume
sweeping. The PDM will remain fixed in its "Normal" state, and
will not change during sweeping. Halt the sweep.
When
the "PDM in
Normal" button
is clicked, the PDM will be commanded to "Inverted",
immediately. The button will change it's
name to "PDM in Inverted". Click "Restart" to resume
sweeping. The PDM will remain fixed in its
"Inverted" state, and will not change during sweeping. Halt the
sweep.
When
the "PDM in
Inverted" button
is clicked, the button will change it's name
to "PDM is Auto". No command is immediately sent to the PDM. Click "Restart" to resume
sweeping. The PDM will return to automatic operation during
sweeping.
As long as the Special Tests Window is open, the PDM
will remain in one of the commanded modes. Operator actions
within the MSA Working Window will not change the state of the
PDM. When the Special Tests Window is exited,
the PDM will revert to automatic operation.
Special Test: Phase Detector Module Sync Test "Sync Test
PDM"
button. This is a special test I wrote for
verification of
the first SLIM VNA. It has no practical use but, it is quite handy to determine the
functionality and quality of the PDM, and it looks cool. For a more
critical test, use the Special
Test: VNA Phase Error Test.
Configure the
MSA/VNA:
Remove
any signal that may be on the input to the MSA.
Select
the Phase Video Selector Switch to "Medium" or "Wide".
In the Sweep Parameters Window, uncheck the "Refresh Screen Each Scan"
[OK].
Open the
Special Tests Window and click the "Sync Test PDM" button.
The following will occur:
The Center Frequency of the sweep will be commanded to 0 (MHz).
The Sweep Width will be commanded to 0 (MHz).
The Wait box will be commanded to "3". The "3" is an arbitrary
number that works well for my computer. It can be changed to a
higher value to plot more sawtooths.
The right magnitude scale is changed to +5 (dB) to -5 (dB).
The left phase scale is changed to 360 degrees (top) and 0 degrees
(bottom).
The markers are removed from the graph.
The PDM is set to "Normal" and not allowed to change inversion states.
Sweeping will automatically start.
The Tracking Generator output will be very close to 0 Hz but it is not
used.
The VNA is now collecting and displaying the phase
differential of the two signals entering the PDM. The two signals
are the Limited I.F. signal from the Log Detector and the Reference
Signal created by the Tracking Generator. In normal VNA
operation, the Tracking Generator Signal is the Reference signal.
It passes through the Device Under Test and enters the MSA input.
The two signals at the PDM are extremely close to the same frequency,
and of course, out of
phase.
In this special test, the Reference signal into the
PDM is still created by the Tracking Generator (PLO 3), but the Limited
I.F. signal is created by the first Local Oscillator (PLO 1).
Both PLO's are commanded to 1013.3 MHz, however, the two signals are
not quite the same frequency. They are usually within 1 Hz of
each other. This creates a "false" phase display, which is
constantly rotating at the rate of the difference in the two
frequencies. When the sweep finishes it's final step, it will not
immediately return to the first step and continue sweeping. It
will wait until the phase differential is approximately 290 degrees and
then restart the sweep at step 0. This is the "sync" part of this
special test. This allows "somewhat" repeatable traces in the
Graph Window. After a few initial sweeps, if the sweep does not
sync, halt the sweep and play with the value in the "Wait" box (Sweep
Parameters Window).
Click [Continue] or [Restart]. Do not move the Mouse during
sweeping. This causes the computer to "housekeep" and the
trace will become quite erratic. Normal computer
"housekeeping" will "mess up" this test, so keep this in mind.
Leave the Special Tests Window open during this test, even if it is
hidden behind one of the other Windows. If it is closed, the test
will work improperly.
What you see on the Graph:
The Phase trace (sawtooths) is the phase differential of the
two
inputs to the PDM, using the left scale. It is indicating the rotations of the
PDM
input signals. The number of rotations per sweep are dependent on
many things but can be controlled by changing the value "Wait" box.
The top of each
sawthooth is flat, and the bottom of the sawtooth does
not reach 0 degrees. With
a "perfect" phase detector, the sawtooth would reach the 360
degree reference line and immediately transition to exactly 0 degrees. This Phase Detector is not
perfect. It has an area of "uncertainty", which is called the
"dead zone". This "dead zone" is indicated by the
phase distance between the first "touch" of the
sawtooth at 360 degrees, to the
transition near the base of the graph. In this case, it is about
15 degrees.
Outside, but close to this "dead zone", we expect the PDM to be
unlinear. Nonlinearity would be indicated when the
sawtooth is not a straight line. Due to the large scale
factor, this unlinearity of the sawtooth is difficult to see.
Therefore, we can decrease the scale factor and use a different trace
to show unlinearity. We can use the "Magnitude" trace for this.
The
Magnitude trace is not magnitude at all. It is the amount of phase change for
each step in the sweep, using the right dB scale as degrees. This value will depend upon
the speed of the
sweep and the difference in the two frequencies entering the PDM.
Prove this by halting the sweep, change the value in the "Wait" box,
and [Continue]. The trace
is quite noisy, due to the phase noise of the PLO's and the Video
bandwidth selection. For best PDM linearity, we are looking for a
flat trace (no trend), discounting the peak to peak noise. A
general slope
or shift in the trace indicates deviation from linearity. In
the graph, the PDM is
showing a very flat trend (linearity) except within the area of the
"Dead zone". The VNA software does not
allow phase
data to be used within the limits of -72 degrees (+288 deg) to +72
degrees. Therefore, this PDM has a great deal of margin.
To return to normal VNA operation, Halt the sweep and close the Special
Tests Window. Open the Sweep Parameters Window and select
your appropriate parameters. The Magnitude and Phase scale
windows must be changed also. It may be quicker to just close
this session of MSA and re-start it.
Special Test: VNA Phase Error Test "Preset
VNA
Linearity" button. This test is a very accurate method of
measuring VNA Phase Error. However, all VNA Phase Error is
factored out during normal VNA measurements when a Reference Line
Calibration has been performed.
Manually configure the MSA/VNA:
Install
a 50 ohm test cable from the Tracking Generator
Output to the MSA Input. A minimal length cable (3 inches) will
test the Phase Error of the complete VNA. A very long cable (10
ft. to 100 ft.) will minimize the error contributions of the components
external to the PDM. Mainly, it
will reduce the error created by the Mixer 4 (AM to PM conversion) and
the PDM will be the major contributor to error. We will use this
configuration to measuring the Phase Error of only
the PDM. I used a 118 feet of RG-188.
Select
the Phase Video Selector Switch to "Medium" or "Wide".
Open
the Special Tests and click the
button called "Preset Phase Linearity". When this button is
clicked,
the following will occur:
The button will change it's name to [Test Transmission Linearity].
The Center Frequency of the sweep
will be commanded to 500 (MHz).
The Sweep Width will be commanded to 1000 (MHz).
The Wait box will be commanded to "22".
The right Magnitude scale is changed to +5 to -5, we will use this as
the error scale.
The left Phase scale is set to +180 degrees (top) and -180 degrees
(bottom).
The PDM (Phase Detector Module) is set to "Normal" and not allowed to
change inversion states.
Sweeping will automatically start.
The Magnitude trace will display the MSA input
power, but it is likely to be outside of the
+5 to -5 graph limits. Disregard the trace, Magnitude is not used
in this test,
anyway.
The VNA will
sweep from 0 to 1000 MHz, and display multiple sawtooths, the number
depending on the length of the cable between the TG output and the MSA
input. Each sawtooth represents 360 degrees of phase change.
Halt the sweep. We now want to choose one of the
sawtooths to analyze. In this example, there are
approximately 100 sawtooths displayed. Position an "L" marker and
an "R" marker spaced by a few sawtooths. See markers in example
above.
Double Left Click the Mouse to place the marker.
Click the "Expand L-R" button.
The sweep will resume, with only a few sawtooths. See example
below.
Halt the sweep.
You will now notice that each sawtooth has a "glitch" in its slope, at
approximately 0 degrees. This is the 0/360 degree "dead
zone". This is caused by having the PDM fixed at "Normal".
Position an "L" marker on one of
these "glitches" and an "R" marker on a "glitch" to the right,
separated by two full sawtooths. See the example below.
Click
the "ExpandL-R" button.
The sweep will resume, with only two sawtooths. See example,
below.
Halt the sweep.
We now want to choose only one of the
sawtooths to analyze. Note that, the center
horizontal scale line represents 0 degrees (left scale). Each
other horizontal
scale line represents 36 degrees separation from 0 degrees.
Select Marker "L" and position the Mouse Curser about 36 degrees left
of the wanted
sawtooth's 180 degree transition. This is the crossover at -144
degrees. Double Left Click the Mouse to place the marker.
Select
Marker "R" and position
the Mouse Curser about 36 degrees to the right of the second sawtooth's
180
degree transition. This is the crossover at
+144 degrees. Double Left Click the Mouse
to place the marker.
Click the "Expand L-R" button.
The
sweep will resume, with only one sawtooth.
Halt the sweep.
You should end up with a
trace in the
Graph Window that looks similar to the following:
The phase trace
indicates the phase differential at the Phase Detector Module.
The phase will change as the
frequency changes, due to the total line lengths within the VNA plus
the length of the external test cable. Somewhat
more than a single sawtooth is displayed, more than 360 degrees.
Imagine a straight-edge ruler on the monitor screem,
directly over the +180 deg to -180 deg slope. In a perfect VNA
and perfect PDM, the slope would be as straight and flat as your
straight-edge ruler. Any deviation from a straight line would
indicate the
total VNA Phase Error. There is an obvious deviation near the 0
degree crossover. This is the PDM's "dead zone". The straight edge test is
impossible to do with such a large scale, so we can add another trace with a
very small scale. This is the next portion.
Before entering
(and during) the next portion of the test, verify
that the Phase trace in the Graph represents more than one full
sawtooth,
and less than two. That is, the trace is at least 360 degrees and
less than 720 degrees. It is also important that the first and
last steps in the sweep are on the linear portion of the
sawtooth. If not, change the Center Frequency and/or the Sweep
Width
until the criteria are met.
Select an "L" marker to be placed. Position the
Mouse Pointer anywhere on the Phase sawtooth except near the "dead
zone". This point
will become
the Reference for the next portion of the test.
Double Left Click the Mouse to place the marker.
Click the "Test Transmission Linearity"
button in the Special Tests Window.
The Graph will clear and
repeat the previous sweep, but now adding a trace to indicate
the Phase Error of the VNA (right scale). The Phase Error is any
deviation from the straight line of a "theoretically perfect" sawtooth
slope. The software calculates this error by measuring the
actual phase at each step and comparing it to a theoretical phase
measurement. When the
sweep is Halted, a phase/frequency
factor will be
displayed in the Message Box, in degrees per
MHz. This value (-72.01) is dependent of the length of test
cable.
The Phase Error will be displayed in the Marker box as S21 dB.
Read the right
(dB) scale as "phase error, in degrees". The Phase error will be
0
degrees at the Reference Point, Marker "L". You will notice an
upward
"trend" in the Phase Error trace. It begins at Marker 3 and ends
at
Marker 4. This is the Error contribution of the VNA components,
outside the PDM. If it were a PDM contribution, the Marker 4
would
revert to the same level as Marker 3.
The Error trace looks have peak to peak
noise of +/- 0.2 degrees. This is not noise, it is the frequency
to phase conversion
that is created within the Final Crystal Filter of the MSA.
During the sweep, the final I.F. deviates a few Hertz, and very narrow
bandwidth Crystal Filters will exaggerate this FM to PM
conversion. This is especially noticeable on the Phase error
trace below the "L" marker.
The "dead zone"
of the PDM is clearly indicated where the Phase trace is
transitioning near zero degrees. The phase error is so large,
it
leaves the boundries of the +5 degree, -5 degree right scale.
This data was taken with the first SLIM
MSA/VNA. It's PDM had been modified to use a "pulse trigger"
scheme in place of a "one shot". The uncertainty of the "pulse
trigger" is indicated by the error trace
to the right of the
"dead zone". The PDM's "non-linearity area", including its "dead
zone", is indicated between Markers 1 and 2. This is from 18.48
deg to
-16.72 deg, a total of 35.2 degrees. This PDM is indicating
excellent
Phase Error. It looks to be about .1 degrees.
The Reference
Point can be changed and a new sweep initiated. Halt
the
Sweep, double left click a different reference point,
and click the "Test Transmission Linearity" Button. You can also halt
and change the
scales, Center Frequency,
or Sweep Width. But, you must click [Restart] Button after any
change. The previously determined phase/frequency factor and
Reference point will be used for all new sweeps if the [One Step],
[Continue], or [Restart] buttons are clicked. Clicking the "Test Transmission Linearity" Button will update the Reference
Point and recalculate the phase/frequency factor.
To return to normal VNA Mode operation, Halt the sweep, then exit the
Special Tests Window by clicking the "X" button. The values that
were
placed in the Graph Window will "stick" there until the operator
changes them.
Even though this test may indicate Phase Errors, those Errors are
factored out when a Reference Line Calibration is performed before a
normal VNA measurement.
