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Keysight Technologies
Techniques for Time Domain Measurements
Using FieldFox Handheld Analyzers
Application Note
Abstract
This application note will introduce time domain and distance-to-fault (DTF)
measurement techniques for identifying the location and relative amplitudes of
discontinuities while operating in the field. This application note will describe
the relationship between frequency domain measurements and time domain
transforms and their relationships to time resolution and range. Also shown
will be VNA configurations for characterizing band-limited devices such as
couplers, filters, antennas and waveguide components, and broadband devices
such as cables and connectors. This note will also discuss time domain
"gating," a powerful feature that effectively isolates discontinuities in the time
domain just as a filter would isolate signal energy in the frequency domain.
Measurement examples will be provided using the Keysight Technologies, Inc.
FieldFox vector network analyzer.
Introduction
Testing and qualifying components and systems that function as part of a communications or radar
system often requires that the electrical performance of these devices achieve a certain level of
specified performance across the operating frequency range. Specifications include Voltage Standing
Wave Ratio (VSWR), return loss and insertion loss to name a few. These specifications provide a
clear distinction when the device under test (DUT) has passed or failed its performance requirements
as a function of frequency. Figure 1a shows the measured VSWR of a system where the VSWR has
exceeded the specification at several places across the measured frequency range of 8.5 to 12 GHz.
The Keysight FieldFox vector network analyzer (VNA) used in this example was configured with limit
lines to help the operator quickly identify whether the DUT has passed or failed the test.
While a frequency measurement provides useful information into the proper functioning of a system,
having only swept frequency may not provide enough information to determine the root cause of the
problem. When a system fails to meet specification, troubleshooting is often difficult as components
would then need to be swapped in and out of the system until performance once again meets the
specified requirements. Fortunately, there is another measurement technique that provides details into
the location and magnitude of any such problems. This technique relies on measurements in the time
domain and a vector network analyzer, such as the Keysight FieldFox, has the capability of displaying
the time domain characteristics of one and two port components and systems. Figure 1b shows the
time domain transformation of the VSWR measurement with the location of individual discontinuities
being displayed as a function of time. In this example, it can be assumed that the largest peak in the
time domain is associated with the component causing the out-of-spec condition for this system.
Knowing the propagation velocity of the signal in the transmission system, the physical location of the
fault can be determined and the system can be quickly repaired.
This application note will introduce time domain and DTF measurement techniques for identifying the
location and relative amplitudes of discontinuities while operating in the field.
(a) Frequency response (b) Time response
Figure 1a. Measured VSWR as a function of frequency (1a), and the time domain response showing several discontinuities as a function of time (1b).
Time domain measurement basics
Time domain analysis is very useful
to observe the effects of mismatches
along a transmission line system.
When an RF or microwave signal
propagates along a transmission
line, a portion of the signal will be
reflected back from any disconti-
nuities encountered along the path.
Using time domain analysis, the loca-
tion of each discontinuity is displayed
as a function of time along the x-axis
and the amplitude of the reflected
signal, or S11, is plotted along the
y-axis. Knowing the propagation
velocity along the transmission line
allows the time measurement to be
scaled to physical distance. It is also
possible to examine the time domain
response of a transmitted signal, or
S21, but this measurement requires
connection at two ports of a system.
As field measurements often limit
access to only one port of a system,
this application note will primarily Figure 2. Configuration and measurement of a time domain response showing three
focus on the time domain response discontinuities along a coaxial transmission line
of reflection, or S11, measurements.
Figure 2 shows a simple example The measurement from any one-port be simultaneously displayed in time
of a time domain response for two or two-port device can be represented and frequency creates a powerful
short sections of coaxial line con- in the time domain and/or the fre- analysis and problem solving tool.
nected together with an adapter quency domain. If a measurement is Fortunately, modern test instrumenta-
and terminated in a 50-ohm coaxial made in one domain, then the other tion, such as a VNA, includes this
load. If the input to the line is excited domain can be calculated using a mathematical transformation as part
with a short impulse waveform, well-known mathematical technique of the firmware allowing the user to
then it is possible to observe the called the Fourier Transform (FT). This display either time domain data or
reflected impulse response from transform provides a universal problem frequency domain data, or both. For
each discontinuity as a function of solving method that allows one to example, Figures 1a and 1b show the
time. In this case, there are reflected examine a particular measurement frequency measurement and time
signals from the input connector (1), from an entirely different viewpoint domain transform recorded using the
the adapter (2) and the termination [2]. If a measurement is recorded Keysight FieldFox VNA.
(3). Excessively large peaks would be using a time domain method, then
representative of problems in one or a FT calculation will result in a
more of these components. It is also frequency domain representation of
possible to excite the line with a the data. Alternatively, if frequency
step waveform and details of this domain data is initially captured, then
technique are shown in the Keysight an Inverse FT (IFT) will result in a
Time Domain Application Note time domain representation of the
1287-12 [1]. data. The fact that the same data can
3
Instruments with time domain capability
There are two basic instruments A vector network analyzer, such as
capable of displaying the time domain the Keysight FieldFox N9918A VNA is
response of individual discontinuities primarily a frequency domain instru-
along a transmission system; the ment with capability of measuring the
time domain reflectometer (TDR) and reflected and transmission charac-
the vector network analyzer (VNA). teristics of one and two-port devices.
A time domain reflectometer uses a Using error-corrected data measured
traditional method of launching an in the frequency domain, the
impulse or a step waveform into the response of a network to an impulse
test device and directly measuring the or step function can be calculated
response as a function of time [3]. using the IFT and displayed as a
Using a step generator and broadband function of time. As the VNA uses
oscilloscope, such as the Keysight narrowband measurement receivers,
86100D DCA with 54754A TDR the dynamic range will be typically
Module, a fast edge is launched into higher than oscilloscope-based TDR
the transmission line. The incident systems. Also a VNA includes time
and reflected voltage waves are moni- domain capability for measuring
tored by the broadband oscilloscope band-limited devices, called bandpass
and the position of each discontinuity mode, which will be discussed later
can be displayed as a function of time. in this application note. Lastly, there
is a configuration of a FieldFox VNA,
known as a cable and antenna (CAT)
analyzer, which performs the same
frequency-to-time domain transforma-
tion but scales the time measurements
to an equivalent physical distance to
aid the operator in quickly locating
faults in RF and microwave
transmission lines in the field.
4
Measurement example using a horn antenna
Figure 3 shows a test configuration In practice, it could be possible that
that will be used to examine the metallic structures placed near an
frequency domain and time domain antenna system could have a nega-
responses of an over-the-air measure- tive effect on the system performance
ment using an X-band waveguide as reflections from these additional
horn antenna and a separate metal structures could create ripples large
plate placed near the antenna. The enough to cause an out-of-spec
high-gain horn antenna is connected condition. As mentioned, examining
to a short length of WR-90 waveguide only the frequency response does
transmission line and the transmis- not provide sufficient insight into the
sion line is attached to a waveguide- root cause of the problem(s). The
to-coaxial adapter for connection to time domain response under these
the VNA. The VNA is calibrated for measurement conditions will be
S11 at the plane where the adapter examined next.
is connected to the instrument port.
The measurements will include the
effects of all the components shown Coaxial Horn
adapter Waveguide antenna
in Figure 3 and also include effects
from other items in the surrounding
environment. The metal plate is a Metal
0.3 meter square aluminum plate plate
mounted to a tripod and positioned
in front of the antenna. The distance
between the metal plate and the
antenna is varied in order to examine
changes in the frequency and time Figure 3. Configuration for measuring the frequency domain and time domain responses
responses. of an over-the-air measurement using an X-band horn antenna
Figure 4 shows the measured
frequency response of the system
under three test conditions; two with
the metal plate near the antenna
and one without. The yellow curve
shows the measurement without the
metal plate. There is a small ripple
in the measured S11 response due
to interaction between the various
transmission lines and reflections
from items in the surrounding
environment. The blue trace shows
the frequency response with the
metal plate positioned at 1.7 meters
from the horn antenna. In this case,
there is a larger amount of ripple in
S11 when compared to the case with
no plate. The orange curve shows
the results when the plate is moved
closer at a 1.2 meter separation. Here Figure 4. Comparison of the measured frequency response of the X-band antenna system
the ripple is even more pronounced under three test conditions; two with the metal plate near the antenna and one without
as the reflected signal from the plate the metal plate
has larger received amplitude.
5
Measurement example using a horn antenna (continued)
Figure 5 shows the time domain
response of the antenna system
using a Keysight FieldFox VNA with
the time domain option. For the case
without the metal plate, shown as the
yellow trace, the peaks associated
with signal reflections from the sur-
rounding environment are relatively
low in comparison to those from the
adapter and antenna. This type of
measurement would provide a good
baseline for a new system installation
and could be used as a comparison as
more antenna components and struc-
tures are added around this system.
Figure 5 also shows the time domain
response when the metal plate is Figure 5. Time domain response of the antenna system under three test conditions;
positioned near the antenna. For the two with the metal plate near the antenna and one without the metal plate
measurement at 1.7 meters, shown
as the blue trace, there is a large As shown earlier in Figure 4, the
peak representing the reflection from environmental reflections introduce
the metal plate and a smaller peak to a ripple in the frequency domain
the right representing the reflection response. It would be useful to
from the legs of the tripod. When the confirm the system performance
plate and tripod are moved closer without the effects from the environ-
at 1.2 meters, shown as the orange ment. Under this test requirement,
trace, the measured time domain the VNA has a gating function that is
response shows an equivalent time configured in the time domain to filter
shift in these peaks. From these out specific reflections and allow the
measurements, you can see that the frequency domain response to be
peaks associated with the adapter viewed without the effects from these
and antenna are static as these reflections. The next section will
components were not changed during introduce the basic concepts of time
the testing. There is also a noticeable domain filtering also known as gating.
increase in the peak amplitude asso-
ciated with the metal plate when the
plate is moved closer to the antenna.
This increase in peak amplitude is
due to the reduction in space loss
as the signal now propagates over a
shorter distance.
6
Gating and frequency response
The basic concept of gating is to To continue with the antenna shown with gated response, there
introduce a time filter to remove example discussed in the previous is a reduction in the ripple across
unwanted reflections from the time section, Figure 7a shows the time the measured frequency span as the
domain response. This time filter domain response with and without reflections from the environment
works in the same way as a fre- a gating function. In this example, have been filtered out from the
quency filter would in the frequency a bandpass gate is configured to measurement. Gating is a very power-
domain. Once the undesired respons- remove the effects of the metal plate ful tool to examine the reflection
es are removed from the time domain, and the environment. The yellow and transmission properties of a
the frequency domain response of the trace is the original measurement component or system by selectively
remaining devices can be observed. including the peak from the metal removing specific responses from the
Figure 6 is a simplified example of an plate reflection. The blue trace shows measurements in any domain.
ideal time domain response having the time domain response with the
three peaks. For this example, the bandpass gate configured to filter any It should be noted that when a
time filter or "gate" is configured to response to the right of the antenna. discontinuity along a transmission
remove the two outside peaks. By With the bandpass gate applied, line reflects energy, that portion of
adjusting the gate's time center and the time response only includes the the energy will not reach subsequent
span, an equivalent bandpass filter effects of the coaxial adapter and the discontinuities further down the line.
can be created in the time domain. antenna. It is also possible to remove This effect can hide, or "mask," the
Once the gate is activated, the upper a single peak, or group of adjacent true response of the later discontinu-
and lower peaks are removed from peaks, using a notch gate. ity. This condition also occurs when
the measurements in the time and the transmission path has insertion
frequency domains. Along with the Figure 7b shows a comparison loss along the length of the line. The
bandpass gate, the FieldFox VNA has between the original frequency gating function does not compensate
another type of gate that can notch response including the plate and for the masking effect and masking
out a single peak, or grouping of environmental reflections, shown in will be discussed in the next section
peaks, leaving behind the remaining yellow, and the response when the of this application note.
peaks above and below this filter gate is applied, shown in blue. As
response. In either case, once the
gate is properly configured in the time
domain, the user can return the VNA
to the frequency domain to examine
the frequency response of the system
with the effects of the filtered peaks.
(a) Time response
(b) Frequency response
Figure 6. Simplified example of an ideal Figure 7. (7a) time and (7b) frequency domain responses of the X-band antenna system
time domain response having three peaks with and without a gate function applied to the measured data
and the results when a time "gate" is
applied to the original response
7
Masking in coaxial lines
Figure 8 shows the time domain the reflected signals could be below It should be noted that the measured
response of a short length of RG58 the dynamic range of the VNA and amplitude on any time domain display
coaxial cable with the end of the would not be observed on a time is actually an average value taken
cable terminated in an open circuit. domain display. This would also occur across the entire measured frequency
The time response shows a single if the transmission path contained a range. Therefore, if the transmission
large peak representing the reflec- very large discontinuity that reflected system includes couplers, filters and
tion from the end of the cable. It is most of the energy and prevented any other items with varying insertion
expected that an ideal open circuit later reflections from being observed loss, then applying a single factor
would have an S11 of 0 dB resulting on the time domain display. to compensate for the insertion loss
from all of the energy being reflected as a function of frequency may not
back to the source. As shown in If the insertion loss of the cable is provide accurate amplitude results.
Figure 8, the measured peak for this known, the Keysight FieldFox VNA This effect will be shown in the next
test configuration has a lower value and CAT analyzers allow the cable section of this note.
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