Trigger Gating: Circulating Loop and
Burst Data Analysis with the Agilent
86100A Infiniium DCA Wide Bandwidth
Oscilloscope
Copyright ã 2000
Agilent Technologies, Inc.
Trigger Gating: Circulating Loop and Burst Data Analysis
with the Agilent 86100A Infiniium DCA Wide Bandwidth
Oscilloscope
There are experiments incorporating wide-bandwidth oscilloscopes that require select
portions of a waveform to be viewed while other portions of the waveform are
intentionally ignored. Examples include circulating loops, or data that occurs in bursts.
In the specific example of the circulating loop, signal propagation through an extremely
long length of fiber, typically in excess of 1000 km, is simulated with multiple
circulations through a shorter length of fiber. For example, a 9000 km trans-pacific fiber
link can be simulated by routing a signal 18 times through a loop of fiber 500 km in
length.
When an oscilloscope is used to view such a signal, the instrument should only sample
the elements that have propagated the correct number of circulations. Thus the
oscilloscope needs to be synchronized with the signals used to control the loading and
circulation within the loop.
Wide-bandwidth oscilloscopes typically require a signal synchronous to the data as a
timing reference. For each trigger edge/event, one and only one data point is sampled.
To acquire a waveform that is composed of 1000 data points, 1000 trigger edge/events
must be responded to by the oscilloscope. The Agilent 86100 Infiniium DCA has an
external BNC connector port called the trigger gate. This port responds to TTL
compatible signals. With the trigger gate feature enabled, a high voltage at the port will
enable the oscilloscope to respond to trigger edges and thus acquire data. When the
signal at the port is low, the oscilloscope will not respond to triggers even if they are
presented to the instrument. They are simply ignored and waveform data is not captured.
An example of the control signals used in a circulating loop experiment is shown in
figure 1. The basic process is to configure the switching to load the loop with data, and
then close the loop so that the data continuously propagates around the loop. Once the
data has traversed the loop the correct number of times, the oscilloscope is gated to allow
it to respond to the always-present trigger signal and acquire only this portion of data. It
is important to note that the signals used to control the loop switches as well as the signal
used to gate the oscilloscope are defined and provided by the user. The oscilloscope
itself does not generate or control any of these signals.
Transmitter TransmitterOscilloscope Oscilloscope
Loop Loop
Load state
Transmitter
switch
Loop
switch
Trigger
gate
Loop state
Figure 1: Timing diagram
The exact timing required for loading the loop and circulating within the loop is
determined by the time to propagate through the loop and the number of circulations
required. It should also be noted that it is ideal to have the duration of the trigger gate
enable be less than the round trip time through the loop. This is necessary to guarantee
that any signals acquired are temporally distant and within the time between the
switching transients as well as to allow for the delay required to enable and disable the
oscilloscope gating. In the example mentioned above, the 500 km loop has a roundtrip
time of about 2.5 ms. The trigger gate was set to about 1.5 ms in duration to avoid the
switching transients. The gating pulse is enabled only after 18 roundtrips through the
loop, so the gating function, loop switch, and transmitter switch have periods of 450 ms.
Figure 2: Measurement results: Output waveform of loop after 18 circulations
If an experiment is to be conducted which requires a greater level of precision in
controlling when the gate is enabled and disabled, it becomes important to consider the
rate at which the instrument responds to both the enable and disable states of the gating
signal. The instrument will be able to respond to a trigger 100 ns after the gate is enabled
(low to high transition). When the gating signal makes the high to low transition,
eventually the oscilloscope will not accept and respond to trigger signals. In the
sampling process, the oscilloscope is triggered and a sample is taken at a time equal to
the delay setting (this is a minimum of 24 nanoseconds and is identical to the time
position on the display where the sample will be located). Once a sample is taken, the
oscilloscope must rearm. This takes approximately 27 microseconds. In the process of
rearming, the status of the trigger gate port is checked. Thus if the gate goes from a high
to a low just after the status is checked, over 27 microseconds will lapse before the
instrument will no longer accept triggers. (In this worst case scenario, only one sample
will be taken before the gate is enabled again). The maximum time that can elapse from
the time a trigger is accepted, the gating signal drops from a high to a low state, and a
data sample is taken is defined by t DISABLE.
t
DISABLE
= 27 us + trigger period + maximum time position on the instrument screen
Signal under
test
Gating signal
Valid data
100 ns
Section of
signal
actually
sampled
< t DISABLE
> t DISABLE
It is important to make sure that these timing issues are considered to guarantee that data
is not acquired outside of the “valid data” region. It is also important to note that the
gating signal has no affect at all on the timing relationship between when the scope is
triggered and when the data is sampled. Trigger gating only controls when the
oscilloscope will accept a trigger signal.
A gating feature exists in the Agilent 83480 option 100 oscilloscope mainframe. Some
significant differences exist in how this feature has been implemented in the Agilent
86100. Trigger gating is a standard feature for all 86100 mainframes. In the 83480
implementation of trigger gating, the instrument was susceptible to false triggering at the
gating transients. These were manifested as vertical streaks across the displayed
waveform. This problem has been eliminated in the Agilent 86100A.
Note: Carl Davidson of Tyco Submarine performed definition and verification of the
experiment example shown above
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