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The difference between hold-off control and delayed sweep

The difference between hold-off control and delayed sweep
The difference between hold-off control
and delayed sweep
In oscilloscope triggering, hold-off is a simple concept and is easy to implement. In contrast,
delayed sweep is somewhat more obscure as to its details and current status in digital storage
oscilloscopes. It was prominent in analog scopes but currently has more limited application.
First hold-off: Hold-off relates to triggering the oscilloscope. The time base exists along a horizontal
line (X-axis) that is not infinite but bounded by the left and right edges of the display. There comes
a time when we run out of scope display to represent time at the right side of the screen. The
obvious solution is to perform a blanked-out trace and begin anew at the left, the analogy being the
process of reading a printed page. In this exercise, the eye performs a mentally blanked out retrace
and begins at the next line down, the reader eventually turning the page.
This scheme is not available to
the oscilloscope because the
vertical space above and below
the X-axis is needed to represent
amplitude. Furthermore, if the
continuation of the trace were to
begin right away, the waveforms
would not coincide and at best
the user would see multiple
conflicting superimposed
graphics and at worse, at higher
frequencies, a meaningless blur
of light.
In this figure from Tektronix, the upper trace represents the sweep
voltage in an analog scope. The lower trace represents the trace
blanking signal. The trace is blanked during the retrace and tail
end of the hold-off period. The trigger is re-armed quickly after the
retrace. In modern digital scopes, the trigger and acquisition
technique differs from than that of the analog scope. But the end
result of the Trigger Holdoff function is the same. In digital
scopes, the input signals are essentially sampled continuously.
The signals are put into memory and sent to the trigger system
which examines them for events that meet the trigger criteria.
When a trigger event happens, the user-requested acquisition is
completed and displayed. Because of the continuous sampled
nature of the system, both pre-trigger and post-trigger data is
saved. This is a major advantage of digital scopes over their
analog predecessors.
Howard Vollum and his
colleagues at Tektronix solved
this problem in 1946 with the
development of a workable
triggered sweep for the
oscilloscope. The basic idea is
that successive traces will
appear, starting at the left of the
display, when a uniform point
along the waveform occurs. The
oscilloscope waits until this
happens, then instantly initiates
a new cycle that is synchronized
with those occurring before and
after. The product is a clear,
stable image that accurately
represents the ongoing signal.
Triggering can take place in
response to various events, including rising edge and falling edge. The user can choose the event
and the amplitude level used for triggering. Triggering behavior is straightforward and easy to
The difference between hold-off control and delayed sweep
implement in most instances. But occasionally the waveform contains or acquires an anomaly that
can cause false triggering. Anomalies can consist of an occasional glitch that rises high enough to
initiate unwanted triggering.
Another source of unintended triggering is
the burst, which is a purposely introduced
brief interval of intense electrical or
electromagnetic activity whose purpose is
to convey information while creating
minimal impact on the primary signal. An
example is the color burst, embedded in
the video signal in TV transmission.
Tektronix set up this example of a 350-μsec bursted
sine wave with a 500-μsec gap between the bursts. By
properly adjusting the Trigger Holdoff so the trigger rearms during the gap between the bursts, a stable nonoverlapped trace results.
False triggering caused by spurious events
as described above can be prevented by
means of a user-implemented tool known
as hold-off. The operator instructs the
oscilloscope to refrain from recognizing any
triggering event until a specified amount of
time elapses.
Assume the hold-off interval is set at 0.001
sec. It is to be emphasized that triggering
does not necessarily arise upon the
expiration of this amount of time. A new
triggering event must also take place, so both conditions must be met.
The procedure for implementing hold-off is similar in most digital scopes, but we’ll use the Rigol
DS2102 oscilloscope as an example. To implement hold-off, first press Acquire. Then, in the
Triggering section in the lower right area of the front panel, press Menu. At the bottom of the list of
choices that appear at the right of the display, press Setting. The third item from the top in the list of
categories at the right of the display is Hold-Off. The small multipurpose knob at the top center of
the front panel permits adjustment of the amount (time interval) of hold-off, and the present setting
is shown. This interval can be set anywhere between 100 nsec and 10 sec.
In the Tektronix MDO3104, as in other Tek 3000 series oscilloscopes, the procedure to set the
hold-off interval is similar. First press Default Setup and press Menu Off until all menus are cleared,
just to get to a good starting place. Then, in the Trigger section next to the numeric keypad in the
front panel, press Menu. In the choices that appear across the bottom of the display, press the soft
key that corresponds to Hold-Off. Notice that in the menu that appears to the right of the display,
the third item from the top is Hold-Off. The amount can be set by using Multipurpose Knob a with
Fine disabled. (It is quicker to use the numeric keypad.) Hold-off intervals range from a minimum of
20 nsec to a maximum of 8 sec.
Another important, related-but-somewhat-different, concept in oscilloscope operation is delayed
sweep. This is a second, auxiliary time base that was widely used in the earlier analog
oscilloscopes. It lets the user take a close look at a small selected segment of the time base of the
signal under investigation. So in this respect it resembles “zoom” in contemporary digital
instruments. We might want to know what this has to do with “delay”.
What happens in delayed sweep relates to the existence of two separate scope time bases, which
may in various formats be shown simultaneously in the display. To the primary time base is
appended a user-defined delay. When the delay expires, the delayed time base is triggered. The
The difference between hold-off control and delayed sweep
delayed time base usually incorporates a much faster sweep, creating a temporal window that
typically magnifies the interval by a factor of 1,000.
The oscilloscope is capable of showing the main time base only, the delayed time base only, or
both simultaneously at different intensities. Thus, the delayed sweep can be used to enlarge a
length of waveform horizontally for the easier viewing of details. The user is able to closely
examine the leading or trailing edge of a pulse. Because the delay is calibrated,it’s possible to
make precise measurements within the waveform, far better than when working with the graticule
within the main time base.
Like many aspects of the analog oscilloscope, delayed sweep was inspired by similar functions in
British World War II radar. Early radar had two varieties, A and B sweep. Using a comparator, a
rising B voltage was juxtaposed against a fixed voltage. When the two levels were equal, the A
sweep commenced. This permitted an accelerated sweep so an expanded and more detailed
picture emerged.
Some instruments reversed A and B, but that was the means for creating a more detailed image in
radar. The technology worked in early analog oscilloscope designs as well.
Newer digital oscilloscopes with storage capability realize the same effect even if the electronics
are totally different. The terminology now is “zoom.” An example can be seen on Tektronix series
3000 Mixed Domain instruments. When the oscilloscope has booted up, press Default Setup and
Menu Off. Then, with a BNC cable installed from AFG Out on the back panel to an analog channel
input on the front panel, press AFG. In the display we see the default sine wave.
Rather than delayed sweep per se, we will access a zoom image of the sine wave. In the Wave
Inspector section on the front panel, turn the large knob at the center clockwise. A split-screen view
displays the overall sine wave at the top and a close-up view, the bracketed portion, at the bottom.
The time interval appears above the brackets. The zoom factor is shown at the left.
At the bottom of the display, below the zoom view, are all the usual metrics including the type of
waveform, frequency, amplitude, offset, period and high and low levels.
At the bottom of the Wave Inspector control on the front panel are the Mark controls. Marks may be
set or cleared in either of the two windows. Just as the inner of the two large knobs in the Wave
Inspector section determines the amount of zoom, the outer knob controls pan. Turning this knob
moves the lower, more detailed view of the waveform from side to side.
The Wave Inspector permits the user to capture details that could not otherwise be discerned in the
waveform of the signal that is being studied.