Using Oscilloscopes on Vehicles

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How does one work with an oscilloscope?

All oscilloscopes have a display on which the waveform is shown. The screen can be a cathoderay tube,
a LCD panel; a PC monitor can also be used. Here a typical oscilloscope screen is shown.  

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There are divisions on the screen; they allow the signals parameters to be visually measured.
With the divisions on the horizontal axis the time parameters of the signal are measured. The
vertical divisions are used to measure the strength of the voltage.
The “time” scale can vary from parts of a second to several seconds. The “voltage” scale can
vary from several mV to several kV.

The graphics which are visualized on the screen are called waveforms. An oscilloscope can only
observe waveforms produced by an electrical voltage. The screen of the oscilloscope shows the
stretched through time image of the electrical fluctuation allowing for the shape and the
amplitude of the voltage to be rendered into account, as well as making phase and frequency
measurements.

For most of the measurements only 2 probes, such as Multimeter are required. The ground cable
of the probe should be connected to the negative side of the car battery or the chassis, and the
other cable should be connected to the cable whose signal we want to check.  

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Basic terminology when working with oscilloscopes



“Zero” line
If no electrical current source is plugged into to the oscilloscope, the waveform is represented by
a straight line. This line is called “zero” line because it represents a level corresponding to a 0V
voltage at the input of the oscilloscope.  







Waveform – there is no signal at the input of the oscilloscope
(The yellow line is the zero line)  

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The position of the zero line can be displaced vertically according to the geometrical center of
the screen. The need for moving the zero line vertically depends on the type and shape of the
signal, as in the cases when using multi-channeled oscilloscope for a better visualization of
signals from more than one channel.


When we connect a direct current (DC) supply to the oscilloscope, the waveform we would
observe would be a straight line. The line itself would be vertically displaced from the zero line.
The difference between the observed waveform and the zero line is proportional to the value of
the electric current.  


The majority of waveforms have a shape different from a straight line.  

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Vertical Sensitivity


The graph on the screen of the oscilloscope shows the relation between the values of the voltage
and time. If the amplitude of the input voltage is greater, a higher range of the vertical amplifier
must be set. Depending on the amplitude of the signal, for a better visual result an appropriate
vertical amplifier is used. The ability to change the scale of amplification of the signal enables
the oscilloscope to show signals with very high amplitudes as well as signals with very low
amplitudes. The appropriate value of the amplification depends on the amplitude parameters of
the signal observed. The same signal would be shown differently depending on the degree of
amplification. A larger range is used when the amplitude of the whole signal needs to be shown.
A smaller range is used when, is need to make a detailed observation of the shape and the
amplitude parameters of separate sections of the signal. In such cases, when the signal is of
higher voltage, only a part of the signal is visible on the screen.

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Timebase Controls


The oscilloscope plots a graph of the voltage left-to-right, starting from the left side of the
screen. These select the horizontal speed of the spot as it creates the trace; this process is
commonly referred to as the sweep. In all oscilloscopes the sweep speed is selectable and
calibrated in units of time per major graticule division. Quite a wide range of sweep speeds is
generally provided, from seconds to as fast as picoseconds (in the fastest 'scopes) per division.
As mentioned above the sweep is measured in seconds. In automobile measurements
milliseconds are more often used (ms) – 1ms=1/1000 s. The value of the sweep can be changed
with the time switch. The same signal is visualized differently depending on the sweep setting
selected. A shorter period of time is selected when a detail observation of the shape and time
parameters of separate sections of the signal is needed. In such cases a very short time fragment
of the signal is shown on the screen. If we need to observe a larger time fragment of the signal
(for example when showing individual impulses with irregular shape of the signal or skipping
impulses) a bigger sweep is used.  

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Basic types of sweeps
Sweep trigger controls - Synchronization
Synchronization is needed for stabilizing the image of the signal on the screen. Synchronization
provides that the plotting of each separate signal starts from the same point on the screen. The
moment, when the plotting of the new screen starts is called a “triggering” moment. Because of
this the image shown on the screen does not move or is relatively stable. When there is no
synchronization active, which may be a result of a wrong synchronization setting, the signal is
seen as mishmash.  






The oscilloscope is not synchronized – a mishmash type of display  

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For a correct synchronization setting the following must be set
? The level control sets the threshold voltage. The threshold voltage is the value of the
voltage at which the oscilloscope starts plotting.
? The slope control selects the direction (negative or positive-going).



Types of synchronization:
Automatic sweep mode – This mode is used when measuring signals that repeat periodically in
time. When using multi-channel oscilloscopes it is necessary to select the signal that will be
synchronized. It is also necessary to select the level of synchronization of the signal - falling or
increasing front.

Single sweeps – This mode is used when observing signals consisting of impulses with identical
shape. The time intervals between the pulses can be identical or can vary. This mode is also used
with single impulse on the input signal. We must again select the level of synchronization –
falling or increasing front.

External trigger – The oscilloscope has to have an additional input used for external
synchronization in order for this mode to be used. Automobile oscilloscopes usually have an
ignition synchronization signal connected to that input (#1 Cylinder).  

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Holdoff control – This function is very useful when complex signals consisting of several
frequencies must be observed. In order for these signals to be synchronized on the screen of the
oscilloscope, the device must have a "Trigger hold-off" function. On the examples shown below
this function is represented by softkey V0 on the control section of the front panel.






An example of a signal that needs a holdoff setting, in order for the display of the signal to be
stable.  

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Pre-trigger
This is a method of visualizing the signal on a digital oscilloscope, which helps for the detailed
observation of certain parts of the signal before the synchronization of the signal (before the
triggering). There is no such concept in analog oscilloscopes. The pre-triggering visualization is
possible because during the conversion of the signal to digital data, part of the values is kept in a
buffer memory. After the synchronization of the signal the values kept in the buffer memory can
be shown on the screen.