Using Oscilloscopes on Vehicles

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Non-periodical variable signal


An example for non-periodical signals is the digital data transfer between the cars differednt
controllers. There is another type of non-periodical signals-single signals. This is a kind of signal
that is represented by a single impulse, which may never repeat or may repeat after a long interval
of time.

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Triangular periodical signalSignals that don’t repeat through equal time intervals are called non-periodical.  

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Square periodical signal
The injector control signal is a square one similar to the one shown above. Its impulses repeat
periodically.

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Periodical and non-periodical signals
A signal is a periodical one if the values of its voltage pulsations and the shape of these
pulsations are identical and are repeated through equal intervals of time.

The time needed for one periodical signal to complete one full cycle is called period. The
number of periods per second is called the frequency of the signal. If the waveform of the
voltage of the periodical signal crosses the ‘zero’ line the signal is called a varying signal. If the
waveform does not cross the ‘zero’ line the signal is a constant one. Example waveforms of
different periodical signals are shown below.

The first signal shown is a sine wave. This signal is characterized by 2 parameters – amplitude
and frequency. In the automobile electronics similar to the sine wave signal are the signals
generated by inductive speed and position sensors. Similar signals are generated by some
crankshaft position sensors (CKP), camshaft position sensors (CMP), vehicle speed sensors
(VSS) and others.





Sine wave periodical signal

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Another example for DC of a much more complex nature is the rectified current of a generator.
This current is positive, but it’s also pulsating. These pulsations are additionally amplified by the
fact that in the shown example below there is a damaged diode in the rectifier bridge.  







Example of complex DC voltage  






Alternating current (AC) - In AC the movement of electric charge periodically reverses
direction. In direct current (DC), the flow of electric charge is only in one direction. The signal
varies around 0V. Its momentary value can be both positive and negative. Such voltages are
represented by almost all signals from inductive sensors: the CKP sensor, the CMP sensor, the
signal from the ABS sensor etc.

The voltage in the electrical circuit is also AC and has sinusoidal shape. All examples viewed in
the “Periodical and non-periodical signals” section are also examples for variable signals.  

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What types of electrical signals are there and what are their parameters?



DC and AC
Direct current (DC) is the unidirectional flow of electric charge. It can be positive or it can
be negative. Direct current is produced by such sources as batteries and electric machines of the
dynamo type. Also may be obtained from an alternating current supply by use of a current switching
arrangement called a rectifier. The picture below shows the current of a car battery.  





The wave produced by a car battery  

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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.  

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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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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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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