Using Oscilloscopes on Vehicles

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Pulse Width and Rise Time Measurements

In many applications, the details of a pulse's shape are important. Pulses can become distorted and cause a circuit to malfunction,

and the timing of pulses in a pulse train is often significant. Standard pulse measurements are pulse width and pulse rise time.

Rise time is the amount of time a pulse takes to go from a low to high voltage. By convention, the rise time is measured from 10%

to 90% of the full voltage of the pulse. Pulse width is the amount of time the pulse stays high. Some scopes will calculate and display

pulse width (measured in seconds) and also duty cycle (the proportion of time that a pulsetrain is high.)

Conclusion

Using a scope gives you a window into a new world. No longer do you just see a static (or more often, flickering around!) voltages

coming out of a sensor or the ECU. Now you can see the shape of that signal - which is a whole lot more illuminating...

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Time and Frequency Measurements

You can make time measurements using the horizontal scale of the oscilloscope.

Time measurements include measuring the period and pulse width of pulses.

Remember that frequency is the reciprocal of the period, so once you know the

period, the frequency is one divided by the period. Like voltage measurements,

time measurements are more accurate when you adjust the portion of the signal

to be measured to cover a large area of the screen. Again, some scopes will do

these calculations for you.

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Scope Measurement

Voltage Measurements

The oscilloscope is primarily a voltage-measuring device. The most basic method of taking voltage measurements

is to count the number of divisions a waveform spans up the oscilloscope's vertical scale. Adjusting the volts/div control

signal to allow the signal to cover most of the screen vertically makes for the best voltage measurements. The more screen

area you use, the more accurately you can read from the screen. Then it's as simple as reading off how many divisions per

volt the scope is set to, and estimating on-screen how many divisions the waveform covers.

With AC signals (eg a sinewave from a speed sensor), you would normally look at the peak to peak voltage.

With a DC voltage, the whole line will be elevated from the zero point. Some scopes will do these calculations for you.

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Scope Systems and Controls

An oscilloscope has three main controls, labelled Vertical, Horizontal, and Trigger.

You need to adjust these three basic settings to accommodate an incoming signal:

a) The attenuation (reduction) or amplification (increasing) of the signal - use the volts/div

(volts per on-screen division) control to adjust the height of the signal to the desired

measurement range.

b) The time base - use the sec/div (seconds per on-screen division) control to set the

amount of time per division represented horizontally across the screen.

c)The triggering of the oscilloscope - use the trigger level to stabilize a repeating signal,

or to trigger on a single event.

These adjustments sound more complex than they actually are: what you want to see

is a steady waveform that fits on the screen. The first point (a) simply fits the waveform

on the screen vertically, (b) sets the bottom axis so that the waveform repeats sufficiently

that you can recognise it, and (c) makes sure that the waveform is clearly depicted.

And as we said, some digital scopes have an 'auto' button that do all of these things for you!

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Digital Oscilloscope

A digital oscilloscope uses an analog-to-digital converter (ADC) to convert the measured voltage into digital information.

It acquires the waveform as a series of samples, and stores these samples until it accumulates enough samples to

describe a waveform. It then re-assembles the waveform for display on the screen.

The digital approach means that the oscilloscope can display any frequency within its range with stability, brightness,

and clarity. It can also easily freeze the waveform, allowing it to be studied at leisure. Digital scopes can usually be

powered by batteries and use an LCD screen. All scope adaptors that are used with laptop PCs are digital.

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Types of Scopes

Oscilloscopes can be classified as analog and digital types.

Analog Oscilloscopes

An analog oscilloscope works by applying the measured signal voltage directly to the vertical axis of an electron beam

that moves from left to right across the oscilloscope screen - usually a cathode-ray tube (CRT). The back side of the

screen is treated with luminous phosphor that glows wherever the electron beam hits it. The signal voltage deflects

the beam up and down proportionally as it moves horizontally across the display, tracing the waveform on the screen.

Analog oscilloscopes are characterised by the large screens used in traditional 'tune-up' machines and the smaller

scopes with the glowing green screens used in electronics. They are excellent tools, however in automotive use they

suffer from major drawbacks - the need for mains power, the greater difficulty in set-up and the absence of a storage

mode that allows the freezing of the on-screen image.

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Voltage

Voltage is the amount of electric potential - or signal strength - between two points in a circuit.

Usually, one of these points is ground, or zero volts. DC signals are measured on a scope as

you would with a multimeter - from ground to the amplitude (height) of the signal.

Automotive AC signals are often measured from the maximum peak to the minimum peak of a

waveform, which is referred to as the peak-to-peak voltage. The peak-to-peak voltage of this

inductive crank sensor is just under 16 volts.

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Waveform Measurements

Many terms are used to describe the types of measurements made with an oscilloscope.

Frequency and Period

If a signal repeats, it has a frequency. Frequency is measured in Hertz (Hz) and equals the number of

times the signal repeats itself in one second. Hertz can also be referred to as 'cycles per second'. A

repetitive signal also has a period - this is the amount of time it takes the signal to complete one cycle.

Period and frequency are reciprocals of each other, so that 1/period equals the frequency and 1/frequency

equals the period.

For example, the sine wave here has a frequency of 3 Hz and a period of 1/3 second. Some scopes can

calculate frequency and display it as a standalone number, while in other cases the period needs to be

read off the scope screen and the frequency then calculated from this.

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Square and Rectangular Waves

The square wave is another common wave shape. Basically, a square wave is a voltage that turns on and off

(ie goes high and low) at regular intervals. An injector waveform is fundamentally a square wave - the injector

is either on or off. A rectangular wave is like the square wave, except that the high and low time intervals are

not of equal length. That is, the 'on' and 'off' times are not equal. Again, this is often the case with an injector,

where at low loads the 'off' time will be much longer than the 'on' time. The waveform shown here is from a Hall

Effect road speed sensor.

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You can classify most waves into these types:

Sine waves

Square and rectangular waves

Triangle and saw-tooth waves

Complex waves

In automotive applications, sine and square waves dominate.

Sine Waves

The sine wave is the fundamental wave shape. It has harmonious mathematical properties

- it is the same sine shape you may have studied in high school trigonometry class. Mains

AC voltage varies as a sine wave. ('AC' signifies alternating current, although the voltage

alternates too. 'DC' stands for direct current, which means a steady current and voltage,

such as a car battery produces.) Many speed sensors produce sine wave outputs - this

waveform is from an ABS inductive speed sensor.