In this section we will try to give you a brief introduction into how the
modern oscilloscope works and its basic controls. It is our intention to
help you to have trust in this universal diagnostic tool. This introduction
can in no way claim to be complete with regards to the electronic details of
such tool, the aim is only to inform you of those things which are of
importance for practical operation and use. Therefore many things are
simplified in order to facilitate understanding of the basic functions.
- What is a oscilloscope: The oscilloscope is
an electronic device with a screen, which is used to measure voltage
against time. With its very fast measuring capabilities it can detect very
short ( in time) and intermittent voltage signals and “freeze” them on the
screen before they disappear. As they say “picture speaks a thousand
words”, the oscilloscope paints a picture for us, on the screen,
representing voltage versus time. Understanding those “pictures” or as we
call them waveforms, is of utmost importance as to understanding the
processes that have created them.
- The Scope screen: The oscilloscope screen has
gone through some degree of “evolution” as the old screens were
cathode-ray tubes and the modern screens can be anything from LCD screens,
picture tubes (as used in TV’s and PC screens) or even plasma screens.
There is a certain degree of difference between those and the way they
represent a waveform. Due to the nature of LCD and PC displays the signal
representation appears to be somewhat not as smooth as with normal tube.
This is what we call “digitizing”. As you probably know, the PC screen is
organized in rows and columns of little squire dots called - pixels. The
very text you are reading now is in fact “made” of series of pixels
organized in such a way as to represent letters. If you magnify those
letters you’ll see the “painted” squire dots (pixels) next to each other.
Most scopes today use this form of representing the waveforms as it is a
small trade-off against the ability to capture, store, roll-back, expand,
and even save the waveforms. So if you are used to see waveforms on your
scope and move to one of another type you may find the same waveform
somewhat strange. Never the less, little practice and basic understanding
is all you need to quickly get used to the new one.
- Painting a picture: In order for us to
understand the waveforms we see on the scope screen we need to know how
are they painted on it. As we have said before, the scope represents the
signal in to directions. Voltage value is represented vertically and the
time taken – horizontally. Therefore the screen is divided in to a grid of
squires called divisions vertically and horizontally. Vertically -
voltage in volts, milivolts, kilovolts and so on, and horizontally – time
in seconds, milliseconds, microseconds and so on. Each direction of
divisions can be set to represent different values. So you can have 5v/div
( 5 volts per division ) representing voltage value of 5v if you have
pattern that reaches to the top of the division, and 1s/div ( 1 second per
division ) if the signal has lasted for 1sec. the pattern will reach the
end of the division. The signal is painted pixel by pixel as it goes and
any change in vertical direction represents the signal voltage value, and
the time it took is represented horizontally.
- The Laboratory Scope: This is where the
scopes were only used in the beginning, in the laboratories. Hence the
name. The old scopes had cathode-ray tubes and limited capabilities of
“conditioning” of the measured signal. If a signal had only occurred once
and it did not repeat for long time, one have to stare at the screen for a
long time in order not to miss it. Now that is painful and time consuming.
With the advance of electronics and computers however, the modern lab
scopes are much more sophisticated as they have much better capabilities
of capturing the signal and displaying it on the screen.
- The Automotive Oscilloscope: This scope is
essentially an ordinary scope, but with added capabilities and specific
signal pick-ups as to make operating with it and using it to diagnose
today’s vehicles faster and easier. One can still use an “ordinary” scope
or a lab scope for such work, but setting time-base and triggers can be
some times tedious and you’ll need those “special” signal pick-ups in
order not to damage the scope inputs. So get yourself the right tool for
- Basic Scope Controls: The two most important
controls in the oscilloscope are the controls for change of time-base
(horizontally) and change of voltage range ( vertically). Setting up those
two properly, makes the difference between seeing a waveform and blank
screen! While doing automotive diagnostics, we usually know (more or less)
what is the voltage of the signal we want measure, setting the time-base (
time/div ) can be more difficult in order to “capture” the waveform we
need. Good place to start is in the middle of the time-base range and then
once you see a waveform appearing on screen, change the controls
accordingly to get a good practical view of the measured signal. For most
purposes setting the time-base to 20ms/div ( 20 milliseconds per division
) and 10v/div for the voltage range will get you a waveform and then the
controls can be readjusted to suit the signal strength and time.
- Channels: We can ( for simplicity) look at
the scope channels as its inputs. One channel scope can display only one
waveform on the screen at a time because have one input. Now if we combine
two channels with one screen, we’ll have two inputs and we’ll get two
waveforms that we can compare on to the same screen. Most oscilloscopes
have two channels, although 3, 4 and even more channels in one scope (
depending on the model ) can be found. Quite frankly the more the better
as it gives the scope more versatility and comparing power.
- Triggering: Another control that we use in
the oscilloscope is the ability to “start measuring when a particular
event occurs”. This is called triggering. As the name suggest triggers are
conditions which when met will trigger the scope to start the measurement.
Now, this is very useful, if we want to separate particular event in time
and also helps to get steady waveform that starts always from the same
time when we measure repetitive events. Most events in the ignitions
system are repetitive as we know already. Triggering can be done to
positive and negative values, and to rising and falling values.
Oscilloscopes usually have internal trigger ability as well as external.
Internal Trigger: Internal triggering is when we adjust the scope
to trigger when the measured signal reaches certain value. i.e. 1v for
instance. So when the signal is under 1v the screen will stay still (no
measurement displayed) but as soon as it reaches the trigger point ( in this
case 1v), the scope will start plotting the waveform on the screen. In other
words it will show us what is happening with the signal after it reaches or
exceeds the trigger level of 1v.. The internal triggering is only done for
the scope channel we are working with.
External Trigger: External triggering is when we use external (
other to the one measured with the scope channel) signal. This external
signal have to be monitored (measured) through another channel of the scope
and when a trigger event occurs in that signal this event will trigger the
first channel to start plotting the waveform of it’s own measured
signal. We may or may not want to see the (external) triggering signal from
the second channel. It is a matter of adjustment what you want to see on the
Free Run (no trigger): This is when we leave the scope to plot
the measured signal in a waveform and don’t interfere with how the waveform
is displayed. This may be ok for some repetitive AC signals but for most DC
signals and especially for ignition related signals we need to trigger as to
see the events occurring from the very beginning. Otherwise you’ll get a
waveform that starts anywhere on the time-base with no guarantee that it
will start again there the next time it occurs.
To conclude: The Oscilloscope is the fastest,
most versatile and the only tool that will let you see the signal and
its variations under measurement with respect to the time it took. Those
abilities make this tool, indispensable, if not the most important of the
arsenal of automotive technician. Ever seen a doctor trying to predict the
sex of a child before birth, without ultrasound machine. He may just get it
right sometimes, but never 100%.
In addition to the ability to
plot waveforms, most modern computer
based oscilloscopes have many other useful features as calculating the
minimum and maximum voltage peak of a the signal, conversions from frequency
in to RPM, saving the waveforms and recording a "movie" of consecutive
waveforms for a later review, etc, etc. Specialised Scopes as the ignition
oscilloscope ( depending on the manufacturer and model ) can have other
improvements as automatically setting the time/voltage bases for
measurements of different signals i.e. "fuel injector" or "ignition primary"
. This saves you time and ensures the waveform is always captured correctly.