OSCILLOSCOPE: BASE GUIDE
Learning the oscilloscope (also scopemeter or scope)
This short and simple reading allows to understand
the basic working concepts and the possible uses of
a scopemeter. It is not depending from
performances and cost of the instrument.
Indeed the oscilloscope is used to observe
slow speed signals, like pulses generated from
cardiac heartbeat, or fast and irregular signals
of electronic equipments like radio and
To reader, beginner or not, who has a basic knowledge about electric
unit measurement and wants to know and learn to use this
great instrument not so complex in spite of the high number
of keys, knobs and selectors impressive at first glance.
Just in case somebody did not tell you at school.
Here you will find explanation of basic principles and working modes
and use of analogue oscilloscope.
All described concepts applies to understand and to use digital
oscilloscope which is created to replicate the analogue one
adding all possible enhancements.
The scopemeter is an instrument that shows graphically the behaviour of an electrical signal in the time scale (T).
It performs qualitative measurement type rather than quantitative.
I mean that the oscilloscope draws the behavior of
a voltage (V) but the absolute measurment has lower
precision than a digital voltmeter (or multimeter).
The latter allows for example to easily appreciate
an exact 4.53V battery voltage whereas the
oscilloscope read out is just around 4.5 Volt.
The same concept applies to time scale,
when I need an accurate frequency measurement
I must use a frequency meter.
WHAT THE OSCILLOSCOPE IS USED FOR ?
The screen has a reference grid
with usually 8 vertical
and 10 horizontal divisions.
Each resulting square has 5 further
subdivisions per axis useful
to better readings.
HOW IT APPEARS
There is at least one input channel
for the Voltage signal V to be shown,
about the two channels (or dual trace) we will speak later.
This signal passes through an adjustable gain amplifier
and the selection knob sets the amplitude value for each
So setting 2 V/Div means that the maximum watching
amplitude of the input signal is 16V (2V multiplied
by 8 vertical divisions) or referring to center (zero)
is 8V positive and 8V negative.
Y AXIS - Vertical - Voltage V
This axis too has a selection knob to set the temporal
base or how long is a division.
For example setting 10ms/Div means that to
trace the whole X axis it spends 0.1 seconds
(10 ms multiplied by 10 divisions
= 100 ms).
We will call scan everyone of this sweeps.
X AXIS - Horizontal - TIME BASE
An electronic beam light up a dot on the screen.
Where the dot is depends on the two deflection
systems, horizontal and vertical.
The vertical axis is driven by the input signal
while the horizontal one by the internal time base.
Without input signal the dot moves from left to
right tracing a flatten horizontal line.
Now suppose to apply at the vertical input
a 10Vpp (peak to peak) triangular wave signal with
25Hz of repetition frequency.
That means 25 cycles per second so a period is:
1/25 = 0,04 seconds = 40ms.
Setting the gain to 5V/Div and
the time base to 10ms/Div
what is traced at every scan on the
time axis appears as depicts this picture:
There is shown a 2 divisions
height signal repeated every
4 divisions on X axis.
Now if I switch the gain, consequently changes
the vertically filled divisions (Y axis).
Switching instead the time base, changes of
course the occupied horizontal divisions (X axis).
So we realize that every
signal with any frequency, voltage
and shape can be shown graphically
by the oscilloscope just setting it
up properly. Within of course max
and min limits reported on selectors.
It is necessary to emphasize that every scan on
the X axis draws a new trace and the time base knob
defines the trace's lenght of time, 0.1 seconds in our example.
At this point we have to understand when a scan begins,
or best, what starts it.
The trigger perform just that function.
We call trigger the event that starts each single scan.
This fundamental section allows two settings:
Practically the above settings define that the
trigger event (beginnig of scan) happens when
the input signal crosses the trigger level in
one of the two possible ways, rising for the
positive edge else falling for the negative one.
- Selecting the edge between positive and negative.
- Presetting the trigger voltage level in continuous range (through analog potentiometer),
not by predefined steps.
The example trace in previous picture starts at
zero voltage level (center of Y axis) hence the
trigger level was preset around zero Volt
while the selected edge was negative.
At the end of scan (end tracing on X axis) the
electronic beam is turned off and brought back
to the left of the screen (starting point).
Then it waits a new trigger event.
With this system happens that, for
constantly repeated waves, an identical trigger
event restarts a new scan that retrace
exactly the previous shape.
In that condition the input signal is
triggered (locked or coupled or hooked up)
so we can see a stable waveform on the grid.
Without trigger instead the input signal is
shifting on the X axis.
Let us make clearer this concept with a picture.
Supposing to have a continuous saw tooth wave
the scopemeter shows just the fraction fitted
in one scan. The beam at end of scan switches
off and go back to the left.
This operation spends a fixed time known as
"HOLD-OFF" time (H).
If now begins a new scan, the new starting point
is different from the previous one, please see this
Here is therefore what is shown without trigger,
our input signal running sliding on temporal axis.
Open animated example (new window)
Animated example (same window)
All oscilloscopes have at least 3 basic trigger modes:
Moreover, on some oscilloscopes there is a special
trigger section. It may allows a delay from trigger
event rather then widen the time base into a scan part.
Since that section is specific and change model by
model it must be seen case by case.
Here let us remain on basic use.
- SINGLE - In this one shot mode, the scan starts
only once at first trigger event.
After that, it must be manually re-enabled by a
push-button to wait for another start event.
So in single mode a trace is drawn just once at first
trigger event and the lenght of time scan depends on
time base preset.
- NORMAL - A scan restarts only on trigger event.
At the end of scan the beam go to the starting point
(left of screen) to wait for next event. Hence in normal
mode, when trigger events lack there is not any trace.
- AUTO - Automatic, the scan restarts automatically at
each end of scan also without trigger event, so a trace
is always shown without input signal too.
When the input signal is small, such to not generate
a trigger event, it is however shown even if sliding
as above described.
Now go ahead with
of many push-buttons, switches and so on.