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OSCILLOSCOPE: oscilloscope icon BASE GUIDE


INTRODUCTION

Learning the oscilloscope (also scopemeter or scope)

This short and simple reading allows the understanding of basic working concepts and the possible uses of a scopemeter. It is not depending on 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 microprocessor circuits.


INTENDED AUDIENCE

For 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 nobody did teach you at school. Here you will find explanation of basic principles and working modes and use of analogue oscilloscope. All described concepts are useful to understand and to use digital oscilloscope which is created to replicate the analogue one adding all possible enhancements.


WHAT THE OSCILLOSCOPE IS USED FOR ?

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.
Click to zoom Standard oscilloscope grid or quadrant, click to zoom


HOW IT APPEARS

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.


Y AXIS - Vertical - Voltage V

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 Y division. 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.


X AXIS - Horizontal - TIME BASE

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.


WORKING PRINCIPLE

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.
Triangular wave, click to zoom Triangular wave 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.


TRIGGER

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:
  • Selecting the edge between positive and negative.
  • Presetting the trigger voltage level in continuous range (through analog potentiometer), not by predefined steps.
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 and falling for the negative one.
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 picture:

Click to zoom Scans with HOLD OFF fixed to minimum

Here is therefore what is shown without trigger,
our input signal running sliding on temporal axis.

Click to zoom Shifting signal without trigger
Open animated example (new window)
Animated example (same window)


TRIGGER MODES

All oscilloscopes have at least 3 basic trigger modes:
  • 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.
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.
Now go ahead with practice description of many push-buttons, switches and so on.

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