A Quick Guide to Using an Oscilloscope

WisdomAugust

1.3 Triggering ControlThis is the tricky bit that no-one understands, so engage your brain…
Imagine that the input signal to an oscilloscope is a sinusoidal voltage. The scope repeatedly1
draws a trace across the screen that represents the time-varying voltage. The trace is drawn
hundreds or thousands of times each second. Now imagine that each time the trace is drawn
across the screen, the drawing begins on a different part of the sine wave. The trace of the
sine wave will flicker horizontally, backwards and forwards across the screen. I am sure that
you have seen something like this. To stop the flickering and “freeze” the trace on the screen,
the scope must start to draw the sine wave on exactly the same part of the wave every time the
wave is drawn. This is what the TRIGGERING control lets you do.



Triggering works by setting a reference (DC) voltage level, which the scope compares with
the input signal. When the input signal voltage reaches the trigger voltage, the scope begins
drawing the trace on the screen. The trace remains rock-solid on the screen, as if by magic!
Actually, what is happening is that every time the trace is drawn (thousands of times per
second) it is drawn in exactly the same place.



Of course, there are a couple of circumstances where this will not give a stable screen trace:

? If the input signal voltage never reaches the trigger voltage, the scope never draws the
trace. I am sure that you have (or have not?) seen this, too.
? If the input signal is non-repetitive. For this, you need a storage scope – see later.  

WisdomAugust

Note that time on an oscilloscope screen moves from left to right (unlike in a graph), so that
older stuff is to the right of the screen.
On some scopes there is also an “XY” setting for displaying two input channels – one
channel drives the horizontal display and the other drives the vertical display. This is useful
for looking at the phase, frequency or voltage relationships between two signals.  

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1.2 Horizontal Control (Timebase)The horizontal control sets the scale in sec at which the trace is drawn on the screen.
There is only one horizontal control for all input channels, as shown in Figure 2. There is also
a HORIZONTAL POSITION control.

CAUTION! There is usually a small knob marked VAR (or VARIABLE) that allows
adjustment between the “clicks” of the HORIZONTAL control knob. When measuring times
from the screen, ensure that the VAR knob is in the CAL (or CALIBRATE) position!





Figure 2: Typical horizontal (timebase) control of an analog
(left) and digital oscilloscope (right).  

WisdomAugust

There are some subsidiary vertical controls that you need to be aware of
? COUPLING: The coupling setting determines which part of the signal presented to the
input is displayed on the screen. The Coupling control has three settings:

o DC: The full signal voltage, including any DC component that may be present, is
displayed. This is the usual setting.

o AC: A coupling capacitor is placed in series with the input, removing any low frequency
component of the signal, including the DC component. The time constant
of this high-pass filter is usually about 0.1 sec. AC coupling is useful if you have to
look at a small signal sitting on top of a large DC voltage.

o GND: The signal is removed from the input, and the input connected to +0V.

? POSITION: This moves the trace vertically on the screen. Use it in conjunction with
COUPLING – GND to set the zero voltage position of the trace.  

WisdomAugust

1 Oscilloscope ControlsNote: The figures show typical controls for both analogue and digital scopes. There will be more
control knobs on an analogue scope than on a digital scope. Many of the controls on a digital scope are “soft”
controls, where the functions activated by a few push buttons are changed in the instrument’s software.


1.1 Vertical Control (Gain)
The VERTICAL control sets the gain (ratio) in Volts/div between the voltage of the input
signal and the vertical deflection of the trace drawn on the screen. There is a separate vertical
control for each input channel, similar to those shown in figure 1. Each vertical control is
usually a large knob.


         
Figure 1: Typical vertical gain controls of a two-channel analog (left) and digital oscilloscope (right).  




CAUTION! There is usually a small knob marked VAR (or VARIABLE) that allows
adjustment between the “clicks” of the VERTICAL control knob. When measuring voltages
from the screen, ensure that the VAR knob is in the CAL (or CALIBRATE) position!