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A Quick Guide to Using an Oscilloscope

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Post time 2018-6-4 10:05:14 | Show all posts |Read mode
An oscilloscope is a test instrument that allows you to “see” how the voltage at a point in a
circuit varies with time. A ‘scope’ samples the voltage and displays it on a screen that is
marked with a grid in division. From the vertical gain in Volts/div we can measure the
amplitude of the signal in Volts, and from the horizontal scale in sec/div (the ‘timebase’), we
can measure the timing of that signal. Scopes always have two or more channels so that the
relative timing or amplitude of two signals can be compared.



Scopes are often a complete mystery to students when they begin a course. Since we expect
you to be proficient in using a scope, we have written this short note to help you get started.
Scopes are actually quite straightforward to use. Regardless of how fancy a scope may appear
to be, any scope really has only three controls – vertical, horizontal, and triggering. These are
described below.  




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 Author| Post time 2018-6-5 10:23:05 | Show all posts
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!  





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 Author| Post time 2018-6-6 13:25:11 | Show all posts
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.  




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 Author| Post time 2018-6-7 08:13:14 | Show all posts
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).  






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 Author| Post time 2018-6-8 08:29:20 | Show all posts
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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 Author| Post time 2018-6-9 10:46:21 | Show all posts
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.  




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 Author| Post time 2018-6-11 08:27:52 | Show all posts
Now that you understand the basics, here are descriptions of all the usual trigger controls:
? LEVEL: This knob allows the trigger reference voltage to be set;
? SLOPE: Sets the slope (+ or –) of the signal that will cause triggering as it crosses the
level of the trigger reference voltage;
? MODE: Allows selection between
o NORMAL: the trace will be drawn on the screen when the trigger source signal
crosses the trigger reference voltage with the correct slope;
o AUTO: the trace “free runs”. This is sometimes useful for looking at signals that
have very little AC component, and also for signals that sometimes get very small.
Both types of signals can be hard to trigger on;
oSINGLE: Once “armed”, the scope will trigger the next time that the input signal
matches the trigger level and slope condition. This triggering option is essential for
viewing non-repetitive waveforms, and is most useful when the oscilloscope is a
digital storage scope.
? SOURCE: Determines which signal is compared to the trigger reference voltage.
Usually selectable between
oCH1, CH2, etc: The various input signals;
oEXTERNAL: some other signal that is injected through a dedicated input;
o LINE: the AC power supply for the scope, and maybe some other options.
? HOLDOFF: is a time delay before the scope can re-trigger. It is sometimes useful when
an input signal is nearly repetitive.  




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 Author| Post time 7 day(s) ago | Show all posts
Typical triggering controls are shown in Figure 3.

                  


Figure 3: Typical trigger controls of an analog (left) and digital
oscilloscope (right).



Fancy scopes often have more, fancy, triggering options. You may come across scopes that
trigger when a particular series or parallel pattern appears on one or more inputs, or a scope
that triggers when a waveform goes outside of a pre-set voltage-and-time envelope.





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 Author| Post time 6 day(s) ago | Show all posts
Edited by WisdomAugust at 2018-6-13 08:36

2 The Importance of Probes

The function of an oscilloscope is to allow you to “see” a time-varying waveform. The
function of a scope probe (see Figure 4) is to transmit the waveform from the circuit node to
the scope input without distortion or added noise. That is, a scope probe is not just any old bit
of wire that happens to be lying around on the lab bench.





Figure 4: a) A scope probe;                                                                                                                           b) Not a scope probe.

WARNING! Scope probes are delicate and expensive. We will not tolerate anyone misusing

scope probes. You have been warned!  






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 Author| Post time 5 day(s) ago | Show all posts
The input impedance of a scope is typically around 1M? in parallel with about 100pF. Even
this may load the circuit under test too much, with the input capacitance distorting the
measured waveform. The usual solution is to use a “10x” passive scope probe which has a
resistance of 9M? in parallel with an adjustable capacitor of about 5pF. When the capacitor is
adjusted to be 1/9th of the capacitance of the scope input plus probe lead, the probe will have
an impedance of 10 M? at all frequencies. Loading of the circuit under test will have been
reduced by a factor of 10, and waveform distortion eliminated through the process of probe
compensation. You should normally use the scope probe in the “10x” mode.



In Figure 4a, the short black lead with the alligator clip is the ground connection, to be
clipped to the +0V reference point in the circuit. The probe hook can be seen just above the
probe itself. This style of probe hook pulls (gently!) off the probe; some styles screw off.  




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