does an oscilloscope do?
oscilloscope is easily the most useful instrument available
circuits because it allows you
see the signals at different points in the circuit.
best way of investigating an electronic system is to monitor
signals at the input and output of each system block, checking
that each block is operating as expected and is correctly
linked to the next.
a little practice, you will be able to find and correct faults
quickly and accurately.
diagram shows a Hameg HM 203-6 oscilloscope, a popular instrument
in UK schools. Your oscilloscope may look different but will
have similar controls.
function of an oscilloscope
is extremely simple: it draws a V/t graph, a graph of voltage
against time, voltage on the vertical or Y-axis, and time
on the horizontal or X-axis.
you can see, the screen of this oscilloscope has 8 squares
or divisions on the vertical axis, and 10 squares or divsions
on the horizontal axis. Usually, these squares are 1 cm in
else may have been twiddling knobs and pressing buttons
before you. Before you switch the oscilloscope on, check
that all the controls are in their 'normal' positions. For
the Hameg HM 203-6, this means that:
push button switches are in the OUT position
all slide switches are in the UP position
all rotating controls are CENTRED
the central TIME/DIV and VOLTS/DIV and the HOLD OFF controls
are in the calibrated, or CAL position
both VOLTS/DIV controls to 1 V/DIV and the TIME/DIV control
to 2 s/DIV, its slowest setting:
ON, red button, top centre:
green LED illuminates and, after a few moments, you should
see a small bright spot, or trace, moving fairly slowly across
the Y-POS 1 control:
happens when you twiddle this?
Y-POS 1 allows you to move the spot up and down the screen.
For the present, adjust the trace so that it runs horizontally
across the centre of the screen.
investigate the INTENSITY and FOCUS controls:
these are correctly set, the spot will be reasonably bright
but not glaring, and as sharply focused as possible. (The
TR control is screwdriver adjusted. It is only needed
if the spot moves at an angle rather than horizontally
across the screen with no signal connected.)
TIME/DIV control determines the horizontal scale of the
graph which appears on the oscilloscope screen.
10 squares across the screen and the spot moving at 0.2
s/DIV, how long does it take for the spot to cross the screen?
The answer is 0.2 x 10 = 2 s. Count seconds. Does the spot
take 2 seconds to cross the screen
rotate the TIME/DIV control clockwise:
the spot moving at 0.1 s/DIV, it will take 1 second to cross
to rotate TIME/DIV clockwise. With each new setting, the
spot moves faster. At around 10 ms/DIV, the spot is no longer
separately visible. Instead, there is a bright line across
the screen. This happens because the screen remains bright
for a short time after the spot has passed, an effect which
is known as the persistence of the screen. It is useful
to think of the spot as still there, just moving too fast
to be seen.
rotating TIME/DIV. At faster settings, the line becomes
fainter because the spot is moving very quickly indeed.
At a setting of 10 µs/DIV how long does it take for
the spot to cross the screen?
VOLTS/DIV controls determine the vertical scale of the graph
drawn on the oscilloscope screen.
that VOLTS/DIV 1 is set at 1 V/DIV and that the adjacent
controls are set correctly:
Hameg HM 203-6 has a built in source of signals which allow
you to check that the oscilloscope is working properly.
A connection to the input of channel 1, CH 1, of the oscilloscope
can be made using a special connector called a BNC plug,
as shown below:
diagram shows a lead with a BNC plug at one end and crocodile
clips at the other. When the crocodile clip from the red
wire is clipped to the lower metal terminal, a 2 V square
wave is connected to the input of CH 1.
VOLTS/DIV and TIME/DIV until you obtain a clear picture
of the 2 V signal, which should look like this:
on the effect of Y-POS 1 and X-POS:
do these controls do?
1 moves the whole trace vertically up and down on the screen,
while X-POS moves the whole trace from side to side on the
screen. These control are useful because the trace can be
moved so that more of the picture appears on the screen,
or to make measurements easier using the grid which covers
have now learned about and used the most important controls
on the oscilloscope.
know that the function of an oscilloscope is to draw
a V/t graph.
know how to put all the controls into their 'normal'
positions, so that a trace should appear when the oscilloscope
is switched on.
know how the change the horizontal scale of the V/t
graph, how to change the vertical scale, and how to
connect and display a signal.
is needed now is practice so that all of these controls