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An oscilloscope employs a cathode ray tube (CRT) similar to the
picture tube in a television. The CRT 'shoots' an electron beam at a
phosphorous coated screen. When the electrons hit the screen they excite
the phosphorous atoms and cause them to give off light. The beam is
constantly scanning across the screen. When there is no input signal, the
beam just scans in a horizontal line.
Feature and common settings
There are a number of switches on a scope which have to be set
correctly for the information on the display to be of any use. I generally
use the scope for trouble shooting car audio problems. These are the most
common settings for me.
Inputs
Most scopes have 2 input 'channels'. They are labeled channel A and
channel B. They can be used individually or together.
Focus
The focus control simply allows you to keep the beam in focus. (big
surprise!)
Vertical and horizontal position controls
These controls simply allow you to center the beam on the display.
Timebase:
The timebase determines the time it takes to scan 1 division (from
side to side). For audio, I generally use 0.2 milliseconds; for switching
power supplies, 2-5 microseconds (depending on the frequency at which the
power supply is oscillating).
Trace intensity
The trace intensity allows you to adjust the beam to a comfortable
brightness level. When the timebase is set for very short times (very fast
scanning speed), the display may appear dim. If the scanning is slow, the
display may be uncomfortably bright. If a very bright display is used
often, it will also reduce the life of the display by burning a line on
the screen.
Volts/division:
Determines the sensitivity of the input amplifiers. It allows you to
adjust for the best resolution. For car audio 10v/div is the most common,
lower settings (more sensitive input) are used for preamp level
troubleshooting
Trigger source
The scope must be 'triggered to display a stable waveform. There are
several options for the trigger source. The most common trigger source is
the signal on the input being used. If you are using the channel A input,
the trigger source would be set to 'channel A'. This is the configuration
which I use most. If you are using both inputs, you can select either
channel as the trigger source.
Trigger level:
For the waveform to be 'locked' on the screen, the signal has to be of
a sufficient level. If you want the scope to be triggered (locked onto the
signal) when the voltage of the waveform reaches a certain point, you can
set the 'trigger level' so that it will trigger properly. For car audio
work, this control is usually set to its center '0' position. It will
cause the scope to trigger on the weakest of signals. This control is more
commonly used when working with video.
Trigger mode
The trigger mode allows the scope to lock onto different types of
signals. My advice... use the trigger mode which gives you the best
results for the waveform being monitored.
AC/DC input coupling
You should remember that we talked about high pass crossovers and the
fact that a high pass crossover blocks low frequencies. You should know
that a crossover is actually a filter. The input to the scope can be
switched to go through a high pass filter or to bypass the filter. When
switched to pass through the filter, the scope is A.C. coupled and the
D.C. component of the signal is removed. When the scope is D.C. coupled,
the signal is not passed through the filter.
Vector input
The vector input is used when you need to compare two signals. When
the scope is in the vector mode, a voltage applied to one of the inputs
will cause the beam to move in the vertical plane. Input to the other
input will cause the beam to move in the horizontal plane. There is no
scanning in the vector mode.
VOLTAGE MEASUREMENT
If the input of the scope has a positive or negative voltage, the beam
will still scan across but will be deflected up or down from the reference
line in proportion to the voltage applied. The volts/division selector
allows you to keep the beam from being deflected off of the top or bottom
of the display. The v/d selector also lets you compensate for a small
voltage so that you may view the signal with better detail. The sine wave
on the display is approximately a 1000 hertz test tone. If the scope is
set for 10 volts/division (and it is), the voltage of the sine wave is
approximately 50 volts 'peak to peak' which is equal to 25 volts 'peak'.
The signal is swinging as high as +2.5 divisions and as low as -2.5
divisions. A total of 5 divisions at 10 volts/division.
TIME MEASUREMENT
The time/division control tells the scope to scan at a predetermined
rate of speed. If it is set at .2 microseconds/division (as it is in the
picture). The time that it takes the beam to scan one horizontal division
will be .2 microseconds. When using the scope for viewing audio waveforms,
it is usually adjusted so that the scanning beam looks like a straight
steady line. For viewing extremely low frequencies, it may be necessary to
adjust the timebase (volts/division) control to a point that you may see
the beam scanning across. If you set the timebase control to 100
milliseconds/division, it will take 1 second to scan across the whole
display (10 divisions*.1 seconds/division). I haven't mentioned it yet but
it is easy to determine the frequency of a sine wave if you know how long
it takes to complete a full cycle. The frequency is the reciprocal of the
time it takes to complete one cycle. If it takes 1/1000 of a second to
complete one full cycle, the frequency of the signal is 1000 hertz. 1/500
of a second (or .002 seconds) for 1 cycle would be 500 hertz.
PERIOD OF A WAVEFORM
If I didn't mention it earlier, the period of a waveform is the time
it takes to complete 1 full cycle. To find frequency of a waveform if the
period is known, simply divide 1 by the period (1/period).
You can also determine the frequency of a sine wave with a scope. The
diagram below is (supposed to be) the screen of a scope. As you can see,
there are there are 10 divisions from left to right (and up and down). If
you know the timebase setting and the timebase, you can calculate the
frequency of the waveform. The text on the diagram indicates that the
timebase is set to .1 milliseconds per division. This means that the
'beam' is scanning across at a rate which will move the beam through 1
division in .0001 seconds. You can see that the wave, from 1 peak to the
next, spans 3 divisions, which means that its period is .0003 seconds. If
we divide 1 by .0003 (1/.0003), we get 3333 cycles per second (3333
hertz). Please note that this is NOT a very precise way to measure
frequency. A frequency counter would be MUCH more accurate.
 Please
note that the waveform would start at the left side of the display and
would extend to the other side of the display. It would NOT begin and end
as I have it drawn.
A.C. coupling
In the picture below, the scope is set for A.C. coupling. Let's say
that it is the speaker output of a high power head unit. You should
remember that the speaker output signal is 'riding' on 6 volts of D.C. The
D.C. is filtered out of the signal so the signal is vertically centered on
the reference.
D.C. coupling
In this picture, the scope is D.C. coupled. You can see that the
signal looks the same but is shifted up. You should notice that the
horizontal center of the sine wave is located at 3 divisions above the
reference. At 2 volts/division, the horizontal center of the sine wave is
at 6 volts (just as it would be on a real scope).
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