The oscilloscope, or scope for short, is a
device for drawing calibrated graphs of voltage vs time very quickly and
conveniently. Such an instrument is obviously useful for the design and repair
of circuits in which voltages and currents are changing with time. There are
also many devices, called transducers, which convert some non-electrical
quantity such as pressure, sound, light intensity, or position to a voltage. By
using a transducer the scope can make a plot of the changes in almost any
measurable quantity. This capability is widely used in science and technology.
The heart of the oscilloscope is a cathode ray
tube or CRT, of the sort you have already studied. Looking at the face of the
instrument, you are viewing the screen that the electron beam strikes.
Electronic circuits in the scope apply voltages to one set of deflection plates
to sweep the beam across the screen from left to right at a constant rate,
thereby providing the time axis. Other circuits amplify or attenuate the input
signal as needed, and apply voltages to the other set of deflection plates to
move the beam vertically, providing the voltage axis.
Fig. 1
Controls are provided to select the time and
voltage scales needed for any given situation. At the end of each sweep, the
beam is shut off and the horizontal deflection voltage is reset so the beam
would start at the left edge of the screen again. Since a scope is usually used
to plot a rapidly changing quantity, one sweep and therefore one plot may last
only a few microseconds. If the phenomenon we are studying can be made
repetitive, we can repeat the sweep sequence many times to get a display
suitable for a more leisurely examination.
A special circuit, called a trigger circuit,
examines the incoming voltage signal and starts the sweep at the same point in
the repetitive cycle for each new sweep. This results in a visually steady
display of the input. Several controls are provided to set the trigger as
needed. The scope you will use is a very flexible instrument, typical of those
available in a research laboratory. It has two channels, so that two different
voltages may be plotted simultaneously for direct comparison, and a variety of
triggering options.
The front panel, shown in Fig. 1, is
correspondingly formidable at first glance. Fortunately, the myriad of controls
can be 2 considered in several independent groups,
which are marked in the figure. In the remainder of this section we will
examine each group in turn, concentrating on the controls we will need in
subsequent experiments. The experimental procedure section will then take you
through a series of measurements designed to demonstrate the operation of each
section.
At the left side of the instrument, the CRT
screen is divided into a one centimeter grid, ruled on the inside surface of
the tube. Each solid line is one division for the horizontal and vertical
deflection. The dotted lines are provided for pulse rise-time measurements.
Moving across the panel, we come to the power switch and the CRT controls. The
trace rotation and probe adjust are used only when repairing the instrument.
The intensity control should be set to give a visible trace, but excessive
brightness will defocus the spot and may damage the screen. Both the intensity
and focus may need to be adjusted when the sweep rate is changed drastically.
The beam finder is provided as an aid to
setting the scope. When pushed, it reduces the deflection voltages enough that
the beam will always appear on the screen. The position controls are then used
to center the spot, and you should obtain a display suitable for final
adjustments when the beam finder button is released. Fig. 1. Oscilloscope front
panel with functional blocks marked.
The vertical system accepts input signals and
develops appropriate deflection voltages for the CRT. Because this is a
two-channel scope there are two identical sets of vertical controls, one for
each trace. The block diagram in Fig. 2 shows the flow of signals in one channel.
Voltages are applied between a grounded terminal labeled GND and either CH 1 or
CH 2 for the channel desired.
The coupling switch allows the input circuit to
accept all signals when set at DC, or only the time-varying part when set for
AC. The middle position, GND, connects the vertical amplifier input to ground,
so that you can see where the zero-voltage height is on the screen. (Using the
GND setting does not connect the external input terminal to ground, so your
circuit will not be disturbed.) The position control allows you to place the
trace on the screen as desired, for example aligning the zero-voltage position
with one of the grid lines.
The vertical sensitivity control, labeled
VOLTS/DIV is used to set the vertical scale factor. For example, when the 50m
marking is next to the 1X symbol, the scope is set for a vertical scale of 50
mV per centimeter, and a deflection of 2 cm, equal to 2 large divisions, would
indicate an input voltage of 100 mV. This control is normally set to make the
vertical part of the signal a convenient size on the screen.
The red knob in the center of the control
allows you to continuously vary the vertical scale factor, rather than using
the fixed settings. This feature is occasionally useful for relative
measurements, but for quantitative work you need to know the calibration, and
you must use the fixed steps. Turning the red knob fully clockwise sets the
control for the fixed steps marked on the main control.
The controls labeled VERTICAL MODE select
several related functions. Starting at the left, you can display the signal
from channel 1, channel 2 or from both channels. The switch at the right
determines how the display is done when both channels are in use. In CHOP mode
the beam LEVEL SLOPE MODE MODE SEC/DIV CH 1 BEAM FINDER FOCUS INTENSITY
COUPLING POSITION MODE VOLTS/DIV SOURCE INT/EXT POSITION Attenuator PreAmp
Delay line Amp Trigger system Sweep generator Amp CRT control EXT INPUT
The CHOP mode is therefore better for slow
sweeps because it draws both traces together, although it cannot switch quickly
enough to handle very fast signals. As a general rule, use ALT mode unless the
display is slow enough to be irritating, and then switch to CHOP. The third
setting, ADD, algebraically adds the input of channel 1 and 2. We will not have
much use for this feature. The BW LIMIT (bandwidth limit) button allows you to
cut off signals with frequencies higher than 10 MHz, so that they do not appear
on the display. Most of our signals are at lower frequencies, so leaving this
button in will cut out some noise without losing any information.
The CH 2 INVERT button inverts the signal from
channel two, so that increasing positive voltages are plotted downward, rather
than upward. This button should be out for normal operation. The horizontal
system controls the time scale of our plots. The main control, labeled SEC/DIV,
works much like the vertical sensitivity controls, with a series of fixed
settings and a red variable control. Note that the scale is divided into
regions for seconds, milliseconds and microseconds per division.
When set for 1 ms/division, as in the figure,
each one centimeter division on the screen corresponds to one millisecond. The
position labeled X-Y disables the timed sweep, and allows you to plot the
voltage applied to channel 2 vs the voltage applied to channel 1. The
horizontal mode switch and the delay time controls are used only for special
tricks, which we will not need. Leave the mode switch at NO DLY for no delay.
The position knob works just like its vertical counterpart.
The trigger system is used to start successive
sweeps at corresponding points on the input waveform on each successive sweep.
This operation is indicated schematically in Fig. 3. The Fig. 3. A continuous
input waveform and four successive sweeps on the scope screen. The trigger is
set for positive slope with the trigger level at the dashed line. LEVEL knob controls the voltage at which the
trigger starts the sweep.
The SLOPE button is used to specify whether the
signal should have a positive or negative slope at the trigger voltage. The
source of the triggering signal is chosen with a set of switches. You may
choose to trigger off the signals applied to the vertical channels, off the
power lines, or off of a signal applied to the EXT INPUT connector. Internal
triggering is most common, and you may choose to use channel 1 or channel 2,
independently of which one you are displaying at the time. VERT MODE triggering
is odd, in that the trigger is obtained, in ALT sweep, from whichever channel
is supposed to start next. This is usually confusing, and we will not use it.
Triggering from the power lines is useful if
you want to study something that might be synchronized with the AC power.
External triggering is useful when your experiment produces a signal that
occurs a fixed time before the signal of interest, since you can then see the
early parts of your signal easily.
The other three buttons set the sweep mode. In
SGL SWP, for single sweep, the trace moves across the screen only once, and
then waits until the button is pressed again. This is an archaic feature
formerly used when photographing the trace. P-P AUTO, for peak-to-peak
automatic is the standard mode, since it will almost always produce a usable
trace. If the trace is not stable in P-P AUTO you can try NORM, for normal
mode. This is useful for slowly varying signals, and for situations where the
triggering is tricky, for example because of noise. The LEVEL control must be
set carefully to get a trace in NORM mode, so a small light is provided to tell
you when the scope has found a satisfactory trigger. If the light is on but you
have no trace, something other than triggering is the problem. Other trigger
modes are provided for television service work, but we will not use them.
The VAR HOLDOFF control is usually left full counterclockwise. Its use will be explained when needed.
Blogger Comment
Facebook Comment