Neurobiology Laboratory GETTING TO KNOW YOUR EQUIPMENT
THE OSCILLOSCOPE
The first thing to know is
that these oscilloscopes are relatively hard to break, and they don't bite - so
don't worry!
We are going to work
generally from left to right across the control panel.
I. POWER AND DISPLAY
Look at the display
screen. On the screen is a graticule
(black grid marks).
Look at the group of buttons
and knobs directly to the right of the display. Plug in the unit, and find and push the power button. You should see a green power indicator light
just below the power button. In a
moment, you should see a trace (green line) appear on the display. If you do not see a trace, don't panic - you will soon learn how
to find it.
The INTENSITY knob adjusts
the brightness of the trace on the screen.
The FOCUS knob adjusts the focus.
If you can see the trace, turn these two knobs and observe how the trace
changes in appearance. (If you still
can't see the trace, you still shouldn't worry!) The intensity should be kept at its lowest visible level to
prevent burn-in of the phosphores and
damage to the display, and
the focus should be adjusted to make the trace as distinct as possible.
II. VERTICAL CONTROLS
Moving further to the right,
look at the section marked VERTICAL.
Our oscilloscopes have two input channels, with an independent set of
controls for each.
Find the vertical POSITION
knob for channel 1. Turn this knob and
observe how the trace moves. Now turn
the position knob all the way clockwise. You should not be able to see the
trace. This is the best way to
illustrate the BEAM FIND function. If
your trace is too far up or down and is out of view, the BEAM FIND function
will allow you to find it. Press the
BEAM FIND button and note that the trace (which is too far up) is now visible
on the screen. You can use this
function to locate a trace, then use the vertical POSITION knob to move the
trace within the viewing area. Use the
BEAM FIND function and the vertical POSITION knob to reposition your trace
within the viewing area.
The section marked MODE has
controls which allow you to select various options for channel display. The left most switch allows you to choose
between a display of Channel 1 only, Channel 2 only, or both channels simultaneously. Move this switch to BOTH and use the BEAM
FIND and VERTICAL POSITION knob for channel 2 to position the channel 2 trace
just below the channel 1 trace. The
middle MODE switch inverts the trace; put it in the NORM (non-inverted)
position. The other control ADDs
together the voltages on channels 1 and 2, ALTernates sweeps between channels 1
and 2, or CHOPs them very quickly so they both appear simultaneously. We will use the CHOP setting most of the
time.
Look at the sections marked
CH1 or CH2 VOLTS/DIV. There is a
separate section for each channel. This
may be one of the most important sections of the oscilloscope. This provides a means for calibrating the
vertical graticule on the screen.
Notice that the screen is divided into squares. By turning the knob under the VOLTS/DIV
section, you can change the amount of voltage that is covered within each
square. For example, if the VOLTS/DIV
is set to 5, then that means that the vertical range of each square on the
screen represents 5 volts. If VOLTS/DIV is set to 0.5, then the vertical
range of each square on the screen represents 1/2 (0.5) volt.
Notice that there are two
brackets on the VOLTS/DIV knob: 1X and 10X.
This is important, because it refers to the type of probe you are using
with the oscilloscope. The probes we
have (the probes are the things that look sort of like a pen attached to the
side of the oscilloscope) are 10X, and so you should read the voltage range
bracketed by the 10X when using the probe.
WE WILL BE USING DIRECT CABLES FROM THE STIMULATOR TO THE OSCILLOSCOPE
FOR ALL EXPERIMENTS SO THE 1X BRACKETED VOLTAGE RANGE IS APPROPRIATE. Note:
make sure that the small knob marked CAL is turned all of the way
clockwise. This knob allows you to
select ranges in between those marked.
You probably will not use this knob.
The AC-GND-DC switches below
the VOLTS/DIV knobs control the way the input voltage is coupled to the
vertical amplifiers. The GND or ground
setting connects the input to the case ground which defines zero volts. Use
this setting when you start so you can position your trace. Once you have set the GND trace position,
then any voltage can be measured relative to that zero position. The DC or direct-current setting, couples
the input to the vertical amplifier directly so that any continuous voltage
will appear as a continuous offset of the trace relative to the GND zero
position. The AC or alternating-current
setting is a little more complicated.
This control places a big capacitor between the input and the vertical
amplifier which effectively removes and long-lasting voltage offset from the
trace, which keeps the trace centered on the GND zero position.
EXERCISES:
1. If you turn the VOLTS/DIV knob all of the way to the right, what
is the reading at the 1X bracket (the number isn't necessarily centered in the
bracket)? HINT: the "m" means
"milli" volt. (What about 10X?)
2. Turn your VOLTS/DIV knob to 5 (1X bracket). What is the maximum deflection (the maximum
amount of voltage) that can be displayed on the screen? In other words, if you
had a signal that went from the bottom of the screen to the top of the screen,
how big (in volts) would that signal be?
3. Set the VOLTS/DIV knob to 50m (50 millivolts). What is the maximum deflection than can now be
displayed on the screen?
ANSWERS:
1. 5 mV; 50 mV
2. 40 V (5 Volts/DIV X 8 divisions in the graticule)
3. 400 mV (50 mV/DIV X 8 divisions in the graticule)
III. HORIZONTAL SECTION
The controls within this
section operate in a similar manner to the VERTICAL section controls. The horizontal controls, however, affect
both channels at once. These control
the time frame that is displayed on the screen.
The MAG switch zooms in on a
brief portion of the time display. You
can use this to closely examine very fast events that are captured with the
storage function (described below). We will always use the X1 MAG setting.
The POSITION knob is just
like the vertical POSITION knob. Move
this and observe the horizontal movement of the traces.
The SEC/DIV (seconds per
division) knob is divided into seconds (sec), milliseconds (ms) and
microseconds (s). It operates just like the
VOLTS/DIV in the vertical section but in this case SEC/DIV controls the amount
of time it takes for the trace to sweep across each horizontal division of the
graticule.
EXERCISES:
1. Set the MAG switch to X1.
What should you set the SEC/DIV knob on in order to have a 50 ms trace
across the entire screen?
2. What is the maximum time that can be displayed on the screen?
What is the fastest time a trace can sweep across the scope?
ANSWERS:
1. The SEC/DIV knob should be set at 5 ms/DIV. (5 ms/DIV X 10 divisions = 50 ms)
2. 0.5 sec X 10 divisions on
the graticule= 5 seconds. .1 ms X 10 divisions = 1 ms.
IV. TRIGGER SECTION
Look at the section marked
TRIGGER. The concept of a trigger will
be more obvious when we look at the stimulator. For now, you need to know only the basics.
Put the SEC/DIV (in the
horizontal section) switch on 0.5 sec.
Now put the trigger MODE switch on P-P AUTO (which stands for
Peak-to-Peak AUTOmatic retrigger).
Observe how the signal moves across the screen taking 5 sec for each
SWEEP across the screen. The trigger refers to a signal telling the
oscilloscope to begin a sweep. As you
can see, in the P-P AUTO mode the sweep is automatically retriggered when it
reaches the right side of the screen so you get a continuous look at the
voltage. In contrast, when the trigger
mode is set to NORM, the sweep is only triggered when a signal arrives at the
SOURCE (bottom section of the trigger panel).
Ordinarily, we will be using an EXTernal trigger source signal coming
from the stimulator in the NORM mode to trigger sweeps during an experiment
(more on this when we get to the stimulator).
But the P-P AUTO mode is also useful if we simply want to watch the
voltage in a continuous fashion.
If you are still confused
about the trigger, don't worry - most everyone else is too. It should be more obvious as we use the
stimulator later.
V. STORAGE CONTROLS
At the top of the control
panel there is a row of four buttons.
These are the storage controls that give the oscilloscope the ability to
save traces for later reference. When
the button is depressed the function listed above the button is active. The STORE/NON-STORE button must be on in order to store a signal and
for the rest of the buttons to work. Depress
this button. The SAVE/CONTINUE button
essentially takes a "snapshot" of the trace on the screen at the
instant the button is depressed. This
is good for when you want to save a response long enough to prove to the
instructor that you did it. Depress
this button and then try to move the VERTICAL POSITION knobs. The traces don't move. Now release this button and try to move the
traces. They will move now. The SAVE/REF/ERASE button SAVEs what is on
the screen (at the instant you depress the button) as a REFERENCE, then
continues to superimpose subsequent traces on top of the reference you just
saved. Push this button and then move
the VERTICAL POSITION knobs to get an idea of how this function works. This is good for saving a baseline response
(e.g. before you apply a drug) and comparing subsequent (e.g., post-drug) responses. If you release the SAVE/REF button, you will
lose your stored reference trace.
However, if instead of releasing the SAVE/REF button you press the
SAVE/CONTINUE button, you will save not only your reference trace, but also the
subsequent superimposed trace.
Now release all buttons except the STORE/NONSTORE button.
The PRE-TRIG switch, when
50% is selected, puts the trigger point in the middle of the screen. This allows you to see what happens before
the trigger (e.g., before the stimulus).
When 0% is selected, the trigger happens at the far left. Then you only see what happens after the
trigger. This function will become more
apparent when we use a stimulator.
THE STIMULATOR
Notice the stimulator has a
sticker which says, "DANGER - HIGH VOLTAGE." This means that although you can hurt
yourself with this machine, you really have to try hard to accomplish it.
Make sure the stimulator is
plugged in and the power is OFF.
The stimulator provides an
electrical source for classical nerve and muscle stimulation. For right now, we will be using the
stimulator to obtain a trace on the oscilloscope.
Notice that below each knob
on the stimulator is a multiplier switch.
The actual setting value of the knob is the knob position TIMES the
multiplier value (e.g. with the VOLTAGE knob set at 5 and the multiplier at .1,
the output amplitude equals .5 volts).
I. CONNECTING THE STIMULATOR TO THE OSCILLOSCOPE
You should have two coaxial
cables to be used to hook up the stimulator.
Each cable has a dual banana plug (the red one with the two "posts"
on it) and a BNC plug (the silver, cylindrical one).
First, we will hook up the
trigger signal to the oscilloscope.
Take one cable and connect the BNC end to the bottom right EXT
INPUT.
Now look at the banana plug
end. Notice that on the side of the
banana plug there is a little "tab" which says GND. The "post" on this side of the
banana plug is the ground post. On the
left side of the stimulator there are five posts, each one a different
color. Connect the banana plug's ground
post to the green post on the stimulator, and the other banana plug post to the
brown post on the stimulator.
(Note: all you have to do is
plug the post into the receptacles which are spaced just right for the dual
plugs to fit.)
Take your other coaxial
cable and connect the dual banana plug to the two terminals directly underneath
the sign that says "DANGER: HIGH VOLTAGE". The ground post of the banana plug should be in the black
(negative) post on the stimulator. Now
take the BNC plug and connect it to the Channel 2 input post on the
oscilloscope. Channel 1 would be just as good, but we want to show you some
special features that occur on channel 2.
Now everything is properly
hooked up. Make sure all of the knobs
and magnitude switches on the stimulator are fully COUNTERclockwise. Now that everything is turned all the way
down you can turn on the power to the stimulator. ALWAYS TURN EVERYTHING DOWN BEFORE YOU TURN ON THE STIMULATOR TO
AVOID SHOCK.
Switch the oscilloscope MODE
switch to channel 2 and position the trace.
In the trigger section of the scope,
set the mode switch to "NORM " and set both SOURCE switches to "EXT"
(external) .
Turn the frequency on the
stimulator to 10 pulses per second (PPS) and slowly turn the LEVEL knob in the
TRIGGER section of the scope until the trig'd ready light flashes on every time
there is a stimulus. The stimulator is
now sending a signal to the scope telling it when to begin a sweep. Now is the time to set the other scope
switches. Set the SEC/DIV to 20ms. The source is only coming in through channel
2, therefore channel 1 controls have no effect. Set the channel 2 VOLTS/DIV to 5 volts. Note: we are not using
the probes, so 1X should be used here.
Set MODE switch to NORMal and set the AC/GND/DC to DC.
On the stimulator, set the
following controls:
FREQUENCY to 10 pulses per
second (PPS) [Note: remember the magnification switches.]
DELAY to 20 microseconds
(all the way down). [Note that DELAY is in milliseconds]
DURATION TO 10 milliseconds.
VOLTS to 10 volts.
Set the stimulus switches to
REGULAR and to REPEAT. The POLARITY
switch should be set to NORMAL and under OUTPUT, set to MONO.
Now the oscilloscope screen
should look like this:
{see
drawing on board}
Notice that two pulses are
displayed on the screen. From what you
learned about the oscilloscope, you should know that each pulse is 10 volts and
is of 10 ms duration.
Now move the FREQUENCY
knob. By increasing the frequency you
will have more pulses displayed on the screen, because frequency represents
PPS.
The scope is receiving two
inputs from the stimulator. One is an
"instruction" (the trigger signal) that tells the scope when to
initiate a sweep. The other input is
the voltage pulse that is generated by the stimulator. Changing the DELAY on the stimulator adjusts
the time between the trigger signal and the voltage pulse.
The DURATION knob on the
stimulator adjusts the amount of time that the pulse is delivered, i.e. how
long the pulse is.
The VOLTAGE knob varies the
amplitude of the stimulus pulse.
Now fiddle with these knobs
and observe how the signal on the scope changes.