In early attempts to record bioelectric events, the recorded potential changes were displayed by instruments which converted the amplified potentials to mechanical movements of indicators as pens. This still remains a convenient method for recording slow bioelectric phenomena (as done with the polygraph, for example). However, since even a string galvanometer has inertia and a relatively low resonance frequency, such electro-mechanical recording systems are not adequate for following electrical events having relatively high frequency components.

The development of the cathode- ray tube has provided us with a virtually inertia-free indicator of potential charges, since the "pointer" consists of a thin, weightless beam of electrons. At the base of the cathode-ray tube, a heated cathode serves as a source of "boiled off" electrons. The electrons are attracted and accelerated by an anode (+3000 volts) occupying the periphery inside the front portion of the tube. Impinging upon the phosphor coating of the inside face of the tube, the electron beam causes a bright spot of fluorescence.

The beam, and consequently the spot, are made to move across the face of the tube by means of two sets of charged electrostatic plates near the cathode. The vertical plates control the horizontal (x-axis) movements of the beam, while the horizontal plates control the vertical (y-axis) deflections of the beam. The plates are made to do this by changes imposed on their charges by the sweep circuit and the vertical amplifier.

SAWTOOTH WAVE-FORM

The sweep circuit causes the beam to move steadily across the screen from left to right (facing the CRO) during the downward slope of each sawtooth. At the end of the ramp the voltage jumps back to the peak level, snapping the beam back to the left side of the CRT screen. Thus the sweep circuit provides the time base on the x- axis. The slope of the ramp and thus the rate of sweeping is determined by the position of the front panel knob SWEEP SECOND/DIV. When the SOURCE control is in the AUTO TRIG position, the "sawteeth" are recurrent, and so the slope of the ramp determines the sweep frequency as well as the sweep rate.

Under many recording circumstances it is desirable to have the frequency of the sweep independent of its rate. In that case the AUTO TRIG position is not used, and each sawtooth must be triggered in some manner. There are several ways of triggering the sawtooth:

1. Recurrent triggering as in AUTO TRIG as mentioned above.

2. LINE synchronized sweeping, in which each cycle of the "house" current triggers one sawtooth. This is useful if the signal being displayed is a function of the 60 cycle line voltage.

3. LEFT or RIGHT triggering in which the signal itself triggers the initiation of the sweep. LEFT refers to a signal on the left amplifier and RIGHT to a signal on the right amplifier. The main drawback of this method is that the signal is underway at the left side of the screen before the sweep begins. The vertical deflection must be at least 0.5 cm before it will trigger a sweep.

4. EXTernal triggering in which a synchronizing signal having a fixed time relationship to the signal is fed into the 'scope and serves to trigger the sweep before the signal begins.

It is generally desirable that a repeating signal be displayed in a constant position on the screen during experiments.

The vertical amplifier amplifies the input signal and feeds it to the horizontal plates. Therefore, the vertical position of the beam is a function of the input signal. The ratio between input voltage level and the vertical deflection of the beam is controlled by setting the gain of the amplifier with the front panel knob, SECONDS/DIV. The base line position is set with the POSITION knob.

PRACTICE WITH OSCILLOSCOPE & STIMULATOR

These directions are for a Tektronix 5111A Storage Oscilloscope. Others are similar.

Screen Controls - Upper right panel

Brightness - Controls brightness of the storage image

Erase - Erases a stored image if upr & lwr are pushed in.

Store - upr Causes traces to remain on the upper half of screen

- lwr Same for the lower half of screen

Intensity - Outer ring controls the intensity of the beam

Enhance - Inner ring enhances storage of trace. Turn to Off (fully CCW).

Focus - Focusses beam. Get the sharpest image possible.

Beam finder - Push. If beam is off-scale high, then beam appears above center. If off-scale low, beam appears below center screen.

Power - Pull to turn the machine on.

Time Base Amplifier - Lower right panel. The time base amplifier sets beam sweep speed and beam trigger functions.

Display - CHOP(pushed in) = all beams move across screen together. ALT(pushed out) = each beam takes alternate turns crossing the screen.

Position - Moves beam sweep horizontally. Set this so that beam starts its sweep at the first reticule mark.

Sweep Speed Selector - Selects the rate of travel of the beam across the screen from 0.1u sec/div to 5 sec/div. A division is also a cm.

Cal - To select speeds in between those marked. Always keep it calibrated by turning fully clockwise.

SWP MAG - Increases beam sweep speed by 10X, but doesn't change trigger. Leave out or off.

Triggering

Level - Sets sensitivity of trigger which starts the beam across the screen. Middle position is most sensitive. Left means a more negative trigger is needed to start the beam. Right means a more positive trigger is needed to start the beam.

Source

Left - An event fed into the left amplifier will start the beam if more than 0.5 div. deflection.

Right - An event fed into the right amplifier

Line - Beams starts in synchrony with 60 cycle current.

Ext - Beam starts when a signal comes into the Ext Input connection. (from stimulator)

Auto trig - Beam starts immediately upon finishing a sweep. (Continuous sweep)

AC coupl (In)- triggering ignores DC drift or offset. + slope (in)- Trigger starts on up slope of a voltage change (Out)- Beam starts on a downward voltage change.

Singl SWP - Beam will cross the screen once. It still needs to be triggered by one of the other sources.

Reset - Sets beam ready to sweep a single trace again.

Voltage Amplifier - Lower left panel (Dual Trace or Differential)

Display - Turns amplifier on.

Position - Moves beam vertically to any desired position.

Bandwidth Limit - Cuts out frequencies above 10 kHz when in. These frequencies have the most interference.

Volts/Div - Selects sensitivity. At 2, a 2 volt change will move the beam one centimeter (div) on the screen. Be sure that cal is fully clockwise.

AC (In) will ignore DC drift, DC offset, or any slow voltage change. Helps keep beam on screen.

DC (out) will measure all voltage changes even slow ones or steady voltages.

Gnd (in) This input will be at ground voltage to chassis = 0 volts.

On the differential amp, leave all of the other buttons out. AC & Gnd are repeated for the (-) input. Grounding one input means that the amp is no longer differential, but compares voltage in the other input to chassis (ground) voltage.

On the Dual Beam amp:

CH 1 (In) Turns on beam for this amp.

CH 2 (In) Turns on a second beam for this amp and controls are duplicated for its control.

OTHER EXERCISES

Turn on the machine with the controls in the following positions:

Time Base Amp

Seconds/div = 5m or 5 milliseconds

Auto Trig = On (In)

CHOP = On (In)

Left Differential Amp

Display = On

Ch 1 = On

Sensitivity = 50 mV/div

Adjust:

Intensity to get a clear, but not an overly bright line.

Focus to get the narrowest line.

All other buttons should be out and all cal knobs fully clockwise. Position the beam horizontally (Time base amplifier) to start the sweep at the left most line of the reticule. Position the beam vertically with the position knob of the left amplifier to line up with the center of the reticule.

Exercise 1. 60 cycle voltage, sweep speed, and DC drift.

Connect a wire to the ground(black) and active(red) input jack of the left amplifier. Set sensitivity at 50 mV/div and sweep speed at 5 msec/div.

With your finger, grab the free end of the ground wire. Let go and grab the free end of the active wire. Explain the traces you see. Remember that the Oscilloscope measures the voltage difference that occurs between the two inputs.

Repeat this paragraph with Gnd pushed in on the black (-) input.

Remove the ground and again, grab the active wire, touch your other hand to the table, grab both wires in the same hand, touch the two wires together. Explain the traces.

Again, grab the active wire and then push in line. Push in + slope. Slowly move level from one extreme to the other. Push auto trig out and move level from one extreme to the other. Explain everything that happened.

Grab the red wire in one hand and black in the other, set sensitivity to 5 mV/div, and squeeze the leads.Do you get DC drift? Push in AC and repeat. Explain. Grab active wire and push in GND. Explain. Turn off line, AC, + slope, and return sensitivity to 50 mV/div.

Play with the controls mentioned above until you know their functions.

Turn on Auto trig and change the sweep speed to 0.5 sec/div. NEVER ALLOW THE BEAM TO STAND STATIONARY (as on 50 mV & .5 V) or it can burn a hole in the phosphors of the screen. Move the vertical position knob while it is sweeping. Voila an Etch and Sketch.

Exercise 2. Using the Stimulator and Oscilloscope

Attach the other end of the (-) input wire you attached to the oscilloscope to the black(- ) output of the Grass Stimulator and the active lead to the red (+) post. Take another lead with banana plugs and connect the chassis ground of the oscilloscope to the GND post of the stimulator. Set the sweep speed to 0.5 sec/div and the voltage sensitivity to 5 Volts/div.

Set the stimulator at the following settings:

Frequency = 4x1 pps (pulses per second)

Delay = 10x1 ms (milliseconds)

Duration = 10x10 ms

Volts = 4x1 volts

Polarity = normal

Stimulus = regular

Output = Mono

Turn on the stimulator and push mode switch to single a few times. What happens on the stimulator? on the oscilloscope? Be sure that the beam is on a reticule line. Stimulate and increase volts on the stimulator until the beam goes up 1 full centimeter. How many volts does the oscilloscope say that it is receiving? How many does the stimulator say it is giving? Which is the most accurate?

Switch the mode to repeat. How many pulses are there /sec? What horizontal distance is one second on the reticule? Speed up the sweep to 20m. How long does one pulse last? Can you adjust stimulus frequency so that the image stays in one place or at least almost? What is this frequency according to the stimulator? according to the oscilloscope? Can you adjust the duration on the stimulator so that it lasts exactly 60 msec? Push out auto trig and push in source left. Adjust level until you get a trace. Push in + slope. Is it easier to adjust the duration to 60 msec? Explain to your partner all that happened above. Slowly move the volts on the stimulator up to 10 and back down to one. What happens? There is a threshold deflection of about 0.5 div necessary to trigger the beam on left. Reset the oscilloscope to left off, + slope off, auto trig on, and Volts/div 5.

Let's play with the delay on the stimulator. Set seconds/div on 20m and stimulator frequency at 2 pps, and delay at 50 msec. Set stimulus at twin pulses. What do you see? Again push in left source and + slope and push out auto trig. One at a time, change every setting on the stimulator and then return it where it was before trying the next setting. Change the sweep speed on the oscilloscope as required and repeat the above. Explain the tracings.

When everything is back to normal or as set in the previous paragraph, change polarity to reverse. Readjust level and + slope until you get a reverse of the above results. What does output Bi do? Think capacitor. Return to Mono and push in AC on the differential amp. Think capacitor again. Return to DC.

External trigger.

Attach a wire from ext input on the oscilloscope to out pulse on the stimulator. Set the stimulator as it was in the beginning of the exercise except mode on repeat, frequency on 0.1 X 10 PPS, duration on 1 X 10 msec, and delay on 10 X 10 msec and the oscilloscope on 50 msec/div, 5 volts/div and source on ext. Adjust level until a sweep appears. Now play a game. Set the frequency, delay, duration, and volts at anything you like and then use the oscilloscope to measure each of them accurately. Change oscilloscope settings if you need to for a good measurement. What happens with delay on regular versus twin pulses?

Storage

Set stimulator at minimum frequency, 10x1 delay, 10x0.1 duration and 4 volts. Set the oscilloscope at ext source, and 5 volts/div. Push in erase upr & lwr and store upr and lwr. Push erase. Push erase. Push erase. Play with brightness and intensity. Return it to a reasonable value and push erase. Slowly move the vertical position. Push erase. Now play your measurement game using store. Isn't life getting easier? NOTE: Time on store reduces the life of the tube. So do not leave it on store unless you are using it.

Exercises - Things to measure for practice

What is the voltage difference between your two hands that is induced by our house current? What is the voltage difference between your hand and ground that is induced by our house current? Without getting rough on the switch, how quickly can you stimulate a second time after tripping the single mode switch? What is the time constant of the capacitor in the stimulator output (Switch output to Bi)? What is the time constant of the capacitor in the oscilloscope input (AC)? You should be able to accomplish this last puzzle both empirically and by calculation. Measure empirically the number of cycles/sec in house current.

Measure the voltage of a battery.

EQUIPMENT & SUPPLIES

For each set-up:

1 Oscilloscope w/ storage 1 BNC to double banana adapter

1 BNC to single banana adapter 1 Dual banana plug - both ends

2 Single banana pug both ends 1 Grass stimulator

1 Battery