Bio 331 - Lab 1

Biological Sciences 300/301, Smith College | Neurophysiology

Lab 1: Using the Oscilloscope

http://www.science.smith.edu/departments/NeuroSci/courses/bio330/labs/L1cro.html

View the video: Using the Oscilloscope.

In our first laboratory, we will review the features of our "hybrid" analog/digital storage oscilloscopes. When you come to lab, you and a lab partner should sit near one of the equipment racks. There should be only one team at each rack.

An oscilloscope is an instrument for displaying electrical signals (the vertical scale) against time (horizontal scale). It does this by sweeping a bright spot across the screen and modulating the position of the spot. Typically, we will use oscilloscopes for viewing electrical potentials from nerves, but today we will look at simple demonstration signals instead.

The overall layout and major controls of our Hitachi VC6020 oscilloscopes are shown here:

layout of Hitachi oscilloscope

Each major group of controls is described below.

"Housekeeping" controls: intensity, focus, illumination

These controls adjust the brightness and focus of the spot, and the illumination of the square grid. You should adjust the controls as needed to create a sharp and bright spot.
(click small figures to see larger images)

Time/Division control

This large knob determines how fast the spot sweeps horizontally across the screen. The white line on the knob points to numbers that tell how much time each box on the screen is worth. In practice, you adjust the knob so the signal you are observing can be seen well, and then you read the knob to see what the time per box is. Notice that the units change, from seconds (S) to milliseconds (mS) to microseconds (uS). We will generally work in the millisecond range.

Two smaller knobs nearby adjust the horizontal position of the sweep and reduce the sweep's speed from its calibrated values. We will always want it calibrated (a red warning light comes on if the sweep is uncalibrated), and we usually will line up the left edge of the sweep with the left edge of the grid.

Channel 1 Volts/Division

The large gray knob controls the vertical size on the screen of the signal on channel 1. In practice, you turn the knob until the signal is big enough to see easily, and then you read the dial to see how many volts each vertical box on the screen is worth. Note that there are two scale ranges, volts and millivolts (mV).

In the center of the big knob is a smaller knob that decreases the apparent size of the signal. A red light comes on to warn us that the vertical scale is now uncalibrated. We will always keep the smaller knob clicked in its calibrated position. Another small knob nearby controls the vertical position of channel 1's trace.

The input signal comes into the oscillscope through a BNC connector to which you will need to attach a cable. To the right of the connector is a toggle switch labelled AC GND DC. Using it in DC position will give you the most complete view of the signal. This is the only setting that shows steady or slowly changing voltages, such as resting potentials. For rapidly changing signals, the AC setting may be more convenient, as it filters out drift artifacts in the baseline voltage. We will typically use the AC setting when observing action potentials. The middle setting, GND, disconnects the internal circuits from the input connector and connects the circuits instead to ground. This allows you to place the trace at a position on the screen that represents 0 volts, after which you can switch back to AC or DC to examine the input signal.

It may seem obvious, but it's worth mentioning that changing the vertical sensitivity does not change the incoming signal. It only changes how big the signal appears on the screen.

Channel 2 Volts/Division

The controls for channel 2's vertical display are identical to those for channel 1, but arranged in a mirror layout.

Mode (Ch1/Ch2/Chop)

This knob controls whether the screen shows one or two traces.

CH1 displays only the trace for vertical channel 1.

CH2 displays only the trace for vertical channel 2.

ALT alternates the two traces, one per sweep. It is not a useful mode for our experiments.

CHOP displays both traces simultaneously. Use chop mode for dual traces.

ADD combines channel 1 and channel 2 into a single added trace. This is not generally useful for us.

.

Trigger Controls

The trigger controls are located in the upper right corner. They determine when the spot should start crossing the screen. The trigger circuits inspect the signal, and when it satisfies the criteria you have set with the trigger controls, the spot brightens and begins its sweep across the screen. At the end of its sweep, the spot darkens, jumps back to the left edge of the screen, and waits for the next trigger event.

Source. The source switch determines which signal the trigger circuits inspect. The choices are:

Internal (the internal signal on channel 1 or channel 2, displayed on the screen). We typically use internal triggering to view spontaneous action potentials.

Line (the 60-cycle power line). This can be useful for diagnosing the presence of electrical noise that we need to eliminate.

External (a signal connected to the nearby BNC jack). We use this to trigger the oscilloscope from the synchronizing pulse provided by our electrical stimulators.

An additional switch that really belongs with the trigger source controls is labelled "INT TRIG" and is located far away, at the bottom between the inputs for channel 1 and channel 2. It determines whether the internal trigger signal comes from channel 1 or channel 2. Make sure this switch is set where you want it to be; it is easy to overlook it.

Mode. There are two settings useful to us:

Auto triggers the sweep based on the criteria we have set with the other trigger controls, but with one exception. If the signal does not meet our criteria after a very brief interval, the sweep is triggered anyway. Auto is useful when we start out or when we don't care about triggering, because it always puts a trace on the screen.

Normal is the strict version: the screen stays dark unless the signal meets the criteria we have set. Use normal to see individual action potentials that occur only sporadically.

The other two settings, TV-V and TV-H, are not useful for our work.

Coupling. A blue pushbutton determines whether the input to the trigger circuits is AC- or DC-coupled.

Level. The major trigger criterion that we set is the voltage level that the signal must meet before the sweep is allowed to begin. In the pictures on the left, the a sinusoidal voltage has been connected to channel 1, and the trigger circuits are set to inspect an internal signal on channel 1. We can see approximately two cycles of the sine wave on the screen.

In the upper picture, the trigger level has been set very low, and the sweep waits until the signal passes through that level before it begins drawing a trace. You can detect the selected level as the starting point at the left edge of the trace. Since the sinusoidal signal is repetitive, the trace always starts at the same moment in the sinewave, and every trace draws over the previous traces. The signal appears frozen on the screen.

In the lower picture, the level has been set higher. The trace still appears frozen on the screen, but each sweep now starts at a relatively high voltage level. Turning the level knob shifts the criterion voltage required for a sweep to begin.

Slope. A signal can pass through a particular voltage level in two directions: upwards (positive slope) or downwards (negative slope). We select which slope we want by pushing in or pulling out the Level control. In selects positive slope, Out selects negative slope.

In the two screen shots, the sinusoidal voltage signal is being viewed with the trigger level set in the middle of the signal's voltage range. In the top picture, positive slope has been selected. The trace waits until the signal voltage passes through the selected level in an upwards direction. In the bottom picture, negative slope has been selected. The trace waits until the signal passes through the same level, but now in a downward direction.

.

Digital Storage

The push-buttons at the top middle control digital aspects of the oscilloscope. The oscilloscope can capture a sweep in its memory as a series of numbers representing the voltage at successive moments during the sweep. When the sweep is over, it is played back repeatedly for ease of viewing.

NON-STORE is like a conventional oscilloscope. Each triggered sweep is displayed only once, in real time. When you turn on the oscilloscope, it starts up in non-store mode.

NORM is a digital storage mode. After all the points for a triggered sweep have been captured, the sweep is displayed repeatedly until it is replaced by a new digitized sweep.

HOLD retains the present digitized sweep, preventing its replacement by a new one.

SINGLE, used in conjunction with HOLD mode, acquires one digitized sweep on the next appropriate trigger event. It then switches automatically to HOLD mode to preserve the captured sweep.

ROLL, selected by pressing NORM and HOLD simultaneously, simulates a slowly-moving chart recorder. It is suitable only for slowly changing signals, not for trains of action potentials, due to the low number of samples taken per second. HOLD will freeze the screen in ROLL mode.

PLOT outputs the digitized traces slowly, through connectors on the rear panel, for writing on a chart recorder. This is the way we will make hardcopies of data from the screen.

PRE-TRIG is a knob that determines how many horizontal divisions on the screen will be displayed prior to the trigger event. This allows you to see such things as the rising phase of a spike when you are triggering high up on the spike's waveform. Pre-trig has no effect in non-store mode.

.

Viewing the calibration signal

To experiment with the controls on the oscilloscope, you need a signal to display. As a convenience, the oscilloscope itself provides a square-wave calibration signal. Select a BNC/double-banana cable (top picture -- one end has a red double-banana plug, the other end a male BNC connector). Plug the BNC end into the input connector for channel 1 (push on and twist to lock). Examine the cable's other end, the red double-banana plug. One side has a tab on it; this is the grounded side. Ordinarily you would connect this to a ground connector on an amplifier or other electronic instrument. However, because the calibration signal is part of the oscilloscope's internal circuitry, the ground connection has already been established internally. There is no need to make another connection, and the grounded side of the double-banana plug will just hang in the air.

The other, non-tab, side of the double-banana plug carries the voltage signal to be displayed. Slip a clip-lead (bottom picture) over this side, and attach the clip lead's other end to the calibration connector. (This is a small tab under the "housekeeping" controls at the right of the screen.) To see the signal, make sure the triggering mode is set to Auto, adjust the trigger level to stabilize the square-wave's display, and adjust channel 1's vertical sensitivity until the square wave almost fills the screen vertically. Set the time/division control so each sweep shows one or two cycles of the square wave signal. Measure the amplitude of the square wave (volts) and the duration of one cycle (seconds or milliseconds). Can you determine whether the square wave is positive-going from a zero baseline, negative-going from zero, or straddles zero? (Hint: channel 1's vertical controls must be set on DC to answer this question, and you must use the vertical position control to set zero at a known location on the screen.)

As the weeks go by, you will become more intuitively familiar with these oscilloscopes. Refer back to this lab as necessary to review the features and controls.

Links

Appendix: Cables and Connectors.

Back to laboratory table of contents page

Back to Bio 300/301 home page

© 2003 by Richard F. Olivo. Permission is granted to non-profit educational institutions to reproduce or adapt this Web page for internal use provided that the original source and copyright are acknowledged.

	Biological Sciences 300/301, Smith College \| Neurophysiology Lab 1: Using the Oscilloscope http://www.science.smith.edu/departments/NeuroSci/courses/bio330/labs/L1cro.html
	View the video: Using the Oscilloscope.
	In our first laboratory, we will review the features of our "hybrid" analog/digital storage oscilloscopes. When you come to lab, you and a lab partner should sit near one of the equipment racks. There should be only one team at each rack. An oscilloscope is an instrument for displaying electrical signals (the vertical scale) against time (horizontal scale). It does this by sweeping a bright spot across the screen and modulating the position of the spot. Typically, we will use oscilloscopes for viewing electrical potentials from nerves, but today we will look at simple demonstration signals instead. The overall layout and major controls of our Hitachi VC6020 oscilloscopes are shown here:

	Each major group of controls is described below. "Housekeeping" controls: intensity, focus, illumination These controls adjust the brightness and focus of the spot, and the illumination of the square grid. You should adjust the controls as needed to create a sharp and bright spot. (click small figures to see larger images)
	Time/Division control This large knob determines how fast the spot sweeps horizontally across the screen. The white line on the knob points to numbers that tell how much time each box on the screen is worth. In practice, you adjust the knob so the signal you are observing can be seen well, and then you read the knob to see what the time per box is. Notice that the units change, from seconds (S) to milliseconds (mS) to microseconds (uS). We will generally work in the millisecond range. Two smaller knobs nearby adjust the horizontal position of the sweep and reduce the sweep's speed from its calibrated values. We will always want it calibrated (a red warning light comes on if the sweep is uncalibrated), and we usually will line up the left edge of the sweep with the left edge of the grid.
	Channel 1 Volts/Division The large gray knob controls the vertical size on the screen of the signal on channel 1. In practice, you turn the knob until the signal is big enough to see easily, and then you read the dial to see how many volts each vertical box on the screen is worth. Note that there are two scale ranges, volts and millivolts (mV). In the center of the big knob is a smaller knob that decreases the apparent size of the signal. A red light comes on to warn us that the vertical scale is now uncalibrated. We will always keep the smaller knob clicked in its calibrated position. Another small knob nearby controls the vertical position of channel 1's trace. The input signal comes into the oscillscope through a BNC connector to which you will need to attach a cable. To the right of the connector is a toggle switch labelled AC GND DC. Using it in DC position will give you the most complete view of the signal. This is the only setting that shows steady or slowly changing voltages, such as resting potentials. For rapidly changing signals, the AC setting may be more convenient, as it filters out drift artifacts in the baseline voltage. We will typically use the AC setting when observing action potentials. The middle setting, GND, disconnects the internal circuits from the input connector and connects the circuits instead to ground. This allows you to place the trace at a position on the screen that represents 0 volts, after which you can switch back to AC or DC to examine the input signal. It may seem obvious, but it's worth mentioning that changing the vertical sensitivity does not change the incoming signal. It only changes how big the signal appears on the screen.
	Channel 2 Volts/Division The controls for channel 2's vertical display are identical to those for channel 1, but arranged in a mirror layout.
	Mode (Ch1/Ch2/Chop) This knob controls whether the screen shows one or two traces. CH1 displays only the trace for vertical channel 1. CH2 displays only the trace for vertical channel 2. ALT alternates the two traces, one per sweep. It is not a useful mode for our experiments. CHOP displays both traces simultaneously. Use chop mode for dual traces. ADD combines channel 1 and channel 2 into a single added trace. This is not generally useful for us.
.	Trigger Controls The trigger controls are located in the upper right corner. They determine when the spot should start crossing the screen. The trigger circuits inspect the signal, and when it satisfies the criteria you have set with the trigger controls, the spot brightens and begins its sweep across the screen. At the end of its sweep, the spot darkens, jumps back to the left edge of the screen, and waits for the next trigger event. Source. The source switch determines which signal the trigger circuits inspect. The choices are: Internal (the internal signal on channel 1 or channel 2, displayed on the screen). We typically use internal triggering to view spontaneous action potentials. Line (the 60-cycle power line). This can be useful for diagnosing the presence of electrical noise that we need to eliminate. External (a signal connected to the nearby BNC jack). We use this to trigger the oscilloscope from the synchronizing pulse provided by our electrical stimulators. An additional switch that really belongs with the trigger source controls is labelled "INT TRIG" and is located far away, at the bottom between the inputs for channel 1 and channel 2. It determines whether the internal trigger signal comes from channel 1 or channel 2. Make sure this switch is set where you want it to be; it is easy to overlook it. Mode. There are two settings useful to us: Auto triggers the sweep based on the criteria we have set with the other trigger controls, but with one exception. If the signal does not meet our criteria after a very brief interval, the sweep is triggered anyway. Auto is useful when we start out or when we don't care about triggering, because it always puts a trace on the screen. Normal is the strict version: the screen stays dark unless the signal meets the criteria we have set. Use normal to see individual action potentials that occur only sporadically. The other two settings, TV-V and TV-H, are not useful for our work. Coupling. A blue pushbutton determines whether the input to the trigger circuits is AC- or DC-coupled.
	Level. The major trigger criterion that we set is the voltage level that the signal must meet before the sweep is allowed to begin. In the pictures on the left, the a sinusoidal voltage has been connected to channel 1, and the trigger circuits are set to inspect an internal signal on channel 1. We can see approximately two cycles of the sine wave on the screen. In the upper picture, the trigger level has been set very low, and the sweep waits until the signal passes through that level before it begins drawing a trace. You can detect the selected level as the starting point at the left edge of the trace. Since the sinusoidal signal is repetitive, the trace always starts at the same moment in the sinewave, and every trace draws over the previous traces. The signal appears frozen on the screen. In the lower picture, the level has been set higher. The trace still appears frozen on the screen, but each sweep now starts at a relatively high voltage level. Turning the level knob shifts the criterion voltage required for a sweep to begin.
	Slope. A signal can pass through a particular voltage level in two directions: upwards (positive slope) or downwards (negative slope). We select which slope we want by pushing in or pulling out the Level control. In selects positive slope, Out selects negative slope. In the two screen shots, the sinusoidal voltage signal is being viewed with the trigger level set in the middle of the signal's voltage range. In the top picture, positive slope has been selected. The trace waits until the signal voltage passes through the selected level in an upwards direction. In the bottom picture, negative slope has been selected. The trace waits until the signal passes through the same level, but now in a downward direction.
.	Digital Storage The push-buttons at the top middle control digital aspects of the oscilloscope. The oscilloscope can capture a sweep in its memory as a series of numbers representing the voltage at successive moments during the sweep. When the sweep is over, it is played back repeatedly for ease of viewing. NON-STORE is like a conventional oscilloscope. Each triggered sweep is displayed only once, in real time. When you turn on the oscilloscope, it starts up in non-store mode. NORM is a digital storage mode. After all the points for a triggered sweep have been captured, the sweep is displayed repeatedly until it is replaced by a new digitized sweep. HOLD retains the present digitized sweep, preventing its replacement by a new one. SINGLE, used in conjunction with HOLD mode, acquires one digitized sweep on the next appropriate trigger event. It then switches automatically to HOLD mode to preserve the captured sweep. ROLL, selected by pressing NORM and HOLD simultaneously, simulates a slowly-moving chart recorder. It is suitable only for slowly changing signals, not for trains of action potentials, due to the low number of samples taken per second. HOLD will freeze the screen in ROLL mode. PLOT outputs the digitized traces slowly, through connectors on the rear panel, for writing on a chart recorder. This is the way we will make hardcopies of data from the screen. PRE-TRIG is a knob that determines how many horizontal divisions on the screen will be displayed prior to the trigger event. This allows you to see such things as the rising phase of a spike when you are triggering high up on the spike's waveform. Pre-trig has no effect in non-store mode.
.	Viewing the calibration signal To experiment with the controls on the oscilloscope, you need a signal to display. As a convenience, the oscilloscope itself provides a square-wave calibration signal. Select a BNC/double-banana cable (top picture -- one end has a red double-banana plug, the other end a male BNC connector). Plug the BNC end into the input connector for channel 1 (push on and twist to lock). Examine the cable's other end, the red double-banana plug. One side has a tab on it; this is the grounded side. Ordinarily you would connect this to a ground connector on an amplifier or other electronic instrument. However, because the calibration signal is part of the oscilloscope's internal circuitry, the ground connection has already been established internally. There is no need to make another connection, and the grounded side of the double-banana plug will just hang in the air.
	The other, non-tab, side of the double-banana plug carries the voltage signal to be displayed. Slip a clip-lead (bottom picture) over this side, and attach the clip lead's other end to the calibration connector. (This is a small tab under the "housekeeping" controls at the right of the screen.) To see the signal, make sure the triggering mode is set to Auto, adjust the trigger level to stabilize the square-wave's display, and adjust channel 1's vertical sensitivity until the square wave almost fills the screen vertically. Set the time/division control so each sweep shows one or two cycles of the square wave signal. Measure the amplitude of the square wave (volts) and the duration of one cycle (seconds or milliseconds). Can you determine whether the square wave is positive-going from a zero baseline, negative-going from zero, or straddles zero? (Hint: channel 1's vertical controls must be set on DC to answer this question, and you must use the vertical position control to set zero at a known location on the screen.)
	As the weeks go by, you will become more intuitively familiar with these oscilloscopes. Refer back to this lab as necessary to review the features and controls.

Links	Appendix: Cables and Connectors. Back to laboratory table of contents page Back to Bio 300/301 home page
	© 2003 by Richard F. Olivo. Permission is granted to non-profit educational institutions to reproduce or adapt this Web page for internal use provided that the original source and copyright are acknowledged.