Oscilloscope and EKG sensor - Department of...

Oscilloscope and EKG sensor

Equipment: Capstone, BK Precision model 2530B oscilloscope, function generator, 2-3 foot coaxcable with male BNC connectors, BNC to female banana adapter, 1 voltage sensor, EKG sensor,EKG Sensor Electrode Patches

Remark: The oscilloscope is a difficult instrument to understand, but it is also a very importantinstrument. Oscilloscopes are ubiquitous in the medical profession because they are the basis ofa heart meter. They are seen next to hospital beds showing the electrical output of a heart, andare used extensively in operating rooms. In this lab, you will study how an oscilloscope works,and observe the various AC waveforms from a function generator. You will also be using an EKGsensor to monitor your heart rate on an oscillscope. Study the write-up carefully before the lab!This lab cannot be approached casually.

1 Introduction

The standard instrument for examining time dependent voltages in a circuit is the oscilloscope, or“scope” for short. Even if you are doing a DC experiment it is a good idea to look at the circuitwith a scope. You might find a large AC signal (symmetric with respect to ground) that does notshow up on DC instruments yet is affecting your experiment in undesirable ways.

An AC voltage varies on time and can be drawn on a graph with voltage in the vertical axis andtime on the horizontal axis. If the voltage is periodic (repeats itself after one cycle) an oscilloscopewill continually present a similar curve. By adjusting the scope it is possible to display part of onecycle or many cycles of the periodic voltage, which can be referred to as a waveform. Properties ofa periodic voltage, such as shape, peak-to-peak amplitude, and period, can be measured. Althoughscopes have other uses, these are the primary subjects of focus for this experiment.

The oscilloscope is a sophisticated instrument. Competent use depends on understanding howthis instrument works. It is often necessary to consult the instruction manual for a particular scopein order to use it effectively. The basic ideas are usually the same but are implemented in differentways.

In this lab you will become familiar with 2 oscilloscopes, the model 2530B made by BK Precision,and the Capstone scope. Both these instruments do very much the same thing except that thecontrols on the BK are mostly knobs and switches and those on the Capstone are icons and buttons.Sorry no touchscreen display! The Capstone scope is digital and the input waveform is digitizedand can be stored, at least for a little while.

In the last section of the lab you will be using an EKG sensor. An EKG sensor monitors yourheart rate by measuring the distribution of dipoles in the heart. The EKG sensor will be used inconjunction with the Capstone graph to see your heart cycle as a function time. You will determinethe time of your P, Q, R, S, T cardiac cycle. Each of these letters represents the wave function ofyour heart. The P wave is the initial pulse that goes through the atrial muscle. The Q, R, S is the

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General Physics II Lab: Oscilloscope and EKG sensor

electrical waveform in the ventricular muscle. The Q wave is down, R is up and the S is down. Thelast part is the T wave form, which represents the repolarization of the ventricles.

2 Description of How a Scope Displays a Single Periodic Voltage

An oscilloscope can create a graph display of an AC voltage. A periodic function in time is onethat repeats itself again and again, such as a sine wave or a square wave. A scope can display avoltage that is periodic in time in exactly the same way. When the periodic voltage is connectedto the vertical input of the scope, and the scope is adjusted correctly, a graph of the voltage as afunction of time will appear as a stationary signal on the screen of the scope as seen in Fig. 1

Figure 1:

Every scope has a sweep voltage. A sweep voltage is a signal that the scope sends out to searchfor the initial voltage being applied. Fig. 2 shows the horizontal sweep voltage as a function oftime. This linear ramp, when applied to the horizontal axis of the scope screen, sweeps the beamfrom the left side of the screen to the right side of the screen. When the horizontal ramp voltageis at VL, the beam is at the left side of the screen; when it’s at zero, the beam is in the center;and when it’s at VR, the beam is at the right side of the screen. We assume that |VL|=|VR|. TheTIME/DIV switch determines how quickly the scope signal is swept across the screen, ultimatelycontrolling the horizontal axis of the screen.

Figure 2:

Usual the mode of operation is the voltage on the horizontal axis held at VL. If a “trigger”pulse is applied to the circuit (source of the trigger pulse will be discussed in a moment), the beamis turned on and the time base oscillator applies a sweep voltage ramp to the horizontal axis. Theinput voltage is applied to the vertical axis and the signal is swept across the screen from left to

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right at a constant speed. A spot of light will be observed moving across the screen if the sweepspeed is slow enough, and will trace the input voltage for a time P. When the ramp reaches VR

the beam reaches the right side of the screen. The sweep voltage is then very quickly returned toVL. During this process the beam is shut off or “blanked” so that a return trace is not seen. Fora stationary trace to be properly observed the trigger pulses must all occur at similar point of theinput voltage waveform. Those similar points are taken to be the same voltage and the same slopeof the input voltage. The vertical part of the signal is proportional to the input voltage.

We’re only interested in one of these signals which will be the voltage waveform you’ll want toexamine.

We consider the case where the trigger pulse is derived from the input voltage, (which is thevoltage that is to be observed). The operator of the scope can control what part of the initial signalhe or she sees by adjusting the “trigger level”. Choose a value of the input voltage at which thetrigger is to occur (we call this voltage VT ). This value can be positive or negative, but must bebetween the maximum and minimum values of the input voltage in order for a trigger to occur andhave a stable signal. The scope operator can also choose whether the trigger pulse occurs whenthe slope of the input voltage (with respect to time) is positive or negative. An additional point isthat once a sweep starts it is always completed. This allows the use of a low enough sweep speedso that a number of cycles of the input waveform can be displayed on the screen.

Fig. 3 shows a sinusoidal input voltage and the horizontal sweep voltage as a function of time.The trigger voltage VT has been chosen as positive. The triggering slope has also been chosen aspositive. As you can see, whenever the input voltage has a positive slope and reaches the value VT ,a trigger pulse is produced. The time it takes for the beam to go from the left side of the screen tothe right is designated by “T.” It does this repetitively, always starting at the “same” point of theinput voltage and always tracing out the same curve. The result is a stationary trace of the inputvoltage on the scope screen.

Figure 3:

You should understand the following statements. If you do not, please reread and study theprevious material.

• For a given input signal, if the horizontal time/div sweep speed is increased, less of the inputwaveform or fewer cycles will be displayed.

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• If the horizontal time/div sweep speed is decreased, more of the input waveform or morecycles will be displayed.

• If the trigger voltage VT is changed, the trace of the input voltage can be shifted left or rightin a continuous fashion (exclude the square wave).

• If the trigger voltage VT is kept constant but the trigger slope is changed, the trace will beshifted left or right. For example, Fig. 3 is drawn for triggering to occur on a positive slopeof the input voltage. If VT is kept the same but triggering were set to occur on thenegative slope of the input signal, at what points of the input signal in Fig. 3might triggering occur? How about a negative VT and positive slope? A negativeVT and a negative slope?

• Fig. 3 shows plots of three ramp or sweep voltages. A given ramp voltage, no matter which ofthe three plots it is associated with, corresponds to the same horizontal position of the beamon the screen.

3 BK Controls for Single Trace Use

This section describes the controls (knobs, switches, and buttons) and connectors (jacks) of the BKscope for single trace use. Fig. 4 is a picture of the front panel of the BK scope, with these knobs,switches, buttons,connectors, etc, all numbered. Some controls are grouped in “boxes” accordingto function. These boxes are also numbered in Fig. 4. The screen on the left side of the scope iswhere the trace will appear. In the pages that follow, descriptions of each control, connector, andbox you will find on the BK oscilloscope are listed and numbered accordingly.

Figure 4:

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1. POWER Button: This turns the scope on and off. Leave the scope off when not in use.

2. RUNSTOP Button: This button will be lit when the scope is on.

3. VERTICAL Box: This box is divided into two halves, since the scope has two possible inputvoltages (Channel 1 and Channel 2). The left half of the box corresponds to channel 1, andthe right to channel 2. The contents of this box are listed below (numbers 4-8). Channel 2and the rest of the right side of the box will not be used in this lab.

4. CH1 and CH2 Buttons: Like the RUNSTOP Button, both these buttons will be lit when the scope

is on.

The following controls for the gain (5 and 6) determine the visual appearance of the signal onthe screen.

5. CH1 VOLTS/DIV Control: This is the vertical sensitivity control for channel 1. It determinesthe amount of amplification applied to the input signal before the input signal is connectedto the vertical deflection plates. The setting of this knob gives the vertical deflection of thetrace for a particular input signal. For example, if this control is set at 50 mV, a 100 mVinput signal will produce a 2 cm vertical deflection of the trace.

6. CH2 VOLTS/DIV Control: This is the vertical sensitivity control for channel 2.

7. CH1 POSITION Control: This adjusts the position of the channel 1 trace vertically.

8. CH2 POSITION Control: This adjusts the position of the channel 2 trace vertically.

9. CH1 and CH2 Input Jacks: These are two connectors for possible input voltages. The jacksare female Baby Type N connectors, or BNC connectors. The jack corresponding to channel1 is on the left and channel 2 on the right. Connect the signal you wish to examine in thislab to the channel 1 jack, the channel 2 jack is not needed.

10. HORIZONTAL Box: This box is time based. It controls the rate at which the horizontaldeflection voltage is changed, which also affects the appearance of the signal. The contentsof this box are listed below (numbers 11 and 12).

11. TIME/DIV Control: This adjusts the rate, in discreet steps, at which the electron beam isswept across the screen.

12. HORIZONTAL POSITION Control: This adjusts the horizontal position of the trace.

13. TRIGGER Box: This box determines the level at which the scope senses the signal. Therotating knob (18) The contents of this box are listed below (number 14).

14. TRIGGER LEVEL Control: This enables you to select and adjust the trigger level voltage,VT , and move the trace left and right.

15. EXT TRIG Input Jack: This is for a possible external trigger.

16. AUTO Button.

17. USB Port: This is used to record and transfer wave forms. Not used in this lab.

18. CALIBRATE Port: This provides a 2 V p-p 1 kH calibration square wave. Not used in thislab.

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As you can see, there are a lot of controls, most of which you will need to use in the followingexperiments, and unless you have a physical picture of what the controls do you will not be ableto use the scope in a reasonable sort of way. Review the previous section and Fig. 4 as necessary.

4 Using The BK Oscilloscope

In this section, you will be able to “see” how the voltage depends on time by using the BK scopeto examine various AC voltages. Measure the amplitude at various times, and measure the timesbetween different points on the voltage vs time curve. It will probably be helpful to refer to theprevious sections during these exercises.

4.1 Preliminaries

Remove any input cables from the scope, and turn on the scope (1). In a few seconds you shouldsee a trace that looks like a horizontal line. If you do not, check your initial control settings andpress the AUTO button. Leave the trace centered. Turn the TIME/DIV knob to 250 ms and notethe light spot move slowly across the screen until it gets to the right hand side of the screen. Whathappens then? Investigate the effect of turning the TIME/DIV knob. Increase the horizontalsweep speed one click at a time and note how the moving spot becomes a solid line. Observe thatthere is a range of sweep speeds for which the trace “flickers.”

When you need to make measurements of an input signal (peak voltage, period, etc.), it isuseful to use the two position controls (7 and 12) to move parts of the trace onto the gridlines ofthe screen that have finer divisions.

4.2 Observing Voltage From The Function Generator

Fig. 5 is an image of the face of the function generator you will use in this section. The buttonsand the output significant to this section are labeled.

Figure 5:

This function generator produces sine, square and triangular waveforms which are selected bythree pushbuttons. The frequency of these waveforms can be adjusted from 0.3 Hz to 3 MHz bymeans of the knob marked frequency and 7 pushbuttons. The output frequency appears in the

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display panel. At the bottom of the display panel to the right appear Hz, with a possible multiplierfor Hz, such as k or M, towards the middle. It is important to check for the k or M to determinewhether the stated frequency is in kHz or MHz. The multiplier appears and vanishes automaticallyas you cross certain threshold frequencies such as 1 kHz. The amplitude of the output is varied bya knob at the right marked AMPL. The knobs have “in” and “out” positions; the functions for theknobs are marked in black for the “in” position and blue for the “out” position. While using thisinstrument as a simple function generator, be sure all the knobs at the bottom of the front panelare in and that the only the green LED’s for the waveform chosen and for the frequency range areon (buttons at top).

Connect the 50 Ω output to the CH1 vertical input (9) of the BK scope using the coaxial cable(pictured below).

Set the AMPL knob full CCW and turn on both the function generator and scope. Adjust thefunction generator for about a 1 kHz sine wave and turn the AMPL knob to the center. On theOscilloscope, adjust the TIME/DIV and VOLTS/DIV knobs until you see a reasonable sinewavesignal.

Vary all of the controls listed below, leaving the parameters of the input voltage constant. Notethe effect on the trace of varying a control and understand why the trace changes in the way it does.In answering the following questions, make sketches of the trace if it will help your explanation.

How does the trace change if:

1. The VOLTS/DIV controls are changed?

2. The TIME/DIV controls are changed?

3. The trigger level voltage is changed? Note that if the trigger voltage is set too high ortoo low the trace does not “freeze” but “runs” to the right or the left if the trigger couplingswitch is on AUTO.

4. Next step is to click on the AUTO button. Then rotate the TRIG LEVEL knob in bothdirections. Can you make the trace disappear? Explain.

5. The triggering slope is changed from + to −? (To change the slope, push the trigmenu button and then push the button next to ”slope” on the screen.)

6. The position controls are changed?

The controls you have just used are the ones most frequently employed in adjusting the trace.

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4.3 Other Waveforms and Functions

Observe square and triangular waveforms from the function generator by pushing the appropriatebuttons. The frequency is not critical, but around 1 kHz works well.

4.4 Measuring Waveform Parameters

In this section you will be looking at AC waveforms again. A useful function of a scope is to measurethe peak to peak voltage and period of a voltage waveform. For both of these measurementsjudicious use of both the horizontal and vertical position controls will enable you to utilize the finermarkings on some of the grid lines on the scope screen. The amplitude of the output of thefunction generator is not known, so you are using the scope to measure this amplitude.

• Set the function generator to have a triangle wave output at 1 khz.

• Turn the function generator amplitude knob fully clockwise.

• Adjust the scope controls so that the trace takes up most of the screen vertically, and is aboutone cycle of the input waveform horizontally on the screen.

• The voltage division of horizontal grid lines is shown in the left bottom corner of the screen.Use the calibration of the VOLTS/DIV control to determine the peak to peak voltage of thetriangular wave.

• Compare the function generator stated frequency with your period measurement.

• Use the calibration of the TIME/DIV control and the vertical grid lines on the screen todetermine the period of the wave.

• The display of the current time scale will be located in the middle bottom of the screen.Repeat for the sine and the square waveforms at 1 khz.

On the function generator turn the the amplitude knob to the middle and determinethe peak and peak-to-peak voltages of all three waveforms at 1 khz.

5 Using The Capstone Oscilloscope

One of the displays available with Capstone is the oscilloscope. This is not actually an oscilloscopebut software that closely mimics an oscilloscope.

In comparison to the Capstone oscilloscope, the BK scope is a much faster scope. The highestfrequency you can examine with the BK scope is 25 MHz, while with the Capstone scope it’s10 MHz. The other main difference is that in the Capstone scope the controls are icon buttons,and an arrow which slides up and down along the y axis of the scope to adjust the trigger level. OnCapstone the trigger needs to be activated by clicking on the button that has a red arrow pointingup with a dash above, located on the top left of the scope screen. A small triangle in a verticalstrip at the very left of the scope screen will appear when you click on the button. The defaulttrigger voltage is 0 with positive slope.

There are only 2 trigger options in the Capstone scope. The default is triggeringthe signal on the rising level, and the other option is the falling level.

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5.1 Setting up and using the Capstone Scope with the Voltage Sensor

In this section you are examining AC waveforms from the Wavetek function generator with theCapstone scope.

• Click on the Capstone icon in the desktop screen to open the software.

• When you’re using the Capstone scope use the voltage sensor to measure voltages in a circuit.

• To set up the voltage sensor, plug the analog voltage senor into the interface, click on thehardware setup button (located below the tools column on the left side of the software), anda picture of the interface will come up.

• Click on the analog channel of the interface that the voltage sensor is plugged into. A dropdown window will appear, select voltage sensor.

• Click on the copper colored tack located on the right corner of the hardware setup windowto resize the screen.

• Next, go to the displays column located on the right side of screen and click on the scope iconand drag it to the center of the white screen.

• Go to the bottom of the Capstone screen. Click on the ”Continuous Mode” button and select”Fast Monitor Mode”. Below is an image of what it should look.

Plug the voltage sensor into the bnc banana adapter. Below is an image of the BNC adapter.

Connect the BNC adapter to the output of the function generator.

1. The input for each trace is selected on the screen of the Capstone Scope. You click on selectmeasurement of the y axis on the screen and scroll down to the input. To add an additionalmeasurement you need to click on the add new measurement of the y axis button. The add

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a measurement button is the one with blue arrow pointing up with a green asterisk on theside.

2. This scope is a storage scope. When you stop monitoring data the last trace is stored forfurther use.

3. There is a data coordinates cursor which is used in a similar way as in the graph display. Thedata coordinate cursor can tell you the exact location on the waveform. It is located on thetop center of the Capstone scope.

A nice feature of the Capstone scope is the Data Coordinates Cursor, located on the top leftcorner of the scope window. It is used to determine the exact voltage or time of a signal, whichdetermines the (X,Y) values of the graph. In this case its (Time, Voltage) and the units are(Seconds, Volts). With a good stable trace investigate this capability.

Below is an illustration explaining the basic functions of the Capstone scope.

5.2 Measuring a waveform using Capstone scope

You will repeat what you have done in section 4.4 of measuring only one waveform using the Casptone scope!Your instructor will select the waveform and frequency that you will measure. Againyou will,

• Set the function generator to have a wave form and output frequency your instructorpicks!

• Turn the function generator amplitude knob fully clockwise. You will use the Capstone scopeto determine the peak voltage, the peak-to-peak voltage, and the period of the waveform. You may have trouble getting a trace on the Capstone scope. This may be due to thefact that the default trigger level is 0. Try adjusting the trigger level for a slightly positivevoltage.

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6 EKG sensor with Capstone software

BEFORE YOU START THIS SECTION OF THE EXPERIMENT RESTART THE CAPSTONESOFTWARE!

This time around you will be using the EKG sensor in conjunction with the Capstone software.Plug the EKG sensor into one of the analog channels. Click on the analog channel that the EKGsensor is plugged into and select the EKG sensor. Scroll down to the bottom of the Capstone screenand you will see a ”Record” button. Next to the record button you will see a ”Continuous Mode”button. Click on the ”Continuous Mode” button and select ”Fast Monitor Mode”. Below is animage of what it should look.

• Click and drag scope icon from the displays window to the center of the white screen.

• Resize the screen size by clicking on the copper tack located in the upper right corner of thehardware setup window.

• Next, click on select measurements on the scope screen and select Amplitude (mV). Now youwill have to adjust the y- axis of the scale of the scope to have a range of +2500 mv to -2500mv range. Then adjust x- axis scale of the scope to have time range of about 4 seconds.

Look at the following figure to understand the controls of the Capstone scope and adjustment ofthe scales.

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You will take a resting EKG reading of either you or lab partner. For this part of the experimentonly one person of the group will be the guinea pig! Look at Fig. 6 to connect the electrode patchesand EKG sensor to your lab partner. Keep in mind before you connect the patches you need towipe all three points of contact with an antiseptic towel. This will help remove dirt and dead skin.Many of you have not showered in a while, so its wise for you to clean each spot thoroughly.

Figure 6:

At this point you should be using the data coordinates cursor to make all your measurementsfor the following activities.

1. Measure the EKG of a person who is at rest. Person whose EKG is being measured shouldrelax, remain still, and breathe normally. Someone else should operate the computer whiledata is collected.

2. Next measure the EKG of the same person while she or he is standing up.

3. For both parts measure the intervals between P-Q, QRS, and Q-T. An example of an EKGsignal is shown in Fig. 7. The following intervals are known as the P wave, the QRS complex,and the T wave, respectively. Explain the significance in relation to the heart. What is thepeak-to-peak value of the voltage between the R wave and the S wave? What isthe definition of this occurrence?

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Figure 7:

6.1 Additional questions and exercise

The sequence from P wave to T wave represents a heart cycle. The number of such cycles in aminute is called the heart rate and is typically 70-80 cycles (beats) per minute at rest. What wasthe frequency of your heart during the experiment? Note: Do not be alarmed if your EKGdoes not correspond to the numbers. These numbers represent typical averages and many healthyhearts have data that fall outside of these parameters. The sensor is NOT intended for medicaldiagnoses. You’re going to measure your EKG cycle before you exercise and after!

• First measure the EKG at rest.

• Print the graph.

• Then disconnect the sensor wires from the electrode patches, but leave the patches on theperson whose EKG is being measured. Have the person exercise for two minutes. Jogging inplace or doing jumping jacks should be sufficient.

• Reattach the sensor wires to the electrodes on the person within thirty seconds after theexercise is done, and measure the EKG. Compare and contrast the EKG after mild exerciseto the resting EKG. Use the coordinate cursor of the Capstone oscilloscope to measure thedifferences. What are the differences and the significances of said differences?

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• Now, reverse the placement of the electrodes (place the green alligator clip on the left electrodeand the red alligator clip on the right one). Measure the EKG again. What happensto the signal? Note and explain briefly the difference. Print the graph.

Create a table as shown below for you when you are at rest, after standing, and after exercisingfor two minutes. Keep in mind you must be still when taking the EKG.

7 Finishing Up

Please leave the bench as you found it. Thank you.

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Oscilloscope and EKG sensor - Department of...

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