Lab 1 - Introductory Experiments and Linear Circuits I

 University of California at Berkeley

Donald A. Glaser Physics 111A

Instrumentation Laboratory

Lab 1

Introductory Experiments and Linear Circuits I

© 2016 by the Regents of the University of California. All rights reserved.

 

General guidelines:

• Always read the entire Lab Writeup before coming to lab.  Lab time is precious.  Don't waste it reading the background material.
• Likewise, do the pre-lab questions before you come to lab.  You are required to do the prelab questions before beginning benchwork.  Information required to answer the prelab questions can be found in the background material at the beginning of the lab, from lecture, in the stated references, and on the web.
• As best you can, formulate a plan to perform the Lab exercises.
• Please ask questions of the GSIs or Professor if you don't understand something (or you think you know something that we don't) at any time during the course!
• Please clean your work station at the end of class or when you leave.   Resort all the prestripped wires, return your components to the proper drawers, place your bnc jacks, minigrabbers, 50 ohm terminators, Tees, etc, back in their box, and neaten all your BNC cables.  
  Note that you can check out and permanently keep one (per person)  portable breadboard, VB-106 or VB-108,  from the 111-Lab . Store these breadboards in the shelves along the west wall.   As you build more complicated circuits, you will find it useful to keep your circuits assembled on these portable boards until you next class.   You must disassemble circuits on lab station breadboards.
• Wikipedia has many useful and informative articles.
• Reprints and other information can be found on the Physics 111 Library Site.

Important safety habits:

• When you are done for the day, make sure you power down all equipment.
• Never place food or drink next to any apparatus. Accidental spills can damage or destroy the equipment and your experiment and give you a shock.  Food can only be consumed on the designated, marked tables.

This lab is unusual in that it has two parts.  In your prelab prep, concentrate on the Part 1 before your first lab day, and Part 2 when you have complete Part 1.  You do not need to have completed the Part 2 prelab questions before beginning Part 1.

Part 1 of this lab introduces you to the equipment you will be using throughout the 111a lab and most of the labs in Physics 111b, particularly the digital multimeter (DMM), breadboard, power supplies, oscilloscope, and the signal generator. The breadboard, power supplies and some other components are integrated into a box found at each lab station.   

Everyone needs to have a working knowledge of this equipment before continuing with the rest of the 111a course.  Note that the manuals for the lab equipment are posted online.   The XYZ’s of scopes contains a general introduction to oscilloscopes.

In Part 2 of this lab you will study circuits made from linear components such as resistors, and capacitors. You will build filters and learn the concept of frequency dependent impedance and its importance in circuit analysis and electrical measurements. You will also learn why we use scope probes and terminators for scope measurements. 

"Student Manual for Art of  Electronics", Hayes & Horowitz	Chapter 1, p 1–31 and p 32–60
"The Art of Electronics", Horowitz & Hill, 3rd edition	Chapter 1.1–1.4, 1.7 and Appendixes A, B, C, D, skim H, and O. (You'll read the rest of Ch. 1 for the next two weeks, so you might want to get started now.)
Look at other Books available	Readings

All parts spec sheets are on the Physics 111 Library site.

Note that you can generally find most of the information that you need for the labs in the lab writeups themselves.  The references listed above are for background and more in depth explanations.

Legend for the lab writeup symbols

 This problem should be done before coming to the 111 lab.

This problem must be checked and signed off by the GSIs or the Professor.  You may continue on to your next question while waiting for a signoff, but keep your setup for the signoff question.

Pre-lab Questions:

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Pre-lab questions (Part 1):

1.      Write a short paragraph each explaining how the breadboard, power supply, multimeter, oscilloscope, and the signal/pulse generator are used.

2.      What is the difference between the common and the ground of a circuit?

3.      Derive the voltage divider equation ( $\frac{V_{out}}{V_{in}}=\frac{R_2}{R_2+R_1}$) for the following circuit:

                                                              

Pre-lab questions (Part 2):

1.      What is impedance? Input impedance? Output impedance?

2.      What are the interesting properties of a coaxial cable and transmission lines? (hint: Wikipedia.org)

3.      What is a low-pass circuit? A high pass circuit? Draw an example of each. Derive the transfer function for each of these circuits.

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Now let’s play with the equipment. video link introduction below;

Keithley 2110 Digital Multimeter (DMM)

Spec Sheet PDF

 

The DMM is used to measure voltages, currents, resistances and several other more complicated quantities. The DMM is a relatively simple instrument.

Turn on the digital multimeter by pressing the power button located to the left on the front panel. To make a measurement with the DMM, first connect a double banana plug to one end of a BNC cable and a pair of mini-grabbers to the other end.   The outer shield of the BNC is traditionally at ground; the inner wire carries the signals.  When the BNC cable is attached to a double banana plug, one of the plugs will be attached to the outer shield, i.e. the ground, and the other to the signal.  Figure out which of the banana plugs is labeled ground.  Look for the little black tab on the banana plug pair; this tab will the ground. 

 

 

Double Banana Plug	minigrabber	BNC Cable

 

 

Voltage

Find the pair of red and black inputs on the DMM that indicate where to insert the banana plugs to make a voltage measurement; remember that red is positive and black is ground and hook up the banana plug accordingly. You have two different voltage measuring options: DCV and ACV. DCV measures voltage created by a DC current and ACV for an AC current.  You can change the DMM’s range to maximize your measurement’s resolution by pressing the up and down arrows labeled “Range +” and “Range – “.

Resistance

Be sure to disconnect the electronics component you wish to measure from the circuit before making the resistance measurement (think about why it is better to do so). You also have two options for measuring resistance. One is the standard 2 wire measurement and the other is a 4 wire measurement. The 4 wire measurement method is used for measuring very small resistances when even the resistance in the copper wire leads is important. Because this lab won’t be dealing with such low resistances, the majority of the measurements you make will be done with the standard 2 wire method. To make the measurement, hook up the banana plug to the correct red/black inputs for measuring resistance, indicated by the \(\Omega\) symbol, and select the 2 wire resistance option indicated by the \(\Omega^2\) symbol.

Capacitance

Same setup as the voltage and resistance measurements. The capacitance measurement option is indicated by the capacitor symbol.

Current

Current measurements also have two options: DCI and ACI. DC current measurements using this DMM are tricky because the appropriate jacks, the black jack on the right and the white jack on the left, are too far apart to have a single banana plug inserted into them. To compensate for this problem, we will be using two banana plugs and only use the center conducting wire of two different BNC cables to perform the measurement. Consult the picture below for how this is done.
 

 

Here we are measuring the current flowing across a resistor. Notice how the banana plugs are inserted in and how we are only using the red grabbers, or the center conducting wires, to make the measurement. Conversely, we could flip the orientation of both banana plugs and only use the grounding (black) wires to make the measurements instead. The important thing to note is that any DC current measurement requires the left white jack and the right black jack to work; you can insert the banana plugs in any orientation you want but you must remember to use the correct BNC grounding or conducting wires.

ACI measurements are easy because we only need one banana plug. The jacks for an ACI measurement are the black and white jacks on the right. Be sure to hook up the banana plug with the correct polarity. 

Alternate method to measure current:

DMM Hookup connections for current measement

Note black and red leads are in series with the resistor to measure current.

     Connect the DMM leads in parallel with a component to measure voltages across it, and in series with a component to measure currents through it.  When measuring the resistance of a component, the element must be isolated from the rest of the circuit. 

 

Fluke 179 Handheld DMM 

Video link introduction above;

The Fluke meter is similar to the Keithley, but less accurate.  It is good for quickly measuring voltages and resistances.  On the  setting, the DMM will beep when there is a low resistance path between the two leads; this "continuity check" setting is used to trace wires paths.

 

 

 

 

 

​Breadboard Box

(B) BSC Laboratory Breadboard Box

Breadboard

Commercial electronic equipment is constructed on printed circuit boards; “wires” are photo-etched onto a sheet of copper, and components are soldered into place. To save time and effort, we will build our prototype circuits on solderless breadboards. A breadboard is an insulating board with a regular pattern of holes that can be used as sockets. The sockets are interconnected with hidden wires, and electronic component leads or wires pushed into the socket holes will make contact with the interconnecting wires below.  The interconnecting wires on our breadboards follow the pattern shown by the heavy lines in the drawing below; the gray squares indicate the positions of the sockets. 

Breadboard Practice:

Use 22-gauge solid (not stranded) wire to make connections. [Wire gauges (thicknesses) are listed at https://en.wikipedia.org/wiki/American_wire_gauge.)]   Use the precut wires, or, for long runs, cut interconnecting wires to the right length. Strip ~3/8” of insulation from each end, and poke the bare wires into the breadboard socket holes until they bottom out. The breadboards are delicate! Forcing wires or component leads into the board can damage the sockets or make a poor connection. Wires or leads that do not fit easily into the breadboards may be too thick. 

·        Use the buses (the long horizontal strips shown in Figure 1) for power and ground connections. Good bus habits will save you lots of time and trouble with complicated circuits by making your circuit wiring more transparent and by removing unnecessary clutter.

·        Build your circuits compactly. Long leads between components introduce stray capacitance and can result in oscillations or high frequency [e.g., radio frequency (RF)] pickup.

·        For clarity, signals should flow from left to right; place input signals on the left side of the board, circuitry in the middle, and output signals on the right side.

·        Use color-coding to make your wiring clear: by convention, red wires are used for power connections, black wires are used for ground connections, and other color wires used for signals. Following this convention, espcially for the long wires that you cut yourself, will help enormously when working with complex circuits later in this course.

 

 

Problem 1.1.1 - Breadboard Layout

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Use the Digital Multimeter (DMM) in resistance-measuring mode to check some of the internal connections in the breadboard. Make sure you understand how the breadboard is set-up: what is connected and what is not. Sketch a simple diagram showing what connects where on the breadboard.   Your sketch should suffice to explain the breadboard to someone using it for the first time.

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Commons and Grounds

Recall that voltage is a measure of the potential difference between two points. Although we say “the voltage at point A”, we really mean “the voltage at point A with respect to the local zero-potential reference point.”

The most useful zero-potential reference point is the earth itself—the “ground.”  A ground in any circuit is defined to be any wire, lead or bus somehow connected to the earth. The power company thoughtfully provides a wire connected to the earth in all three-pronged power outlets, and that ground wire is often connected to a ground lead in electrical equipment. Thus, the potential of a point in a grounded circuit is the same as the potential difference between that point and the earth. Why do most electrical wall sockets have three leads? The electric company intends current to flow between the hot and neutral wires in the wall socket, the two rectangular slots. No current should flow in the ground wire. The electrical company arranges its transformers so that the hot lead is approximately 120V from the neutral, and the neutral is approximately at ground. But things are rarely perfect, and the neutral lead is often a few volts from ground. As for the ground lead itself (the horseshoe shaped hole), the electric company grounds the lead by actually attaching it to a long conducting rod stuck into the earth. Look for the ground wire the next time you walk by a transformer on a pole! Other good grounds are available. Cold water pipes, for example, are well connected to the earth, and are often used as grounds. The electric company doesn’t supply the ground as a courtesy for electronic circuits builders; they supply it for shock prevention. Electrical shocks occur when a sufficiently high voltage drives a sufficiently high current through the victim’s body. Most dangerous are shocks in which currents travels through the victim’s heart; only 50mA can be lethal. Grounding the outer case of a piece of equipment greatly reduces the chance of shocks by shielding the user from any high internal voltages. It is difficult (but not impossible!) to get a serious shock with voltages less than about 50V.  While you should always think before touching a bare wire, shocks should not be a problem with any of the circuits in the BSC lab. Caution:  Standard electronic circuit construction always uses black for ground, red for power and other colors for signal leads.  BUT North American building wiring always uses white for neutral, BLACK FOR HOT (the dangerous lead) and green for ground.

North American Power Outlet http://hyperphysics.phy-astr.gsu.edu/

While grounding a circuit is usually beneficial, it is not actually necessary. Cell phones, for instance, are not grounded. For these circuits we define a “common” for the voltage—a local point that all measurements on the circuit refer to. 

 Power supply

Each station has two power supplies built into the box carrying the breadboard. The first supply (bottom two terminals) has an output voltage of 5 V and is used mainly with digital circuits. The common of the 5V supply is marked 0V, and is connected to the metal chassis of the breadboard box, which is in turn connected to ground. Thus the common of the 5V supply is also a ground.

The other supply, the top three terminals, supplies 12V between adjacent terminals. This power supply “floats”, i.e. it is not connected to ground.  A floating supply maintains its rated voltage difference between its terminals, but its absolute potential can float up or down. Usually ground is attached to the 0V terminal, but it may be useful to attach it to either of the other two terminals. Watch out: leaving the power supply floating, or inappropriately grounding the supply, can lead to subtle circuit failures. If you use both the 5V and the ±12V power supplies in a circuit, make sure that the ±12V supply is referenced to the 5V supply.  

Note that the voltages labeled ±12V are actually closer to ±13.1V, but throughout this class we will refer to them by their nominal value, namely as the ±12V supplies. 

 

 

 

 

 

 

 

 

 

 

 

 

The following Breadboard Box components will be occasionally useful throughout this semester.  For now, read the descriptions and locate these components on the Breadboard Box, but there are no exercises that require them in this lab.

Offset Adder	Potentiometers Potentiometer Internal Construction After http://www.eeweb.com/electronics-quiz/mechanical-potentiometers	Buttons and Switches
The Offset Adder adds a constant voltage to the input signal. The constant voltage can be adjusted by turning the Offset Adjust knob, and can be either positive or negative.	There are two 25k and one 1M potentiometers on the breadboard. Potentiometers are resistors whose resitance can be varied. Potentiometers have three terminals; turning the knob changes the resistance between the red terminal, called the wiper, and the green terminals.  The resistance between the red terminal and one of the green terminals will vary from 0 to the designated resistance, while the resistance between the red terminal and the other green terminal varies inversely, i.e. between the designated resitance and 0.   The resistance between the two green terminals  is constant at the designated resistance.	There are two normally open buttons, two normally closed buttons, and one SPDT switch on the Breadboard Box. Normally open means the circuit is open until pressing the button closes (shorts) it. Normally closed means exactly the opposite; pressing the button opens it. The SPDT (single-poll double-throw) connects the green terminal to the left white terminal when the switch is to the left, and to the right white terminal when the switch is to the right.

Logic Switches	Printed Circuit Boards (PCB)
The eight logic switches are all referenced to a single logic common. Throwing the switch puts it in either a HIGH or LOW state. These can be connected to a circuit to easily switch the voltage from high to low or vice versa.	There are two PCBs on the breadboard.  These boards contain digit displays that are no longer commonly used in this class.   Each PCB has two 9368 chips and two FND357 chips along with various other components. The displays are wired as specified in the silkscreen text at the PCB bottom.

 Problem 1.1.4 - Voltage Divider: General Calculation

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Calculate the voltage at point A with respect to 0V in the general case shown here. You will use this “voltage divider” formula in every lab in the course!

Estimate the error (uncertainty) using the example below, which comes from the Kiethley DMM manual:

Accuracy (error) =  0.012% of value + 0.004% of range (i.e. 1mV, 10mV, 10 volts)
As an example of how to calculate the actual reading limits, assume that you are measuring 5V on the 10V range. 
     Accuracy = 0.012% of value + 0.004% of range
     0.012% * 5 V + 0.004% * 10 V
     0.0006 V + 0.0004 V
     0.0010 V
     Thus, the actual reading range is 5 V ± 1 mV or from 4.999 V to 5.001 V.
DC current, AC voltage, AC current, and resistance errors calculations are performed in a similar manner using the pertinent specifications, ranges, and input signal values.

Does the voltage at point A agree with the value calculated in Section 1.1.9?  Comment on the  closeness of your calculated and measured values. Do they agree to within the measurement uncertainty?

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Problem 1.1.11 - Voltage Divider: Current Measurement

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Measure the current through the resistors. Is your result consistent with the calculated values? (Specify your measurement's uncertainty.)

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Problem 1.1.12 - Voltage Divider: Power Calculations

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Using the nominal values for simplicity:            a) Calculate how much power each resistor dissipates.  Are your resistors rated for this power? (Hint: these are 1/4 watt resistors.)            b) Keeping the same 24V supply and keeping the ratio of the resistors the same, would you increase or decrease the resistor values to approach the maximum power rating of the resistors?            c) If you were to change the resistors as suggested in b), which resistor would reach its max power rating first?            d) Calculate how much larger or smaller (5 times smaller? 100 larger?) you would have to make the resistor values until one resistor first exceeded its maximum power rating.

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Tektronix MSO 2024 Digital Oscilloscope

Video link introduction above; Spec sheet PDF

The oscilloscope is the most important instrument in any electronics lab, as well as in many physics labs. To do well in this lab and in 111b you must be fluent with its operation.

 

 

 

 

 

 

 

 

 

Trace image overlay will only happen if the time it takes the trace to move across the screen is an integer multiple of the period of the waveform being viewed.   This is a rare.   Normally, this doesn't happen, and one gets traces that look like those shown at right.  Notice how the phase of the signal jumps around. With normal trace draw speeds, this signal would be completely jumbled as numerous traces would be overlaid with random start phases.  To stop this from happening, scopes are "triggered".   Before each trace starts from the left, the scope waits until the signal reaches a specified amplitude.  Only when the signal reaches this amplitude does the trace begin.  This will align the scope traces at the same phase, and we will see the desired quasi-static image. Scope triggering is tricky and frustrating...it is probably the most difficult thing about using a scope.  With practice, you will get good at it.

Improperly Triggered Scope

A scope is a very complicated instruments. The 111a scopes have over sixty controls…and these are relatively simple scopes. Fortunately most of these controls are rarely or never adjusted while taking routine measurements; only a four controls determine the scope’s basic operation. These primary controls are indicated on the panel drawn below.

 The four important controls, which will be discussed in more detail later, are:

1. The Vertical Scaling Knob adjusts the vertical size of the image on the scope screen; a typical setting would be 1V/div.
2. The Vertical Position Knobs move the scope traces up and down.
3. The Horizontal Scaling Knob adjusts how fast the scope draws the image across the screen; a typical setting would be 1ms/div.
4. The Trigger Level Knob adjusts the triggering voltage. 

The signal itself is connected to one of the input BNC jacks on the scope.  Our scopes have four channels, and can display up to four signals simultaneously.

Turning on the scope

The power button is located on the front in the lower left hand corner. The scope will take some time to start up it has a computer processor; do not press any buttons during this time.

Viewing an electronic signal

Using a BNC cable, connect the signal to one of the channels on the scope. The channels are located in the front on the bottom right of the scope and are labeled “1, 2, 3, and 4”. 

Do not plug the signal into the “Aux In”; the use for this input will be discussed later. After you hook up the signal to the scope, you should see a trace of it appear on the screen. If you do not, play around with the scope’s viewing controls. 

Vertical and Horizontal Scaling and Position Controls

The vertical scale for each of the scope’s channels is set with the knobs labeled “Scale” in the Vertical section of the scope controls and has units of volts/division. The horizontal scale for all of the channels is set similarly in the Horizontal section of the scope controls and has units of seconds/division. For example, a 100Hz 10 volt peak to peak triangle wave will take up two divisions on the 5V/division scale and one period will take up 10 divisions on the 1 ms/division scale. The current value of the vertical and horizontal scales is displayed at the bottom of the scope’s screen.

You can also adjust the position of the signal on the screen with the position controls. These are the smaller sized knobs located in the Vertical and Horizontal sections of the scope controls. The arrows on the left side of the screen indicate where ground is for each channel.

XY mode can be accessed by pressing the “Acquire” button in the Horizontal section of the scope controls and then turning on the “XY Display” option.  XY mode will display two signals at the same time, the amplitude of one of the signals will make up the x-axis and the amplitude of the other will make up the y-axis. Consequently, if you put an equal amplitude sine wave as the y-axis and cosine wave as the x-axis, in XY mode the resulting trace would be a circle.

The sec/div (time-base) knob is the primary horizontal control, and controls the rate at which the  scope trace sweeps (from left to right) across the screen. A scope trace of a 1ms period sine wave, displayed on the 0.5ms/div scale, would show five complete cycles of the wave.

Try this exercise: turn on the power switch, set the trigger to AUTO, to CH1, and both channel’s input switches (AC GND DC) to ground (GND). Set the horizontal time-base to 0.1ms/div. Move the vertical "position" knob slowly for channel 1 until you obtain a line at the center of the screen. Adjust the "focus" and "intensity if needed." Now slow the time-base to 0.1s/div.  See how the trace sweeps across the screen?

Look at the scope's "TRIGGER" section. The "trigger circuit" determines when the scope starts its horizontal display sweep, but it needs an input. The source for this input is set by the position of the TRIGGER SOURCE switch, and the operating mode by the “A TRIGGER” switch. Look up what each switch does.

               One other control deserves special mention: the Autoset button. If you cannot find the scope trace, just push this button and the trace will be pulled onto the screen and set to the correct voltage setting.

 

Individual Channel Configuration

You can turn individual channels on/off for viewing by pressing the corresponding channel buttons which are color coded yellow, blue, magenta and green. The menu for each channel is also accessed through these same buttons. In the menu you can configure an individual channel’s coupling, bandwidth, label, etc. To exit out of the menu, press the “Menu Off” button. (Note: A common mistake is to measure the amplitude of a low frequency signal with the channel on AC coupling. AC coupling is mainly used to measure small AC signals on top of large DC signals.  Always use DC coupling unless there is a specific reason to use AC coupling.)

Triggering

Triggering on the scope will be one of the more difficult controls to understand. Recall that the scope displays a trace of the signal by constantly redrawing the signal on top of itself. The scope does this by selecting a certain amplitude for the signal to reach and once the signal reaches that amplitude the scope begins drawing the signal trace from left to right on the screen. If the signal is periodic, then the scope produces the same trace every time it draws. If the scope had selected random points along the signal to begin drawing, the resulting image would be a smear of all the differently timed traces.  As a result, the scope needs to be “synched” to produce a readable signal which can be done with the trigger controls located to the far right of the scope controls. 

The triggering circuit needs to know at what amplitude the signal needs to reach to begin drawing. This can be set with the “level” knob; an arrow on the right of the screen indicates the trigger level. Additionally, it needs to know which signal it is using to trigger, a.k.a. the “source”, and it must know whether to trigger when the signal has an increasing or decreasing slope. Other features include the triggering coupling, the mode, type, etc. which can all be configured in the trigger menu.

Furthermore, the scope does not need to rely on the input signal itself to tell it when to trigger. It can accept an auxiliary signal to tell it when to trigger instead. This auxiliary signal is fed into the “Aux In” channel and the source must be set to “Aux” in the trigger menu. With these settings the scope will begin drawing the trace whenever the auxiliary source triggers the scope to do so.

Making Measurements

The digital oscilloscope is capable of making measurements of the input signals such as measuring the frequency. You can configure the scope to perform a measurement by pressing the “Measure” button at the top of the scope controls. From there, select “Add Measurement” and then use the multipurpose A knob (located to right of the screen) to indicate the measurement type and the source.  Afterwards, select “OK Add Measurement” and the scope will continuously perform the measurement and display the result at the bottom of the screen. 

Other mathematical operations on the signals can be accessed by pressing the red “Math” button. Selecting the “Dual Wfm Math” option lets you add, subtract, or multiply two signals. The “FFT” option performs a Fourier transform. The data used to perform the transform is less than one screen width of the signal and the range can be viewed by turning on the “Gating Indicators” in the “FFT” menu; use the horizontal position control to move the boundaries over the region of the signal you want the transform to be performed. The horizontal scaling (in units of Hz/division) of the transform is indicated in the “FFT” menu to the right of the screen and can be adjusted using the multipurpose B knob.

Helpful tips for when you can’t get a good signal trace

-         Sometimes the signal you see may be ridiculously larger than expected due to the “Probe Setup” being configured to “10X” which multiplies the signal seen on the scope trace by a factor of 10.  Access the menu for that channel and take a look at the “Probe Setup” option to see if it is set to “10X” and change it accordingly using the Multipurpose A knob.

-         If you have accidentally changed one of the settings on the scope which messed up the signal trace and do not know how to reset it, you can always return the scope to its default settings by pressing the “Default Setup” button located below the scope screen.

-         If you have been playing around with the viewing controls for a while and cannot get a clear trace to appear on the screen, then press the “Autoset” button located above the Trigger controls. This will have the scope configure itself to what it thinks are the best settings for viewing the signal. (Note: The “Autoset” settings may not always be the best settings to use. Use Autoset to first get a readable signal on the screen and from there configure the viewing controls yourself to improve the trace further.)

-         More information on the scope’s controls can be found on the scope's online manual. and from "The XYZ's of Using a Scope," also posted on the course website.

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Problem 1.1.13 - Scope Practice

Connect a BNC cable from the T1 output (a 1 kHz sine wave) on the Signal Distribution Box (Shown at right) to channel 1 on the scope.  Connect a second BNC cable from the T2 output (a 2kHz triangle wave) to channel 2.  Adjust the scope so that the image on the scope is similar to the image below (the signal amplitudes may be different.).     Practice this several times; take turns with your partner - one scrambling the controls of the scope - and the other recovering the signal. When you are ready, a GSI will scramble the controls and sign you off when you recover the signal. You do not need to write anything down.