How to Create a Bode Plot

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A Bode plot is a graph that describes how a circuit responds to different frequencies. This tells us, for example, that an amplifier has poor bass (low-frequency) response.
Engineers use these plots to better understand their own designs, to choose components
for a new design, or to determine whether a circuit can become unstable when the
wrong frequencies are applied.



1. Extended Definition


As previously mentioned, a Bode plot is a graph that describes how a circuit responds to
different frequencies. A Bode plot specifically shows a circuit’s gain with respect to frequency.
It actually consists of two graphs: a magnitude response, and a phase response. To illustrate
this, a sample Bode plot is shown below:

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Part Six: Creating the worksheet to record your data


1. On your lab computer, open Excel and start a new spreadsheet.  Label the columns
“Frequency,” “Vin,” “dV,” “Vout,” “Delay,” “Phase” and “Gain.”


2. Below “Frequency,” enter each frequency you plan to test (refer to your lab procedures).


3. Below “Vout” in cell D2, enter this formula:  =B2+C2


4. Press return. The “=B2+C2” turns into a zero since we haven’t entered anything into B2 or C2.


5. Press Ctrl+D, then return.  The formula gets copied from D2 into D3, with Excel changing its
formula into “=B3+C3” automatically.  Keep pressing Ctrl+D then return until you fill the column for
each of your frequencies.

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Voltage gain in decibels is defined as:
G_dB=20*〖log〗_10 (Vout/Vin).
Positive gain means amplification, and negative gain indicates attenuation. So if a circuit had a Vout of 1 volts and a Vin of √2 volts (a voltage drop), its gain would be:
G_dB=20〖*log〗_10 (1/√2)=-3 dB.
This -3 dB mark is important, as it indicates where the circuit’s output power (not
voltage!) is exactly half of its input power.
The phase plot describes how different frequencies take relatively shorter or longer
time to travel through the circuit. Any frequency with a phase reading of -180? or –π
radians will be unstable at that frequency.
To create a Bode plot from an existing circuit, test the circuit with a range of frequencies.
This range depends on the application at hand, such as audio or data transmission.
Stimulate the circuit’s input with a simple sine wave at the frequencies of interest.
Measure the input and the output with an oscilloscope, and compare the difference
between the two. Record these differences in a spreadsheet, then graph them to see
the final Bode plot. The data can be tabulated and plotted by hand instead, if desired.

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Connecting the equipment

1. Check that the function generator and oscilloscope are connected to the closest AC outlet.


2. Connect the first probe to the “50 Ω OUTPUT” connector on the front lower-right corner of the function generator.


a. Connect the red positive lead to the input terminal of your circuit.
b. Connect the black negative lead to the ground terminal of your circuit


3. Connect the second probe to the “CH 1” connector on the front of the oscilloscope


a. Connect the red positive lead to the input terminal of your circuit
b. Connect the black negative lead to the ground terminal of your circuit.


4. Connect the third probe to the “CH 2” connector on the front of the oscilloscope


a. Connect the red positive lead to the output terminal of your circuit
b. Connect the black negative lead to the ground terminal of your circuit, (unless instructed otherwise by the lab TA).

5. Make sure that the cables are not hanging over the edge of the workspace.

6. Double-check the connections. They should be as shown in Figure 1.

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Turn on the equipment


1. Press the power button (labeled “O/I”) on the top side of the oscilloscope.
2. Press the “POWER” button on the front upper-right corner of the function generator.


Set the function generator frequency and amplitude


1.Press the button below “FREQ.” on the function generator.  The light above the button turns on.
2. Adjust the frequency to the lowest frequency you wish to test, your starting frequency.  
This can be done with the large dial on the function generator, or with the four soft-keys below the display.  
The buttons marked “- val +” change the digit under the cursor, and the “< cur >” buttons move the cursor.
3. Press the button below “AMPL.” on the function generator.  The light above the button turns on.
4. Adjust the amplitude to the voltage specified in the lab procedure for the circuit being tested,
using the same dial or soft-keys.  Notice that this is Vpp, the peak-to-peak voltage. The maximum (positive)
and minimum (negative) voltages of the wave will be half the peak-to-peak voltage.
5. Press the button “OUTPUT” on the function generator. The light to the left of the button turns on.

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Turn on the equipment


1. Press the power button (labeled “O/I”) on the top side of the oscilloscope.
2. Press the “POWER” button on the front upper-right corner of the function generator.


Set the function generator frequency and amplitude


1.Press the button below “FREQ.” on the function generator.  The light above the button turns on.
2. Adjust the frequency to the lowest frequency you wish to test, your starting frequency.  
This can be done with the large dial on the function generator, or with the four soft-keys below the display.  
The buttons marked “- val +” change the digit under the cursor, and the “< cur >” buttons move the cursor.
3. Press the button below “AMPL.” on the function generator.  The light above the button turns on.
4. Adjust the amplitude to the voltage specified in the lab procedure for the circuit being tested,
using the same dial or soft-keys.  Notice that this is Vpp, the peak-to-peak voltage. The maximum (positive)
and minimum (negative) voltages of the wave will be half the peak-to-peak voltage.
5. Press the button “OUTPUT” on the function generator. The light to the left of the button turns on.

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Set the oscilloscope window


1. Press the “DEFAULT SETUP” button on the upper-right corner of the oscilloscope.  
The wave may appear on the display, or it may only show noise. The next steps will bring it
into focus.


2. Press the “AUTOSET” button on the upper-right corner of the oscilloscope. The waves
should appear in focus.


3. Press the second soft-key from the top. This tells the oscilloscope to display a single period
of the wave.

• The oscilloscope soft-keys are located to the right of the display.
  

4. Press the “MEASURE” button on the top middle of the oscilloscope.  The default measurement
screen will be shown.


5. Press the top soft-key on the oscilloscope to select the first measurement.  Press the top soft-key
labeled “Source” until “CH1” is listed.  Press the second soft-key from the top labeled “Type” until “Freq”
is shown.  Press the bottom soft-key to go back.

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6.
Press the second soft-key from the top to select the second measurement.  
Press the top soft-key labeled “Source” until “CH1” is listed.  Press the second
soft-key from the top labeled “Type” until “Pk-Pk” is shown.  Press the bottom
soft-key to go back.


7.
Press the third soft-key from the top to select the third measurement.  Press
the top soft-key labeled “Source” until “CH2” is listed.  Press the second soft-key
from the top labeled “Type” until “Freq” is shown. Press the last soft-key (fifth from
the top) to go back.


8.
Press the fourth soft-key from the top to select the fourth measurement. Press the top soft-key labeled “Source” until “CH2” is listed. Press the second
soft-key from the top labeled “Type” until “Pk-Pk” is shown. Press the bottom
soft-key to go back.

9.
Turn the “HORIZONTAL SEC/DIV” knob counterclockwise a little until it clicks
once.  Your display should now show more than one period.  The CH1 and CH2
frequency measurements should change from “?” to the true reading.


10. Press the “CURSOR” button on the top middle of the oscilloscope.  


11.
Press the top soft-key next to “Type” until “Time” is listed.  The middle of the
right column of the screen shows us the readings we are interested in:  Δt and ΔV.  
Below that, the readings for Cursor 1 and Cursor 2 are shown.

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6. Below “Phase” in cell F2, enter this formula:  =2*pi()*A2*E2
7. Press return.  Press Ctrl+D, then return as you did before to fill the column.

8. Below “Gain” in cell G2, enter this formula:  =20*log10(D2/B2)

9. Press return.  Press Ctrl+D, then return as you did before to fill the column.  
Ignore the errors for now.

10. Save this worksheet to use as a template.  You can use it the next time
you need to create a Bode plot, letting you skip Part 6.

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Part Seven  Getting the data for the Bode plot


1. Your oscilloscope should still be on the cursor display from the end of Part 5.  
If it isn’t, press the “CURSOR” button on the top middle of the oscilloscope.   


2. Turn the “HORIZONTAL SEC/DIV” knob to zoom in on the wave, so that a single period is shown.


3. Push the fourth soft-key from the top to select Cursor 1.


4. Turn the “multi-function” knob located on the center top of the oscilloscope to move the cursor.  
The knob is unlabeled and is located just above the “PRINT” button.


5. Position the cursor so that it lines up with the very top of the CH1 (upper, in orange) wave.


6. Push the last soft-key (fifth from the top) to select Cursor 2.


7. Turn the “multi-function” knob located on the center top of the oscilloscope to move the cursor.  
Position the cursor so that it lines up with the very top of the CH2 (lower, in blue) wave.


8. On your spreadsheet, record your data:

• a. Vin – the voltage reading under Cursor 1 (820 mV in the example above; record this as 0.820 in your spreadsheet)
• b. dV – the reading next to ΔV (20.0 mV in the example above; record this as 0.020 in your spreadsheet)
• c. Delay – the reading next to Δt (160.0 ?s in the example above; record this as 0.000160 in your spreadsheet).