EET 222 Equalizer Project

Introduction

Operational amplifiers are complex circuits. They comprise many complex functions to create a very robust black box that can be configured to create a wide array of circuits and systems. Op ampsprovide essential building blocks for creating simple gain stages or to perform mathematical functions, such as addition, subtraction, and integration. Filters, oscillators, and voltage regulators all benefit from the use of op amps as component in the design.

This project will use op amps to create a two band equalizer. The input to an equalizer is an AC signal composed of a range of frequencies. The equalizer then splits this signal into different frequency components using filters. The output of each filter is then passed through a variable gain stage. This allows the user to independently adjust the gain for each frequency range. He/She may decide to amplify the low frequencies while doing nothing to the high frequencies or vice versa. The signals are now combined back into one signal, via a summer circuit. This combined signal is now output, typically to a speaker. Figure 1 shows how all of these blocks fit together in a simple two band equalizer. LPF stands for Low Pass Filter, while HPF stands for High Pass Filter. “A” is the gain for each path. These gains are variable and independent from each other.

In this project you will build and test all of the blocks shown in Figure 1. The function generator willbe used to create a signal with multiple frequencies in it. For our testing purposes, the function generator will be set up to alternate between two discrete frequencies, 500Hz and 10KHz. The variable gain stages will be controlled by potentiometers.

Read the entire project before starting.

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Signal_In

LPF

HPF

A

A

Filter Blocks Gain Stages

Summer Speaker

Figure 1: Block diagram of two-band equalizer

EET 222 Equalizer Project

Task 1-- Gain Stages

The first task is to build and test our variable gain stages. The schematic for the gain stages is shownin Figure 2. R1 is a 10K Pot connected in such a way as to provide a variable resistance. U1 is a LM741 op-amp from your kit. Be sure to consult the data sheet for the LM741 before you begin to construct the circuit.

Educational Outcome: After completion of this task you should be able to explain how the gain stage works and understand BW and slew rate. If you don't understand these concepts or how they were demonstrated, go back and rework parts of this task.

Preparation and Planning:

• Calculate the max and min gain of this amplifier.

• Calculate the minimum BW of the amplifier.

• What gain produces this minimum BW?

• Will this amplifier work at the frequencies of interest?

• Calculate the BW of the amplifier when the gain is 1?

• The maximum AC input signal will be 100mVpp. Will this input cause a slewing problem at our frequencies of interest?

• Develop a test plan to measure the small signal BW of this block. This is the same as the Gain BW product. Remember that f2(cl) occurs when Voutdb=Voutmaxdb-3db. (You don't need to convert into db, but remember what happens to the gain at the -3db point.)

Build and Test: Now build your circuit. Be sure to consult the data sheet and the tips provided in this document, to help with this. Remember you will be placing 5 op-amps on your breadboard so it is imperative that you create a compact neat circuit layout. Also don't forget that the op-amps won't work without the +-15V supply provided.

Hook a 1V DC signal up to the input. Measure the max and min Vout. Is the gain correct? Now check the max and min gain with a 1KHz, 100mVpp SINE wave from the function generator.

With the amplifier set to max gain, use your test plan to find the BW of this block. Does this match your calculated BW? The error here may be as large as 15-20% but it should be in the same ballpark. If you think about graphing this on semi-log paper, the difference should be less than 2 minor divisions. Now find the BW with the amplifier set to a gain of 1. Does this match your calculations?

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U1

R1

10K

R2

4.7K

+1

5-1

5

INOUT

Figure 2: Gain Stage Schematic

EET 222 Equalizer ProjectWith the gain of the amplifier still set to 1, increase the input voltage to 1Vpp and change the frequency to 400KHz. What happens to the output? What non-ideal characteristic of the op-amp areyou observing. Is this expected? Why? You will have to do some calculations to explain the why.

Now build your 2nd gain stage. Verify that it is correct, i.e. has the same gain and BW as the first gain stage. Build this stage close to your 1st stage to conserve space on your board. Make sure you record data for this 2nd stage.

Task 1 should take you about 1 week.

Things to include in report: All measurements and calculations. Why does BW change when gain changes? Why does BW change when input amplitude increases? Details of how BW measurements were taken.

Task 2-- Adder

Our next task is to build the summer. The summer schematic is shown in Figure 3. Once again U1 is a LM741 op-amp. When asked to measure the Vout, measure it at the output of the op-amp that is labeled OUT.

Educational Outcome: This task expands on the basic concepts of how op amps work. You should feel more confident analyzing op amp circuits. You should also have a better understanding of some of the limitations of op amps and how to read data sheets.

Preparation and Planning:

• Calculate the value of Vout in terms of VA and VB.

• Develop a plan to test and verify that the calculations are correct.

• If a 100mVpp, 1KHz SINE wave is connected to both VA and VB what is the expected output?

Build and Test: Build the schematic show in Figure 3. Use your test plan to test the circuit and confirm that it performs as expected. Make sure you collect enough data to demonstrate that it is working properly. This will likely involve several oscope screen shots.

In this circuit the load resistor is 100Ω. We ultimately want to drive a speaker that is only 8Ω. Lets test our summer with a more appropriate sized load. Setup the circuit so that the output is a 1Vpp sine wave. Now replace R4 with about an 8Ω resistor. Observe and record how the output changes for this new R4. Why does Vout change? A hint to this is to think about what limits are in

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U1

R1

10K

R2

10K

R3

10K R4

100

+15

-15

A

OUTB

Figure 3: Summer Schematic

EET 222 Equalizer Projectthe data sheet that might affect the output.

This task should take about 1 week. After completing this task you should write your first report.

Things to include in report: All measurements and calculations. This should include oscilloscope waveforms showing

the correct output signal.

Task 3-- Speaker Driver

The circuit shown in Figure 4 can be used to fix the problem. It uses a push-pull amplifier, like in EET221, to create a high current that can drive a low impedance load.

Preparation and Planning:

• What is a typical DC current that this driver can produce.?

• How much current is needed to drive the 8Ω speaker?

• Will this circuit be able to drive the speaker?

Build and Test: Build this circuit and confirm that it does drive an 8Ω load. Make sure you have data that can be easily used to show the difference between 8Ω load with and without the high current driver.

This task should take less than 1 week. Once finished begin working on the filter stages.

Things to include in report: All measurements and calculations. This should include oscilloscope waveforms showing

the correct output signal. What is the purpose of Q1 and Q2 in Figure 4

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Figure 4: Summer with high current speaker driver output

U1

R1

10KR2

10K

R3

10K

R4

8

Q1

2N3904

Q2

2N3906

R5

1k

R6

1k

D1

D

D2

D

+15

-15

A

OUTB

Q1B

Q2B

-15

-15

+15

+15

Diodes are both 1N914 or 1N4148

EET 222 Equalizer ProjectTask 4-- LPF

Task 4-- LPF involves building and testing the Low Pass Filter (LPF). The schematic for this task is shown in Figure 4.

Educational Outcome: Task 4-- LPF and Task 5-- HPF introduce filters. You should be able to analyze filter circuits. You should also begin to understand what parameters are critical for a filter circuit and how to measure those parameters.

Preparation and Planning:

• What is the gain and cutoff frequency of this filter?

• Develop a test plan to experimentally create the frequency response of this filter.

• What order filter is it?

• What type of response will this circuit have, i.e. Butterworth, Elliptical, Chebyshev? Why?

Use LTspice to create a simulated frequency response of this filter. You will need to run an AC simulation. Set the input signal as 1V AC.

Build and Test: Build the schematic shown in Figure 4. Once again U1 is a LM741 op-amp. Verify that this filter performs as designed. Use your test plan to find the frequency response of the filter. Compare the experimental frequency response to the simulated and calculated response.

Things to include in report: All measurements and calculations. Frequency response plot from LTspice. Oscilloscope images of the input and output waveforms in the passband and at the cutoff

frequency. Explain what measurements were taken and how you determined BW for your circuit.

Task 5-- HPF

Task 5-- HPF involves building and testing the High Pass Filter (HPF). The schematic for this task is show in Figure 5.

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U1R1

10K

R2

10K C1

4.7n

C2

10.0n

+15

-15

OUTIN

Figure 5: Low Pass Filter (LPF) Schematic

EET 222 Equalizer Project

Educational Outcome: Task 4-- LPF and Task 5-- HPF introduce filters. You should be able to analyze filter circuits. You should also begin to understand what parameters are critical for a filter circuit and how to measure those parameters.

For this task repeat all of the calculations, simulations, and build/test that you did for Task 4-- LPF, using Figure 6.

Task 4-- LPF and Task 5-- HPF should take about 1.5 weeks.

Things to include in report: All measurements and calculations. Frequency response plot from LTspice. Oscilloscope images of the input and output waveforms in the passband and at the cutoff

frequency. How is this filter different from the LPF? Explain what measurements were taken and how you determined BW for your circuit.

Task 6-- Equalizer

Now all the blocks are built and it is time to put them all together. Figure 1, the block diagram, shows how all of the pieces should connect.

Educational Outcome: Task 6-- Equalizer provides experience with placing many blocks into one system. You should become comfortable with more complex circuits and layout. You should recognize how to test individual blocks in a system along with testing the complete system.

Build and Test: The first step is to double check each individual block. Visually check that all of thesub-blocks are correctly wired. Make sure you know where the input and output of each block is located. Now connect the output of the LPF to one of the gain stages and output of the HPF to the other gain stage. Connect the outputs of the gain stages to the summer. Connect the function generator to the input of both the LPF and the HPF.

The first step is to test each path individually. So to test the low frequency path, set the function generator to 100mVpp, 500Hz, sine wave. Create screen captures showing the proper signal at the output of each stage. You should test this with the low frequency path set to maximum and minimum gain. Verify that the magnitudes at the output of each stage are correct. Each setting will require 6 screen captures to verify that the stages are all working properly.

Repeat this test for the high frequency path. You should determine and document the function generator settings.

Now set up the function generator to produce a 100mVpp FSK sine wave. FSK stands for frequency

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U1

R1

4.7K

R2

2.7K

C1

22n

C2

22n

+15

-15

OUTIN

Figure 6: High Pass Filter (HPF) Schematic

EET 222 Equalizer Projectshift keying. The function generator will alternate between producing a 500Hz and 10KHz SINE wave. Read and follow the instructions in Appendix A for setting up the FSK operation. Verify the output voltage of the function generator using the oscilloscope. Use the FFT of the oscilloscope to verify that the function generator is producing the proper frequencies. Appendix B explains the FFT,how to set it up and how to read the graph.

Leave channel 1 of the scope on the input. Now use channel 2 to check output of LPF and HPF using the FFT. Do you see what you expect? Do you see anything strange or unexpected?

Replace R4 in Figure 4 with a speaker. Display the output, node labeled OUT in Figure 4, of the equalizer on the oscilloscope using the FFT. Can you change this output by changing the high frequency and low frequency gains? Does the audio sound change as you change the gains?

Drive the equalizer input from the computer or your iphone. As you adjust the gains of the LPF (Bass) and HPF (Treble) do you hear a difference in the output. You may need to use a better speaker.

Don't forget to demonstrate your circuit to your instructor. Make sure you know how you obtained any data that you have taken so far. Your instructor may ask you to duplicate certain measurements.

Task 5 should take less than 1 week

Things to include in report: When doing your full system test and verification, you should think about what

measurements and data to show the user to verify that your circuit works as planned. Remember we are putting a SINE wave into our system, so we should get a SINE wave out. FFT oscilloscope screen shots.

Input to Equalizer. Output of Equalizer with combinations of LPF and HPF at different gains. Output of LPF and HPF blocks. These should be measured between Filter and Gain

stages. Audio observations of what you heard from the speaker. Does the Equalizer FFT output change as expected when you change the High Frequency and

Low Frequency gains?

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EET 222 Equalizer Project

Task 7-- Report

Now it is time to write the report. Remember to read the report guide, sample report, and rubric. Look at your last report for ideas of what you should change. Make sure you include all of the things listed in each task. These are not exclusive lists. You are welcome to include other details that you feel are pertinent to explaining your project.

Remember that a picture is worth 1000 words. When trying to convince someone that you have the correct data, it is usually helpful to have a screen shot of the correct data. Don't forget to explain to the reader what they should be seeing and how to interpret the data. If you don't explain this, then the reader can interpret the data however he/she sees fit. That interpretation may not be in your favor, so direct them where to go.

Circuit Building Hints and Tips

You may have to calculate or measure quantities that are not explicitly called for to answer some questions. This is expected.

Place a 10uF capacitor between +15 rail and GND and one between -15 rail and GND. This will help cut down on the noise, just like it does in digital.

Make one of your red rails on the breadboard +15 and the other -15. Use two diodes (1N4001or equiv.) to prevent you from hooking up the +15 and -15 incorrectly. There are multiple ways to do this. Figure 7 shows one possible solution.

Always reference the data sheet for the 741 when hooking it up. You don't want to blow up chips.

Place all 741 in the breadboard with the same orientation. This reduces the chance of hooking the power or signals backwards

Trim wires and component leads to keep all connections neat and close to the breadboard. Remember we are putting a SINE wave into our system, so we should get a SINE wave out. Prepare and fill out your data tables as you go in Excel. This is an easy way for you to ask or

answer any questions from you instructor regarding the completed tasks.

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Figure 7: Power supply connections to equalizer

D1

1N4001

D2

1N4001

V1

15V2

15

BreadboardEqualizer

+15

-15

EET 222 Equalizer ProjectAppendix A – Setting up FSK in function generator

FSK stands for Frequency Shift Keying. This is normally a method for digital transmission. The transmitter sends out one frequency that represents a digital '0', while another frequency represents a digital '1'. The transmitter is only capable of sending two frequencies.

We are going to use the FSK function in our function generator to produce a signal that contains multiple frequencies for our system.

FSK setup

1) Press “Shift” and then “Menu” button. The “Menu” button and “Enter” button are the same. This will put you in the Menu function and the display should read A:Mod Menu2) Press “V” (down arrow) once. This causes you to enter into the Mod Menu3) Scroll through this menu by pressing “<” 3 times. The screen should display 8: FSK FREQ.4) Enter this option by pressing “V”.5) Set Frequency to 500Hz. This is the 2nd frequency the function generator will use. The first frequency will be the one you set in the normal method.6) Press “Enter”. This will save the FSK frequency, and kick you out of the programming menu7) Repeat Steps 1 and 2 above to get back into the programming menu8) Press “<” 2 times. Screen should display 9: FSK RATE9) Enter this option by pressing “V”.10) Set the rate to 100Hz. This determines how often the function generator will shift between the two frequencies. With this setting it will switch 100 times in 1 second.11) Press “Enter” to save this setup

You are now ready to use the FSK function. Set function generator to produce a 10KHz SINE wave with an amplitude of 100mVpp. Verify that this is what you see on the oscilloscope screen. Now press “Shift” and “FSK”. The “FSK” button is above the triangle waveform button. The FSK annunciator should now be turned on in the display.The triggering on your waveform will no longer be good. However you should be able to adjust time scale of the display so that you can see two sine waves overlapping each other. Figure 6 shows what the display should look like if you have it set up properly.

The FFT function is useful way to observe a signal with multiple frequencies in it. Appendix B explains how to take the FFT of this signal.

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Figure 8: Oscilloscope of FSK in time domain

EET 222 Equalizer ProjectAppendix B – Using Oscilloscope to display FFT of a signal

FFT stands for Fast Fourier Transform. It is a method for displaying the frequency components of a complex signal.

To measure the FFT of a signal on the Oscilloscope you need to add a Math waveform. This can be done by pressing the Math Menu button. It is located between the CH1 and CH2 controls. Select FFT from the options of Math Type. Math Type will be displayed on the screen menu that uses the buttons along the right side of the screen. Select the appropriate channel to display as well.

Figure 7 illustrates what the FFT looks like for our FSK signal. Notice that the time/division is 1.25KHz, and the magnitude is 10.0dB. The bars show the relative magnitude of the signal at different frequencies. Zero HZ is on the far left side of the screen and then each division is 1.25KHz. The first major peak of our signal is at 500Hz. The 2nd set of peaks is at 10KHz. These are the two frequencies that the FSK is set for.

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Figure 9: FFT of Function generator producing 500Hz and 10KHz FSK signal