Lab 1 Mix it UP BEFORE YOU BEGIN OBJECTIVES FRESHMAN CLINIC FALL 2009 CREATED BY P. MEASE, This being the first lab, I will offer a few prelims:

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1 Lab 1 Mix it UP FRESHMAN CLINIC FALL 2009 CREATED BY P. MEASE, 2009 BEFORE YOU BEGIN This being the first lab, I will offer a few prelims: - Come prepared. The means reading the lab (this document) BEFORE coming to class. The labs are paced moderately fast but are straightforward, however, to connect the concepts to the hands-on, you must THINK! If you are seeing the material for the first time, you will be narrowly focused on the procedure. Read over the lab a few times (should only take 15 min) and it will start to sink in. Your lab-session will go much smoother and you ll understand better. - If you need help, first try and figure the problem out yourself. Developing problem-solving skills is a key part of being an engineer. If you are still stumped, feel free to send me an or stop by. - If other team members are not pulling their weight, try and politely work it out as a group. Everybody has a unique set of skills, which lends itself to specialization. If there are problems that cannot be resolved, please come see me. - Layout: The labs are organized so that they flow sequentially. Because of this, some of the theory may be presented outside the Introduction & Background category so be watchful. The objectives will give you the general goals and should be in the back of your head throughout the process. OBJECTIVES In this lab, you will learn about a few simple resistor networks and create a passive mixer. You will then spice it up with some active electronics and decode an unusual radio transmission. You will learn how to prototype simple circuits, use a power supply (PSU), and digital multi-meter (DMM). P.Mease 2009 Freshman Clinic Lab 1 1

2 INTRODUCTION & BACKGROUND A resistor is a very boring component; however, it is the most common passive component in just about any electronic device. Through this lab, you will hopefully find resistors are kind of cool A resistor resists the flow of current. Simple! The amount that the device resists current flow is measured in ohms, which has the symbol omega (Ω). The higher the resistance, the more current it will resist. Using Ohm s law, you can find the current, voltage, or resistance of any circuit, given any 2 of the three parameters. Ohm s law is given by: E = IR Where E = voltage (in volts), I = current (in amps), and R = well, you already know this one. A voltage divider is a 2 resistor network: The output voltage is defined by the equation: V out = R2 R1 + R2 V in ASIDE: Resistance is the term used when we re only concerned with DC (direct current). With time-changing signals (AC), Impedance (Z) is used. Impedance is just like resistance, except it is a function of frequency. At DC, frequency = 0 so Z(f) = Z(0) = R. Impedance is composed of both Real and Imaginary parts: Z = R + jx Where R is the Real part, X is reactance, and j = i = 1. The impedance of a reactive circuit may look something like: Z = 5 + j3 For DC analysis, the imaginary part is 0, therefore DC Impedance is purely Real. P.Mease 2009 Freshman Clinic Lab 1 2

3 In part 2 of this lab, you ll build the cheapest mixer possible. A mixer is a device that combines multiple signals into one. Mixers can be found in many fields, including: Live/Recording Audio mixing consoles are often used to adjust the proportion of the amplitudes of all of the audio signals of a band or even different microphones on a single instrument. The generally contain many channels next time you re at a concert sit next to the sound engineer and you can see one first hand. Communications mixers are used in the down-conversion of radio signals to baseband like the circuitry in your radio set. Cooking mixers are used to combine various ingredients into one mixture, allowing flavors to collide and interact allowing a chef to create new tastes for his or her masterpiece. Operational amplifiers are ACTIVE devices (they require external power) that can perform a variety of tasks. Let s describe an ideal operational amplifier (as simply as possible). Here is what the schematic symbol for an op-amp looks like: - + It has 2 inputs, one with the plus sign, called the non-inverting input, and the other with the minus sign, called the inverting input. It has one output. It has (at least) 2 power ports (not displayed above). Whenever given a schematic for active devices, always check the datasheet! The datasheet contains all pins as well as any other important information about the device. For single sided operation, we tie the lower (V ss ) power pin to ground and V dd (the upper power pin) to the positive power supply. For dual-supply operation, you will have 3 power pins to connect: V+, V-, and GND. The power port voltages should remain stable and fixed and should be set AT LEAST as high as the maximum voltage swing you need at the output (or input, depending on whichever is greater). Again, be sure to always check the datasheet, particularly the Absolute Maximum Ratings section to find what the limits P.Mease 2009 Freshman Clinic Lab 1 3

4 of the chip our. If you go beyond these ratings, stand clear as your chip may explode! Even worse, is a less-thanspectacular failure: one you can t see. This op-amp has some really awesome features listed below. Pay attention: 1. The input current (the current going into the input pins) is ZERO 2. Zero output impedance 3. Input voltages at the inverting and non-inverting pins are EQUAL. If you put 5V at the non-inverting pin, there will be 5V at the inverting pin. You can worry about this last one when you get to Networks I. Of course this is the ideal op-amp, but a real one is not too far off for design purposes. Later, we ll see why this device will save our ears from being mutilated by a particularly lo-fi circuit. EQUIPMENT & SOFTWARE Breadboard Asst. resistors Keithley 2000/2100 DMM HP/Agilent E3631A PSU Hookup wire (solid 24ga.) OP-AMP: LM324xx DIP14 package Small speaker (>= 8Ω) 3.5mm L-R breakout cable Special file A corrupted radio transmission PROCEDURE Part 1 Voltage Dividers sigh - Build a simple voltage divider that reduces the input voltage by 75%. To build this circuit, you will need 2 resistors (you ll have to figure out their values) and a breadboard. You will have to select one of the values first, to find the other (2 unknowns, 1 equation). Would you want a high (or low) valued resistor network? What would the difference be even though the divider ratio is the same? HINTS: what s that gold band on the resistor body? Also, calculate current through the network - NOTE: Be sure to have each team member check over your wiring before applying power. The circuits here are very simple, but the habit will be much appreciated when you are working on a much more complex circuit. o The input voltage for the circuit will be provided by the E3631A power supply. Use the 6V output: connect a lead or wire from the red post (+) to Vin of your circuit and the black post (-) to the ground of the divider network. Select the +6V button and turn it up to 5V or so. Leave the output on/off OFF! o Take a set of leads and connect the upper-right most ports of the DMM to your circuit. In voltage mode (DCV), the DMM will measure the voltage difference across the leads. Measure the following voltages: From Vout to GND From Vin to Vout o What do these voltages show? o This is pretty boring stuff, so let s move on P.Mease 2009 Freshman Clinic Lab 1 4

5 Part 2 Passive Mixer - On the breadboard, build a passive mixer, using only resistors, that combines 3 signals into one. It might look like this, but what do I know: - To implement your inexpensive mixer, you want to pick the same resistor values for R1, R2, and R3. Try a 4.7k resistor. - To test out your mixer, we have a special file radio waves - pure fractal flame by Cory Ench 2004 o o o This can be downloaded from my course website (download it now). This is a very special file: a lost radio transmission. It is your job to figure out the message. Not so fast! This transmission is corrupted and will not be legible by itself. We were able to recover a de-corruptor file, which, when played at exactly the same time as the radio transmission, cancels out most of the noise (if you re interested in how this works, ask!). To hear the transmission you ll need to mix both of these data. The data files (the radio transmission and de-noiser) are each on separate right and left channels. You will use a 3.5mm stereo cable and apply each of these 2 channels to a separate input of the mixer, which will combine them as one MONO audio signal. P.Mease 2009 Freshman Clinic Lab 1 5

6 o NOTE: you only need to play ONE file (I ve already placed both the transmission and denoiser channels in this file for you). - Using the special 3.5mm jack breakout cable, connect one lead (right or left, doesn t matter) to one of the mixer inputs. The ground of the 3.5mm cable should be grounded (Captain-O strikes again). - Connect the output of your mixer to the speaker. The speaker has two pins. Connect one of the pins to ground on your breadboard and the other to the output of the mixer. o Open VLC Player and drag the lost transmission file into the program. Listen to the one track through your mixer. It is going to be really faint! - Now take the second lead of the special 3.5mm breakout and put it on another input of the mixer. o Press play again. Listen. Can you make out the transmission? Probably not back to the drawing board. Part 3 Active Mixer - The mixer you just built technically works, but you can to do much better because you re good enough, smart enough, and dog-gone it, people like you. look - One major problem with the simple passive mixer in Part 2 is that it really loads the source (here, the source is the computer s soundcard). We won t go into details, but essentially what is happening is that the passive mixer is drawing too much current from the source. The amplitude, as you hopefully heard, was cut down quite a bit when run through the passive mixer to the point where it could barely be heard. The more signals you want to mix, the more the signals get attenuated. The solution is to somehow reduce this loading effect and reduce the amount of current that the network pulls away from the source. This makes sure that the source isn t bogged down. - Read the above paragraph again and see if you can recall a device that would reduce the amount of current fed into the mixer think back sometime today something you read. A: The Op Amp! - How can we configure the op-amp so that nearly 0 current is drawn from the source? P.Mease 2009 Freshman Clinic Lab 1 6

7 - Build the active mixer shown above on a breadboard. The LM324 op-amp is a quad type which has 4 individual op-amps inside. You only need to use one of the amps (pins 1, 2, and 3 plus power). - The LM324 is in a DIP14 package (Dual-Inline-Package, 14 pins). You can tell where pin 1 is located by the little notch between pins 1 and 14: - For power you will need V+ tied to the positive output (RED) of the +/- output of the E3631A and V- to the negative (RED -) output of the PSU. The black jack on the PSU between the two reds is the ground (where your 3.5mm cable ground and speaker grounds go). V+ should be set to about 9V and V- should be set to about -9V. - Apply BOTH the inputs as before and have a listen to the radio transmission again. Can you make it out? o For you lab report, let me know the jist of the transmission (if you can t make out all of the words, don t worry). - We can do even better. The active mixer you built above has a gain (G) of 1. This means whatever is at the input is presented, unscaled, at the output. This configuration is sometimes called a buffer. A buffer is used to isolate the input signals from the output load. In our case here, the input is the P.Mease 2009 Freshman Clinic Lab 1 7

8 audio from the soundcard and the output load is the mixer and speaker. While this is a huge improvement over the passive mixer, we can ADD gain to the circuit to amplify the signal (i.e. make it louder). Using the same schematic as in the G=1 case, we have: V out = R4 V in1 R1 + V in2 R2 + V in3 R3 if we want constant gain for all inputs, then we simplify (R1 = R2 = R3 = ) and have: V out = R4 V in1 + V in2 + V in3 which simplifies to: V out = R4 V in1 + V in2 + V in3 since we can control R4 and, we chose this ratio to select the gain we want: V out = R4 V in1 + V in2 + V in3 where: G = R4 NOTE: Gain is unitless as are other ratios - the units cancel. - Choose values of R4 and such that your circuit has a gain of 3. Apply these changes to your circuit. R1,R2, and R3 can be around 10kΩ. - NOTE: Remember from before that the op amp needs power at least as high as the signal it is outputting. In the G=1 case, the output voltage = input voltage. Here, we have G=3, which means our output has to swing 3 times higher! Adjust the power supply to accommodate this. Assume the input voltage will not exceed 3V. - Run the transmission through the boosted mixer. Notice any change? Congrats, we re finished! P.Mease 2009 Freshman Clinic Lab 1 8

9 DELIVERABLES - Formal Lab Report (see website for an example of an ideal lab report) including: o Answers to any questions throughout the lab o Any notices of problems, uniqueness or odd behavior explain! o List of contributions by each team member o Units! Use the SI system. There will be severe penalties for any quantity given that does not have a proper unit. See lectures for proper conventions. o I am interested in YOUR understanding of the material, not Google s so explain WHY things are happening and your analysis in your own words. o Comments on the lab material and pre-lab lecture only for the lab periods (Monday s inclass lecture comments should NOT be given here) this can be a sentence or paragraph (as you like), submitted on a separate page. If you would like to leave anonymous comments, leave it unnamed in my mailbox, located in the second floor office. These comments will not affect your grade whatsoever (so feel free to be honest!). These comments are used for future development of the labs. o EXTRA CREDIT: locate the original source of the lost radio transmission your completed report to: mease@rowan.edu. Be certain to CC all lab partners. It is due: SAFETY & LAB PROTOCOL - High volumes can easily permanently damage your hearing. Always increase the volume slowly, starting from the lowest settings. - Be sure to turn down any headphone volumes BEFORE putting them on your head! - Return all cabling neatly to the racks - Clean your workspace when finished your experiment - No food or drink allowed - No long jewelry or conductive rings/bracelets, etc - Use safety glasses and fume extractors when soldering P.Mease 2009 Freshman Clinic Lab 1 9