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AM Radio Receiver (Part II) ENGN/PHYS 207—Fall 2019

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AM Radio Receiver (Part II) ENGN/PHYS 207—Fall 2019 Assignment # 5: Escape from the Deserted Island AM radio receiver (part II) ENGN/PHYS 207|Fall 2019 Figure 1: Escape the deserted island. Build an AM radio to receive AM radio transmissions from your rescue crew! Image credit: 30seconds.com Circuits You'll Build • AM radio receiver: tuner + demodulator. The main objective of this lab is to complete the design, construction, and proof-of-concept testing of your very own person AM radio receiver. You'll turn your radio into a message that holds the key to your being rescued from being marooned on a distant, deserted island. Skills and Concepts You'll Learn • Construction of capacitors and inductors • RC low pass filter (smoother) • Diodes as rectifiers • Amplifiers (voltage gain!): Become acquainted with wiring op-amp circuits. 1 1 Tuning Circuit: LC resonance The objective of this assignment is to build a full, working AM radio receiver|a real working radio to tune into music, talk shows, sports, etc! This Circuit journey begins back at Lab 4. Complete section 4 of Lab 4 (if you haven't already), in which you build, test, analyze the tuner of the AM radio. The tuner section allows you to listen into one specific radio station. Once that is complete, time to build the demodulator! 2 AM radio receiver: Demodulation Figure 2: Principle of Amplitude Modulated radio transmission. The amplitude of the carrier wave is modulated according to the audio signal envelope. Image credit: Horowitz and Hill. The Art of Electronics, 3rd ed. As we've discussed in class, Amplitude Modulated radio transmission works by forming an audio frequency envelope around the AM carrier wave (Figure 2). Watch this excellent explanation of quick AM radio theory of operation. The fundamental job of the AM demodulator circuitry is to recover the audio frequency waveform (thick trace in Figure 2 from the AM signal (\modulated radiofrequency carrier" in Figure 2). The carrier wave frequency for the AM band in the United States spans 560 kHz to 1.2 MHz. These are high frequency oscillations compared to audio content concentrated frequency band of 20 Hz - 5 kHz (bass to treble). The higher the ratio of the carrier frequency to the audio content, the more precisely the audio encoding will be. 2 Figure 3: Block diagram of the AM radio receiver. In brief, we tune into a certain AM station (1), amplify the signal (2), recover the audio waveform (3), and plug into an audio amplifier/speaker (4) 3 AM Radio System Overview Before plunging ahead, let's check out Figure 3, which shows a block diagram of an AM radio receiver. This is the 30,000 ft overview of the system. There 4 functional block indicated: 1. LC resonator: tunes into a specific station. It selects one station's AM frequency to listen to. 2. Carrier Wave Amplifier: provides voltage gain to boosts the amplitude of the original signal. 3. Diode rectifier + RC smoother: recovers the audio waveform. Rectifier grabs the positive swings in oscillation, RC smoother mathematically approximates the AM envelope. 4. Buffer Amplifier : allows us to connect audio playback device (speaker) downstream without affecting the circuit upstream. 4 Amplify the AM carrier wave We'll study amplifiers more formally in the coming weeks. Spoiler alert: They are used to amplify small signals to have an amplitude that is larger and easier to use. For instance, say the incoming radio wave has an amplitude of 100 mV = 0.1 V. And let's say we want to amplify that wave to an amplitude of 3V. Then we need an amplifier with a voltage gain of 3V/0.1V = 30. Easily done with an op-amp! 4.1 Op-amps: what's inside the proverbial black box First let's take a quick look at op-amps in practice. Study Figure 4. The 8 electrical contacts extending out of the integrated circuit (IC). They offer easy connections two separate amplifiers, labeled \1" and \2" accordingly. The amplifier is an active element|it must be powered to work. Power connections labeled Vcc+ and Vcc− are shared by both amplifiers. External resistors must be added to complete any op-amp design. Today, we'll build a classic non-inverting amplifier. 3 Figure 4: TL082 op-amp (A) and pinout (B). There are 8 individual electrical contacts{just in time for Halloween! Spider decorations, anyone? Note carefully the half-circle at the \top" of the IC that serves as an orientation mark. Just to the left of this mark is pin 1, as indicated in (B). ICs often show a small dot next to pin 1, as is the case for the particular TL082 package shown in (A). 4.2 Non-inverting Amplifier Speaking of a non-inverting amp, check out Figure 5. To get a first taste of op-amps, we'll wire up this circuit and quickly look at the input and output to understand how it functions. But first, we interrupt this broadcast for an public safety announcement! Figure 5: Non-inverting amplifier configurations. The voltage gain of non-inverting amplifier is Gv = 1 + Rf =Ri: Power connections are ±9 V. First, do NOT power up the op-amp circuits until they are fully wired. Otherwise you run the risk of having something mis-wired, which can lead to a nicely toasted op-amp. The corollary is that you should always power down your circuit before unwiring it and returning components to the bins. Also, you'll quickly notice that the pins extending from the chip are easily bent. When the pins are bent, getting the chip into and out of your board can be a pain. So, take care of your chip, be careful not to bend the pins. Use a small screwdriver to gently wedge underneath the IC then gently rock it free. Easy does it. 4 And we now return to our regular programming... 1. Build the non-inverting amplifier shown in Fig. 5. Use Rf ≈ 5 kΩ, Ri ≈ 500Ω, and R3 ≈ Rf jjRi. For Vin, use a 0.2 V p-p, 100 kHz sine wave. For power supply connections, use±Vcc = ±9 V. Display the input and output voltage sinusoids Vin and Vout simultaneously on the oscilloscope. Carefully sketch what you see. 2. Measure and report the voltage gain. We'll learn in a few weeks that the voltage gain is given by a simple ratio: Gv = 1 + Rf =Ri: How did your measured voltage gain compare to the theoretical value? 3. What is the phase relationship between the input and output? 4.3 AM carrier wave amplifier Now it is time to integrate the non-inverting amp into your AM radio! The job of the amplifier is to boost the signal output from your LC resonator to an amplitude that is compatible with the downstream diode, which requires ≈ 0:6−0:7 V to \turn on." So the voltage gain of the amplifier should be sufficient to achieve an amplifier output signal of ≥ 2 V amplitude (4 V pk-pk). 2 V leaves a little headroom to make sure your diode can go into forward conduction at about 0.6 V. Your task is to choose suitable resistors and build a non-inverting amplifier with an appropriate voltage gain for your AM radio system. For instance, if you previously measured the output signal of your LC resonator is, say, 100 mV, then you need to amplify by a factor of ×20 such that your amplifier outputs ≥ 2V signal. In your design, make sure to use a TL082 variety of op-amp. You should know that there are many amplifier models out there|picking out an op-amp is sort of like picking out a car. The TL082 is a speedy little guy with a high gain-bandwidth product. We need a fast amplifier to handle the radio frequency signals in this case. The gain-bandwidth product specifies the maximum voltage gain that be achieved at a certain frequency. GBW = (maximum voltage gain achievable) × (signal frequency) So let's say we've got a carrier wave at a frequency of 100 kHz and we want to amplify by a factor of ×20. This implies a gain-bandwidth product of GBW = 100kHz × 20 = 2 MHz. The TL082 is spec'd for a 4 MHz gain-bandwidth, so it will be fast enough! 5 5 Buffer Zone: Last stage amplifier Figure 6: Buffer Amplifier. Vout = Vin. The last section of the AM radio is a buffer amplifier built around the \second half" of the TL082 op-amp (remember: there are 2 op-amps in every little 8-leg package). The buffer is shown in Figure 6. We'll see in the coming weeks that for the buffer Vout = Vin. In other words, the buffer’s voltage gain is 1. And you immediately object \What a waste{that's no amplification at all!". Fair enough, my Circuits friends. However, the magic is that this configuration prevents current from being siphoned away from the smoother circuit upstream. This ensures the smoother actually, well, smooths the signal regardless of what it connected further downstream of the buffer. Go ahead and build this little puppy standalone|don't connect it to any of the other AM radio building blocks yet. Verify using your oscilloscope and function generator that, in fact, Vout = Vin. 6 6 Smooth move: Demodulation with Diode rectifier and RC smoother Recall that the job of the diode and RC smoother is to extract the audio signal from the AM waveform. We'll play a few fun circuits tricks (treats?) to do this in a two-step process.
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