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 ||Ri. 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. Firstly, we pick off only the positive-swings using a diode. The diode acts as an electric value allowing current to flow only one way. Hence, the positive voltages pass through, but the negative swings are blocked sine no current can flow in that direction. Second we use an RC smoother (low pass filter) to extract the audio envelope from the AM signal. Before integrating into the AM radio, let’s have a quick play with the diode + RC smoother separately to understand how it works.

6.1 Diode Rectiﬁer

Build the circuit shown in Fig. 7, and measure the input and output (across RL). Make absolutely sure Ch1 and Ch2 coupling mode are set to “DC.” (Press the “Ch1” button to access menu to choose the coupling mode.)

Figure 7: Diode rectiﬁer. Set the input to be a 5 V amplitude (10 Vpp) sine wave at a frequency of 100 kHz. Use RL ≈ 5 kΩ. Use a 1N4148 didoe. Alternatively a 1N400x will work too.

You should hopefully see a picture on your scope like the one in Figure 8. Two things to note: 1) Firstly, only the “tops” (the positive swings) make it through; the bottoms (negative swings) are cut off. That’s because a diode only let’s current flow in one direction (as you saw in Lab 0)! Secondly, note that the peak of the diode rectified output is about 0.6 V less than the input. That’s because silicon diodes require about 0.6 V to “turn on.” You can think of this diode action sort of like a sticky door. It takes a little bit of energy to open the door, but once open, you and everyone else can freely pass through. Verify you see this action.

6.2 RC Smoother

Next, augment the rectifying circuitry by adding a capacitor in parallel with RL, as shown in Figure 9. The capacitor value C should be chosen such that the time constant τ = RLC ≈ 0.2ms = 200µs.

“Why 200 µs?” you ask. Great question! This value for τ is a compromise. It is fast enough to capture audio waveforms up to about 5 kHz (T = 1/200µs implies f = 1/T = 5 kHz). It is also slow enough to NOT track individual oscillations of the AM carrier wave itself at ≈ 100 kHz (oscillations on order of 10 µs). Hence, C is something referred to as a smoothing capacitor in this case because it acts to “smooth over” the fast AM carrier oscillations.

7 Figure 8: Diode rectiﬁer. Rectiﬁed sine wave outputs positive swings only. There is also 0.6V lost due to the forward diode voltage drop.

Figure 9: Rectify and smooth. The capacitor does the smoothing. It stores energy and releases it when the voltage starts dropping.

Back to the RC smoother. Verify you can see this smoothing action on your scope. Viewing input and output signals, you should see something similar to Figure 10.

Figure 10: The eﬀect of the smoothing capacitor. When the time constant τ = RLC is much greater than the period of the input wave T , then you can see the smoothing in action. The ripple voltage is deﬁned as the peak-to-trough dip in voltage over time.

Now try one last thing: try varying the amplitude of the input wave by hand. Just turn the knob on the function generator back and forth. What do you see? What does this have to do with audio waves?

8 7 Playback Time: Full AM Radio Receiver

Finally, time to put all of the pieces of this circuits puzzle together! If all has gone according to plan, you have 4 major building blocks of the radio:

1. LC resonator tuning circuit

2. Op-amp based carrier wave non-inverting ampliﬁer

3. Demodulator with diode + RC smoother

4. Op-amp based buﬀer ampliﬁer

Connect these “functional blocks” together in this order. Once properly connected, you have a properly working AM radio receiver!! Lastly, connect the output of the buffer amplifier to an Pyle-brand audio amplifier and speaker for playback.

8 The Great Escape—Getting oﬀ the Island

Next step: make your own AM broadcasting booth/transmitting station! See Section 9 for directions on how to turn your function generator plus an audio device (computer or cell phone) into the broadcast booth. Sports, talk radio, pop radio, classical, EDM. Your call...for now!

If all goes according to plan, you should be able to see a picture on your oscilloscope like the one in Figure 11. The yellow wave is the AM carrier signal. The blue trace shows the output from the RC smoother, the audio signal itself!

With your broadcast booth up and running, listen in with your new radio–built by your very own hand! Of course, you need to tune the transmitter and receiver to the same frequency. Connect the output of your AM radio receiver into an audio ampliﬁer to playback through a speaker.

Now to get oﬀ this bedeviling thicket of a deserted island, you’ve got to receive the “secret message”—the rendezvous point for the boat that will rescue and return you to civilization at long last!

Download the sound ﬁle DesertedIsland.wav on the course website. Play it on repeat so you can be sure to hear the message. Follow the directions carefully. Present the evidence that you’ve successfully gotten oﬀ the island!

Till next time, this is JE signing oﬀ...

9 Figure 11: AM radio demodulation. Yellow trace is the input to the diode = output from the carrier wave ampliﬁer. Blue trace is the output measured across the smoothing cap and resistor parallel combo.

10 9 Make your own AM broadcast station with function generator

Lexington is beautiful (especially during autumn!), but is not exactly blessed with heaps of AM radio stations. In fact, there is only 1 on the dial. Never fear - we’ll make our own broadcast! How to do this? You need but two items: 1) BK precision 4014B function generator and 2) audio playback source (smart phone, laptop, etc.)1. Follow the step-by-step picture directions in Figure 12. The output of your function generator is now an Amplitude Modulated signal with known carrier frequency. You can connect a coil (inductor) between the two function generator leads to act as an antenna. In case you are wondering “what’s inside the box” to make an AM station, check out Section 11.

Figure 12: Create your own AM station using the BK Precision 4014B functional generator. (A) Plug in your audio playback source to the Modulation In jack. (B). Set the carrier frequency to your resonance frequency. (c) Click the “MODUL” button (highlighted in red square), then select Amplitude Modulation (F2; red arrow).

1Thanks to Simon Marland and Daniel Rhoades for their keen observations leading to this easy broadcast method

11 10 The Report - What you Need to Turn In

Your report should describe the system, explain design principles and demonstrate proof of concept of how your completed AM radio. To this end, be sure to:

1. Circuit schematic for your full ﬁnal AM radio system. Carefully list all actual values used in the circuit and show how each functional block is interconnected.

2. Rationale for circuit components selected. Show relevant calculations and brieﬂy explain how relevant theory was applied.

3. One paragraph description of system overview. In your own words, state/briefly explain the role of each section in the AM radio system. Clearly illustrate input vs. output for each of the “functional blocks.” A series of photos/screenshots would be sufficient. Make sure your photos are of good quality! (Avoid glare, off-angle shots, etc).

4. Suggest at least one substantial possible improvement to the system—design, construction, better choice of voltage gain and/or other component value choices, user-friendly operation, etc.

5. Proof of concept. Sound/Music is best live. So, take a short video of your AM radio in action. It would be incredibly helpful to have a walk-through of what’s on the breadboard, then a live demonstration of music playing....icing on the cake! Best video wins a small circuits-appropriate prize!

12 11 Appendix B: DIY AM broadcast station from scratch

The curious cat may be wondering: how do you make an AM signal in the first place. What’s the magic inside the function generator? The trick is to use an op-amp and JFET as shown in Figure 13. If you stare at this circuit for a second, you’ll probably think Hey, this is similar to the non-inverting amp I built today. Indeed, it is! The trick, however, is to replace the resistor we normally label as Ri with a JFET (junction field effect transistor). The JFET is a device that allows more or less current to flow based on the voltage driving the gate. If we use our audio signal to drive the gate, then the effective resistance of the JFET changes according to the audio signal. In effect, we have made a variable gain amplifier, whose gain varies with the audio signal. Voila–and amplitude modulated signal!

One last detail: you might also notice the battery in series with the audio source driving the JFET. This is called “biasing” the FET and basically puts it in conducting state. Basically, we have to turn on the FET to ﬂow some current, then add the audio signal in series to ﬂow a little more or a little less current through the FET.

Figure 13: AM signal generation. This conﬁguration is similar to non-inverting ampliﬁer, except one resistor is replaced by a FET driven by the audio signal.