AM RADIO TRANSMISSION

6.101 Final Project

TIMI OMTUNDE
SPRING 2020
AM Radio Transmission
6.101 Final Project

Table of Contents

1. Abstract

2. Introduction

3. Goals
a. Base
b. Expected
c. Stretch

4. Block Diagram

5. Schematic

6. PCB Layout

7. Design and Overview

8. Subsystem Overview
a. Transmitter
i. Oscillator
ii. Modulator
iii. RF Amplifier
iv. Antenna
b. Receiver
c. Battery Charger

9. Testing and Debugging

10. Challenges and Improvements

11. Conclusion

12. Acknowledgments

13. Appendix

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AM Radio Transmission Abstract

Amplitude modulation (AM) radio transmissions are commonly used in commercial and
private applications, where the aim is to broadcast to an audience - communications of any type,
whether it be someone speaking, Morse code, or even ambient noise. In AM radio frequencies
(RF) transmissions, the amplitude of the wave is modulated and encoded with the transmitted
sound, while the frequency is kept the same. Furthermore, AM radio transmissions can occur on
different frequencies, with broadcasts of the same frequency interfering with each other.
Therefore, it is important to select a unique – or as unique as possible – frequency band in order
to transmit clean, interference free, communications. And currently, many 1-way and 2-way
radios, with transmission ability have some digital component in order to improve different
aspects of the radio. Thus, for my 6.101 final project, I have built a fully analog circuit, in
keeping with the theme of the class, that can be used to broadcast to specific AM frequencies.
Thus, the goal is to broadcast as far as possible while minimizing the power consumed.
Moreover, I have been able to receive these AM transmissions through an existing receiver and a
receiver I built alongside this project. Yet, I have run into difficulties when extending the
reception range as it arises from the antenna length, the RF transmission power, and the number
of transistor stages. Moreover, I expected noise to be an issue when tested outside of the virtual
environment, since the message is encoded in the amplitude and any type of noise will distort the
amplitude.

Introduction

For my 6.101 final project, I endeavored to build an AM transmitter. Furthermore, since

time permitted, I also constructed a matching AM receiver, in order to put together a walkie-
talkie and test the transmitter. For this project, I directly built the oscillator, charging circuit, and
amplifiers as my main design challenges. As for the other design options, I plan to use standard
BJT designs for my modulator and buffers. The inspiration behind my project comes from my
previous two internships, in which I dealt with serial communication. In both cases, I was using
radio technology to communicate digital information. However, in this project, I wanted to get a
deeper intuitive sense of how the technology that I had been building protocols around worked.
Whereas, the radio technologies that I previously used relied on various modulation transmission
schemes, I used a pure AM transmission scheme due to the low frequencies that it is transmitted
over. With the AM transmission scheme I will be able communicate over distance with vocals,
Morse code, and more. AM transmission will also be an interesting topic to delve into because
AM signals are falling out of favor and are no longer being used. Leading me to hypothesize, I
may not encounter AM signals in a professional or school setting again due to increased
reliability and prevalence of frequency modulation (FM) transmissions.

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Goals

Commitment:

The minimum commitment for my 6.101 project was a device capable of transmitting a single
frequency Amplitude Modulated (AM) signal with a minimum of a 50mW output at the RF
amplifier stage. Additionally, the device is usable with a standard 9-volt battery that can be
bought at most stores. With these requirements, my project uses a crystal frequency oscillator to
transmit an AM signal with or without a shortened antenna on a power source of a standard non-
rechargeable battery.

• Find a suitable crystal oscillator

• Design and simulate the buffer for the crystal oscillator in LTspice
• Simulated 1MHz frequency output from the Oscillator
• Simulate modulator stage with the crystal oscillator and buffer
• Simulate the RF output stage on by itself
• RF output stage wattage of 50mW

Expectation:

My expectation for this project was to have a tunable AM transmitter for the range of 535 –
1705kHz that can obtain a minimum of a 100mW output at the RF amplifier stage. Additionally,
to alleviate some of the power constraints, I planned to utilize a rechargeable battery with an
on/off switch in order to charge the device on the go as well as turning it on and off. The charger
circuit also needed a voltage doubler to increase the 5V, from the USB, to the 9V required by the
battery.

• Simulated 1MHz frequency output from the Oscillator

• Simulate the tuning of the Oscillator from 535 – 1705kHz
• Simulating the entire transmitter circuit from 535 – 1705kHz
• Finding a suitable USB rechargeable battery IC circuit online
• RF output stage wattage of 100mW
• Use a voltage doubler to increase the 5V to 9V

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Stretch:

The final goal of this project was to build a user friendly device that utilizing numerous
techniques learned in and out of 6.101.

• Designing and simulating a USB charger circuit with a 9V output

• RF output stage wattage of 150mW
• Matching AM receiver circuit
o 510 – 1705kHz bandwidth
• Simulating the AM receiver circuit
• Include on the PCB the transmitter and either the receiver or the
charger
o Use a switch to toggle between transmitter and receiver
• Using an amplifier circuit in order to play music or use different
microphones

Block Diagram

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Schematic

Overview:

Figure 1: Schematic Overview

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Transmitter:

Figure 2: Transmitter Schematic

Receiver:

Figure 3: Receiver Schematic

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Battery Charger:

Figure 4: Battery Charger Schematic

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PCB Layout

Layout:

Figure 5: Entire PCB Layout

3D Front View:

Figure 6: PCB Front Layout

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3D Back View:

Figure 7: PCB Back Layout

Design and Overview

The design for my transmitter is simple. I use three transistors and an oscillator as the
base. The first stage modulates the incoming sound, the second stage acts a buffer for the
oscillator, and the third stage amplifies the signal into the antenna. The antenna is LC circuit that
feeds into both the antenna and the oscillator buffer. In this way, I only need to tune one
capacitor or inductor to tune the antenna and the circuit. From these essentials, many other
aspects can be incorporated into the design so that you could use multiple audio sources, a longer
antenna, or more transistor stages.

When I first looked at two-way radios, the transmitter and receiver were embedded inside
of each other. At first, this was the way that I also wanted to achieve my project. However, I
quickly realized an intermingled transmitter and receiver were indicative of superior knowledge
of how both circuits worked. Yet, I also realized that it would be better to separate the two
systems for testing and debugging as well presenting my final project to others, especially at the
final checkoff meeting. Moreover, when going into the project, it is important to note that we had
less time than we traditionally would have. So it was important to fix into stone what and how I
wanted to achieve my project.

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Furthermore, in the design, I also aimed to achieve low power consumption – or as low as
I could drive the power down. In my final implementation, I was able to achieve a total system
power consumption of ~2.15 watts with the ~1.44 watts being consumed at the antenna. Outside
of the antenna, the primary power consumers were the three transistor stages consuming a total
of 0.5826 watts.

Subsystem Overview

Transmitter

The transmitter required the majority of my project time as it was the main
commitment of the project. The transmitter – like most AM and FM radios – can be
broken down into four main sections: the oscillator, the modulator, the RF amplifier, and
the antenna. In the following sections, I will break down why, how, and what of the
section. Why the block is necessary; How the block works, and what you need to do and
have to make it operational.

Oscillator:

The oscillator, in my opinion, is one of the most important pieces of a

transmitter. Moreover, it allows communication of different frequencies –
effectively modifying who can and cannot listen to you. Consequently, the
oscillator provided the most difficulty in getting the device to work properly in
part because oscillators need ambient noise to kickstart them. That means you
need to program noise into LTspice and the oscillator.

Moving forward, my oscillator works through a combination of two

inductors and one capacitor in order to create an oscillating signal. The frequency
at which the signal oscillates depends upon the values of the inductor and
1
capacitor. For this paper, the frequency will be given by 𝑓 = 2𝜋√𝐿𝐶 as I will be
discussing tank circuits and Hartley oscillators. I do want to mention there were
other oscillators such as Butler, Clapp, Colpitts, Gate, and Pierce oscillators that I
considered in my circuit but ultimately decided against due to incompatibility or
design preference.

In my implementations of the oscillator, I made three designs. One design

using an oscillating crystal. A second design using a Hartley oscillator. And a
final, third, design with an LC tank connected to both the antenna and oscillator
buffer.

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In the first design, I used an oscillating crystal to test that my circuit

worked. It was a good way to validate that my circuit did exactly what I expected
it to do.

In the second design, I applied the Hartley oscillator to the transmitter as I

wanted to avoid using an oscillating crystal. The main advantage of using a
custom-built oscillator was that I would be able to slide across a set frequency
spectrum. The disadvantage is that the signal is not as clean as it could be as
Hartley oscillators are harmonic rich devices. For my Hartley oscillator, I
experimented with both a transistor and an op amp configuration. However, I
ultimately settled upon an op amp configuration so that I could more directly
control the gain. The main downside with an op amp is that you need to verify
that its Gain-Bandwidth (GBW) product is rated above your needs and that you
understand how to bias it properly. If the GBW is below your desired frequency
then you will not see your output signal.

However, at some point, I realized that I could approach the problem with
more tact by adding a connection between the antenna tuning circuit and the input
to the oscillator buffer. With this solution, I would turn the circuit into a power
oscillator and only need to tune one circuit instead of two. I was also able to see a
cleaner output signal as the input was closer to a pure sine wave. Moreover, since
I am about to build the device, I understand that such a large range, 510kHz –
1710kHz, may not be possible due to the large voltages involved. However, there
alternatives such as motorized capacitors or Atwater Kent capacitors that were
used in old AM radios.

Modulator:

I used a single BJT due to its simplicity. And because of that the
modulator is a section is relatively straightforward. Modulators are necessary
because they encode the audio input into the amplitude of the carrier frequency.
You can observe the modulated signal at either the collector or emitter. To create
my modulator, I used a single BJT in a common collector configuration. In this
configuration, I could adjust a resistor at the emitter to control the amount of
current at the collector of the amplifier BJT. I put a resistor at the emitter to adjust
the current due to U.S. government regulations restricting the output power at an
antenna. Furthermore, I would advise some format to control the current crossing
the amplifier circuit or antenna so that you can alter the transmission distance and
power. There may be restrictions for broadcasting in your area.

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Once I had completed my modulator, I connected my audio input signal to

the base of my BJT, with a capacitor in between to act as a filter. In my setup, the
audio input controls the on and off cycles of my BJT modulator so that the signal
can continue to the amplifier.

RF Amplifier:

I used one npn BJT for my RF amplifier which still proved to be powerful
in the voltages and currents that I could emit at the antenna. However, the RF
amplifier can be as complicated or as simple as you would like it to be. You can
even omit the amplifier, if you do not want a strong signal. For my project, I had
government restrictions on how strong my antenna output power could be, so I
wanted a way to limit power. I did this through my modulator, which you can
observe how in the previous section. In the U.S.A., non-registered broadcasting
devices must exist in the range of 510kHz – 1710kHz and not emit more than
100mW. Officially, my circuit meets all of these specifications. Unofficially, you
can tweak the system to broadcast at frequencies outside of this range and at
greater power. The output wattage that I intend to use my transmitter would be
~1.5W. However, I have been able to increase my transmitter above this wattage.
In practice, if you want to increase the power across the amplifier, it depends
upon the voltage rail, the BJTs employed, and the modulator emitter resistor.

As a last note, I would caution not to go outside of the specified range as

various government functions are carried out at those frequencies. But, you can
transmit for miles if desired.

Antenna:

LTspice, does not have a built-in antenna, so I did not use an antenna.
Rather, I built a band filter that would filter antenna transmissions and double as
my antenna for the simulations. Furthermore, the antenna can be the simplest
section of the device if you allow it. Just like the RF amplifier, you could choose
to omit it. However, if omitted your transmission results just as in the RF
amplifier would be seriously endangered. As both components serve to amplify
the signal and boost transmission distance.

When building the antenna, I considered multiple options. I decided

between including or omit a band filter and using a stub or whip antenna.
Ultimately, I chose to use a band filter and a whip antenna.

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For using a band filter, this gave me trouble as I needed to make sure that
the bandwidth and quality factor of my antenna overlaps with the rest of the
circuit. If they are different, then you make encounter a shunted signal or no
signal at all. Not necessarily a bad thing, but still not ideal.

Moreover, for the physical antenna, I prefer the whip antenna for when I
build the system as it will yield the best results in terms of transmission power
and distance. Yet, it is less portable as it is can be long and unwieldly. This type
of antenna is ideal for long-range, reliable communication. If I were to use the
stub antenna, then I may be cutting my transmission power and distance by up to
30% with the bright side of having a portable radio. This antenna is ideal for
short-range, easy-use communication.

In short, the choice of which, if any, type of antenna is completely up to

you and your needs.

Receiver

While one of my big inspirations behind the project was to craft a fully functional
walkie-talkie, I did not have the required time to fully create my own receiver due to the
ongoing COVID-19 situation. Thus, I resorted to pulling a bare bones AM receiver
schematic online that I could tweak, improve, and present.

The receiver schematic follows a similar layout to the transmitter. In truth, it is

not too much more that a transmitter in reverse. And, this makes sense – you catch with
the same hand you throw. As you would not want to add more complexity than is needed

Thus, in the receiver, you will see 3 BJTs, the same number as in the transmitters.
The first and second npn BJTs act as a Darlington pair buffer so that the tank circuit does
not overload the rest of the circuit. Whereas the third npn BJT acts with a dual purpose.
The primary and secondary purpose being to demodulate the signal and amplify the
output signal, respectively. Additionally, the resistor reaching from the bottom of the tank
circuit to the base of the third transistor serves to create regenerative feedback – boosting
the signal in order to improve circuit reliability and performance. I set the resistor value
at 120k, but if you were to use a 33k fixed resistor in series with a 100k potentiometer,
then you would be able to filter the selectivity and sensitivity of the device.

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Battery Charger

Building the battery charging circuit proved to be one of the most interesting
aspects of the project as I needed to take in the power consumption of the project and
decide what supply voltage I needed to use and whether to restrict or increase current
consumptions in different parts of the circuit.

I ultimately decided upon a rechargeable 9-volt battery. I chose a rechargeable

battery as it would be more convenient to fuel the system on demand rather than
searching for new batteries whenever I wished to use the system. Additionally, adding a
rechargeable battery added an additional challenge in order to build a circuit to charge the
battery.

When I decided I wanted to use a rechargeable battery, I wanted to use a 12V

battery. However, the only available rechargeable 12V batteries are car batteries. So, I
decided to go with a 9V battery in order to maintain portability.

The charging circuit may look somewhat intimidating, but it is surprisingly

simple. The 5V input voltage is applied to the NE555 so that the output voltage is
doubled to 10V. Proper biasing of the NE555 can be seen on the NE555 data sheet. It
should also be noted that the NE555 is a dexterous IC chip that can act as an oscillator
and numerous other functions. From the output of the NE555 chip, the new 10V is fed
into the actual charging portion of the circuit. While the battery is charging one LED will
be lit. when the battery is fully charged, two LEDs will be lit. This is due to two
MOSFETs or SCRs acting as voltage regulated switches. That is the entire explanation
for the charging circuit. Moreover, the battery can still be used while charging. Moreover,
in the charging circuit the there are two LEDs. One LED is lit by the USB power and the
other LED is lit when the battery reaches 9 volts as it is above the Zener breakdown
voltage. This second LED helps alleviate overcharging, but is not fully if the battery is
charged for an extended period.

As a side note, I want to reiterate that you can use the NE555 to create a greater
range of voltages. In fact, you could even take the 9V from the battery and boost the
voltage to 12V or 15V in order to increase the range of the radio

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Testing and Debugging

All testing was done primarily by hand and LTspice. After testing, I laid out a schematic
with KiCad and turned the schematic into a PCB board layout.

I approached testing in LTspice in a linear fashion. I went section by section. First, I

completed the battery charging circuit as it was the most straightforward. Then, I built the three
transmitter circuits. The three transmitter circuits are the exact same with the only difference
being their oscillator component. Finally, I built the receiver.

For the voltage multiplier portion of the battery charger, I made two designs – both with
the NE555 chip. And, I made two to see which multiplier would reach 10 volts quicker with the
goal being to have a quick circuit. Below you can see the results of the two circuits in Figure 8.
Both starting at 5 volts.

Figure 8: Battery Charger Simulation

From the voltage multiplier, the output voltage is connected to the actual charger circuit.
Testing this piece of the project is not too tricky. All that needs to be done is place voltages
ranging from 0V to the desired battery voltage and measure how the circuit reacts to those
different voltage levels. To do this, take out the battery, and place your varying voltage
connection to where the battery belongs.

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When building the transmitter, I first verified that the crystal oscillator would be working,

Figure 9: Poor Filtering Example 1 - Crystal

and then moved onto the Hartley oscillator portion. But what I want you notice right now is that
while the signal is modulated, there are some rough edges on the back end of the signal in
Figures 9 and 10, crystal oscillator and Hartley oscillator, respectively. This comes about due to
improper antenna band filtering. And, I chose these photos specifically to display what happens
when the two oscillating circuits are not in tune.

Figure 10: Poor Filtering Example 2 – Hartley Oscillator

However, in both situations, while the signal is distorted the message is still allowed to
propagate. I do want to emphasize that these two signals were not my end product, but what I
first encountered before correctly tuning my circuit. If you want to see my finely modulated

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signal see Figure 11. I only included one photo of the modulated signal from the three circuits as
once the signal is fully modulated, they appear identical. Furthermore, I realized more than one
identical photo would be redundant. To see the functional circuits as well as the rest of the
working circuits, see the Appendix.

Moreover, the only solution to this problem is too have the two tuning circuits match,
which as I have showed is difficult to do. Yet, I was able to simply this process by combining
the two oscillators into one. Thus, turning the circuit into a power oscillator. In the image below,
you will see a finely tuned circuit for the full range of 510kHz – 1710kHz.

Figure 11: Ideal Filtering - Power Oscillator

At one point when I was still developing the Hartley oscillator, I realized that my
oscillator was not behaving properly in LTspice due to the lack of noise – oscillators need
random noise in order to start oscillating. I was conflicted on how I was to navigate the situation.
On one hand, I could ignore the problem as I knew the circuit would work in real life. However,
on the other hand, I could try to input random noise to jump start the system. I opted to jump
start the system in order to demo the Hartley oscillator. This is just to illustrate an example of
how using LTspice as a sandbox can quickly become a problem if you are not intimately familiar
with the nuances of theory and practicality. I would like to add that after this project, I plan to
build the full system upon my return to MIT – fingers crossed for the Fall of 2020. Yet, LTspice
is not full of negatives.

LTspice also helped me realize that I could fully omit the Hartley oscillator by feeding
the antenna tank circuit back into the buffer npn BJT in positive feedback. When I did this the
modulated circuit became much cleaner as I was not depending upon two oscillating circuits for
transmission, but one. As a last note about the oscillators, I used two inductors and coupled them
together. I did not initially couple the oscillators, but I eventually did as they would be coupled

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in practice due to mutual inductance. Thus, the total inductance increases. Be aware of this
phenomenon or use a transformer.

When testing the modulator, I ran into the trouble of inputing a square wave. While it is
an easy way to increase power is to input a square wave. You will see sharp peaks due to the
rapid change in voltage that the inductors will try to fight. The mistake I made was not realizing
that my square wave had a 0s rise and fall time. When this was rectified, I was able to see non-
lethal voltage spikes.

For the antenna, I have already highlighted the issues that involve the oscillators I
encountered when building the three transmitter versions. Moreso, I still want to reiterate some
of the options I had when testing and debugging. Originally, I did not think that the antenna
would yield any problems. But, I was underestimating the simplicity of what I was doing. First, I
want to admit that you do not necessarily need to tune an antenna. But, antenna tuning results in
better transmissions signal due to the omission of excess noise. I made the mistake of thinking
that it was enough to set the inductor and capacitor values to the same values. However, when
you do this, you are forgetting about the bandwidth and quality value. Depending on how wide
you want your transmission bandwidth, you need to be pre-meditated in what specific values you
use. Furthermore, I would recommend the quality factor be wide if you want to sweep a
frequency band and narrow if your device is set to one frequency. Since we are on the topic of
bandwidth and quality factor, I should also mention that your oscillator will also have a
bandwidth and quality factor. You want these values to be as close as possible, if not at least
have a sizeable overlap territory.

Finally, I tested the receiver last – see Figures12 & 13 input and output signals. This was
only natural as I needed to produce a modulated signal first to feed into the device. However, if
you wish only to make the receiver, there is a way to simulate a modulated LTspice signal. You
do this by taking two independent voltage sources – one controlling carrier frequency and the
other the signal source – and multiplying them by using a dependent voltage source. From there
you, you can either feed in that signal or the signal attained from your transmitter signal. Now, if
you look at the output –voila – you should see your newly transferred demodulated signal.
Lastly, I want to say that I included a capacitor and resistor at the output in order to help simulate
a speaker. These two components are not need for a physical implementation. I would encourage
you to remove them upon building.

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The receiver input and output, respectively.

Figure 12: Receiver Input

Figure 13: Receiver Output

That concludes my discussion of major hurdles in testing and debugging. While there are
more minor bumps, they do not affect the reliability and performance of the end device. I have
included photos of my LTspice simulations in the case you are interested in recreating my
project.

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Challenges, Improvements and Reflections

For this section, I want to begin with a general discussion and then dive right into
specifics. I hope my experience in the spring of 2020 is a one-off experience for this class. But, I
believe that all this advice would be relatable to any student in the future.

The most important advice is too start early! I cannot emphasize this enough. Hopefully,
you have picked a project that truly interests. And, if the project does interest you, it will take
more than one or two weeks to finish. Really try to utilize the full six weeks even with the
presentations to fully flesh out your ideas and implementations.

Second, now that I have used LTspice extensively, I can say that nothing beats debugging
by hand or a breadboard. While breadboarding has many non-idealities that you encounter.
Those are the problems that you are trying to fix. When you debug through LTspice, it is not the
same and not as enjoyable. You can try to program noise and other potential problems into
LTspice, but those might not be all or any of the potential bumps in the road. An engineering
teacher once told me there are three types of knowledge in this world: what you know you know;
what you know you don’t know; what you don’t know that you don’t know. Often, the third type
of knowledge is the most dangerous as there is no way to prepare or defend against these pitfalls.
Thus, while my circuit works perfectly in theory, there will most definitely be aspects that it can
be found lacking.

Yet, one of the things that I liked about LTspice was how quickly you could discover the
root of a problem. When you debug in real life, you cannot press a button to verify all of the
wires are connected, quickly verify currents and voltages, or connect different modules. LTspice
makes circuit designing easier and better when used in conjunction with building the circuit in
real life. I would truly recommend LTspice as a tool to other engineers for its ease of speeding
up developmental processes.

Funny enough, there was one mistake that I kept committing, not increasing the LTspice
timeframe scope. This was a fix that I caught with relative ease and speed. But, I do want to
emphasize that different aspects of the project run at different times so one section may take 1µs
to run whereas another section may need 10µs to run. I spent fifteen minutes more than I needed
wondering why my voltage multiplier output voltage had not increased beyond the input voltage.
Consequently, I encourage you to be aware of the dichotomies in runtime.

Additionally, one of the biggest challenges that defined my project was the oscillator.
However, it did not have to be the big challenge that it was. If I could go back, I would test all of
the modules in the transmitter individually to my satisfaction and then combine them. With
oscillators there is so much going on with them and really taking the time to understand the hard
math behind them would speed your discovery exponentially.

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For the battery charger, I really wanted to build my own DC-to-DC voltage doubler with
a solar panel connection. Yet, I was short on time. Adding a solar panel is not too hard, but I was
unable to obtain parts in a timely fashion. Furthermore, I would really recommend building your
own voltage multiplier. It is an interesting topic and you could even boost the transmitter overall
signal from 9V to 12V, 15V, or more as that increase would create a stronger signal.

With the receiver, I wish I had more time on this topic as it would have been neat to
control the entirety of the aspects in the circuit. However, time did not allow for this. I did still
create a reasonable receiver, with some help, that can drive headphones or a loudspeaker.

As you can probably tell, time was my biggest challenge. I started the day after spring
break, but that goes to show time is tight.

Moreover, the one thing that I would like to improve as a whole about the circuit is the
energy efficiency. This was a priority coming into the project as my device is battery powered
and can only last so long. I wanted to use energy efficient parts and control the current at
different stages in the circuit. While some of my circuit has energy efficient capabilities, the
whole circuit does not.

Conclusion

Overall, I would say that this project was a success! I really enjoyed learning the
fundamentals of circuit design and how to build devices from scratch. The final implementation
of these skills into the final project really illustrated to myself how much I learned over the
course of the semester. Now that I have a fully operational transmitter and receiver, I might just
open a radio station, who knows!

I would say for my device, I tried to incorporate as many different aspects of

functionality so that anyone, interested or not, can pick up the product and use it intuitively as I
do not want it to have a high learning curve.

When going into this project, I realized that I wanted to build something that would
typically require at least a two-man team. So, at the advice of the teaching staff, I scaled down
the commitment and expected goals to reflect what a one-man team could realistically do and
orient my stretch goals for what a two-man team would be able to do. And, the plan worked well.
I was able to achieve for the base and expected objectives and stretch just enough to reach the
entirety of my goals.

As for parting advice to future classes, I would really encourage them to start early and
pick something that truly interests them. I can only imagine how painful this project can be if

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you picked a topic that you did not like to work with a friend. On the flipside of this is to pick a
partner or team that you can function with and that you can rely to do their sections. It may seem
ironic that I say this since I did my project alone, but I want to emphasize that I did my project
alone as the class was moved online. I also did not want to run into different issues such as time
zone and other potential reasons as there are not too many ways to keep people responsible over
the internet.

If another student were to re-attempt this project, I would say try to work on creating an
ultra-efficient receiver. One that could pick up 1mV and below signals and losslessly amplify
them while getting rid of the potential noise. I say this because I was recently inspired by the
deep space missions of the twentieth century, where these far away satellites would transmit
signals that would be attenuated to almost insignificant level when reaching Earth. I plan to work
on this after the class ends.

Finally, I want to add that I also connected an audio amplifier to both the input of the
transmitter and the output of the receiver, but I was not able to include into the LTspice
simulations as there was not an adequate model in the libraries. However, I was able to include
the amplifier and its two different gain configurations into the PCB layout. 50 and 200 gain for
the receiver and transmitter, respectively.

Lastly, I want to reiterate how grateful I am to see the class through as it showed me how
and electrical engineer would go through an original design process, taking an idea from thought
to a fully functional application. Innovation is not a straight path. Rather a path that turns and
twists like a ravine that an engineer should be ready and excited to venture down. I would
emphatically recommend 6.101 to other students.

Acknowledgements

I found 6.101 to be a truly enjoyable class, and it would not have been possible without
the amazing staff. Specifically, for our teaching team, Gim, Negar, and Mark who have been
supportive and encouraging in person and online for this term. Additionally, I would like to
acknowledge various online websites and forums that have been helpful.

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6.101 Final Project

Appendix

1st AM Transmitter

Figure 14: Crystal Oscillator

2nd AM Transmitter

Figure 15: Hartley Oscillator

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3rd AM Transmitter

Figure 16: Power Oscillator

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6.101 Final Project

AM Receiver

Figure 17: Receiver

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6.101 Final Project

Battery Charger

Figure 18: Battery Charger

1st Voltage Doubler

Figure 19: Voltage Doubler 1

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6.101 Final Project

2nd Voltage Doubler

Figure 20: Voltage Doubler 2

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