How does AM broadcasting/receiving work + a project for HS students?

[BiGyElLoWhAt]

TL;DR Summary

A few questions about AM radio

Hi, I'm working a day for a physics camp in about a month. I have about 6 hrs (less for lunch) with about 10 or so high-achieving high school students. I need to come up with a project that can be done in that time. I can do prep work ahead of time (like making pcb boards, etc) to make things go smoother. The goal for each day is for them to learn something and take something home that they made.

My idea is to make an AM radio receiver.

I've already discussed this a little bit with some other instructors that either are teaching or have taught this camp in the past. Overall consensus seems to be that it's doable. Community thoughts?

Questions:
1)
I've not found a clear answer, but I've played around a bit and it seems like the most sensible way to broadcast AM would be something like $F(\omega) sin(\omega_0)$ where F is the signal (like a song) and $\omega_0$ is the carrier frequency *2pi. Expanding in Fourier series and using trig you can see that it's equivalent to having all of the frequencies in the signal nested around the carrier frequency. Is this mathematically what we're working with?

2)
I came up with a design before I looked one up and had nearly the same thing as what I found. The big difference is that all the circuits online have a diode. I've been told that the diode is extremely important, although I don't understand why. Can someone explain why we need it?
Here is an example. https://www.circuitbasics.com/what-are-am-radios/ (my tool bar is bugged? I can't insert a link or use any other tools, bold, align, etc.)

3)
Given that I realistically have about 5-5.5hrs, how much background info should I give? I plan on going in this weekend and building one, although I suspect it won't take too terribly long for the actual assembly. I would like to cover circuits a bit and maybe do an ohms law experiment to see the linear relationship between V and I in dc, then talk about inductance and solve an LC circuit. Then they would at least have an idea of why they're building what they are. I also might add in a battery and an amp to the circuit if I have trouble getting the volume out of it. I want to be able to pick up a local station.

Potential itinerary:
What is Voltage, What is current (10-20m?)
Measure current as you vary voltage and plot (20-30m?)
Inductance intro, solve LC, find resonant frequency (30m?)
Optional Fourier tl;dr; what is it. (5m)
Give coefficients for sawtooth, have them plot more and more terms in excel, (30-40m? do they know how to use excel? not sure.)
Give schematic, build... (rest of day)

Any suggestions for my timeline? Are my time estimates somewhat reasonable? I'm condensing a lot and I know they're not going to leave a 1 day session experts. I am just trying to find a good balance of doing things vs. knowing what you're doing that works in the allotted time.

 

[berkeman]

My idea is to make an AM radio receiver.

That seems like a good project, and a do-able goal.

Others will explain why the envelope detection diode is needed for AM demodulation, but I have a question about where this "camp" will actually be held. Will it be in a city/urban setting, or an actual "camp" out in the woods somewhere? I ask because it would help the project a lot if there were a couple strong AM radio broadcast stations near enough to be heard by the project radio receivers.

 

[jedishrfu]

It seems like you may want some more slack in your plan.

You could replace excel with a free software tool like freemat and show them how to plot data. Freemat is a lot like MatLab a software tool they wind up using in college.

Thread 'Freemat --> Plotting Two Curves on the same Plot'

Mar 15, 2025

Here's a simple MATLAB program to plot the sine and cosine curves for the interval 000 to 2π2\pi2π:

Code:

% Define the interval from 0 to 2π
x = linspace(0, 2*pi, 100);

% Compute sine and cosine values
y_sin = sin(x);
y_cos = cos(x);

% Plot the sine and cosine curves
figure;
plot(x, y_sin, 'b', 'LineWidth', 2); % Sine curve in blue
hold on;
plot(x, y_cos, 'r', 'LineWidth', 2); % Cosine curve in red

% Add labels, title, and legend
xlabel('x (radians)');
ylabel('Function value');
title('Sine and Cosine Functions');
legend('sin(x)', 'cos(x)');

% Grid for better visualization...

• jedishrfu
• Replies: 0
• Forum: Programming and Computer Science

You could add in why radio moved away from AM to FM and maybe even show how radio technology can be disrupted by lightning or power lines both buried or overhead.

There was also a crystal radio kit of bygone days that used no electricity to receive and convert radio energy into sound energy.

https://en.wikipedia.org/wiki/Crystal_radio

As a kid in the 1950s, I had a rocket radio which was a tunable crystal radio with a simple earpiece.

 

[tech99]

I realise things are different in the USA, but kids in the UK don't seem to know what a radio is, and don't understand how music can be received by anyone for free. It is really hard work doing this topic with this background. In addition, AM broadcasting is closing down here, and FM is next.

 

[DaveE]

This sounds like a great project.

The diode (aka detector) is a critical element in any AM receiver. By rectifying the received RF waveform it down converts it to the audio frequencies we can hear. Without it all of the signals in the receiver would be near the carrier frequency (something like 1MHz). In general, this would be a mixer, which has to have a non-linear response to work. Every radio receiver has a mixer of some sort, usually several. The diode here is the simplest version of that.

Your question makes me think you should study a bit more about AM radios before you proceed. This seems like a good series of videos, although I haven't listened to much of it. In any case the web is full of information about this subject.

 

[jedishrfu]

Another thought is to show how radio technology was used in WW2:

1) As an early form of radar called RDF or Chain Home system

https://en.wikipedia.org/wiki/Chain_Home

2) Directional radio beams to guide German bombers to targets in England and how the English outsmarted them using redirected radio signals to drop their bombs in the Channel.

https://en.wikipedia.org/wiki/Battle_of_the_Beams

3) The illfated mission of a radio controlled bomber, the Allies sent into France to bomb a German artillery site that was terrorizing London. It was piloted by JFK's brother who died when an errant signal triggered the explosives onboard.

https://www.aviationmuseum.net/JoeKennedy.htm

 

[BiGyElLoWhAt]

Thanks all for the responses.
To address @berkeman , camp is loose. It's on a college campus in the physics department.
@DaveE I'll check this series out. Thanks.
@jedishrfu I just made up those numbers based on what I feel like it will take for me to gloss over a lot of the stuff. Maybe explain something, give them 5m to try to work through it themselves, then do it on the board. Something like that. I'll keep the ideas in your 2nd post in mind for something to do if we get through the project and have extra time.

 

[sophiecentaur]

TL;DR Summary: A few questions about AM radio

and it seems like the most sensible way to broadcast AM would be something like F(ω)sin(ω0) where F is the signa

If you Google "Amplitude Modulation" you will see that there is more to it than just multiplying two signals together. To 'generate' AM you need a Modulator, which is something you really need to get familiar with before you launch on teaching the subject to young minds. If you are aiming at high achievers then you could have some egg on face if you don't have the basics yourself. Wiki has all you need if you have basic trig and algebra. You should find the time to do this yourself ASAP.

But I would recommend avoiding too much theory (stick to what you are competent with) and aim at a practical approach with ready made working modular parts and plenty of graphical displays that can be connected up on a breadboard. Again you need to have someone on board who already has practical experience of this sort of topic. There's nothing worse than a room full of students with circuits they can't get to work and a tutor who can't get them all working either.

It depends on how much money you have to spend on this project but there are 'enthusiast' kits available with all the parts supplied and labeled.

Electronics has a habit of not being 'alright on the night'. Preparation and practice is essential.

 

[berkeman]

If you Google "Amplitude Modulation" you will see that there is more to it than just multiplying two signals together. To 'generate' AM you need a Modulator, which is something you really need to get familiar with before you launch on teaching the subject to young minds.

Could you expand on this a bit please? Can you compare and contrast a multiplier versus a modulator? What other circuit components are you thinking are needed to effectively create the AM transmit waveform?

 

[sophiecentaur]

Could you expand on this a bit please? Can you compare and contrast a multiplier versus a modulator? What other circuit components are you thinking are needed to effectively create the AM transmit waveform?

The difference is that there is a 'DC component' of the carrier envelope in AM so a zero modulation signal input still produces a mean value of output carrier level. I mis-remembered the Wiki article on AM and how (in the foundation section) it ignores / assumes that this is included in the 'A' in :

.
A link with the better description re-states it as follows.
s(t) = Ac[1 + kam(t)] cos ωct
the "1" is the unmodulated carrier level. When the unmodulated carrier level is too low, you get over-modulation distortion or even 'suppressed carrier AM' which is not suitable for a simple diode detector (crystal set)

TL;DR Summary: A few questions about AM radio

Given that I realistically have about 5-5.5hrs, how much background info should I give? I plan on going in this weekend and building one, although I suspect it won't take too terribly long for the actual assembly. I would like to cover circuits a bit and maybe do an ohms law experiment to see the linear relationship between V and I in dc, then talk about inductance and solve an LC circuit.

Do you have any test equipment available? An oscilloscope and variable frequency signal source are essential ( and more if the student groups are to have one each). RF can be hard to work with; how familiar with this stuff are the students? It's not the same as digital devices. Bear in mind that an Ohm's Law lesson takes a full hour and any exercise with reactive circuit elements (just the basics) takes even longer. The action of a frequency selective tuned circuit is non-trivial; have you considered what your message would be here and what level?
One problem is what do do when a student group can't make something work. I have loads of experience of chasing faults in simple circuits and remember endlessly hopping from one group of students to another, 'mending' small connection errors. My point is that, in just six hours, everything needs to be very modular and totally reliable.
I may be being too pessimistic here. Your students may all be very capable and take everything in, first time and you may be a seasoned circuit constructor. That would be great.
If you stick to a simple crystal set and can generate an AM signal in the lab then you are more likely to be sucessfull but I have to repeat my concern about getting kids (all of them) to assemble and operate even the simplest electronic systems.
I remember a student commenting (not about practicals in my EE lessons ) that "these things never work". They had been given lessons by teachers who were just not familiar with EE experiments.

 

[tech99]

You could get hold of an old telephone with a carbon microphone and show how it modulates the current from a battery (zero frequency), listen with an earpiece and show it on the 'scope. Then try it with a AC 50 kHz or something like that. Nothing now heard until a diode is put in circuit as a demodulator. I would be inclined to leave the generation of side frequencies for a second lesson; I am grateful I am not having to use computers and Excel when on a camp.

 

[sophiecentaur]

I would be inclined to leave the generation of side frequencies for a second lesson;

I would say the same about any demonstration / group activity / topic which the tutor is not totally familiar with and can do it blindfold. The students deserve a level of fulfilment on this camp. It would be nice to know the level of experience and knowledge of the OP. Many of his comments are a bit vague.

 

[BiGyElLoWhAt]

The difference is that there is a 'DC component' of the carrier envelope in AM so a zero modulation signal input still produces a mean value of output carrier level. I mis-remembered the Wiki article on AM and how (in the foundation section) it ignores / assumes that this is included in the 'A' in :

.
A link with the better description re-states it as follows.
s(t) = Ac[1 + kam(t)] cos ωct
the "1" is the unmodulated carrier level. When the unmodulated carrier level is too low, you get over-modulation distortion or even 'suppressed carrier AM' which is not suitable for a simple diode detector (crystal set)


Do you have any test equipment available?

I see that there might be a difference, but the differences listed in the powerpoint shouldn't affect how the circuit works.

Yes, I have access to everything in the physics department, including function generators, o-scopes, etc.

 

[BiGyElLoWhAt]

I would say the same about any demonstration / group activity / topic which the tutor is not totally familiar with and can do it blindfold. The students deserve a level of fulfilment on this camp. It would be nice to know the level of experience and knowledge of the OP. Many of his comments are a bit vague.

I'm pretty well versed in electronics, including filtering. I would not claim to be an expert, but analyzing the circuits with the students will be no problem. I'm also pretty well versed (again, no expert) with designing/building/troubleshooting circuits.

I have only officially taught DC circuits classes, although I have tutored and worked with other students on AC, for example in the Engineering 'Dynamics' class.

 

[DaveE]

I have access to everything in the physics department, including function generators, o-scopes, etc.

I'm not sure about the logistics (like number of students, AV capability, etc.), but I think it would be great to show them oscilloscope images in real time of the "pieces"; the carrier, the baseband Tx, the modulated carrier, the demodulated Rx baseband signal. Maybe let the see and hear an overmodulated Tx?

A spectrum analyzer would be great too, but that's probably an overreach for one day. You'd have to teach about the frequency vs. time domain.

I think one significant problem with teaching electronics is that it's all invisible, not relatable to the students previous experience IRL. Oscilloscopes are the first choice to "see" real electronics.

 

[DaveE]

Also, the heart of this receiver, IMO, is the LC tank tuning part. This is something I don't think you can deal with in one day with students that haven't studied calculus, EE or physics. So you'll have to figure out how to skirt around the real functioning there. They won't be ready for issues like resonance or impedance matching done right.

 

[BiGyElLoWhAt]

I think simple LC resonance is easy enough to teach, especially if I do a low-pass on the board. You get the multiply by 1, complex conjugate "trick". Then it's just a matter of 1/10 vs 1/100 vs 1/1000 vs 0. You don't really need calculus to see what's happening there intuitively.

 

[sophiecentaur]

I see that there might be a difference, but the differences listed in the powerpoint shouldn't affect how the circuit works.

Yes, I have access to everything in the physics department, including function generators, o-scopes, etc.

The available equipment will make all the difference . What you say makes me feel more encouraged, too. I'm sorry about my pessimism but so many people post such over baseless optimistic hopes for practical demos and I assume the worst until reassured otherwise. But, as they say "Practice, lady" (New York Cop's answer to a woman who asks him how to get to the Metropolitan Opera House)

If you actually want to produce AM (normal Double Sideband AM (DSSAM) ) as broadcast, you cannot just use a multiplier. Without it you will just get a 'string of beads' on your scope. You have to add a DC component to the AC modulating signal. I don't think you'll have a problem with a suitable circuit but you can't get away without.

No carrier. Can only be detected if a (synced) local carrier is added in the receiver. A crystal set will produce no output or, at best, squawking distortion.

Added carrier - can be detected with a simple diode which will detect just the upper or lower envelope. Compare your circuit with what you will find on line.

 

[vela]

Potential itinerary:
What is Voltage, What is current (10-20m?)
Measure current as you vary voltage and plot (20-30m?)
Inductance intro, solve LC, find resonant frequency (30m?)
Optional Fourier tl;dr; what is it. (5m)
Give coefficients for sawtooth, have them plot more and more terms in excel, (30-40m? do they know how to use excel? not sure.)
Give schematic, build... (rest of day)

Any suggestions for my timeline? Are my time estimates somewhat reasonable? I'm condensing a lot and I know they're not going to leave a 1 day session experts. I am just trying to find a good balance of doing things vs. knowing what you're doing that works in the allotted time.

Have you taught these topics before? What is the background of the students? My gut feeling is your timeline is pretty optimistic unless you're covering everything superficially. I'd opt for focusing on fewer topics more deeply.

Maybe you could split the day in two. First part, have them drive an RLC circuit with a sine-wave input and measure the output voltage as a function of frequency. They'll actually see the resonant behavior which provides the motivation for your explanation of what's happening. In the second part of the class, you can show how to use this behavior practically to build the AM radio.

I assume your answer to Berkeman's question about where you are meant that there are indeed AM radio stations nearby that will provide a strong signal.

What are you going to use for an antenna for the radios?

 

[Averagesupernova]

This is pretty much wide open concerning what could be taught here.

-

I think it's important to have a good grasp of signals and systems theory before delving into AM.

-

As has been discussed here in the past the actual carrier of the AM signal does not vary. This not typically taught to beginners.

-

In my opinion the best thing you can do for someone who wants to learn about AM is to explain about the frequency domain and how we can filter for the frequencies of interest. We don't necessarily need to explain how filters work, but impress on the students that we can do this. The sooner the student can visualize signals in the frequency domain, the better. Explain that no matter what, anything done to a pure sine wave will generate new frequencies.

-

I would have a basic signal generator capable of AM, scope, and spectrum analyzer for demonstration. The spectrum analyzer could be a laptop running audacity or something similar. We don't have to use actual radio frequencies for demonstration. The frequency can be turned up for listening on an AM receiver for that part of the demo.

-

Then I would explain how the carrier is multiplied by the modulating signal by using a spreadsheet with some values to cover several cycles of the modulating signal. Graph it to display the composite signal. Have your formulas set up so it's easy to add DC offset to the modulating signal to show how the carrier represents the DC portion of the modulating signal.

-

This approach won't teach them how to build an AM transmitter with parts and a breadboard but it's a lot of info that can be set up ahead of time. I think it's more important to understand it first than to build it and say "Wow, I built a transmitter. Wonder how it works."

 

[sophiecentaur]

"Wow, I built a transmitter. Wonder how it works."

That would be the ideal outcome. I'd love for that outcome but in such a short time this could be very optimistic. Very few kids seem to have the same skill set that our / my contemporaries took for granted. They now learn from computer simulations and, at best, assemblies of modular components (the LEGO approach). Any short experience just has to involve mostly ready-assembled equipment. If there's enough time to obtain / build the stuff then bright kids will surely make the learning connections.

Then I would explain how the carrier is multiplied by the modulating signal

As I've already pointed out, "Multiplication" is really not the best term for modulation in general. OK for arm waving but hardly the way to "explain" how normal AM work. The envelope of an AM signal is the best approach (pictorial and looks like a scope trace) A good way to get the idea of AM across is to use the amplitude control of a signal generator and wobble it fast enough for the scope to show it. (A digital scope could be best). Would Maths actually be needed?