Frequency Modulation tutorial & FM radio transmitter circuit

Frequency Modulation tutorial & FM radio transmitter circuit


In this video I am going to talk about Frequency
Modulation, or FM, and show you a simple FM radio
transmitter circuit that can transmit your voice about 20 meters. This video builds upon material I covered
in my amplitude modulation tutorial so make sure you’ve
watched that first. Remember that modulation is the process of manipulating a carrier wave
to carry useful information, and amplitude modulation
is when you change the amplitude of a waveform in
order to transmit analog or digital information I have an example here with my waveform generator
that is putting out a 1MHz sine wave that is
being amplitude modulated at 1Hz. Now this is what a frequency modulated waveform
looks like. The amplitude stays the same, but we
are changing the frequency. In this example the carrier wave is 1MHz, but we are slowly
changing the frequency so it’s transitioning between 0.9Mhz
and 1.1MHz. We are doing the modulation here at 1Hz
so it’s nice and easy to see. And this is what it looks like at a more realistic frequency
of 1kHz. The shape of the wave on your oscilloscope will
depend on your trigger settings. Now in real life, no one is going to frequency
modulate a 1MHz waveform, and we aren’t transmitting any information that is actually useful, so
let’s move on to a practical example. Consumer FM radio is
usually transmitted around 87.5 to 108MHz. That’s a much higher frequency than most people’s
benchtop waveform generator can handle, so we are going to have to come up with our own
circuit to accomplish this.
What we want is a circuit that oscillates, and produces a sine wave of around 100MHz.
But we also want to be able to control the oscillation
frequency so we can do frequency modulation. The type of
circuit that we want here is called a “voltage controlled oscillator”, or “VCO”. There are
many different types of voltage controlled oscillator circuits
out there, but not all of them are suitable for FM radio.
And there are many FM transmitter circuits out there, but in my experience most of them
are very unstable, so I made my own. This circuit requires a PCB, and I got mine
from OSH Park. It cost me $3 for 3 of them including shipping.
I’ll put a link in the video description section. The first part of the circuit is just a 3.3V
linear regulator to create a nice stable 3.3 volt supply.
The main part of the circuit is a MAX2606 voltage controlled oscillator IC. This inductor sets the approximate frequency
range that the MAX2606 will oscillate at. If you read the
datasheet, you’ll see that this chip can oscillate from 70 to 150MHz, and we just want it to
put out roughly 100MHz for the FM band. 390nH takes
care of that. There’s actually a small amplifier inside
this oscillator chip, and these resistors form part of it. I want
you to know that this isn’t the optimal way to set up a radio amplifier, I’m just showing
you something that’s low powered and will work across a
wide range of frequencies. For the antenna, you can just use a piece
of wire about a meter long, but only add it after you are
done testing things. This capacitor forms a high pass filter to make sure that no DC
voltages can accidentally reach the oscillator.
Now this pin – the tuning pin, is where the fun starts. By applying a voltage here, we
can control the exact frequency that the oscillator spits
out. So let’s do that. Let’s add a simple potentiometer voltage
divider circuit to apply an adjustable DC voltage to the tuning pin. Now if you adjust the potentiometer, you change
the voltage at the tuning pin, which controls the
oscillation frequency. And we can choose pretty much any frequency in the consumer FM radio
band. Now let’s add a microphone. It’s the same
basic microphone circuit you’ve seen in many of my videos
before. This will add a tiny, millivolt level voltage swing on top of the larger DC tuning
voltage. This will modulate our carrier wave just enough to
carry audio. Solder the microphone onto the bottom side
of the PCB and add your antenna. Now set your FM radio to a blank station,
and use the potentiometer to adjust the frequency of your
transmitter until you hear silence. And that’s it! Your FM transmitter is complete. (You broke the law!) Actually I didn’t. This design is low powered
enough that it’s legal to use in most countries. However
what is absolutely not legal anywhere is intentionally trying to transmit on someone else’s frequency
and drown out their station. It’s pretty easy to catch people who do this, so don’t do it. Ok, now you know what FM is and how to use
it to transmit audio. Don’t forget to thumbs up if you
liked this video and check out my channel for more videos about electronics!

100 thoughts on “Frequency Modulation tutorial & FM radio transmitter circuit

  1. You mentioned it was easy to catch people trying to drown out FM signals. Would you consider doing a video about how they catch those people? I want to believe they use some totally rad high tech tools

  2. Just wondering if you use a oscillator driving a 74AC74 in such a way to generate two signals 90 degrees out of phase then use one of the two signals to drive a 74HC4053 configured as a double balanced modulator then put the other signal you generated with the 74AC74 and filter that and add that to the output of the 74HC4053 balanced modulator using capacitor coupling then triple up the resulting signal then feed it through an RF amp then a bandpass filter then et voilà you have an FM transmitter, BTW you need a frequency of around 40MHZ to start with as the 74AC74 phasing generator divides the signal by four so if you want 10.7mhz so you can use a 10.7mhz crystal filter use a 42.8mhz crystal.

  3. wow thanks. didn`t know osh shipped for free to Norway. what a great company. Good work on the pcb. Do you have a parts list?

  4. So if you modulate the frequency and thus making it inconstant, how is the FM signal from your favourite radio station a certain frequency and not a frequency range?

  5. So…what is the actual output in watts/milli-watts..? Anything like the stuff I made as a kid in the1950's?…AM…restricted to 50 milli-watts…good for a few 100 feet….

  6. hey Afrotechmods,
    Thanks for sharing the pcb design on oshpark. could you please post or share the parts list? Especially the source for the smt variable resistor, and the other parts as well?
    A good place might be over at oshpark along with the pcb.

    thanks
    dwight

  7. how does FM modulation works? I mean, I set the frequency to 97mhz on my radio and the frequency from the station keeps changing since its FM. how my radio knows to with frequency it should oscillate to receive RBS signals?

  8. Excellent…. I've been wondering about this sorta jam for a while…. I kinda want to use radio waves to power small battery operated devices and AM has voltage spike issues.

    Utilizing FM band might be ok, provided I'm close to a radio tower. (I live in a city so… s'alright.)

  9. +Afrotechmods I'm attempting to figure out how to build an interior light dimming circuit for automobile applications. In some cars, when you shut the door, the interior light dims all the way to off which is a feature I'd like to incorporate into my pickup truck. My interior lighting has all been replaced with LEDs, so I shouldn't have much current to work with. Would a capacitor or two with a substantial microfarad rating be sufficent, or would I need some extra circuitry?

  10. I believe transmitters over 300 watts is banned in the U.S and also phasing out other station's is a definite no-no, high power antenna's and/ or boosters. You can get 150 watt or so studio quality transmitters online that's purposely made for personal ham radio use. Also since analog tv is pretty much phased out, new law enforces you can do UHF/VHF transmission with strict policy's as well for hobby reasons. Don't forget courtesy law, every curse or inappropriately used word is a $500 fine.

  11. Japanese commercial FM band is 76 – 90MHz.
    In Japan, no license is needed for devices with a signal strength of less than 500µV/m at 3 metres.

  12. I don't know much about "electronics". Could you just hook up a high gain op amp to boost the carrier wave? If they can run at that frequency that is.

  13. Back then in early 1980s a wireless microphone that can be receive on a regular FM receiver was common. No need a dedicated receiver like today. Some of them are factory set at 90 MHz while others 92 MHz. You can change the frequency by adjusting the potentiometer. Most of them have rage about 100 meters (according to manual). I can't find those type of wireless microphone these days. All wireless microphone that I saw today have a dedicated receiver.

  14. Can you make any good book recommendations that would help fortify the knowledge obtainable on your channel in regards to electricity/electronics?

  15. Crazy how I just learned more from this videos then I did in my University class where we built an analog kit version of this.

  16. Another cool way is to oscillate a transistor were the C B junction forms part of the resonant circuit. The C B junction exhibits the varicap efect. By introducing an AF voltage on the collector you end up with modulated RF with a single transistor.

  17. What's the purpose of having the DC bias on the Microphone's + line? Isn't it just immediately stripped out by the capacitor?

  18. Best tutorial ever; now I'm going to go jam radio receivers with the sound of my gas generator I'll be using to power the transmitter xD

  19. You know just the way to explain electronics. Describing stuff, giving examples, illustrations. Explaining why we can use a certain component and why we must avoid one. I can't believe my teachers made all this look so boring back in college. I am learning more now than I did in college. Thank You! 🙂

  20. Can Oshpark make PCBs out of copper clad boards from DigiKey (that is, boards designed for a specific enclosure)? If not, are there any PCB services that do?

  21. Another stellar instructional video. I will be building this myself to better understand the ideas you put forth. $3.00 boards are impressive. I have been spending WAY more than that so thanks for the great tip!!!
    I see you used surface mount components. Tough to do on my bench. I assume the surface mounting reduces stray capacitance and inductance. My breadboard versions usually are a bit tricky to get working especially in the FM range. Anyway, Great video. Thanks!!!!

  22. Mentioning that "you will be caught" by using a high powered transmitter, could you do a video on ADF? Automatic direction finder most commonly used in aviation as an old form of navigation. The system is now used in most planes today to catch an AM baseball game.

  23. So what would stop a car stereo to stop picking up fm,I am trying to figure out why a 40 year old radio just stopped getting all fm stations but 1,and it comes in just barely. I was listening to it and the stations just faded away. Sad thing is I have a ham certification but you don't really learn anything in the classes other than how to cram for the test

  24. I've made this circuit but my Max2606 ic is getting really hot, also I'm not receiving any signal on radio, kindly help…!

  25. afro I love your vids and I would really love to meet you. p.s my name is Will cooper not Nigel. By the way I highly recommended Colin Cunningham's channel which has a playlist marked as Collins lab, Circuit skills and Make presents which you must watch. yours sincerely William Cooper.

  26. I can't find a Max2606 in my country.I also can't find an internal diagram of the Max2606(I am thinking of making one myself).Please help!

  27. can youplease explain how video transmisson works ? for exemple for drones they use rf transmission for video but i couldn't finde any information about that ! thank you

  28. What would be interesting is describing how the IC works. Its not interesting to use IC out of the box and not explain how they work.

  29. I have tried LC tank based fm transmitters although i did not fully understand the feedback section of those this one seems way more easy to work on and build. Without frequency drifting off it would be fun to play with but here no online market sells this chip i found one but its tagged like this MAX2606 +AABC never seen this +AABC as explanation for any chip. Is this the same one?

  30. Is there a way to modify this circuit to get a larger range? I'm looking to transmit just a mile or two on an open frequency

  31. Interested in learning about wireless power? Subscribers can get up to 80% off my course Wireless Power to the People – Wireless Charging 101 on udemy using the coupon code "YOUTUBE"
    http://www.udemy.com/wireless-power-to-the-people-wireless-charging-101/?couponCode=YOUTUBE

  32. Hi, your videos are the first place I look when i want to know how to connect and use certain electronic components, thank you so much. I was looking at your AM transmitter circuit as well and I have the same question. What IC can I use so that the circuit doesn't necessarily have to be low powered. I want to obtain a longer range, I visit my grand parents very often at their ranch where cellphone reception is difficult to obtain so I think it would be fun to design a circuit for communicating. By the way, I'm an engineering student, I'm mentioning this in case it modifies in any way your response.

  33. How about hho generation on demand using 27 MHz Maxwell bridge plates to trigger isolated gases further into others using half rectified 120v. Electrolytes with membranes to seperate

  34. Just ran across your channel. Loved the presentation style and the technology images, you tie this all together very well. I expect I'll be watching some of you other videos. Keep broadcasting!!

  35. For the inductor, would a couple of turns of wire do the trick? I don't have an LCR and my scope can't measure 100mhz.

  36. I've build one of this unstable transmitters and I have a lot harmonics. Is there a way to prevent that?

  37. Neat but on eBay the have already put together modules fm tranmitter,for about the same.price,because your board came with out componets.I no this is a older video ,but I think both ideas are valid

  38. On eBay just type in fm modules,they are cheap they have both transmitter and fm receivers just a thought

  39. Hi anyone know why the Tune Pin pad has a wire soldered to it rather than an SMD Cap and ground connection on the board? Do you desolder it once it's tuned? Thanks for the video!

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