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in this video i am going to talk about frequencymodulation, or fm, and show you a simple fm radiotransmitter circuit that can transmit your voice about 20 meters. this video builds upon material i coveredin my amplitude modulation tutorial so make sure you'vewatched that first. remember that modulation is the process of manipulating a carrier waveto carry useful information, and amplitude modulationis when you change the amplitude of a waveform inorder to transmit analog or digital information i have an example here with my waveform generatorthat is putting out a 1mhz sine wave that

isbeing amplitude modulated at 1hz. now this is what a frequency modulated waveformlooks like. the amplitude stays the same, but weare changing the frequency. in this example the carrier wave is 1mhz, but we are slowlychanging the frequency so it's transitioning between 0.9mhzand 1.1mhz. we are doing the modulation here at 1hzso it's nice and easy to see. and this is what it looks like at a more realistic frequencyof 1khz. the shape of the wave on your oscilloscope willdepend on your trigger settings. now in real life, no one is going to frequencymodulate a 1mhz waveform, and we aren't transmitting

any information that is actually useful, solet's move on to a practical example. consumer fm radio isusually transmitted around 87.5 to 108mhz. that's a much higher frequency than most people'sbenchtop waveform generator can handle, so we are going to have to come up with our owncircuit 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 oscillationfrequency so we can do frequency modulation. the type ofcircuit that we want here is called a "voltage controlled oscillator", or "vco". there aremany different

types of voltage controlled oscillator circuitsout 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 themare very unstable, so i made my own. this circuit requires a pcb, and i got minefrom 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 3.3vlinear 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 frequencyrange that the max2606 will oscillate at. if you read thedatasheet, you'll see that this chip can oscillate from 70 to 150mhz, and we just want it toput out roughly 100mhz for the fm band. 390nh takescare of that. there's actually a small amplifier insidethis oscillator chip, and these resistors form part of it. i wantyou to know that this isn't the optimal way to set up a radio amplifier, i'm just showingyou something that's low powered and will work across awide range of frequencies. for the antenna, you can just use a pieceof wire about a meter long, but only add it

after you aredone testing things. this capacitor forms a high pass filter to make sure that no dcvoltages can accidentally reach the oscillator.now this pin - the tuning pin, is where the fun starts. by applying a voltage here, wecan control the exact frequency that the oscillator spitsout. so let's do that. let's add a simple potentiometer voltagedivider circuit to apply an adjustable dc voltage to the tuning pin. now if you adjust the potentiometer, you changethe voltage at the tuning pin, which controls theoscillation frequency. and we can choose pretty

much any frequency in the consumer fm radioband. now let's add a microphone. it's the samebasic microphone circuit you've seen in many of my videosbefore. this will add a tiny, millivolt level voltage swing on top of the larger dc tuningvoltage. this will modulate our carrier wave just enough tocarry audio. solder the microphone onto the bottom sideof the pcb and add your antenna. now set your fm radio to a blank station,and use the potentiometer to adjust the frequency of yourtransmitter 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 poweredenough that it's legal to use in most countries. howeverwhat is absolutely not legal anywhere is intentionally trying to transmit on someone else's frequencyand 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 useit to transmit audio. don't forget to thumbs up if youliked this video and check out my channel for more videos about electronics!