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!