FM Modulation/de-modulation Circuit

Picture of FM Modulation/de-modulation Circuit

I got the idea for this circuit from one of my professors. That design wasn't meant for passing frequencies high enough to be able to pass an audio file, e.g. 500Hz+, so I built this by modifying the carrier and signal frequencies, using only the Digilent Analog Discovery and the Analog Parts Kit. It should be noted that this circuit is primarily for educational purposes. Also note that there is no radio transmission here either. FM doesn't necessarily mean radio waves have to be involved.

Throughout this Instructable I will be going through some of the functions and features of the Analog Discovery, but it will not be an exhaustive tutorial.

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Step 1: Parts and tools

Picture of Parts and tools
op482 pin_out.gif
ad654 pinout.jpg

All of the parts listed below are provided in the Analog Starter Kit, except for the 9V batteries and clips. They should also be available through various online suppliers, local shops, or the salvage bin on your workbench.

The Parts:

- 2 OP482 quad op-amps (datasheet)

- AD654 voltage to frequency converter (datasheet)

- 9 ceramic capacitors: 2 - 39pF (39), 2 - 100pF (101), 2 - 1nF (102), 2 - 4.7nF (472), 1 - 10nF (103)

- 17 carbon film resistors: 1 - 68, 3 - 1k, 1 - 6.8k, 3 - 10k, 1 - 20k, 1 - 47k, 2 - 100k, 5 - 470k

- 1 1N4001 rectifier diode

- 1 speaker

- solderless breadboard

- jumper wires




- 2 9V batteries with clips

The Tools:

- Digilent Analog Discovery

- Waveforms software (free but required to run the Analog Discovery)

- short, free .wav file download. I used this one because it was awesome.

Pin-out diagrams are included for the OP482 quad op-amp and AD654 voltage to frequency converter, also called a voltage controlled oscillator or VCO.

what a cool project! is it used for transmitting digital binary signals or analogue waveforms only?
brmarcum (author)  smooth_jamie28 days ago

It's designed for analog waveforms, but you could probably send digital binary signals through. Your VCO would go from low to high frequency with hardly any transient stuff in between, so as long as the filters passed the data through while filtering the garbage, I don't see why not.

sweet! I've got a good project for this one. Looking forward to writing my own instructable ;-)
brmarcum (author)  smooth_jamie26 days ago
Sounds great. Be sure to let me know when you post, I'd like to see it.

i can not download the file
help me please
thank you.......................

brmarcum (author)  bahram11871129 days ago

Which file? The .wav? It's called force_be_with.wav at this site.

no wav file
this is a all file of the site(instructables)
brmarcum (author)  bahram11871129 days ago

You can only download a PDF of the whole Instructable if you have a Pro account. If you click on the "Favorite" tab at the top it will save it in your account info so you can find it easier if you want to come back to it.

i can not download the file
help me please
thank you.......................

i can not download the file
help me please
thank you.......................

i can not download the file
help me please
thank you.......................

i can not download the file
help me please
thank you.......................

FM modulation is what the human body uses to send signals on nerves to and from your brain and your muscles and sensory systems. A number of Neurologists and Neurosurgeons have the Amateur Radio Handbook (ARRL) on their shelves.

Cool looking project!


brmarcum (author)  John T. McF. Mood1 month ago

That is very fascinating. One of my professors is currently researching how to map the brain as a series of cascaded low-pass filters, and that fits with what you mentioned.

I had to have a nerve conduction test, and it was fascinating to watch, but not a really 'fun' procedure. I was alert, and slightly numbed by a pill they gave me, but I clearly remember the oscilloscope and the impulse they sent down my arm. Funky to see your hand move when you didn't tell it to! That Doctor had the book... Turns out I have carpal tunnel syndrome as well as some minor nerve damage, but not too bad, I work around it.

brmarcum (author)  John T. McF. Mood1 month ago
That sounds interesting and sort of crappy at the same time :/ Do you remember what frequency ranges we are sensitive to?

Particularly to frequencies that are harmonic to our body length or skull size, or lung size. I'm almost 2m tall, so 144 MHz to 148 MHz for me. I'd recommend minimalizing exposure to ANY frequencies, we're really susceptible to a wide range of radiation & frequencies. Radiation exposure is cumulative, and RF is radiation. So is sunlight for that matter, Suncreen at all times....

brmarcum (author)  John T. McF. Mood1 month ago

So that's why you can extend the range of your car keyless-entry fob by putting it up to your skull. I guess I'll stop doing that then...

Thanks for the info.

That is probably due to elevation more than your noggin. But there's no point in exposing your head unnecessarily. Elevation almost without exception extends range on transmit and receive.

If like me, you lose your car in a parking lot, raise it well above your head then push the button. . Next to your head it would not resonate, reflect or re-radiate, it would absorb and not be good. The FCC publishes RF exposure safety level information, as does the ARRL.

When handheld RADARs became used by police some would sit it on the seat on in between their legs - [pointed at their privates], and a few guys ended up with cancer of their private parts. Yeah, WAY not good.

Whole point being, unneeded or unnecessary exposure to radiation of any type or frequency is not a good thing. They put a lead shield over your groin & chest when x-raying your teeth. (If they don't TELL THEM TO!) If they won't find a different dentist. They stand behind a lead lined wall... I had to gripe at an orthopedist's x-ray tech who said you're older and not fathering kids. I mentioned their lead wall, and said give me the shield blanket, or I'm out of here. The jerks wear dosimeters and after they get exposed to a certain level they have to change fields and cannot work around x-ray equipment any more. Remember cumulative.

This project is more likely to be used in audio frequency ranges though, and probably would be rather innocuous. Magnetic field exposure hasn't been shown yet to be a problem except at extreme levels.

As the ARRL books say, DC to Daylight...

eburman1 month ago

What a great project for teaching fundamental analog electronics! I'm inspired to try it myself. And that USB oscilloscope looks great too. One thing, I'm having trouble with the link to the schematic. It seems to be broken. Is there another way to get it?

brmarcum (author)  eburman1 month ago

I have no idea why but the file won't load properly, so I just removed it. It was just a Word document that had the two schematic images from step 2, so nothing is lost. Thanks for the heads up.

bergerab1 month ago

Awesome! I have so much to learn from this instructable. Great diagrams, schematics, and pictures.

brmarcum (author)  bergerab1 month ago
Thanks bergerab. It is meant to be more educational than anything, but the individual sub-circuits are very useful.