Introduction: 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.

Step 1: Parts and Tools

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

(optional)

- 1 TIP31C NPN BJT

- 1 TIP32C PNP BJT

- 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.

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.

Step 2: Designing

The schematic for this can be daunting to say the least. When I first started working with it, I got lost all the time and had to keep going back over it again and again. You'll notice in the schematic that there are breaks throughout the design. Each of those breaks represents a good stopping point for verifying the circuit function. Take it slow and add one section at a time. The OP482 ICs are quad op-amps, meaning four op-amps on one piece of silicone. That can make it tricky to hook everything together as components can get crowded. This project can be built using individual op-amps if that is easier for you, but I would recommend using the OP27 and not the LM741. The LM741 should work, but I have had problems with it when it comes to needing fast, reliable response times. The specs are just a bit too slow for some applications, specifically the skew rate when you need high gain at a relatively high frequency.

For clarification the schematic image is broken up. Point A matches to point A, B with B.

Step 3: Frequency Modulation

I won't go into the details of FM or AM technology too much here. I would refer you to Wikipedia for that (note the animation on the right side of the page). In a nutshell, for FM signals, the frequency of the signal varies based on the voltage level coming in. So a high voltage gives a high frequency, and vice versa. With AM, the frequency stays the same, but the amplitude goes high or low depending on the input voltage. As you will see, this circuit actually uses both.

The higher frequency signal is the carrier frequency, while the original signal is the data that you want to relay. We will use the data signal to modulate a much higher frequency carrier wave. Later we will filter out the carrier and recover the data.

The first three sections of the schematic are the modulation circuit. We first modulate the carrier wave, then amplify it. The AD654 changes the frequency of the output based on the voltage level of the input. The timing resistors and capacitors give a base frequency value for the IC to work with based on the equation found in the datasheet.

Using pages 1 and 4 of the datasheet for the AD654, connect the components as shown. Note that 'Vs' indicates voltage supply, so '+Vs' is +5V and '-Vs' is ground. '+Vin' is the signal coming from the Analog Discovery AWG Channel 1 pin, labeled 'input' in the schematic.

Turn on the Voltage function from the Waveforms main window (image 2). Click V+ to 'ON', connect the red V+ wire from the Discovery header to pin 8 on the AD654, then turn it on. Using the Discovery's AWG channel 1, output a 1VDC signal (image 3) and plug it into pin 4 of the AD654. You should get a readout on the scope like image 4.

Next we add the first OP482. The first part is a simple voltage follower or buffer. Since op-amps are active components, we will get a boost in current from this part. Next we add an active low-pass filter set to cutoff at approximately 50kHz, which is high enough to allow our signal to pass through but low enough to filter out random high frequency noise we don't want. When connecting the OP482, connect the Discovery's red V+ wire to pin 4 and the white V- wire to pin 11. Turn on the power supply (like in image 2, but both channels this time) before turning on AWG 1 with the signal. Set AWG 1 to an amplitude of 500mV this time. Your scope should look something image 5.

Step 4: Differentiator and Envelope Detector

The theory behind most of these parts is a bit much to get into here. Again, check out wikipedia for the differentiator (note that this one is active because of the op-amp) and the envelope detector explanations. In short, the differentiator, believe it or not, differentiates the signal (think calculus and it makes total sense). As the input signal rises or falls quickly (high frequency), the output amplitude increases. As the input rises or falls slowly (low frequency), the output amplitude decreases closer to 0. This has the interesting effect of turning the FM signal into an AM signal. See the o-scope display in image 2 for a comparison (scope CH 1 to pin 14 of the OP482 and CH 2 to pin 8).

The envelope detector detects only the top edge of the signal, so as the amplitude rises and falls, so will the signal coming out of the envelope detector. See image 3 for an example (CH 1 to pin 8 of OP482 and CH 2 to cathode of 1N4001 diode).

Step 5: Filtering

The signal from the envelope detector doesn't have much amplitude and contains quite a bit of noise. So we add several filters to both clarify and boost the signal. Look at the o-scope display in image 2. The red CH 1 signal is a 25kHz carrier with a 1kHz modulation frequency, amplitude of 500mV and index of 10%. These settings are all under the advanced tab on the Analog Discovery AWG tool (see image 3) if you would like to generate the signal yourself. This signal then is inserted into the input of the differentiator, bypassing the modulation circuit for now. The blue CH 2 is the output of the envelope detector, before any filtering. Do you see the scale on the left of the display in image 2? Each division is 20mV for CH 2. Most scopes will let you choose different scales for each channel, and here the scale for CH 1 is 200mV/div, larger by a factor of 10. With these two signals on a scale of 200mV/div, you would barely be able to see the output of the envelope detector because it is so weak. You can also see the noise in the signal as well in image 2. We need to filter that out and amplify the signal by a fair amount. We will also be running the signal through a band-pass filter to only allow audible frequencies between 200Hz and 2kHz to pass.

The first part is the band-pass filter. It allows only a small range of frequencies to pass through, and in our case 200Hz to 2kHz is more than enough. Note that because the resistors match, there is no gain here, only filtering. We don't want to amplify noise, so let's filter it out first.

After the band-pass filter we have two low-pass filters, the first with gain of approximately 23.5 and cutoff of about 8.6kHz, and the second with unity gain of 1, which means no amplification, and cutoff of about 3.4kHz. Even after the band-pass filter we still have some noise from the carrier wave (filters aren't perfect after all) and the signal is weak. We filter out noise above 8.6kHz and then amplify it 23.5 times. Yep, it's high but it needs it (you can change it by changing the resistor ratio. Click the low-pass filter link above for the equation). The second low-pass filter then further cleans up that signal by filtering noise above 3.4kHz, which is higher than our target range of 200Hz-2kHz, so we're good. The second filter is not absolutely necessary, but it did make a difference in the end.

Image 4 shows the difference in the signal between the output of the envelope detector and the output of the last filter. The scale is not exactly the same as before, but the data was just outside of range so I shrunk it down a little bit. Did you notice the measurement window at the bottom right? That's the 1kHz modulation value we saw in the settings in image 3 above. It's not a perfect sine wave, but it's a far sight cleaner than it was before, and it's good enough that you won't notice the difference.

If you don't want to add the class B amplifier, you're done here. Otherwise, get yourself some 9V batteries and turn the page.

Step 6: Amplifying the Signal

The 9 V batteries and connectors are not included with the Analog Parts Kit, and it is not absolutely necessary to amplify the signal coming from the last low-pass filter. However, as you can see from comparing images 2 and 3, the signal will clip when you add the speaker and form more of a square wave than a sine wave. A small 8-Ohm speaker will clip even more. This happens because the speaker itself also has a transfer function that must be accounted for, which is a fancy way of saying that it is trying to draw too much current at peak voltage, so it clips the voltage. It will actually sound louder, but won't be as clear. Adding the amplifier allows more current to be available and that allows the voltage level at the speaker match the output level at the last filter. Image 4 shows the signal with the amplifier in place and the speaker attached. The difference is minimal this time. I've used this on a 4-inch computer speaker I salvaged and even a 6-inch woofer and they all worked with this amplifier. Just remember that larger speakers require more current so you will drain your batteries faster.

When putting the BJTs on the breadboard it is very easy to get the pins mixed up. I've built this circuit, start to finish, at least half a dozen times, and I still did it. Again. I've included the pin-out diagram for them (they have the same B/C/E pin assignments), so good luck.

Step 7: Testing

Now let's do some testing to verify. If you haven't yet downloaded a short .wav file, do so now. Go into the AWG tool settings and check both channels to turn them both on. Click on the 'player' tab, then on 'add'. A window will come up to allow you to find the downloaded .wav file. Double click on it to load it. Notice the playback frequency in the window to the left? My file is set at 11.02kHz. For now, set the AWG channels to playback at this frequency. You won't break anything if you change it, it just changes the playback speed of the file. Load the file for both AWG channels. There are notes on the image that will help clarify this step.

Set the channels according to image 1, making sure that the offset for CH 1 is 1.25V. It will make a big difference and took me a while to troubleshoot the first time. The amplitude (volume) settings are not super critical, but remember that we are amplifying the signal by quite a lot in the circuit, so start smaller and work your way up as desired.

Start with CH 2 on the AWG and plug the yellow/white wire directly into the speaker, bypassing the entire circuit. This way you can hear the file playback on the speaker before passing it through the circuit. Once you have it playing on the speaker using CH 2, plug CH 1 into pin 4 of the AD654. Unplug AWG CH 2 from the speaker and connect the output of the class B amplifier to the speaker. You should hear the same playback either way.

And there you have it. Please don't hesitate to ask questions, either in the comments below or PM. Have fun building!

Comments

author
Shreyasi_n (author)2017-07-08

Hi! The modulation ckt consists of 3 sub blocks..so is it the last block which generates the carrier freq?? N if yes than what is its time constant or simply what is the carrier freq?? Kindly reply asap its urgent.

author
brmarcum (author)Shreyasi_n2017-07-09

The AD654 voltage controlled oscillator is what generates the carrier frequency based on the values of the timing capacitor and timing resistor. The equation can be found in the datasheet.

author
Itssalunkhe (author)2016-07-01

ThankYou sir, that's very nice of you, and hha im sorry but I'll disturb you again for further query's. ThankYou

author
Itssalunkhe (author)2016-07-01

You are connected analog discovery as a input, so i can definitely use instead of that any common frequency generator, and another question is is this device only works on radio frequency because radio frequency is very small and i want to do it with normal frequency ie 20kHz - 25 kHz

author
brmarcum (author)Itssalunkhe2016-07-01

Yes, you can use any frequency generator.

This circuit is not working at radio frequencies. The capacitor and resistor values for the various filters are set to pass frequencies between 200Hz and 2kHz. If you want to pass frequencies of 20kHz-25kHz, you will need to modify the pass-band limits of the filters. You will also need to increase the frequency modulation. You want your carrier wave to be at least 10X faster than your data frequency. Mine was assuming a max data frequency of about 2kHz, so I set the AD654 to output ~20kHz at 1VDC. If you want to pass 20kHz-25kHz, you need a carrier frequency of about 250kHz minimum.

author
Itssalunkhe (author)2016-06-30

Hello, is there any other option with analog discovery. Its costly. Plz help

author
brmarcum (author)Itssalunkhe2016-06-30

Yes, you can get a separate oscilloscope and signal generator, but not for anything near as low price as the Analog Discovery. For an all-in-one, USB powered, pocket size instrument, you won't find a better value.

If you are a student at an accredited university, both the Analog Discovery and the new Analog Discovery 2 are available at academic price points.

author
Abhi909 (author)2016-01-31

great project

can i place your project on my website.

i'm working on a website which is related to electrical projects.

i also mention your name, link and other info.

plz reply

author
brmarcum (author)Abhi9092016-06-30

Sorry for the late response. I don't mind as long as you attribute. Thank you.

author
ShekharV (author)2015-09-14

Can i use this circuit to send signles for wireless bell or circuits??
Please let me know as soon as possible

author
brmarcum (author)ShekharV2015-09-18

This circuit is not designed for wireless transmission as it is. It is primarily for educational purposes to show how each part works, and how they interact together.

author
ShekharV (author)2015-09-15

Plzzz... Can anybody tell me?

author
LianashaL (author)2014-12-26

Helo, im trying to find a project about FM Modulation, can u give some suggestion about how to do the circuit , please reply me or email me

author
smooth_jamie (author)2014-07-02

what a cool project! is it used for transmitting digital binary signals or analogue waveforms only?

author
brmarcum (author)smooth_jamie2014-07-02

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.

author
smooth_jamie (author)brmarcum2014-07-04

sweet! I've got a good project for this one. Looking forward to writing my own instructable ;-)

author
brmarcum (author)smooth_jamie2014-07-04

Sounds great. Be sure to let me know when you post, I'd like to see it.

author
bahram118711 (author)2014-07-01

hi
i can not download the file
help me please
thank you.......................
bahram_mooud@yahoo.com

author
brmarcum (author)bahram1187112014-07-01

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

author
bahram118711 (author)brmarcum2014-07-01

no wav file
this is a all file of the site(instructables)

author
brmarcum (author)bahram1187112014-07-01

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.

author
bahram118711 (author)2014-07-01

hi
i can not download the file
help me please
thank you.......................
bahram_mooud@yahoo.com

author
bahram118711 (author)2014-07-01

hi
i can not download the file
help me please
thank you.......................
bahram_mooud@yahoo.com

author
bahram118711 (author)2014-07-01

hi
i can not download the file
help me please
thank you.......................
bahram_mooud@yahoo.com

author
bahram118711 (author)2014-07-01

hi
i can not download the file
help me please
thank you.......................
bahram_mooud@yahoo.com

author
John T. McF. Mood (author)2014-06-29

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!
John

AA4PC

author

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.

author

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.

author

That sounds interesting and sort of crappy at the same time :/ Do you remember what frequency ranges we are sensitive to?

author

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....

author

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.

author

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.

author

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

author
eburman (author)2014-06-29

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?

author
brmarcum (author)eburman2014-06-29

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.

author
bergerab (author)2014-06-25

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

author
brmarcum (author)bergerab2014-06-25

Thanks bergerab. It is meant to be more educational than anything, but the individual sub-circuits are very useful.

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Bio: I've always loved to figure out how things work, so hacking and making just fits for me. I'm a husband, a father, an ... More »
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