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Topic: Frequency Modulation Using Arduino (Read 1 time) previous topic - next topic

Cyclicz

I am working on a circuit to take the place of the transmitter side of a IR Microphone system that operates using Frequency modulation at 2.08MHz and I have run into some roadblocks.

The receiver wants a pilot tone of 32.768KHz and a carrier frequency of 2.08MHz along with the audio signal. I have taken a guess that I can just use software to put 32.768KHz on one of the analog pins using pwm and just connect it to the audio output wire (which is connected to a transistor that is controlling the IR LED). [A poorly drawn visual showing my current setup can be found here]

Now the actual question:
Can the carrier frequency be treated the same way that I dealt with the pilot tone or does it require something a little more complicated? If so do I need external components to modulate the frequency or can it be done through software on the Arduino?

Any information or sources that I can take a look at will be helpful.

(The microphone that I am replacing has a sort of "datasheet" with minimal information that I am referring to here)


MarkT

You need an RF FM transmitter circuit, not an Arduino, which is a microcontroller.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Cyclicz

Thank you so much for the insight MarkT!

I'm not sure if you would be able to inform me on the proper way to use an IR LED with such a circuit as all of the ones that I can buy would be for transmission over RF using an antenna and it can't be as simple as adding a LED and a resistor/current limiter where the antenna connection is.

I found a instructables article here explaining the topic to an extent but i'd rather just be able to buy a kit or something rather than building everything from scratch.

Even a simple direction to a site where I can find more information would be helpful as I haven't had much luck in finding other people transmitting FM over IR.

Grumpy_Mike

Quote
and a carrier frequency of 2.08MHz
You will not have the frequency resolution to do this on a 16MHz processor. At the least you will need an external oscillator that you can "pull" with an audio signal.

By and large Instructables are crap, this one seems no exception. I got to this line at the start:-
Quote
Turns out that ambient light created a sort of feedback that made my project very inefficient.
Which shows the author has not got a clue about what he is doing.

Cyclicz

Grumpy_Mike,
I appreciate the additional input, are you able to direct me to a source of information that might be a better direction for creating or buying such a FM IR transmitter circuit? (Since you mentioned that the instructables was a bad source to go off of)

Grumpy_Mike

#5
Feb 26, 2018, 09:48 am Last Edit: Feb 26, 2018, 10:48 am by Grumpy_Mike
I would look at ham radio sites for details of pullable oscillators. What you want is close to an RF transmitter but with an LED being driven by the output.
Normally a PLL that is phase locked loop is locked to a referance crystal oscillator through a divider chin to give you the base frequency. Then your audio and pilot tone are mixed and applied to the VCO, that is voltage controlled element of the PLL.

I can't see an Arduino being of any use to you unless you use an external chip. For example the AD9833BRMZ is a waveform generator, that is accessed through the SPI interface. You could use this to generate your FM. You could sample an audio signal at the input and apply the samples to change the frequency of this chip. However the Arduino's A/D is only 10 bits at a slowish sample rate so the audio quality would not be very high.

MarkT

Thank you so much for the insight MarkT!

I'm not sure if you would be able to inform me on the proper way to use an IR LED with such a circuit as all of the ones that I can buy would be for transmission over RF using an antenna and it can't be as simple as adding a LED and a resistor/current limiter where the antenna connection is.

They are usually designed for an AC coupled 50 ohm load, this is true, but a few passive components ought
to be able to combine that with a DC bias onto an LED.  You don't have to worry about distortion because
harmonics on IR modulation are not a problem and FM doesn't care about anything but the frequency.

32kHz is easy to generate with a 555 or with a watch crystal, 2.08MHz
is a little fiddlier as its rather high for a 555, and you want that oscillator to be a VCO with fairly good
stability and a small frequency range (but larger than a quartz crystal can be pulled).  A colpitts oscillator
with varactor might be one way, there's probably an obscure single chip solution though (one for
modulating FM stereo might be adaptable (the pilot needs to be 32k, not 19k, and the stereo multiplexing
isn't needed, and the VCO might be rather high in frequency and need down-converting...). 
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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