Frequency Modulation

PFloyd36069

Is there any way to do frequency modulation with the prop? like take the input from the mic or a wav file and transmit it to a radio.

Thanks,

Bryan

Mike Green

Look at the application note on the counters (AN001) in the downloads section. You'll need an assembly routine to adjust the counter value to change the carrier frequency according to the amplitude of the audio. The same routine can also do the analog to digital conversion.

PFloyd36069

hi mike, thanks for the fast reply... I am very new to the prop (just got the starter kit on tuesday) so i dont know too much about writing my own code quite yet. What will the code look like to do this?

Mike Green

You will need to use assembly language to do this and that will take some time to learn. There are some tutorials in the "sticky" threads at the start of this forum's thread list. Start with this one (http://forums.parallax.com/showthread.php?p=663640).

Ken Peterson

I generated some audio tones and sent them to an FM radio some time ago. I just connected a foot of wire to one of the prop pins. I used the counter in NCO mode to generate the carrier and then changed the FREQ setting in real time to generate the audio tone.· That much was all done in SPIN.· As·Mike said, to·modulate the carrier with digitized audio would probably require assembly programming, both for speed and timing precision.

Keep in mind that the FM you generate in this way will be very noisy and most likely impossible to get approved with the FCC for a product. With my setup I could pick it up at several points up and down the FM dial. If I had a TV on at the time I probably would have seen interference there too.

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Beau Schwabe

Ken Peterson,

You can greatly·reduce those artifacts and improve your distance·with a properly tuned resonate LC filter on the output pin.· In my earlier tests I achieved·500 feet·with·a tuned LC, and maybe 20 feet without it.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Ken Peterson

I'm sure you can. I was just playing around and proving the concept to myself at the time. Even with an LC circuit, you would need zero clock jitter to avoid stepping on adjacent channels. From what I understand, meeting FCC requirements is not a trivial endeavor.

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Beau Schwabe

Ken Peterson,

Absolutely agreed.··... jitter that·jumps out and then back into phase is considered a noise event on the carrier, jitter that jumps out and stays out is considered drift.· Neither one is desirable and FCC is very strict depending on·the circumstance.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

VIRAND

Is the Propeller ever stable enough for communication on some frequencies, such as ham radio?

(I mean for using it as a frequency synthesizer in a transceiver.)

Post Edited (VIRAND) : 7/7/2008 10:47:49 PM GMT

Mike Green

"amateur radio frequencies" covers a huge range of frequencies.
You're probably referring to the "high frequency" range from 3.5MHz
to 28MHz.

The Propeller is never going to be able to synthesize frequencies in this
range without some problems with jitter. The possible exception would
be if you used crystal control with the Propeller running off a crystal for
the operating frequency or a multiple of it. That kind of limits the range
to under 6MHz. There's also switching noise from the rest of the Propeller.
Any kind of resonant tuned circuit does wonders.

Leon

I can't see it being suitable for anything other than a rudimentary receiver, there would be too much phase noise. It could be used like any other MCU for controlling a radio, of course, but it wouldn't be any better than the usual PIC or AVR.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Post Edited (Leon) : 7/7/2008 11:17:39 PM GMT

Leslie (HA4WV)

We tested the Propeller chip with an electrical engineer (Name: Zsolt, ham radio callsign: HG2ECZ) to frequency-synthesis in the Technical University of Budapest.
His synthesiser is unsuitable for radio frequency aims unfortunately.
We measured it with a panorama receiver, and the jitter was so big that continuing the measurement was not his sense…
The Propeller is nice device, but did not prepare for a radio frequency aim.
We may not expect us to be allowed to build a good RF-synthesiser out of him because of this.

I decided it in such a way after these that the Propeller controls it excellently the AD9951 synthesiser and any LCD display, and with this experiment in the future.

Leslie


Ham radio callsign: HA4WV

VIRAND

That's too bad about the jitter.
I thought it would be wonderful if the Propeller could be used to make a really nice radio like an Icom IC-7800.