Fm transmitter can you make it with prop chip?
purplemonkey
Posts: 89
I would quite like to make a short range (just inside house) fm transmitter for my mp3 player. Has anyone done this and if so can you point me in the right direction for schematic and spin code?
Comments
If you don't mind using some extra components, you could use this FM transmitter from SparkFun.
Not sure about this question as the propeller can output 3.3V at about ~40ma that will = .132Watts. I don't think this is what you are after though.
My first test indicated that you effectively get a wideband transmitter, with sidebands a few Khz away. This would basically be a jammer, and I personally postulated that with the 16 counters you could effectively blackout the entire broadcast FM radio band for a considerable distance.
I gave some power figures, even low power without a proper antenna, will go far.
In short, buy an FCC registered CD/MP3 broadcaster at your favorite cheap store. I got one with a couple of Belkin chargers at Walmart.
Also if i wanted to use a different cog for your code how would i cahnge it in the spin file?
5 MHz (frqa == $1000_0000) NCO:
5 MHz (frqa == $1000_0000) PLL:
Notice the minor jitter sidebands. These probably result from voltage noise in the PLL's VCO.
7.5 MHz (frqa == $1800_0000) PLL:
8.75 MHz (frqa == $1C00_0000) PLL:
9.375 MHz (frqa == $1E00_0000) PLL:
9.6875 MHz (frqa == $1F00_0000) PLL:
9.6875 MHz (frqa == $1F00_0000) NCO:
Just to demonstrate how much worse things can be without the PLL.
100 MHz PLL:
Pretty much covers the entire FM broadcast band.
Also, don't forget that there are odd harmonics of all these signals, since the output is a square wave. Although the jitter sidebands cannot be filtered out after the fact, a good lowpass filter will take care of the harmonics.
-Phil
Phil, I was looking at that output, and it seemed to me that if you attenuated the output by 30 or 40db, then amplified it, you'd get a decent signal with low noise and sidebands. Is that too simplistic a view of RF?
-Phil
It's programmed EEPROM ok but still nothing
So would that be 21.2 DBM transmitting power? What does that equate to in miles or km covered?
I can tell you this, though: no purely Propeller-pin-based transmitter should ever be allowed to transmit signals that escape the workshop or lab. In the spectral analyses I've done, any Prop transmissions designed for VHF propagation are so dirty that they threaten to interfere with aviation, marine, and perhaps other emergency radio frequencies.
If you want to explore this realm, do it with a proper sine-wave oscillator or low-jitter IC designed for the purpose. The Prop is neither.
-Phil
Communication over thousands of miles on Earth can be done with power levels on the order of tens of milliWatts with a good quality signal and a good antenna well matched to each other. You could also have a 100W transmitter coupled poorly into a very bad antenna and be lucky to be heard a few miles away.
The Propeller operates at 3.3V and an I/O pin can provide maybe 20-30mA of current. That would require a load of 100 Ohms to 150 Ohms. You'd need some kind of antenna tuner, both to remove some of the garbage in the output signal and to provide a load of this magnitude for the Propeller's output stage. You must have a good antenna tuner and frequency specific antenna to do some filtering, otherwise you will just have a radio interference generator. If this causes interference to legitimate users, the FCC can seize your equipment and fine you. At a minimum, they can require you to dismantle your equipment and agree never to interfere again.
Based on the experiments I've done, it will take more than an antenna tuner to remove the out-of-band interference from the Propeller output. I've tried quarter-wave coaxial traps connected serially and both parallel and series-resonant LC tuned filters. None reduced the spurious signals to a low-enough level to be suitable for propagation. And that's just the out-of-band stuff. The in-band birdies are simply impossible to remove via filtering. That's why I've been discouraging this approach to Propeller RF generation, in favor of more analog approaches.
-Phil
I'm sure you're correct. You've discussed various aspects of this before. I would rather see people use some basic filtering and matching than none at all. That at least will reduce out-of-band interference. We had all hoped that we could use the Prop-1 for frequency synthesis, but the best we can do is controlling an external DDS chip. I know Chip has put more work on this front into the Prop-2, so maybe that will come to pass.
Maybe I should come up with a simple analog transmitter circuit that can be controlled by a Prop, so that people will have something clean to work with. There's a plethora of one-transistor wireless FM mic circuits on the web that could be adapted. A varactor to control the frequency and a frequency divider in the feedback loop could be the core of a Prop-centric DFLL/DPLL controller. Like you, I'm very anxious to see what Chip has in store for RF generation in the Prop II.
-Phil