Radio Transmittor using FPGA - Page 1
Forum section: electronics / projects

author: tiofilo75
author_rank: Contributor
timestamp: October 18, 2013, 10:21:57 pm
content: Hello everyone. I am currently in my last year of school so I have to come up with a possible senior project to do for my last year and have been thinking about some projects that I could do. One of them is making a transmitter with my Spartan 3E FPGA board. I wanted to know if this would be possible? I want to transmit bits so I would have to do some AM modulation and come up with the receiver which would also be an FPGA. I want to start small so I would be happy just being able to transmit the bits down a small distance. Thanks.

author: Odysseus
author_rank: Regular Contributor
timestamp: October 19, 2013, 02:14:37 am
content: I'm going to assume you want to do as much as possible directly with the FPGA. Also, I know barely anything practical about RF, but here's some links that should help get you started:http://en.wikipedia.org/wiki/On-off_keyinghttp://en.wikipedia.org/wiki/Whip_antennaPick a frequency of operation that your FPGA can easily generate, but not so slow that your antenna has to stretch across the room.You might be able to get away without an amplifier for the TX (i.e. directly drive antenna with I/O pin), but you'll definitely need some sort of front end amplification for the receiver.

author: Whuffo
author_rank: Regular Contributor
timestamp: October 19, 2013, 05:41:42 am
content: I'd suggest looking for another project idea. This one is fraught with peril; the output pin of your chip provides a square wave which if put to an antenna would cause interference on a wide range of frequencies. You'd need at least one stage of RF amplification, a tank circuit, and an output filter between that chip and the antenna. The difference between a RF amplifier and a RF oscillator is experience; they're simple on the schematic, but in the real world they get a lot more tricky. The higher the frequency, the worse the challenges become.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 19, 2013, 05:44:02 am
content: It's possible, but keep in mind that Tiffany Yep spent years learning that sort of stuff. (And she doesn't even custom build the analog front ends!)

author: SeanB
author_rank: Super Contributor
timestamp: October 19, 2013, 05:51:31 am
content: You can quite easily make an AM transmitter in the FPGA with only thing being analogue is the output filter ( at low power the FPGA will be able to provide the power needed direct) and then you feed in digital audio ( or use an external audio ADC driven by the FPGA) and do the modulation in digital, then feed the digital output into a DAC which drives the antenna direct after a low pass filter.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: October 19, 2013, 08:57:56 am
content: Quote from: Whuffo on October 19, 2013, 05:41:42 amMay I know how a square wave would cause interference on a wide range of frequencies?

author: Balaur
author_rank: Unknown
timestamp: October 19, 2013, 09:16:06 am
content: Quote from: Abstr7ct on October 19, 2013, 08:57:56 amCertainlyHint: look at the harmonics plot.

author: Abstr7ct
author_rank: Regular Contributor
timestamp: October 19, 2013, 09:39:15 am
content: Quote from: Balaur on October 19, 2013, 09:16:06 amIn such a case, what should be done to remove the unneeded harmonics? a LPF with a cutoff frequency of 1KHz (the fundamental frequency)?

author: awallin
author_rank: Frequent Contributor
timestamp: October 19, 2013, 11:49:07 am
content: Quote from: tiofilo75 on October 18, 2013, 10:21:57 pmIf you want up the challenge level (stand on the shoulders of giants...):Get in touch with one of the many SDR projects out there, such ashttp://openhpsdr.org/Talk to those people, ask them what new/better/novel stuff their project would need, and do that.

author: Nerull
author_rank: Frequent Contributor
timestamp: October 19, 2013, 03:25:17 pm
content: Quote from: Abstr7ct on October 19, 2013, 09:39:15 amBy not using square waves.Their very nature causes the problems. A square wave has a square shape because it goes through many, many different frequencies. The way to take those out is to make it a sine wave. A LPF will tend to nudge a square wave towards a sine shape, ending up with who knows what in between.

author: tiofilo75
author_rank: Contributor
timestamp: October 19, 2013, 05:49:02 pm
content: Thanks everyone. I looked around the web and it seems as though someone did it through FM. They were transmitting SOS into an FM station.http://youtu.be/4YbDjc3Xb1EThis sounds like a fun project to do.

author: madshaman
author_rank: Frequent Contributor
timestamp: October 19, 2013, 05:59:22 pm
content: Quote from: tiofilo75 on October 19, 2013, 05:49:02 pmI hope it was *very* low power, you know, if I were broadcasting SOS on *any* frequency using *any* kind of modulation, even if I was using a spark gap, I would hope someone would investigate and not assume I was just goofing around. ^^'

author: tiofilo75
author_rank: Contributor
timestamp: October 19, 2013, 06:38:03 pm
content: Yeah it was not a good idea to transmitting SOS, but the proof of concept is pretty cool.

author: Nerull
author_rank: Frequent Contributor
timestamp: October 19, 2013, 06:47:32 pm
content: That's a federal felony in the US, up to 6 years in prison.False SOS/Mayday/etc. calls are no joke. They can cost a lot of money and even put rescuers at risk before it's determined to be false. Automated systems can pick up a SOS call even if no one is actively listening to the station.

author: Whuffo
author_rank: Regular Contributor
timestamp: October 20, 2013, 06:31:57 am
content: Quote from: SeanB on October 19, 2013, 05:51:31 amWow, that's really using a sledgehammer to drive tacks. For this kind of output level with AM in the broadcast band, the analog solution is a big winner.http://www.richardmcwhorter.com/AM_Transistor_Transmitter/Engineering is coming up with the most appropriate solution - one that does the job at the lowest cost.

author: SeanB
author_rank: Super Contributor
timestamp: October 20, 2013, 09:56:34 am
content: I have used an old PC to replace a box of 555 timers.....................It was better for the job.

author: Whuffo
author_rank: Regular Contributor
timestamp: October 20, 2013, 11:09:36 am
content: Quote from: SeanB on October 20, 2013, 09:56:34 amMaybe, but how long can you run it on a 9 volt battery?

author: SeanB
author_rank: Super Contributor
timestamp: October 20, 2013, 05:57:43 pm
content: Quote from: Whuffo on October 20, 2013, 11:09:36 amLess than a single mains cycle, and the box of 555 timers would do about the same time on the 9V battery anyhow.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 21, 2013, 01:18:29 pm
content: Article about making an RF receiver with an FPGA, using a diff input as a comparatorhttp://issuu.com/xcelljournal/docs/xcell_journal_issue_84/28?e=2232228/4052461