Software Defined Radio Megathread

[rektide]

I'm not expecting much right now, but I'm looking at software defined radio (SDR) for controlling RC planes (electrics atm) and I'm going to start this thread to gather whatever information is available.

The only rcg reference thusfar is someone asking about a 15 minute freq monitor (http://www.rcgroups.com/forums/showthread.php?t=474441&highlight=software+defined), and being told open source SDR is catching up.

Well, it should be here by now, and I'll do my best to keep this thread ticking with whatever information I gather.

[matttay]

SDRs have been the dream of some since at least the early 1990's when ADCs started to show enough dynamic range to make things interesting. But the primary reason they never caught on is that the cost of dedicated radios fell faster than the technology required to implement SDR.

Consider, for example, a modern cellphone really as a GSM radio, an EDGE radio, a UMTS radio, an HSDPA and HSUPA radio. It also has a bluetooth radio, and WLAN radio and usually has GPS. It might have DVB-H, too. There are, in most cases, 10+ RF bands used. In all cases, there is dedicated silicon devoted to each radio standard. If SDR were really going to have caught on, you would have thought it would have been for the ultra-high-volume, super-high-performance, super-low-cost cellphone market (1B handsets per year) where there was a serious problem to solve. Instead it has completely failed to make a dent in that market.

So, I'll ask the obvious: why specify SDR? What benefits do you perceive?

The one area it might make sense is if the FCC opened new frequencies to radios that knew how to coexist. That might make things interesting. But prior to that, it's not clear what SDR brings to the party.

[rektide]

Theory one: ADC's are now fast enough to direct sample 72mhz.
Result: Confirm, National Semi has a 155 mhz 14 bit direct sampler. Nyquist theorum states you need twice the direct sample rate of what you want to listen to, and 155 > 2 * 72.

[rektide]

Part of it is just DIY mentality, but most of it is that I simply do not trust someone elses gear to behave in degraded mode. I believe I can squeak out more range in worse situations by going DIY. Even if I dont perform at the same spec, I'll have the digitized recording of what was recieved so I can better my system latter. I also get freedom over my input control selection, versus having to use someone elses controller.

There is some 50 MHz space available for remote control in the Ham bands-- if you are a licensed Ham. I'd originally thought Hams could operate in 72 mhz spectrum if they accepted interference, but it doesnt look like ham's get any Part 97 operating privledges there (Part 97 is the "no intentional harm" rule governing homebrew ham gear).

The really unfortunate side of things is that what I'd like to tune is the reciever, but any electronics I would build have to be both low powered and small to fit on a RC plane.

[matttay]

Theory one: ADC's are now fast enough to direct sample 72mhz.
Result: Confirm, National Semi has a 155 mhz 14 bit direct sampler. Nyquist theorum states you need twice the direct sample rate of what you want to listen to, and 155 > 2 * 72.

Of course, the 14-bits provides you very modest rejection from interferers that are within your nyquist band. If you sample at 150 MHz without any hardware filtering, then TV channel 2, 3 or 4 at ~60 MHz render your 14-bit ADC deaf and useless because the interferers overwhelm the ADC input.

And therein lies the rub: To avoid that, you need to use hardware filters. But once you use hardware filters, you have pretty much given up every advantage you might find from a SDR.

[Ron W3FJW]

There is some 50 MHz space available for remote control in the Ham bands-- if you are a licensed Ham. I'd originally thought Hams could operate in 72 mhz spectrum if they accepted interference, but it doesnt look like ham's get any Part 97 operating privledges there (Part 97 is the "no intentional harm" rule governing homebrew ham gear).

The only privileges Hams have outside of the Ham bands are the same ones you have and have to follow the same rules that you do....
Part 97 does apply to hams operating on certain frequencies or bands of frequencies within some of the Ham bands