easy narrowband receiver? - Page 1
Forum section: electronics / projects

author: djacobow
author_rank: Super Contributor
timestamp: April 29, 2014, 06:50:08 pm
content: I've been working on a software radio project that has hit a brick wall because of limitations in the cheap wde-band receiver hardware I've been using (TV dongles).The signals I am trying to decode are old school aviation VOR transmissions, which are actually quite narrow band. However, I want to decode several simultaneously, or at least be able to jump round-robin to several very quickly. (The goal is to calculate a position without the pilot's effort.)The cheap SDR doesn't work well because it has crappy front end analog filters designed really for TV reception and also because it only has 8b samples, making it impossible to set the gain properly to receive two stations of widely varying power. Better SDR receivers (12b and 14b) with wide bandwidth (20 MHz or so)willwork, but they are just too expensive.So I've gradually come to the conclusion SDR hardware is not the way to go here, and that a simple narrowband AM receiver is the way to go. Except I have nonexistent RF-circuit-design-fu. I was really hoping to avoid the hardware side of things on this particular project.So, my question to the group is: are their any chips or modules, or subsystems, or whole radios that make this relatively simple to read. Optimally, what I'd prefer is- something I could tune digitally, and which tunes and settles quickly- channels are ~25kHz wide, 50 kHz spacing fro 108-118 MHz.- am demodulation in hardware to analog out- AGC and controllable gain with some adjustable parameters.From here forward, I'll do ADC and handle further demodulation in the digital domain.Cheap is the watchword, as I'm trying to build an airborn receiver that I might eventually sell or turn into an OSH project.Hints would be appreciated!Regards,Dave J

author: copper dog
author_rank: Newbie
timestamp: April 30, 2014, 03:03:54 am
content: My first question is why would you want to triangulate with VOR's in the first place? That's what DME and VOR radial is for not to mention GPS or any number of RNAV systems available on the market. That tech went out with LORAN. I'm assuming of course this is a navigation system you're talking about here.

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2014, 05:07:40 am
content: If it can be done cheaply, it could be a nice, ground-based backup for GPS. I'm pretty sure it could be integrated into a portable GPS and could maybe even aid in initial satellite acquisition, and at the very least could provide a somewhat degraded position for times when GPS goes out. I've actually seen GPS go out on more than one occasion. GPS is a weak signal, it can be jammed intentionally and accidentally, etc.DME is a non-starter for a cheap positioning adjunct, as it requires transmitting a high power pulse to the DME station. Basically, it would have to be built in. Honestly, if LORAN still existed, I probably wouldn't even consider this project. I think it was a good system, and there were very reasonable plans to upgrade its accuracy. If the FAA decommissions most ground-based navaids as they promise to eventually, I believe that air navigation will be somewhat at risk. I'm actually surprised that more people don't think so.And anyway, I think an automated system that works on VORs would just be a cool toy. Airliner FMSs already do multi-sensor positions that include VORs. They have the benefit of DME; I wouldn't. So that does mean that there will be times when the geometry does not enable the calculation of a position (only a line) but I'm willing to accept that.I'm curious what other RNAV solutions on the market your are referring to? I'm not aware of any for light GA that are not based on satnav.Quote from: copper dog on April 30, 2014, 03:03:54 am

author: vk6zgo
author_rank: Super Contributor
timestamp: April 30, 2014, 07:28:37 am
content: Quote from: djacobow on April 29, 2014, 06:50:08 pmThere are better SDR designs around--check some Ham Radio sites.You may be able to get the required selectivity with a better quaiity PC Soundcard---the Hams do this for digital modes.Old style analog design would use a VHF/HF converter & several narrow band receivers each with its own AGC system..

author: mark03
author_rank: Frequent Contributor
timestamp: April 30, 2014, 03:18:52 pm
content: Quote from: djacobow on April 30, 2014, 05:07:40 amAccording tohttp://gpsworld.com/glonass-blackout-coincides-with-loran-authorization-in-progress/, LORAN (eLORAN actually) may be coming back before long. How soon do you plan to have your gizmo on the market?

author: G7PSK
author_rank: Super Contributor
timestamp: April 30, 2014, 03:20:50 pm
content: Have a look at the Fun cube dongle, it's designed for amsat.http://www.funcubedongle.com/?page_id=1073

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2014, 05:04:08 pm
content: Hmm, back when I started on this, I did not look at the FCD because I was thinking of the wideband approach with simultaneous software channelizing. But I see how this could work as a narrowband receiver, and it certainly seems to have a simple enough interface.I must say from the block diagram I don't quite see how it works very well. There are front end filters that are 10+ MHz wide, basically for bands, then a mixer, then an IF LPF at 200 kHz and then sampling at 192 kHz. I don't see how that rejects images of nearby signals at 192 kHz intervals. Should there not be some narrowband filter in front of the the mixer?Of course, I know HAMs use this device and it works nicely, so I must be missing something!It's not too expensive for some experimentation.Quote from: G7PSK on April 30, 2014, 03:20:50 pm

author: djacobow
author_rank: Super Contributor
timestamp: April 30, 2014, 05:07:33 pm
content: Quote from: mark03 on April 30, 2014, 03:18:52 pmOh, well, I have most of the DSP code mostly working now, so I think with better HW I could probably have a prototype clunking along in a few weeks. Using some USB based hardware, one could imagine being able to do a phone app in a few more months, working full time.One of the complications of this project, annoyingly, is that VOR signals being line-of-sight, I can really only test this in the air. That is expensive and difficult; it's not easy to fly and operate a laptop simultaneously.I'd be really thrilled to see LORAN lit up again. I'd be happy to work on a LORAN receiver, which I suspect would be fascinating project in itself!