How hard: a totally digital VHF radio (aviation COM & NAV bands i.e. 100-335MHz) - Page 1
Forum section: electronics / rf-microwave

author: peter-h
author_rank: Super Contributor
timestamp: February 28, 2020, 12:24:13 pm
content: This is a project I looked at some years ago. None of the existing players seem to be doing it this way. All are building basically analog stuff, perhaps with DSP on the NAV on which the modulation is in the kHz range so easy enough. And some of the newcomers to the market are really struggling to build radios which outfits like King were doing decades ago...Set aside the usual certification discussionI am just interested in feasibility.One would need to amplify the RF and sample it with a ~1GS/s ADC. The resolution of this ADC is interesting. I doubt 8 bits will be enough and that makes it a lot more expensive, I think.Then one would need some serious DSP stuff, maybe in an FPGA.Maybe one issue is that for a COM+NAV radio you need two of everything, but that's a side issue.

author: m98
author_rank: Frequent Contributor
timestamp: February 28, 2020, 01:30:14 pm
content: I guess the actual SDR-part is easily realised with an AD9361 + Zynq-7000 Soc. Forget dual-watch, you could actually monitor the whole airband including navigation frequencies at once, and use the spare channel to implement a Mode-S transponder.The analog frontend could still be quite a challenge, the narrow channel spacing requires rigorous filtering.

author: awallin
author_rank: Frequent Contributor
timestamp: February 28, 2020, 06:52:20 pm
content: Quote from: peter-h on February 28, 2020, 12:24:13 pmwhat's the advantage of a totally digital approach? processing and storing all those 1GS/s samples becomes a problem real fast...If ~50MHz of real-time bandwidth is enough then Ettus B210 and similar are good enough?

author: David Hess
author_rank: Super Contributor
timestamp: February 28, 2020, 08:04:14 pm
content: Aircraft radios operate in close proximity to other in-band transmitters and powerful out-of-band broadcast FM transmitters so I would worry about overload performance.Also nothing requires such a high sample rate. The entire voice band is only 18 MHz wide so with suitable preselection, helical resonators would work well at those frequencies, the sample rate could be only 100 MS/s or lower.

author: peter-h
author_rank: Super Contributor
timestamp: February 29, 2020, 10:11:50 am
content: Would you then down-convert to say the usual 10.7MHz and do the A-D on that?

author: 9aplus
author_rank: Regular Contributor
timestamp: February 29, 2020, 04:28:24 pm
content: Made some tests on that matter with Lime SDR USB and LimeNET micro... it is feasible butneed at last two boards+RPI 3 or 4 and use of Soapy Remote to one Linux or Win10 user front end.

author: David Hess
author_rank: Super Contributor
timestamp: March 02, 2020, 08:57:41 pm
content: Quote from: peter-h on February 29, 2020, 10:11:50 amThat will work and then because of the very limited bandwidth through the 10.7 MHz filter, the ADC only needs to operate at sample rate sufficient for the 10.7 MHz filter bandwidth so 100s of kSamples/second. A higher IF like 50 MHz would probably be better but the result is the same.

author: radiolistener
author_rank: Super Contributor
timestamp: March 02, 2020, 09:20:23 pm
content: Quote from: David Hess on February 28, 2020, 08:04:14 pmHigh sample rate allows to get processing gain, it will improve SNR. Also high sample rate allows to eliminate mirror channels and reduce cost of input bandpass filter and cover very wide bandwidth. But at the same time high sample rate requires high performance FPGA which consume a lot of power and cost a lot of money. So, such solution will not be energy efficient, but allows to get better performance.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 03, 2020, 01:55:08 am
content: Quote from: peter-h on February 28, 2020, 12:24:13 pmBecause they have come up with designs that work well, are reliable and proven. Not much has changed in the types of signals that need to be handled, it's still plain old AM, so why reinvent the wheel.I don't think there's really much to be gained from an all-digital SDR/DSP approach.Also the power consumption and heat dissipation of such an approach is something that must be considered.

author: peter-h
author_rank: Super Contributor
timestamp: March 03, 2020, 06:54:59 pm
content: They do have a huge component count, however. Look inside a KX165A or similar. It is totally packed with circuitry; most of it signal processing.

author: David Hess
author_rank: Super Contributor
timestamp: March 04, 2020, 01:25:10 am
content: Quote from: radiolistener on March 02, 2020, 09:20:23 pmHigh sample rate does all of that but selectivity is needed to protect against overload so that the receiver will function correctly in a hostile environment.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 04, 2020, 04:47:36 am
content: Also, what would be the point of sampling the entire band of 100-335MHz in one fell swoop? Even from 108-136 (COM and NAV), there would be no point. Different antennas, so entirely different RF paths and thus individual receiver circuitry for each.COM band is vertically polarized, while NAV band uses a different style of antenna offering horizontal polarization (some antennas are two-in one but they are still separate antenna elements built into one, with two separate coax connections).You also want to be able to transmit with the COM part of the radio while still receiving signals on the NAV side uninterrupted.Quote from: David Hess on March 04, 2020, 01:25:10 amIndeed, since on many aircraft you're going to have several antennas in relative proximity to each other, operating simultaneously on different frequencies in the VHF com band.You don't want, for example, the ACARS transmitter overloading or desensitizing your COM radio while you're receiving a transmission from ATC a hundred miles away!Pretty tough to get that kind of performance while sampling the entire band without selectivity in the front end.