[hpsdr] Hermes Interest

[Jeremy McDermond]

On Aug 7, 2011, at 2:28 AM, Abhi Arunoday wrote:

> ***** High Performance Software Defined Radio Discussion List *****
> 
> Hello,
> 
> I agree completely with Jeremy, however, while a single Hermes Card may not be able to do Diversity reception, i'm sure multiple boards can be run together to allow this feature.
> 
> The only Caveat is the higher cost and development of appropriate FPGA and PC code.

We've had some discussions on this before and it seemed rather impractical to do so.  First, the clocks would have to be synchronized on both boards such that they're completely in phase.  See the discussion earlier this week on why merely phase locking clocks to the master 10MHz clock on Atlas based systems doesn't work so well for diversity.  

Second, you would have to find some way to present that data to the computer in a way that it can determine the proper time relationships between the samples.  You could "slave" one Hermes to another in some way, but you'd need an appropriate connector to do so.  I'm not sure there is such a connection.  Even if there were, you're getting in the realm of "why" since you're approaching the complexity of an Atlas system anyhow.  It would make much more sense in this case to develop some sort of daughter card with just the ADC on board.

You could also try to use the Ethernet connections for both of them, but this has also been discussed before.  You would have to have ultra-accurate timestamps on the data to be able to correlate them correctly.  You can't be guaranteed of timely delivery of the ethernet packets (or even that they'll be delivered at all) so you may have to buffer samples coming from both boards significantly.  There's also the questions of where you would get the ultra-accurate timestamp from, and whether those timestamps are going to be accurate enough to capture differences in phase relationships of a 50MHz signal.

The Atlas systems accomplish this by the controller card (Metis or Ozy/Magister) correlating the data from the different Mercuries and interleaving them in the packets to the computer.  We then know that adjacent samples were taken at the same time.  Again, this could be done if you had a "master" FPGA that did all the communication with the network, but then again, this starts to look a whole lot like an Atlas system without a backplane.

All in all it seems like a *LOT* of hacking to come up with a system that we have working relatively well on Atlas.  Hermes was really designed to be simple and self contained, and therefore had compromises to make it so.  Maybe it makes sense at some point in time to come up with a "Diversity Hermes" that has two ADCs on board.  You need some more entertainment on how to cram more components on the board, right? :)  Either that, or the idea of a daughter card with an extra ADC is interesting.  Being the software guy, I'm not sure how hard it would be to do something like that and keep all of the data lines properly shielded and such.

> regards,
> 
> Abhi

--
Jeremy McDermond (NH6Z)
Xenotropic Systems
mcdermj at xenotropic.com




 1312711187.0