[hpsdr] Gibraltar fanning out clock and 1ppsto HORTON/CASMIR/MERCURY/..

[Eric Ellison]

Lyle!

Really eerie. Was going to jump in and respond to the same messages you did
with the same points, as I read them. (smile) Also looked at the 100 ns and
wondered where that came from???

Obviously this IS a very important, topic which again, I mistakenly thought
we had decided, at least for the 10 mhz sigs. Apparently no lines were
dedicated to this.

Since the top of the current proposed boards are 'open' to the inside world,
and equidistant, could we possibly reserve a small area at the top of each
board for the lvds signals to be passed from board to board, via a suitable
medium. Is it possible to even create a lvds pair this closely spaced? If so
it would have to be defined on the least lengthy board (100 mm?).

Just a non-techie asking!

This HAS ALWAYS been exciting. Like mental flypaper! Now we have the experts
gathered from many disciplines. I also agree, I'd love to fool with some
'non-amateur-radio' projects for the basic backplane and Ozy!

Thanks Everyone!

Eric - AA4SW (guess we are going to need numbered Erics again! (smile))
 

-----Original Message-----
From: hpsdr-bounces at hpsdr.org [mailto:hpsdr-bounces at hpsdr.org] On Behalf Of
Lyle Johnson
Sent: Monday, July 10, 2006 11:19 AM
To: Eric Blossom
Cc: Achim Vollhardt; hpsdr at hpsdr.org
Subject: Re: [hpsdr] Gibraltar fanning out clock and 1ppsto
HORTON/CASMIR/MERCURY/..

***** High Performance Software Defined Radio Discussion List *****

Hello Eric!

>> I would like to remind people about the coherent system approach Bob and
>> others mentioned some while ago:
>>
>> IF we manage to derive all signal path clocks from a single source
>> (let's assume a 10 MHz GIBRALTAR for now), this system will be able to
>> PRECISELY time any recorded signal to the sub-100 ns level!
> 
> Coherent processing will be very useful.  However, sub-100 ns is way
> too sloppy if you're dealing with 100 MHz sampling clocks.  That could
> be 10 samples!  Accounting for aperture jitter at those rates, the
> specs get even tighter.
> 
> It seems to me that for any serious coherent processing, clock
> distribution is *not* going to happen across the ATLAS bus, but rather
> through equal length coax or LVDS/LVPECL in some kind of star
configuration.
> 
> If I'm wrong, please set me straight, and explain to me how you plan
> on handling a system with say 4 MERCURY's, or 4 HORTONs for that matter.

I suspect that for most uses, distribution via the backplane will be 
sufficient.  We need to think about the way in which we input the 10 MHz 
clock to minimize jitter to reasonable levels.

For really precise applications -- such as multi-node coherent systems 
-- we'll need something better.  Perhaps one or more 50-ohm output ports 
is the right way, or perhaps an LVDS or similar connector is appropriate.

I suspect we should be able to get much, much better than 10 nsec on the 
Atlas, and the increasing from slot to slot should be on the order of 
100 picoseconds.  One could put two Mercuries in the backplane, on one 
either side of Gibraltar, and have very little skew between the two 
boards,and a fairly stable -- and measurable - offset from Gibraltar.

If we use a 50-ohm system, we can use splitters to get as many nodes as 
we need, as long as we calibrate the splitter.  We could conceivably use 
an extra port from it to measure the phase shift and then compensate for it.

Exciting stuff!

73,

Lyle KK7P

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