[Premium-Rx] Cubic CDR-3280 Digital I + Q output

[David I. Emery]

On Mon, Jan 17, 2011 at 11:52:36PM -0800, Mack Rogers wrote:

> Indeed that is the problem. This data format isn't something that a
> soundcard would be able to process, since those process the I&Q on the
> L&R audio input channels. As far as I know any of the commonly available
> SDR programs could process the data, IF some way of inputting the data
> stream to a PC could be found.

	Someone with reasonable modern hardware skills could pretty
easily create some FPGA programming for one of many moderately priced
development boards for FPGAs of the sort that include USB ports and
import the bit stream through USB.   This obviously requires USB
programming experience at the driver and chip level but lots of the code
required is available in development kits for the chip involved.

	Another approach, which would also work, is to use the older
Cypress FX2 chip along with a much smaller FPGA (or discrete logic if
you like that) - one basically needs to clock the bits into a register
(or for that solution 4 byte wide registers) using the clocks on the
RACAL interface and the framing signal and then clock the registers into
the FX2.    Cypress supplies most of the required software and driver
code as part of a development kit.   There may even be some development
boards with the FX2 on them along with the other required support
glue... though there are now some large FPGAs with the equivalent of the
FX2 in them in addition to a large number of programmable logic blocks.

	I might add that TenTec has a similar digital output on the
RX-331 and RX-340 (no idea exactly HOW similar at a low level) and WJ in
the 90s standardized on a parallel 16 bit wide bus and lots of their
equipment from that era has the 50 pin parallel bus for outputting
audio, I and Q and other things...

	The right FPGA based design could easily handle several of these
different buses with reconfiguration of the FPGA by downloading
different versions of the logic.   And USB 2.0 is reasonably future
proof... not going away  in any real hurry.


> I've been like a dog chasing his own tail on this situation for about
> 4 years now. I dunno what the gov uses this format of I&Q data stream
> for but it sure would be interesting to find out!

	All this output is is a better than (most) soundcards A/D which
converts analog I and Q from some place internal to the radio to the
same stream of 16 bit I and Q samples as a computer sound card would.  
But  because there is no audio processing between the I and Q demods and
the A/D the quality of the sampling is likely better than what one might
get out of a typical soundcard with its audio oriented input circuit -
and the actual sample rate clock may also be more stable and accurate.

	
> To complicate the matter even further each active output on the J7
> connector has an inverted output also.

	That means it is designed for differential transmission, which
would be a very natural interface.   Probably more research will show
this will work quite well with some particular differential receiver
chip family. Also one would likely want to use twisted pair cables for
these signals if one intends to run them over any distance, and
differential driving is what is normal for twisted pairs.


> Who knows, maybe one day I'll be scrounging in some surplus dealers
> junk at a hamfest and there will be a strange looking box with Cubic
> Communications on it that cures this problem for me.

	There may be some PCI cards that input this signal... probably
highly specialized (and thus VERY expensive to buy new) but the sort of
thing that shows up in the almost free whazit bins at fleas... only
problem is recognizing them (and finding drivers and documentation).

	My USB solution suggestion would likely yield an open source
design that would not require unobtanium based proprietary stuff... but
does of course need some engineering talent to implement.



> 
> N4VGB
> Mack Rogers

-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."