On Sat, Oct 14, 2006 at 02:06:01PM -0400, Robert McGwier wrote:
> David I. Emery wrote:
> >
> > This implies that the USRP with a 6 to 8 mhz bandwidth might be
> >able to successfully demodulate the SCPC DVB-S QPSK video transmissions
> >at symbol rates like 3.9876 megasymbols/sec (5.5 mbs transport stream at
> >FEC 3/4) commonly used by satellite trucks for news feeds. Most of
> >these signals carrying single channel NTSC video (720 by 480 MPEG 2) run
> >at either 3.9 megasymbols/sec or 4.2 megasymbols/sec.
>
> As is typical of many signals of this type (geostationary satellite
> signals), it needs very little dynamic range, mostly enough to
> accomodate weather and multipath fades but not what would expect to have
> to accommodate on a different type of FDM channel where you could see
> serious near-far problems. I suspect the 8 bit DDS might work nicely
> indeed here.
In fact many of the chip sets used to implement DVB modems (and
other satellite QPSK demodulators) in the early days before highly
integrated mixed signal ASIC stuff took over used actual 6 or 8 bit A/Ds
feeding the digitry - either digitizing IF or I and Q from a
downconversion. If I remember correctly some of the early Stanford
Telecom chips only accepted 6 or even 4 bits of I and Q.
Obviously typical satellite receiver designs which work over an IF
power range from around -65 to -70 dbm to around -30 dbm or so include
AGC - usually controlled by the signal processor digitally to set the
power at the constellation points to a nominal value (in vector space).
And equally obviously, the USRP has the hardware on board to do just that.
The loop bandwidth for this AGC is uncritical - satellite
carrier powers change slowly with weather fade and antenna motion
(mostly unintended of course) and satellite motion - one needs hundreds
of milliseconds here, not hundreds of hz.
There are some of us looking at using the USRP for making
measurements on, characterizing modulation parameters of, and
demodulating satellite BPSK, QPSK, OQPSK, and 8PSK signals. There are
obviously very many of these, as few other modulations are employed on
the bent pipe repeaters of modern communications satellites. And using
the DBSRX daughter board allows coverage of the IF range of most modern
LNBs - for S, C, X, Ku, and Ka and K bands. And it also directly
covers the 1.5 and 1.6 ghz L band frequencies used for mobile satellite
service (INMARSAT, Iridium, Globalstar).
--
Dave Emery N1PRE, [EMAIL PROTECTED] 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."
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