[hpsdr] Blackfin

[James Courtier-Dutton]

michael taylor wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> 
> On 7/12/07, Philip Covington <p.covington at gmail.com> wrote:
>> The intermediate CIC stages all get trucated  to 24 bits.  I can spit
>> out any rate I/Q data from about 8 MSPS to <8kSPS over USB.   With PCI
>> Express I can DMA into memory at up to 250 MBytes/sec.
> 
> Is there an affordable way to utilize PCI-express?
> 
> Or would SATA or SAS (Serial Attached SCSI) be viable / affordable to
> consider as transports, from an external device to a generic (ITX or
> ATX) computer?
> 
> -Michael, VE3TIX

One might also want to use an interface that is optical, to help isolate
the computer from the RF input board. Once could use 1 Gig Ethernet, and
just put the data in ethernet frames, without any UDP or TCP.
This would give you about 975Mbits/sec data throughput.
By using UDP, this would drop to 957Mbits/sec data throughput.
One could use Optical GigE for the optic isolation.
So, using UDP would give one:
957/8 =  119 Mbytes/sec.
For 16bit samples, this would be: 59MSPS
For 24bit samples, this would be: 39MSPS
For 32bit samples, this would be: 29MSPS

There might be some added overhead, because one might want to include a
timestamp, or sequence counter in the data inside the UDP, in order for
the receiving end to know if any packets had been lost.
If the RF input board was using a GPS clock time source, once could time
stamp it with GPS accurate time. One could truncate this timestamp using
the following method.
Let the computer do NTP to get better than +-1 second accuracy, and use
the microsecond timestamp truncated to +-1 second accuracy from the GPS
accurate time, then merge the two times. Some care must be taken when
merging the two times, but it can be done.

Once can also use two GigE interfaces if one needed to double the
traffic, and just multiplex alternate UDP frames down each one, and
re-order them in the PC by using the timestamp.

Another option is external PCIe. It has an optical cable option.

There are trade-offs with all options, GigE has the advantage of long
distances and is cheap. PCIe is shorter distance, but faster.

Once the bandwidth between the RF input board and the computer
increases, one might be able to separate the FPGA from the ADC on the
input card. So, have an external RF input board with just Direct ADC
feeding to computer, and possibly use a hardware accelerator card, e.g.
Cell processor, within the PC to do the digital filtering at those high
rates. This would make the RF input board cheaper and more flexible. I
wonder how much CPU power one would need to run dttsp at 125MSPS instead
of the current 96SPS? This is also another reason why one might still
need the FPGA.

Kind Regards

James

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