[Discuss-gnuradio] DSP based SDR

[Darrell Harmon]

I am planning to build a DSP based SDR board.  I will be using the 
Analog Devices Blackfin DSP (ADSP-BF532) and the PLX NET2272 USB 2.0 
interface.  The DSP has a PPI interface for a high speed parallel ADC 
which can accept 16 bit input at up to 65MHz.  I doubt that much data 
could be processed, but 20MHz samping should be possible.  The PPI 
supports DMA to L1 memory or SDRAM.  The Blackfin is a 16bit 400MHz dual 
MAC fixed point DSP.  There is a port of GCC for the Blackin and no FPGA 
will be used, so only free software will be needed for development.  The 
board will be able to operate without a PC attatched.  The board will be 
4 layers and about the size of a credit card.  I intend to release the 
schematics (both pdf and gSchem formats).

I am considering the ADS5520 ($20, 12 bit) and the LTC2227 ($11, 12 
bit). The LTC part has the advantage of pin compatible 12 and 14 bit 
versions.  Does anyone have ADC suggestions?  Low power is important as 
I would like to power the board from USB.  I may be able to get a dual 
LTC229x ADC to multiplex the data onto the PPI, but that would result in 
a lower data rate.

The PPI will be used for the ADC, so adding a high speed DAC will not be 
possible.  A slower DAC could be attatched to the I2S serial ports.  
Would a 192KHz 24 bit audio DAC such as the PCM1753 be useful to 
anyone?  I could also interface a 24 bit 192KHz audio ADC such as the 
PCM1804 for weak signal recieve.

I am still unsure how I will handle the analog input.  I had considered 
putting a transformer on the board and providing a SMA or Hirose U.FL 
connector.  I don't like this option because it eliminates the option of 
DC coupling.  I could use a differential amplifier, but that will 
introduce distortion.  The THS4509 is expensive but low distortion.  
Another option is to use a surface mount connector and provide 
differential connections to the ADC as well as SPI and GPIO.  This would 
require an additional board, but would be the most flexible.

Also any advice on how to interface this with gnuradio would be welcome.

--
Darrell Harmon
100x100mm SBC running GNU/Linux:
http://dlharmon.com/sbc.html