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Re: [Discuss-gnuradio] DSP based SDR

From: John Gilmore
Subject: Re: [Discuss-gnuradio] DSP based SDR
Date: Thu, 26 May 2005 07:50:08 -0700

Have you considered putting a USRP-compatible daughterboard connector
on your DSP-based board?  That would let you mostly ignore radio front
ends.  You and anyone else could piggyback on the work done to make
each radio front end receiver board made for the USRP.  You'd just use
the cheap connector/transformer board for direct connection to
external radios.  It seems like building the radio front ends is a
significant pain (they're coming out more slowly than expected), so
you can save yourself that pain.

Matt and Eric claimed during the USRP development that it wasn't
possible to build a USB-powered board that didn't (1) draw too much
power, and (2) couple too much noise into the sensitive analog
circuitry.  Of course, their board has four hungry chips on it; yours
will have only two.  You might try building your first prototype
boards with jumpers or solder bridges that will let you test with USB
power and with wallwart power -- and see if the radio performs

In the USRP I had proposed that the clocks to the ADC's be
programmable, so the whole processing chain could run more slowly than
the maximum rate.  Matt was unable to find clock generator parts that
meet his jitter specs (which are critical for oversampling).  That's
why the USRP runs the ADC at top speed and downconverts using math in
the FPGA.  If your Blackfin can't accept data at top speed, you'll
have to improve on the USRP's clocking.

You prefer USB2 over firewire or gigabit Ethernet?  It would be nice
to have a radio spectrum interface that didn't have USB2's bandwidth
as its bottleneck.  Gig E is always full duplex (no collisions between
transmit and receive), and offers higher bandwidth.  GigE switches are
now in the $100 range and dropping fast (or you can plug the board
directly into a computer's GigE port, if it isn't on the Internet at
the same time, e.g. for mobile laptop use).

        John Gilmore

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