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Re: [Discuss-gnuradio] Low-cost hardware options

From: John Gilmore
Subject: Re: [Discuss-gnuradio] Low-cost hardware options
Date: Sat, 15 Jan 2011 18:56:46 -0800

> Also upgraded to the next speed-grade of the ADC, to give 40Msps...

You spec'd only a 12-bit ADC.  In my naive view, resolution seems like
it's more important to SDR than samples per second.  Resolution is how
you avoid losing weak signals when you are of necessity sampling a
wide band.  Can we improve on this to get a 14- or 16-bit ADC, perhaps
with a lower sampling rate, at a reasonable price?

As I recall from early USRP days, clock jitter makes a real mess of
doing anything important with an SDR.  If you can't trust your clock,
you don't know when your samples happened, which makes all the
computation a lot fuzzier.  That's why the USRP didn't synthesize its
sampling clock -- nobody back then built a synthesized clock that had
low enough jitter.  Does this ADF4351 qualify?  And what kinds of
interactions are there between that clock and the clock on the ADC?
Shouldn't the downsampler and the digitizer both be using the same
clock, or a clock that's derived from the same clock?

Is there a way to use i2c programmable width filters instead of the 20
MHz lowpass filters, to narrow the bandwidth of the signal being
digitized down to just the range of interest?  This would help
ameliorate the low-res ADC by filtering out nearby
loud-yet-uninteresting signals.

Finally, don't assume that a USB3 chip will easily support downgrading
to a USB2 connection.  It ought to be that way, but might not be.  The
EZ-FX2 chip does support both USB1 and USB2, but in the last ten
years, nobody ever got around to programming the USRP firmware to
actually make it work with USB1.  That became somewhat moot as USB2
became standard in everything.

Similarly, the GigE interface on the USRP2 has never supported
downgrading to 100 Mbit Ethernet, even though that's part of the GigE
spec.  However, now that they've switched to using a UDP-based
protocol rather than an Ethernet-frame-based protocol (finally -
hooray!), you can plug the USRP2 into a switch.  Switches all allow
10, 100, and 1000 Mbit/sec Ethernet to communicate.  If you're going
to put 1GE on this device rather than USB2 or USB3, I suggest
including a switch chip too, so it will transparently talk to any
speed of Ethernet.  GigE is still too uncommon on laptops these days.


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