On Wed, Feb 27, 2013 at 11:52 AM, Tom Rondeau <t...@trondeau.com> wrote: > On Wed, Feb 27, 2013 at 11:38 AM, Marcus D. Leech <mle...@ripnet.com> wrote: >> On 27/02/13 10:08 AM, Tom Rondeau wrote: >>> >>> On the other hand, one of the major areas of work that we are still >>> pursuing lies in the RF front end. We have wideband systems. Ettus has >>> produced a number of daughterboards that cover multiple GHz of >>> spectrum, which is fantastic. But through that, we suffer a bit on the >>> amplifiers and filters needed for some kinds of communications tasks. >>> What Ettus has done is produced very good IP3s, NFs, gains, etc over >>> these large bandwidths, but that doesn't exactly compare to having a >>> filter and amplifier specific to a small bandwidth for something like >>> cellular communications. Or even, for that matter, antennas for >>> various waveforms. >>> >>> Even today's wideband RFICs tend to have a lot of tweakable/tunable >>> parameters to meet specific needs of different areas of spectrum. Are >>> there software solutions that could be used to automatically adjust >>> these parameters? Or an RLC matching circuit? Some of this, I know, >>> requires advances in the materials and components to make any sense, >>> in other cases the feedback loop could be a bit long to make any >>> significant impact. But it's fun to think about. Goes back to my >>> dissertation days, actually :) >>> >>> Tom >>> >> In some sense, what we're talking about here is the difference between >> Software *Controlled* Radio, and >> Software *Defined* Radio. >> >> A chain of DSP blocks applies a series of mathematical transforms to a >> digitized signal, in similar ways >> to the way a series of R/L/C/Gain components do to an analog signal. >> One can think of the R/L/C approach >> as performing rough *approximations* to a transform that is defined in >> strict mathematical terms. The DSP >> approach, in general, is able to achieve a higher "fidelity" of those >> transforms with a higher degree of >> flexibility and reconfigurability than could possibly be achieved with >> analog hardware. Although, at some >> considerable cost--a simple FM demodulator chip is $0.35 in bulk, >> whereas the amount of compute-gear you >> require to do the same thing is considerably more costly. But DSP/SDR >> doesn't require that you break out >> the soldering iron and parts bin every time you want to >> tweak/repurpose things. >> >> Now, having said that, the notion of having some kind of "tracking" >> filtering isn't a bad idea, the problem comes >> down to implementation, and the cost trade-offs involved. >> Considering things like daughter cards covering >> 30Mhz to 4.4Ghz, exactly how many "cuts" across that bandwidth do you >> make, and how much are people >> willing to pay for additional dynamic range implied by band-limiting >> at the RF end of things? The technology >> is mostly there -- GaAsFET RF switches, LTCC filter modules, saw >> filters, dielectric filters, etc, are all out there. >> But almost any "hard" decision made by the manufacturer in such things >> is likely to be "wrong" in enough cases >> that perhaps all of that should be done externally to a daughtercard, >> with provision for a generic switching >> interface (like the existing GPIOs on many Ettus daughtercards). > > Well said. > >> In an ideal world, you wouldn't need much front-end filtering. Your >> gain stages would be uber-linear up to ridiculous >> input powers, and you'd have a 24-bit ADC sampling at several Gsps. >> We aren't anywhere near there yet. > > Yeah.... that's a hard one to swallow. The near-far problem in some > bands makes even what you're considering here problematic. ~140 dB > dynamic range does sound pretty good for most things, though :) > > There are other proposed solutions out there that take this concept > even farther but require much greater investment in hardware cost > (we're talking multiple ADCs, DSP units, etc.). > > But we're still going to need front end filtering and amplifiers for a > while longer. Oh, and let's still not forget the antenna, though there > are decent solutions for narrowband signals over large bandwidths > (that is, good performance over a large bandwidth with varying group > delays along the way, so you can only get away with narrowband signals > or clever correction algorithms). > > Tom > > >> -- >> Principal Investigator >> Shirleys Bay Radio Astronomy Consortium >> http://www.sbrac.org
For those interested, this DARPA project was pointed out to me: https://www.fbo.gov/index?s=opportunity&mode=form&id=7c438631d57659b7b9f932df6d3da484&tab=core&_cview=1 Here's a brief write-up that summarizes the effort (apologies for the in-your-face ad at the top): http://www.militaryaerospace.com/articles/2012/08/darpa-rf-fpga.html Tom _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
