Re: [Discuss-gnuradio] Resistance? Capacitance? Inductance?

[Tom Rondeau]

On Wed, Feb 27, 2013 at 11:52 AM, Tom Rondeau <address@hidden> wrote:
> On Wed, Feb 27, 2013 at 11:38 AM, Marcus D. Leech <address@hidden> 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