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

[Marcus D. Leech]

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).

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.



 

-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org