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

[Tom Rondeau]

On Wed, Feb 27, 2013 at 3:04 AM, Martin Braun (CEL)
<address@hidden> wrote:
> On Tue, Feb 26, 2013 at 04:10:55PM -0600, Joel Mayer wrote:
>> Dear GNU Radio aficionado's-
>>
>> Whatever happened to resistance, capacitance, and inductance?
>> When I joined this thread I was hoping you would once in a while
>> talk about ways of using the software in the computer to modify
>> the resonant circuit in the hardware radio by making adjustments
>> to the resistors, capacitors, and inductors.
>
> Oh, one of my favourite topics :)
>
> I'll have to point out that I'm completely biased here:
> When I started playing around with radio, the thingy between antenna and
> the computer was a total black box, just producing the bits and bytes I
> needed. In fact, that's what got me attracted to the SDR world, because
> I could already program, but those Smith charts... but that's not the
> point.
>
> First, I'll give you a practical answer: This mailing list is about GNU
> Radio, which is in a sense very hardware-independent. So there you go.
>
> But it's also very important to understand that Software Radio is not
> about modelling hardware. Using software allows the use of DSP, which
> means we can do stuff like linear-phase filters etc. (good luck tuning
> your R/C/I to do that), and we can create transceivers for digitally
> modulated signals very easily, which is really awkward in discrete
> hardware (although analog modulations are also easily coded).
>
> So, perhaps I just misunderstood your question. But despite not knowing
> how you would modify an inductor through software, this wouldn't help in
> building a transceiver that, upon sensing a change in the radio
> environment, would reconfigure itself and change the waveform used--
> which is one goal of GNU Radio. Of course, this comes with some
> abstraction, which means that changing the resonant circuit of my USRP
> is nothing more than a library call to set_center_frequency() or
> whatever it's called.
>
> MB

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