Re: [Discuss-gnuradio] using external front ends

[Marcus D. Leech]

> You could, but the issue is that the LNA on these boards serve an
> important purpose in the sensitivity of the overall front-end.  If you
> are going to place another front-end in front of an existing one, you
> will have to keep in mind the characteristics of the existing
> front-end.  Yes, you can do it, but in practice it seems like a bad
> idea; it depends on what you want to do, really.
> ~Jeff
>
This type of thing is done all the time "in industry" -- not a bad idea
at all.  It's true that if you want to preserve overall Tsys, you should
  make certain that your front-end is at least as good as the device
that you're "fronting".

Use of transverters, both commercially, and in amateur radio is very
common.   In fact, the DBS_RX card uses a downconverter that is
  "normally" used "in behind" another down-converter--namely a C-band or
Ku-Band LNB, which downconverts the appropriate satellite
  band (C: 3.7-4.2GHz  Ku: 11.750 - 12.250GHz) to L-band (950MHz to
1450MHz).

The WBX and other cards do, it's true, have a low-noise amplifier "out
front", and in some sense it's a "shame" to "wreck" that.
  But usually down-converter/transverter front-ends use low-noise Rx
chains anyway.  Further, in any situation where there will
  be significant amounts of feed line between the Rx part of the
daughtercard (WBX, DBS_RX, etc) and the antenna, you are
  *forced* to put in your own LNA anyway, right up at the antenna, in
order to preserve system noise figure.  Any significant loss
  in front of a low-noise amplifier will render the low-noise figure
meaningless.

Consider for example, a situation where you have a LNA with a 0.8dB
noise figure (none of the daughtercards are quit that
  good, but let's go with this example).    Let's say you have 10ft of
feedline between your daughtercard and the antenna, and that you're
  operating at 1.0GHz.  You're likely looking at almost 1dB of insertion
loss for that cable, which makes your effective 0.8dB noise figure
  now a 1.8dB noise figure.  Put in terms of linear noise temperature,
that's an increase from 60K noise temperature to 152K noise temperature.
  In reality, if ultimate sensitivity (which is strongly related to
system noise figure/temperature) is important to you, you have to deal with
  that right up at the antenna.


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