Using external front ends

Hi

I wanted to find out if it is possible to use daughterboards other
than the basic rx and tx with external front ends. Ie is it possible
to use the WBX daughterboard with an external front end?

Regards

Stacey

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

On 9/30/2010 4:50 AM, Stacey Rukezo wrote:

Hi

I wanted to find out if it is possible to use daughterboards other
than the basic rx and tx with external front ends. Ie is it possible
to use the WBX daughterboard with an external front end?

Regards

Stacey


~Jeffrey L., K1VZX

Yes, that would be possible. Keep in mind that the WBX already has
substantial gain, so you’ll
likely need to attenuate the output of your own front-end to keep the
signal levels in range
(below -20dbm) for the Rx on the WBX board.

This kind of thing is done all the time in the strictly-analog-radio
world. So-called transverters
are used to convert from one frequency domain to another. In the
amateur radio world it’s
often the case that people operate on the microwave bands by using
their 144MHz-range
transceiver through a “transverter” to convert to higher frequency
operation on other bands.
Absolutely no reason you couldn’t do the same thing with cards like
the WBX.


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

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

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