On 09/06/2013 03:33 PM, Ian Buckley wrote:
Filters will all to some degree attenuate your signal of interest, but
by how much varies dramatically depending on the type and design of
the filter, it could be 0.5dB or 20dB, but the point is that it
attenuates potential interferers and noise by a great amount. An LNA
cascaded into a bandpass filter as close as possible to the antenna is
generally an ideal setup for this type of weak signal work.
LNA into filter is the line-up we use in radio astronomy, which might be
described as the "limiting case" of weak-signal work.
A circular-waveguide feedhorn designed for the band of interest will act
as a high-pass filter with a "knee" at the design frequency of the
feedhorn. This,
combined with directionality of a dish is often enough to eliminate
the worst of the interferers *before* the LNA, and then apply a filter
after the LNA.
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