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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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