Real analog-to-digital converters lack the
discrimination to pick up sub-microvolt, nanowatt
radio signals. Therefore a low-noise amplifier must
precede the conversion step and this device
introduces its own problems. For example if spurious
signals are present (which is typical), these compete
with the desired signals within the amplifier's
dynamic range. They may introduce distortion in the
desired signals, or may block them completely. The
standard solution is to put band-pass filters between
the antenna and the amplifier, but these reduce the
radio's flexibility - which some see as the whole
point of a software radio. Real software radios often
have two or three analog "channels" that
are switched in and out. These contain matched
filters, amplifiers and sometimes a mixer.
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