Editing Software-defined radio (section)

== Operating principles ==
[[File:SDR et WF.svg|thumb|right|upright=1.8|Software defined radio concept]]

[[Superheterodyne receiver]]s use a VFO ([[variable-frequency oscillator]]), [[Frequency mixer|mixer]], and [[Filter (signal processing)|filter]] to tune the desired signal to a common IF ([[intermediate frequency]]) or [[baseband]]. Typically in SDR, this signal is then sampled by the analog-to-digital converter. However, in some applications it is not necessary to tune the signal to an intermediate frequency and the radio frequency signal is directly sampled by the [[analog-to-digital converter]] (after amplification).

Real analog-to-digital converters lack the dynamic range to pick up sub-microvolt, nanowatt-power radio signals produced by an antenna. Therefore, a [[low-noise amplifier]] must precede the conversion step and this device introduces its own problems. For example, if [[Spurious emission|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 filter]]s  between the antenna and the amplifier, but these reduce the radio's flexibility. Real software radios often have two or three analog channel filters with different bandwidths that are switched in and out.

The flexibility of SDR allows for dynamic spectrum usage, alleviating the need to statically assign the scarce spectral resources to a single fixed service.<ref>{{Cite journal |url=http://www.spectrum.ieee.org/mar04/3811 |archive-url=https://archive.today/20120910051620/http://www.spectrum.ieee.org/mar04/3811 |url-status=dead |archive-date=September 10, 2012 |first1=Gregory |last1=Staple |first2=Kevin |last2=Werbach |title=The End of Spectrum Scarcity |journal=[[IEEE Spectrum]] |date=March 2004|volume=41 |issue=3 |pages=48–52 |doi=10.1109/MSPEC.2004.1270548 |s2cid=1667310 |url-access=subscription }}</ref>