Editing Heterodyne (section)

===Up and down converters===
In large scale [[telecommunication network]]s such as [[telephone network]] trunks, [[microwave relay]] networks, cable television systems, and [[communication satellite]] links, large [[bandwidth (signal processing)|bandwidth]] capacity links are shared by many individual communication channels by using heterodyning to move the frequency of the individual signals up to different frequencies, which share the channel. This is called [[frequency division multiplexing]] (FDM).

For example, a [[coaxial cable]] used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency, so they do not interfere with one another. At the cable source or [[Cable television headend|headend]], electronic upconverters convert each incoming television channel to a new, higher frequency.  They do this by mixing the television signal frequency, ''f<sub>CH</sub>'' with a [[local oscillator]] at a much higher frequency {{math|''f<sub>LO</sub>''}}, creating a heterodyne at the sum {{math|''f<sub>CH</sub>''&nbsp;+&nbsp;''f<sub>LO</sub>''}}, which is added to the cable. At the consumer's home, the cable [[set top box]] has a downconverter that mixes the incoming signal at frequency {{math|''f<sub>CH</sub>''&nbsp;+&nbsp;''f<sub>LO</sub>''}} with the same local oscillator frequency {{math|''f<sub>LO</sub>''}} creating the difference heterodyne frequency, converting the television channel back to its original frequency: {{math|1=(''f<sub>CH</sub>''&nbsp;+&nbsp;''f<sub>LO</sub>'')&nbsp;−&nbsp;''f<sub>LO</sub>'' =&nbsp;''f<sub>CH</sub>''}}.  Each channel is moved to a different higher frequency.  The original lower basic frequency of the signal is called the [[baseband]], while the higher channel it is moved to is called the [[passband]].