Editing Adjacent-channel interference

{{Short description|Interference caused by a signal in an adjacent channel}}
{{more footnotes|date=July 2018}}
'''Adjacent-channel interference''' ('''ACI''') is [[Interference (communication)|interference]] caused by extraneous [[power (physics)|power]] from a [[Electromagnetic radiation|signal]] in an [[adjacent channel]]. ACI may be caused by inadequate filtering (such as incomplete filtering of unwanted [[modulation]] products in [[frequency modulation|FM]] systems), improper [[tuner (radio)|tuning]] or poor frequency control (in the reference channel, the interfering channel or both).

ACI is distinguished from [[crosstalk]].<ref>{{Cite web |title=Federal Standard 1037C |url=https://its.ntia.gov/research-topics/federal-standard-1037c |access-date=10 May 2024 |url-status=live |archive-url=https://web.archive.org/web/20240331152240/https://its.ntia.gov/research-topics/federal-standard-1037c/ |archive-date=31 March 2024}}</ref>

==Origin==
The adjacent-channel interference which receiver A experiences from a transmitter B is the sum of the power that B emits into A's channel—known as the "unwanted emission", and represented by the ACLR (Adjacent Channel Leakage Ratio)—and the power that A picks up from B's channel, which is represented by the ACS (Adjacent Channel Selectivity). B emitting power into A's channel is called adjacent-channel leakage (unwanted emissions). It occurs for two reasons. First, because RF filters require a [[roll-off]], and do not eliminate a signal completely. Second, due to [[intermodulation]] in B's amplifiers, which cause the transmitted spectrum to spread beyond what was intended. Therefore, B emits some power in the adjacent channel which is picked up by A. A receives some emissions from B's channel due to the roll off of A's selectivity filters. [[Selectivity (radio)|Selectivity]] [[Filter (signal processing)|filters]] are designed to "select" a channel. Similarly, B's signal suffers intermodulation distortion passing through A's RF input amplifiers, leaking more power into adjacent frequencies.

==Avoidance procedure==
Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacent-channel interference. For example, in North America, [[FM radio]] stations in a single region cannot be licensed on adjacent frequencies — that is, if a station is licensed on 99.5&nbsp;MHz in a city, the first-adjacent frequencies of 99.3&nbsp;MHz and 99.7&nbsp;MHz cannot be used anywhere within a certain distance of that station's transmitter, and the second-adjacent frequencies of 99.1&nbsp;MHz and 99.9&nbsp;MHz are restricted to specialized usages such as low-power stations.<ref name=fcc>[https://www.govinfo.gov/content/pkg/CFR-2019-title47-vol4/xml/CFR-2019-title47-vol4-sec73-807.xml "Code of Federal Regulations: Minimum distance separation between stations"]. [[Federal Communications Commission]], October 1, 2019.</ref> Similar restrictions formerly applied to third-adjacent frequencies as well (i.e. 98.9&nbsp;MHz and 100.1&nbsp;MHz in the example above), but these are no longer observed.<ref name=fcc/>

==See also==
*[[Adjacent channel power ratio]]
*[[Federal Standard 1037C]]
*[[MIL-STD-188]]

==References==
{{Reflist}}

[[Category:Interference]]
[[Category:Electrical parameters]]