Therefore, high selectivity is crucial in environments where adjacent channels are close together as high selectivity enables RF system designers to most efficiently use the available bandwidth. If a filter has high selectivity, smaller transition bands are needed, which means smaller guard bands are necessary and less bandwidth is wasted implementing these features. Thus, a filter’s selectivity can tell us how much of the total bandwidth needs to be dedicated to transition bands. Selectivity is also sometimes described by talking about the size of the transition band necessary to get from the pass band to a certain rejection level, and often the size of the transition band is expressed as a percentage of the center frequency. Selectivity is a measurement of a filter’s ability to pass or reject specific frequencies closer to the band of interest. More specifically, in their recommendation document on “Unwanted Emissions in the Out-of-Band Domain,” the International Telecommunications Union (ITU) defines out-of-band emissions as “Emission on a frequency or frequencies immediately outside the necessary bandwidth which results from the modulation process, but excluding spurious emissions.” Selectivity These out-of-band emissions are far away from the band of interest (refer back to Figure 1) but can still interfere with the signals within the passband through effects such as aliasing. Therefore, your filter needs to have the ability to reject (attenuate) out-of-band emissions. Out-of-Band RejectionĪ passband filter cannot allow interference from signals outside the bandwidth of interest. It is important to note that you should consider insertion loss as a specification on both the Tx side since power is a system cost driver, as well as on the Rx side because loss impacts the overall noise figure of the receiver.
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