Why New Narrowband Channel Assignments Impact Filter Q and Channel Isolation Requirements
Narrowbanding has been a congestion mitigation challenge since the early 1990’s, and in 2013 the FCC required all current licensees to be fully operational with 12.5 kHz equipment. This step allowed the conversion of preexisting channel spectrum to be further divided, creating additional channels. However, there is a cost to making spectrum more efficient. Denser channel assignments also mean that there are twice as many potential interference vectors from adjacent channels, and the requirement for channel-to-channel isolation must be more stringent.
Moreover, it is likely in the next several years that the FCC will begin another round of narrowband that will require all licensees to meet new 6.25 kHz channel spacing requirements. VHF private land mobile narrowband already employs 7.5 kHz spacing.
Radios made after the 2000’s were incorporated with programmability which allowed channel assignments to be changed. So, some may wonder why filter quality factor or channel isolation has anything to do with narrowbanding. The radio equipment may have only been specified under the larger bandwidth requirements and, to be competitive, newer radios and service providers need to opt for higher quality factor filters and radio equipment with higher channel isolation to avoid illegal interference into the increasingly narrow VHF and UHF channels.
Caption: The quality factor of an oscillator or filter resonator is directly measured as a ratio of the center frequency to the 3dB bandwidth of the resonator or oscillators frequency response. Hence, the higher the Q, the “sharper” the resonant response.
Since filter quality factor is the ratio of the center frequency to the 3dB bandwidth of the resonator response; in order to cut the 3dB bandwidth in half, the filter Q will have to be doubled to offer the same performance in the new narrowband channel assignments. With the upcoming divide down to 6.25 kHz, this means that for comparable performance, the filter Qs will have to be doubled yet again. There are physical limitations, material limitations, and design constraints that limit filter Qs, and these factors must be accounted for in order to provide high performance filters that meet these new Q requirements while also being reliable and offer high power handling. Channel isolation follows in much the same way, and is impacted by filter Q and the attenuation properties of the filter outside of the passband.
With narrower channels, the requirements for frequency stability are more stringent. Users must pay more attention to proper tuning of all components in a radio system and the radio’s frequency controls must be more accurate. Temperature or aging could cause frequency response drift that could interfere with nearby channels.