Section 90.539 - Frequency stabilityTransmitters designed to operate in 769-775 MHz and 799-805 MHz frequency bands must meet the frequency stability requirements in this section.
(a) Mobile, portable and control transmitters must normally use automatic frequency control (AFC) to lock on to the base station signal.(b) The frequency stability of base transmitters operating in the narrowband segment must be 100 parts per billion or better.(c) The frequency stability of mobile, portable, and control transmitters operating in the narrowband segment must be 400 parts per billion or better when AFC is locked to the base station. When AFC is not locked to the base station, the frequency stability must be at least 1.0 ppm for 6.25 kHz, 1.5 ppm for 12.5 kHz (2 channel aggregate), and 2.5 ppm for 25 kHz (4 channel aggregate).(d) The frequency stability of base transmitters operating in the wideband segment must be 1 part per million or better.(e) The frequency stability of mobile, portable and control transmitters operating in the wideband segment must be 1.25 parts per million or better when AFC is locked to a base station, and 5 parts per million or better when AFC is not locked.63 FR 58651, Nov. 2, 1998, as amended at 65 FR 53646 , Sept. 5, 2000; 72 FR 48861 , Aug. 24, 2007