Section 73.1540 - Carrier frequency measurements(a) The carrier frequency of each AM and FM station shall be measured or determined as often as necessary to ensure that they are maintained within the prescribed tolerances.(b) In measuring the carrier frequency, the licensee may use any method or procedure that has sufficient precision to establish that the carrier frequency is within the prescribed departure limits.(c) The primary standard of frequency for radio frequency measurements is the standard frequency maintained by the National Bureau of Standards or the standard signals of Stations WWV, WWVB, and WWVH of the National Bureau of Standards.43 FR 32783, July 28, 1978, as amended at 48 FR 44805, Sept. 30, 1983; 65 FR 30004 , May 10, 2000