47 CFR 76.612 - Cable television frequency separation standards.
(a) In the aeronautical radiocommunication bands 118-137, 225-328.6 and 335.4-400 MHz, the frequency of all carrier signals or signal components carried at an average power level equal to or greater than 10−4 watts in a 25 kHz bandwidth in any 160 microsecond period must operate at frequencies offset from certain frequencies which may be used by aeronautical radio services operated by Commission licensees or by the United States Government or its Agencies. The aeronautical frequencies from which offsets must be maintained are those frequencies which are within one of the aeronautical bands defined in this subparagraph, and when expressed in MHz and divided by 0.025 yield an integer. The offset must meet one of the following two criteria:
(1) All such cable carriers or signal components shall be offset by 12.5 kHz with a frequency tolerance of ±5 kHz; or
(2) The fundamental frequency from which the visual carrier frequencies are derived by multiplication by an integer number which shall be 6.0003 MHz with a tolerance of ±1 Hz (Harmonically Related Carrier (HRC) comb generators only).
(b) In the aeronautical radionavigation bands 108-118 and 328.6-335.4 MHz, the frequency of all carrier signals or signal components carrier at an average power level equal to or greater than 10−4 watts in a 25 kHz bandwidth in any 160 microsecond period shall be offset by 25 kHz with a tolerance of ±5 kHz. The aeronautical radionavigation frequencies from which offsets must be maintained are defined as follows:
(1) Within the aeronautical band 108-118 MHz when expressed in MHz and divided by 0.025 yield an even integer.
(2) Within the band 328.6-335.4 MHz, the radionavigation glide path channels are listed in Section 87.501 of the Rules.
The HRC system, as described above, will meet this requirement in the 328.6-335.4 MHz navigation glide path band. Those Incrementally Related Carriers (IRC) systems, with comb generator reference frequencies set at certain odd multiples equal to or greater than 3 times the 0.0125 MHz aeronautical communications band offset, e.g. (6n 1.250 ±0.0375) MHz, may also meet the 25 kHz offset requirement in the navigation glide path band.