Sec.  73.757   System specifications for single-sideband (SSB) modulated emissions
in the HF broadcasting service.

   (a) System parameters—(1) Channel spacing. In a mixed DSB, SSB and digital
   environment (see Resolution 517 (Rev.WRC–03)), the channel spacing shall be
   10 kHz. In the interest of spectrum conservation, it is also permissible to
   interleave SSB emissions midway between two adjacent DSB channels, i.e.,
   with 5 kHz separation between carrier frequencies, provided that the
   interleaved emission is not to the same geographical area as either of the
   emissions between which it is interleaved. In an all inclusive SSB
   environment, the channel spacing and carrier frequency separation shall be 5
   kHz.

   (2) Equivalent sideband power. When the carrier reduction relative to peak
   envelope power is 6 dB, an equivalent SSB emission is one giving the same
   audio-frequency signal-to-noise ratio at the receiver output as the
   corresponding DSB emission, when it is received by a DSB receiver with
   envelope detection. This is achieved when the sideband power of the SSB
   emission is 3 dB larger than the total sideband power of the DSB emission.
   (The peak envelope power of the equivalent SSB emission and the carrier
   power are the same as that of the DSB emission.)

   (b) Emission characteristics—(1) Nominal carrier frequencies. Nominal
   carrier frequencies shall be integral multiples of 5 kHz.

   (2) Frequency tolerance. The frequency tolerance shall be 10 Hz.

   Note 1 to Paragraph (b)(2): The ITU suggests that administrations avoid
   carrier frequency differences of a few hertz, which cause degradations
   similar to periodic fading. This could be avoided if the frequency tolerance
   were 0.1 Hz, a tolerance which would be suitable for SSB emissions.

   Note 2 to Paragraph (b)(2): The SSB system adopted for the bands allocated
   exclusively to HF broadcasting does not require a frequency tolerance less
   than 10 Hz. The degradation mentioned in Note 1 occurs when the ratio of
   wanted-to-interfering signal is well below the required protection ratio.
   This remark is equally valid for both DSB and SSB emissions.

   (3) Audio-frequency band. The upper limit of the audio-frequency band (at—3
   dB) of the transmitter shall not exceed 4.5 kHz with a further slope of
   attenuation of 35 dB/kHz and the lower limit shall be 150 Hz with lower
   frequencies attenuated at a slope of 6 dB per octave.

   (4) Modulation processing. If audio-frequency signal processing is used, the
   dynamic range of the modulating signal shall be not less than 20 dB.

   (5) Necessary bandwidth. The necessary bandwidth shall not exceed 4.5 kHz.

   (6) Carrier reduction (relative to peak envelope power). In a mixed DSB, SSB
   and digital environment, the carrier reduction shall be 6 dB to allow SSB
   emissions to be received by conventional DSB receivers with envelope
   detection without significant deterioration of the reception quality.

   (7) Sideband to be emitted. Only the upper sideband shall be used.

   (8) Attenuation of the unwanted sideband. The attenuation of the unwanted
   sideband (lower sideband) and of intermodulation products in that part of
   the emission spectrum shall be at least 35 dB relative to the wanted
   sideband signal level. However, since there is in practice a large
   difference between signal amplitudes in adjacent channels, a greater
   attenuation is recommended.

   [ 70 FR 46677 , Aug. 10, 2005]