Sec. 21.122  Microwave digital modulation.

    (a) Microwave transmitters employing digital modulation techniques 
and operating below 15 GHz shall, with appropriate multiplex equipment, 
comply with the following additional requirement: The bit rate, in bits 
per second, shall be equal to or greater than the bandwidth specified by 
the emission designator in Hertz (e.g., to be acceptable, equipment 
transmitting at a 6 Mb/s rate must not require a bandwidth of greater 
than 6 MHz), except the bandwidth used to calculate the minimum rate 
shall not include any authorized guard band.
    (b) For purposes of compliance with the emission limitation 
requirements of Sec. 21.106(a)(2) of this part and the requirements of 
paragraph (a) of this section, digital modulation techniques are 
considered as being employed when digital modulation contributes 50 
percent or more to the total peak frequency deviation of a transmitted 
radio frequency carrier. The total peak frequency deviation shall be 
determined by adding the deviation produced by the digital modulation 
signal and the deviation produced by any frequency division multiplex 
(FDM) modulation used. The deviation (D) produced by the FDM signal 
shall be determined in accordance with Sec. 2.202(f) of part 2 of this 
chapter.
    (c) Transmitters employing digital modulation techniques shall 
effectively eliminate carrier spikes or single frequency tones in the 
output signal to the degree which would be obtained without repetitive 
patterns occurring in the signal.
[ 44 FR 60534 , Oct. 19, 1979, as amended at  46 FR 23451 , Apr. 27, 1981; 
 49 FR 37775 , Sept. 26, 1984;  58 FR 49226 , Sept. 22, 1993;  61 FR 26676 , 
May 28, 1996]

                     Subpart D--Technical Operation