ITU-R Recommendation ITU-R SM.329 ‎[4] on Unwanted emissions in the spurious domain provides a definition of dBc unit, which is defined as “Decibels relative to the unmodulated carrier power of the emission. In the cases which do not have a carrier, for example in some digital modulation schemes where the carrier is not accessible for measurement, the reference level equivalent to dBc is decibels relative to the mean power P. From this recommendation the following example is presented:

A land mobile transmitter, with any value of emission bandwidth, must meet an attenuation of 43 + 10 logP, or 70 dBc, whichever is less stringent. To measure the emissions the use of a reference bandwidth of 100 kHz is recommended for spurious emissions within the frequency range 30 to 1000 MHz.

With a measured total mean power of 10 W:

Attenuation relative to total mean power = 43 + 10 log(10) = 53 dB

The 53 dBc is less stringent than 70 dBc, so the 53 dBc value is used. Therefore, emissions must not exceed 53 dBc in a 100 kHz reference bandwidth, or converting to an absolute level: 10 dBW – 53 dBc = – 43 dBW in a 100 kHz reference bandwidth.

With a measured total mean power of 1 000 W:

Attenuation relative to total mean power = 43 + 10 log(1000 ) = 73 dB

The 73 dBc is more stringent than 70 dBc limit, so the 70 dBc value is used. Therefore, emissions must not exceed 70 dBc in a 100 kHz reference bandwidth, or converting to an absolute level: 30 dBW - 70 dBc = -40 dBW in a 100 kHz reference bandwidth.

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