Last week the FCC released a Public Notice outlining two methods for determining compliance with the emission mask requirements when using equipment that can't be set to 500 kHz RBW:
- "Method 1: Scale the measured power to a 500 kHz bandwidth based on 10 log (500 kHz/equivalent noise bandwidth of the resolution filter) and compare each point on the spectrum to the emission mask; or, equivalently, scale the measured attenuation by 10 log (equivalent noise bandwidth of the resolution filter/500 kHz) and compare each point.
- "Method 2: Sum the power across a 500 kHz window in order to implement an effective measurement bandwidth of 500 kHz; this power level is then compared to the mask value computed at the center frequency of the 500 kHz band."
FCC rules for out-of-channel emission masks for both full-service and low power DTV transmitters are based on a measurement bandwidth of 500 kHz and are defined as attenuation relative to the total in-channel power. Because the 500 kHz resolution bandwidth (RBW) may not be available on many spectrum analyzers, the FCC allows RBWs narrower than 500 kHz to be used if appropriate correction factors are applied.
Either or both methods could be used when making emission mask measurements, but the FCC warned that resolution bandwidth should not be greater than 500 kHz in any case.
The Public Notice provides guidance on setting sweep span and resolution bandwidth on spectrum analyzers with digital displays. When using these spectrum analyzers, the FCC cautions "care must be taken to ensure that the frequency bin width on the analyzer [span /(number of points in the spectrum - 1)] is less than or equal to the RBW." It also warns that depending on the type of spectrum analyzer used, "additional correction factors may be necessary to accurately represent the average power of the noise-like DTV transmission."
The Public Notice mentions the detailed procedure for measuring DTV emission spectra under development by the IEEE Broadcast Technology Society. Information on IEEE broadcast standards can be found at http://www.ieee.org/organizations/society/bt/stnds.html.