The EarthCARE satellite's Broadband Radiometer (BBR) consists of three telescopes and a rotating chopper drum (CD). Together they yield alternating measurements of totalwave (TW) (0.25 - >50 um) and shortwave (SW) (0.25 - 4 um) radiances with point-spread functions that translate to ~0.6 km diameter pixels. The mission requires that SW and TW radiances be averaged over ~100 km² domains. Corresponding average longwave (LW) radiances are differences between TW and SW averages. It is shown that impacts on domain-average nadir radiances due to alternating samples of TW and SW signals for realistic cloudy atmospheres are sensitive to: variance of cloudy-sky radiances; CD rotation rate; and along-track length of averaging domains. Over 5 x 21 km domains and at 50% rotation rate, uncertainties reached up to 3.2 W/m²/sr and 4.1 W/m²/sr for SW and TW radiances, respectively. The BBR's design allows for in-flight alteration of CD rate. An approximate method is provided for estimating SW and LW uncertainties due to CD rate. While the nominal rotation rate meets EarthCARE's mission requirements, reducing below ~75% of that rate will lead to uncertainties for domain-average LW radiances that will often exceed mission requirements. This could be mitigated by increasing the size of averaging domains, but that would compromise the BBR's role in EarthCARE's radiative closure assessment programme. Uncertainties for off-nadir radiances are largely free of impacts arising from changes to CD rotation rate.