  Curvature radiation is a natural candidate for the emission mechanism of FRB.
However, FRB spectra have structure with $\Delta \nu/\nu \sim
\text{0.03--0.2}$, inconsistent with the very smooth spectrum of curvature
radiation. Although this spectral structure might be attributed to chromatic
scintillation or lensing, in four FRB high spectral resolution data indicate
scintillation decorrelation bandwidths much narrower than the observed $\sim
\text{30--300}$ MHz spectral structure. Some of the observed structure may be
intrinsic to the radiation mechanism. I suggest that the observed spectral
structure reflects the spatial structure of a clumpy radiating charge
distribution, and that the characteristic curvature radiation frequency may be
much higher than the observed frequencies. In this coupled plasma-curvature
radiation process the radiated spectra are the product of the spectra of the
plasma wave and that of incoherent curvature radiation. The argument applies to
all coherent radiation processes, including those that produce pulsar
nanoshots. The implied FRB "clump" charges are large, and produce electrostatic
potentials that suggest electron Lorentz factors $\gtrsim 10^2$. The result
applies generally to coherently radiating sources.
