  The formation pathways of deuterated species trace different regions of
protoplanetary disks and may shed light into their physical structure. We aim
to constrain the radial extent of main deuterated species; we are particularly
interested in spatially characterizing the high and low temperature pathways
for enhancing deuteration of these species. We observed the disk surrounding
the Herbig Ae star HD 163296 using ALMA in Band 6 and obtained resolved
spectral imaging data of DCO$^+$ ($J$=3-2), DCN ($J$=3-2) and N$_2$D$^+$
($J$=3-2). We model the radial emission profiles of DCO$^+$, DCN and
N$_2$D$^+$, assuming their emission is optically thin, using a parametric model
of their abundances and radial excitation temperature estimates. DCO$^+$ can be
described by a three-region model, with constant-abundance rings centered at 70
AU, 150 AU and 260 AU. The DCN radial profile peaks at about ~60 AU and
N$_2$D$^+$ is seen in a ring at ~160 AU. Simple models of both molecules using
constant abundances reproduce the data. Assuming reasonable average excitation
temperatures for the whole disk, their disk-averaged column densities (and
deuterium fractionation ratios) are 1.6-2.6$\times 10^{12}$ cm$^{-2}$
(0.04-0.07), 2.9-5.2$\times 10^{12}$ cm$^{-2}$ ($\sim$0.02) and 1.6-2.5 $\times
10^{11}$ cm$^{-2}$ (0.34-0.45) for DCO$^+$, DCN and N$_2$D$^+$, respectively.
Our simple best-fit models show a correlation between the radial location of
the first two rings in DCO$^+$ and the DCN and N$_2$D$^+$ abundance
distributions that can be interpreted as the high and low temperature
deuteration pathways regimes. The origin of the third DCO$^+$ ring at 260 AU is
unknown but may be due to a local decrease of ultraviolet opacity allowing the
photodesorption of CO or due to thermal desorption of CO as a consequence of
radial drift and settlement of dust grains.
