  Meson Green's functions and decay constants $f_{\Gamma}$ in different
channels $\Gamma$ are calculated using the Field Correlator Method. Both,
spectrum and $f_\Gamma$, appear to be expressed only through universal
constants: the string tension $\sigma$, $\alpha_s$, and the pole quark masses.
For the $S$-wave states the calculated masses agree with the experimental
numbers within $\pm 5$ MeV. For the $D$ and $D_s$ mesons the values of $f_{\rm
P} (1S)$ are equal to 210(10) and 260(10) MeV, respectively, and their ratio
$f_{D_s}/f_D$=1.24(3) agrees with recent CLEO experiment. The values $f_{\rm
P}(1S)=182, 216, 438$ MeV are obtained for the $B$, $B_s$, and $B_c$ mesons
with the ratio $f_{B_s}/f_B$=1.19(2) and $f_D/f_B$=1.14(2). The decay constants
$f_{\rm P}(2S)$ for the first radial excitations as well as the decay constants
$f_{\rm V}(1S)$ in the vector channel are also calculated. The difference of
about 20% between $f_{D_s}$ and $f_D$, $f_{B_s}$ and $f_B$ directly follows
from our analytical formulas.
