  We present an analysis of the different types of noise from the detection and
interrogation laser in our strontium lattice clock. We develop a noise model
showing that in our setup quantum projection noise--limited detection is
possible if more than 130~atoms are interrogated. Adding information about the
noise spectrum of our clock laser with sub-$10^{-16}$ fractional frequency
instability allows to infer the clock stability for different modes of
operation. Excellent agreement with experimental observations for the
instability of the difference between two interleaved stabilizations is found.
We infer a clock instability of $1.6 \times 10^{-16}/\sqrt{\tau / \mathrm{s}}$
as a function of averaging time $\tau$ for normal clock operation.
