Variance form of the large sieve

For coefficients $b_n$ supported on an interval of $H$ consecutive integers, set $B=\sum b_n$ and $B_p(a)={\sum }_{n\equiv a(\mathrm{mod}p)}{b}_n$. ThenThe orthogonality of roots of unity gives $p{\sum }_a|B_p(a)-B/p{|}^2={\sum }_{r=1}^{p-1}|{\sum }_n{b}_{n}e(rn/p){|}^2$. The distinct fractions $r/p$ have circular spacing at least $Q^{-2}$; now apply the exponential-sum large sieve.