M. Stadler \emph{et al.}, "Development of the SwissFEL Undulator BPM System", in Proc. 3rd International Beam Instrumentation Conference, Monterey, CA, USA, paper WEPD13, pp. 675–679, ISBN: 978-3-95450-141-0, \url{http://jacow.org/IBIC2014/papers/wepd13.pdf}, 2014.