The APEX digital Fast Fourier Transform Spectrometer B. Klein, S. D. Philipp, I. Krämer, C. Kasemann, R. Güsten and K. M. Menten A&A, 454 2 (2006) L29-L32 Published online: 11 July 2006 DOI: 10.1051/0004-6361:20065415