pulsewidth FTICR piQund4 genac 3dB heterodyne ELISPOT 13Qp56 omegaQund0 DSFN microvolts colorburst masker 5060Qp194 whitework modulated 98Qp356 bandpass 20kHz subharmonics DCF77 fQundo radic midband kilohertz lowpass dBc Nivkhi 0Qp7mm demodulated modulation Fif 6MHz superheterodyne sinusoid