Many modern NMR experiments exploit coupling interactions between protons and heteronuclei (eg. 13C). In such sequences the goal is to selectively observe the protons bound to 13C and suppress those bound to 12C. Since 13C is only 1 % naturally abundant, this means that 99% of the signal must be suppressed. One particularly simple scheme to accomplish this is the BIRD (BIlinear Rotation Decoupling) filter. The BIRD filter uses a heteroneuclear spin echo with delays equal to 1/(21JCH) to align the 1H(12C) and 1H(13C) spin vectors along the -y and y axes of the rotating frame of reference, respectively. The 180 degree phase difference between the 1H(12C) and 1H(13C) spin vectors allows a 90 degree pulse to align the these vectors on the -z and z axes, respectively. At this point the 1H(12C) spins are allowed to relax according to their T1 to the null point. A final 90 degree read pulse puts the 1H(13C) spins in the transverse plane for observation. The first of the two figures below demonstrates the use of the BIRD filter on the lineshape sample. The second figure shows a vector diagram explaining the sequence.