Besides the TPPM sequence, the efficiencies of the parent [SW.sub.f]-TPPM and some other phase modulated swept-frequency decoupling schemes derived from [SW.sub.f]-TPPM by decreasing the number of TPPM blocks, namely, [SW.sub.f]-TPPM-7 , [SW.sub.f]-TPPM-6, [SW.sub.f]-TPPM-5, and [SW.sub.f]-TPPM-4, which contains 7, 6, 5, and 4 TPPM blocks, respectively, are compared.
The scheme for two-pulse phase modulation (TPPM)  consists of two pulses of equal length [tau] with phases [phi] and -[phi].
The [SW.sub.f]-TPPM sequence  was formulatedwith TPPM as the building block.
The multiplicative factors of pulse duration given in [SW.sub.f] TPPM-11 follow a tangential path and it is possible to make new variants with less number of TPPM blocks keeping the pulse length variation tangential.
Experiments were performed with the parent [SW.sub.f]-TPPM-11 and its variants, [SW.sub.f]-TPPM-7, [SW.sub.f]-TPPM-6, [SW.sub.f]-TPPM-5, [SW.sub.f]-TPPM-4, and TPPM.
In TPPM, [SW.sub.f]-TPPM5, and [SW.sub.f]-TPPM-4 the decoupling bandwidth is smaller than for sequences with more number of elements as shown in Figure 2(a).
Experiments were conducted to study the effect of phase angles on the decoupling bandwidth in [SW.sub.f]-TPPM sequences with 4, 5, 6, 7, and 11 TPPM elements.
In the following sections, the trajectories of nuclear magnetization vector of the abundant nuclei in a simple spin system during TPPM and [SW.sub.f]-TPPM decoupling are traced out on the Bloch sphere and compared with those obtained from WALTZ-16, MLEV, and GaRP decoupling sequences which are commonly used in liquids for removing heteronuclear couplings.
Figure 5(a) shows the trajectory of proton magnetization vector on the Bloch sphere subjected to TPPM decoupling with a phase angle of 15[degrees] at a radiofrequency offset corresponding to the proton resonance in a simple spin system, benzene.