Constant-time period General scheme

A constant-time (CT) period involve one spin undergoing chemical shift evolution while simultaneously building up or refocusing antiphase coherence with other spins. Thus, during a CT period of lenght 2T (or 2*d5), chemical shift of the selected X spin evolves only during a variable t₁ period whereas homonuclear couplings, J_XX, only evolves during the fixed 2*d5 period (see general scheme for CT period in a two-spin system (IX) ).
 
On the other hand, in a three-spin system, IXY, in which :

• X undergo chemical shift evolution.
• J_IX>J_XY>0 and J_IY=0
• I is always a heteronuclear spin with respect X and Y (they can be homo- or heteronuclear spins). 

several schemes can be clearly designed as a function of the evolution of the corresponding J_IX and J_XY coupling constants into the Ct period:
 

	Case A: No J  coupling evolution	Case B:	Case C	Case D: Evolution of both coupling constants
JIX	0	0	0	2*d6
JXY	0	2*d5	2*d4	2*d4
JXX'	2*d5	2*d5	2*d5	2*d5