14N NMR echo in NH4ClO4 after the pulse sequence (theta1)x-tau-(theta2)y-t

We have derived a closed-form expression for the solid echo signal of quadrupolar I = 1 nuclei after the pulse sequence (theta1)x-tau-(theta2)y-t for arbitrary values of the RF nutation frequency omega1 = gammaB1 and the quadrupolar frequency omegaQ. In the case of single crystals both the true echo term of this expression and its induction-signal-like terms are important as shown by experiments on 14N nuclei in NH4ClO4 crystal. Conditions for obtaining the maximal echo in powder samples are presented. A very low B1 field together with long RF pulses may distort even the central part of the spectrum, resulting in strange looking apparent spectra.