  The experimental study of the modulation of the envelope of spin-echo signals
due to internal and external fields is an important spectroscopic tool to
detect very small internal magnetic fields. We derive the free induction decay
and the frequency spectrum and amplitude of spin-echo signals for arbitrary
orientation of fields with respect to crystalline axis for nuclei in a crystal
of orthorhombic symmetry. Results reproduce the results that no modulation
should be observed in tetragonal crystals for fields either along the c-axis or
any direction in the basal plane and give details of the signal as a function
of the orthorhombicity parameter. They are used to discuss recent experimental
results and provide guidelines for future experiments.
