def __new__(cls, fx_or_grid, fy_or_field=None, fz=None, dtype=complex, freq
    =None):
    """Initiate a new Source Field."""
    if freq is None:
        print(f'* ERROR   :: SourceField requires the frequency.')
        raise ValueError('SourceField needs `freq`.')
    if freq > 0:
        dtype = complex
    else:
        dtype = float
    return super().__new__(cls, fx_or_grid, fy_or_field=fy_or_field, fz=fz,
        dtype=dtype, freq=freq)
