def METHOD_NAME(gain, nf_expected, setup_edfa_fixed_gain, si):
    """=> unitary test for fixed gain model Edfa._calc_nf() (and Edfa.interpol_params)"""
    edfa = setup_edfa_fixed_gain
    si.signal /= db2lin(gain)
    si.nli /= db2lin(gain)
    si.ase /= db2lin(gain)
    edfa.operational.gain_target = gain
    si.pref = si.pref._replace(p_span0=0, p_spani=-gain)
    edfa.interpol_params(si)
    assert pytest.approx(nf_expected, abs=0.01) == edfa.nf[0]