Exact and approximate spectra for trapezoidal FM

The intensity spectrum for an FM signal modulated by a trapezoidal waveform is given in terms of Fresnel integrals. The numerical results, as observed with a finite resolution filter, are compared with the quasi-stationary model, which is asymptotically valid for large values of the FM index. The size of the discrepancy relative to the asymptotic model depends on the resolution of the spectrum analyzer viewing the FM signal.