N9918AU/355 FieldFox analog demodulation has two parts: (1) Tune and listen, and (2) AM/FM metrics. Tune and listen is available with the purchase of the spectrum analyzer option 233 (required). AM/FM metrics become available when Option 355 is purchased. AM/FM metrics provide the user with RF spectrum view, demodulated baseband signal waveform, carrier power, frequency deviation, SINAD and more.