Explanation: F1B FEC mode utilizes Frequency Shift Keying (FSK) for digital data transmission. In FSK, data is represented by shifting the carrier frequency between two or more discrete, predefined frequencies (mark and space). Measuring "frequency deviation" with a spectrum analyzer, as described in option C, is a technique primarily used for analyzing analog Frequency Modulation (FM) signals, where the carrier frequency varies continuously in proportion to the amplitude of the modulating signal. Since F1B FEC is a digital FSK mode that uses discrete frequency shifts, measuring frequency deviation in this context is not a relevant or valid troubleshooting method for its modulation quality. Options A, B, and D are universally valid troubleshooting steps for any electronic equipment, including transmitters: A) Visual inspection is always the first step to identify obvious physical damage or incorrect settings. B) Proper power supply voltages are critical for correct operation of all circuits. D) Using an oscilloscope to check signal integrity at various stages can pinpoint where the signal becomes degraded or lost.