Subelement B: MF-HF-DSC-SITOR (NBDP) Equip. & Operations— Topic :
Question 14B3
Element 9 (GMDSS Maintainer)You are able to hear signals in the voice/SSB mode but not in the SITOR (NBDP) mode. What is the most likely cause?
Explanation
SITOR (NBDP) is a narrow-bandwidth digital mode, typically requiring a very precise and narrow Intermediate Frequency (IF) filter for proper reception and decoding. A 500 Hz IF filter is designed for exactly this purpose, isolating the narrow digital signal from noise and adjacent interference.
If the 500 Hz IF filter becomes defective, the radio's ability to process these narrow signals is severely impaired or lost, preventing reception in SITOR (NBDP) mode. However, voice and SSB modes use a much wider IF filter (e.g., 2.4 kHz or 2.7 kHz). The failure of the *separate* 500 Hz filter would not affect the functionality of the wider SSB filter, allowing you to still hear signals in voice/SSB mode.
* **A) The phase locked loop (PLL) circuit has failed:** A PLL failure would affect frequency stability and reception across *all* modes, not just one.
* **C) The R.F. amplifier has failed:** An RF amplifier failure would significantly reduce or eliminate reception for *all* modes equally.
* **D) The B.F.O. circuit has failed:** A BFO failure would prevent proper demodulation of *all* SSB and CW-type signals (including SITOR), making voice/SSB unintelligible or nonexistent.
Related Questions
14B1 The voice is garbled and unreadable on MF/HF SSB channels. What is the most likely cause?14B2 All signals sound normal on an MF/HF receiver except one that has a very high pitched voice barely readable. What is the most likely cause?14B4 The phase locked loop circuit in an MF/HF receiver appears to be faulty. What is the most likely cause?14B5 You are able to hear signals in the SITOR (NBDP) mode but not in the voice/SSB mode. What is the most likely cause?14B6 The AGC function of an MF/HF receiver is inoperative. What is the most likely cause?