Subelement A: VHF-DSC Equipment & Operation— Topic :
Question 7A5
Element 9 (GMDSS Maintainer)What condition may indicate voltage controlled oscillator failure in the synthesizer of a VHF transmitter?
Explanation
A Voltage Controlled Oscillator (VCO) is a critical component of a frequency synthesizer in a VHF transmitter, generating the actual RF carrier frequency. Its frequency is precisely controlled by a Phase-Locked Loop (PLL) circuit, which compares the VCO's output to a stable reference and adjusts a control voltage to keep it locked to the desired frequency.
An **out-of-lock condition in the Phased Locked-Loop circuit (B)** directly indicates that the PLL can no longer stabilize the VCO. This could happen if the VCO fails to oscillate, drifts excessively, or its control input circuitry malfunctions, preventing the PLL from maintaining phase lock. This is a primary diagnostic symptom for VCO failure.
**A) Low transmitter power output in the high power mode** is usually indicative of issues in the power amplifier stage, antenna, or impedance matching, not directly the VCO. While a completely failed VCO would lead to no power, "low power" isn't the most specific symptom.
**C) Excessive deviation in the frequency modulated output signal** relates to the modulator or audio processing stages, not the stability of the carrier frequency generated by the VCO.
**D) A constant transmitter power output at various modulation levels** suggests problems with the modulation input or the power amplifier's linearity, not the VCO itself.
Related Questions
7A3 A mismatched antenna system can degrade marine VHF radio performance. Antenna system matching can be analyzed by determining the Standing Wave Ratio (SWR). Which of the following is the best test procedure to determine the VHF antenna system SWR?7A4 What may be indicated if a VHF radio operates normally but the transmitter produces nearly zero RF output in the high power mode on all marine channels?7A6 What condition may exist if a VHF radio is operational in receive mode but unable to transmit?8A1 What condition may be indicated if a VHF radio cannot contact nearby stations on any channel, an inline wattmeter indicates 25 watts output (HIGH power mode), and no modulation is heard when the transmitted signal is monitored with an external receiver?8A2 What condition may be indicated if a VHF radio cannot contact nearby stations, an inline wattmeter indicates less than 1 watt output (HIGH power mode), and nominal voltage (e.g., 12.8 vdc) is applied to the VHF radio power input line?