What is a common cause of overmodulation of AFSK signals?
The correct answer is D: Excessive transmit audio levels. A common cause of overmodulation of AFSK (Audio Frequency Shift Keying) signals is excessive transmit audio levels. When the audio input to the AFSK modulator is too high, it causes overmodulation.
AFSK signals are created by feeding audio tones (representing digital data) into a transmitter's microphone input. If the audio level is too high, the transmitter is overmodulated, causing distortion, splatter, and interference to adjacent frequencies. The audio level needs to be adjusted so the AFSK tones modulate the transmitter properly without exceeding the maximum modulation level. This is similar to voice overmodulation - too much audio input causes problems. Proper audio level adjustment is essential for clean AFSK operation.
Exam Tip
AFSK overmodulation = Excessive audio levels. Remember: Overmodulation of AFSK signals is commonly caused by excessive transmit audio levels. Reduce audio input to prevent overmodulation.
Memory Aid
"**A**FSK **O**vermodulation = **E**xcessive **A**udio (think 'AO = EA')"
Real-World Application
You're transmitting AFSK, and other operators report your signal is splattering and interfering with adjacent frequencies. The problem is your audio level is too high - the AFSK tones are overmodulating the transmitter. You reduce the audio input level, and the overmodulation disappears. Your signal is now clean and properly modulated.
FCC Part 97.307Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Excessive retries don't cause overmodulation. Retries are a protocol function, not related to modulation levels.
Option B: Incorrect. Excessive frequency deviation might be a symptom, but the cause is excessive audio levels driving the deviation too high. The root cause is audio level.
Option C: Incorrect. Bit errors in the modem don't cause overmodulation. Bit errors affect data integrity, not modulation levels.
题目解析
The correct answer is D: Excessive transmit audio levels. A common cause of overmodulation of AFSK (Audio Frequency Shift Keying) signals is excessive transmit audio levels. When the audio input to the AFSK modulator is too high, it causes overmodulation. AFSK signals are created by feeding audio tones (representing digital data) into a transmitter's microphone input. If the audio level is too high, the transmitter is overmodulated, causing distortion, splatter, and interference to adjacent frequencies. The audio level needs to be adjusted so the AFSK tones modulate the transmitter properly without exceeding the maximum modulation level. This is similar to voice overmodulation - too much audio input causes problems. Proper audio level adjustment is essential for clean AFSK operation.
考试技巧
AFSK overmodulation = Excessive audio levels. Remember: Overmodulation of AFSK signals is commonly caused by excessive transmit audio levels. Reduce audio input to prevent overmodulation.
记忆口诀
**A**FSK **O**vermodulation = **E**xcessive **A**udio (think 'AO = EA')
实际应用示例
You're transmitting AFSK, and other operators report your signal is splattering and interfering with adjacent frequencies. The problem is your audio level is too high - the AFSK tones are overmodulating the transmitter. You reduce the audio input level, and the overmodulation disappears. Your signal is now clean and properly modulated.
错误选项分析
Option A: Incorrect. Excessive retries don't cause overmodulation. Retries are a protocol function, not related to modulation levels. Option B: Incorrect. Excessive frequency deviation might be a symptom, but the cause is excessive audio levels driving the deviation too high. The root cause is audio level. Option C: Incorrect. Bit errors in the modem don't cause overmodulation. Bit errors affect data integrity, not modulation levels.
知识点
AFSK, Overmodulation, Audio levels, Digital modes
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.