What term describes the use of a sub-audible tone transmitted along with normal voice audio to open the squelch of a receiver?
The correct answer is D: CTCSS. CTCSS (Continuous Tone-Coded Squelch System) is a sub-audible tone (typically below 300 Hz) that's transmitted along with normal voice audio to open the squelch of a receiver. This allows repeaters and receivers to only respond to signals that include the correct CTCSS tone.
CTCSS tones are inaudible to human ears but are detected by the receiver's squelch circuit. When a repeater is configured with CTCSS, it only retransmits signals that include the correct tone. This helps filter out interference and unwanted signals. Common CTCSS tones range from 67.0 Hz to 254.1 Hz, with 100.0 Hz, 103.5 Hz, and 107.2 Hz being very common.
Exam Tip
Sub-audible tone = CTCSS. Remember: CTCSS uses a continuous sub-audible tone (below 300 Hz) that you can't hear but the receiver detects to open squelch.
Memory Aid
"**S**ub-**A**udible **T**one = **C**TCSS (think 'SAT = CTCSS')"
Real-World Application
Your local repeater requires a CTCSS tone of 100.0 Hz. You program your radio to transmit this tone along with your voice. When you transmit, the repeater detects the 100.0 Hz tone (which you can't hear) and opens its squelch to retransmit your signal. Without the correct CTCSS tone, the repeater won't respond, even if your signal is strong enough.
FCC Part 97.205Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Carrier squelch opens the squelch based on signal strength, not a sub-audible tone. It doesn't use tones at all.
Option B: Incorrect. Tone burst is a brief audible tone used for repeater access, not a continuous sub-audible tone.
Option C: Incorrect. DTMF (Dual-Tone Multi-Frequency) uses pairs of audible tones for signaling, not a single sub-audible tone.
题目解析
The correct answer is D: CTCSS. CTCSS (Continuous Tone-Coded Squelch System) is a sub-audible tone (typically below 300 Hz) that's transmitted along with normal voice audio to open the squelch of a receiver. This allows repeaters and receivers to only respond to signals that include the correct CTCSS tone. CTCSS tones are inaudible to human ears but are detected by the receiver's squelch circuit. When a repeater is configured with CTCSS, it only retransmits signals that include the correct tone. This helps filter out interference and unwanted signals. Common CTCSS tones range from 67.0 Hz to 254.1 Hz, with 100.0 Hz, 103.5 Hz, and 107.2 Hz being very common.
考试技巧
Sub-audible tone = CTCSS. Remember: CTCSS uses a continuous sub-audible tone (below 300 Hz) that you can't hear but the receiver detects to open squelch.
记忆口诀
**S**ub-**A**udible **T**one = **C**TCSS (think 'SAT = CTCSS')
实际应用示例
Your local repeater requires a CTCSS tone of 100.0 Hz. You program your radio to transmit this tone along with your voice. When you transmit, the repeater detects the 100.0 Hz tone (which you can't hear) and opens its squelch to retransmit your signal. Without the correct CTCSS tone, the repeater won't respond, even if your signal is strong enough.
错误选项分析
Option A: Incorrect. Carrier squelch opens the squelch based on signal strength, not a sub-audible tone. It doesn't use tones at all. Option B: Incorrect. Tone burst is a brief audible tone used for repeater access, not a continuous sub-audible tone. Option C: Incorrect. DTMF (Dual-Tone Multi-Frequency) uses pairs of audible tones for signaling, not a single sub-audible tone.
知识点
CTCSS, Sub-audible tone, Squelch control, Repeater access
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.