Extra exam: signals and emissions
AC waveforms, sampling, modulation, digital signals and spread spectrum. 48 questions, 10 at a time, starting with ones you haven't seen.
Rules to remember10 key signals and emissions rulesShow the rules
- Unprocessed speech has high peaks and low average power, so SSB phone typically has a PEP-to-average ratio of about 2.5 to 1. (p. 91)
- OFDM is used by amateur digital modes, sending data on many closely spaced subcarriers. (p. 93)
- Deviation ratio is the maximum carrier deviation divided by the highest audio modulating frequency. (p. 93)
- QAM sends data by varying the amplitudes of two carriers on the same frequency that are 90 degrees apart in phase. (p. 94)
- Shifting a PSK signal's phase at a zero crossing avoids sudden jumps in amplitude, which keeps splatter and bandwidth to a minimum. (p. 94)
- PSK31 shapes its data pulses as smooth sinusoids, avoiding sharp transitions and keeping its bandwidth very narrow. (p. 95)
- In Gray code only one bit changes between consecutive values, which avoids errors when reading encoders. (p. 95)
- Nodes in an amateur mesh network, such as AREDN, use Internet Protocol (IP) addresses. (p. 96)
- Baudot uses 5 bits per character and two shift codes, LTRS and FIGS, while ASCII uses 7 or 8 bits and needs no shift codes. (p. 98)
- ASCII has enough codes to include both uppercase and lowercase letters, unlike Baudot. (p. 99)
Taken from the explanations to checked practice questions; page numbers link to the official handbook.
Sample signals and emissions questions
1.What is digital time division multiplexing?
- ATwo or more signals are arranged to share discrete time slots of a data transmission
- BTwo or more signals are quadrature modulated to increase bandwidth efficiency
- CTwo or more data streams are assigned to discrete sub-carriers on an FM transmitter
- DTwo or more data streams share the same channel by transmitting time of transmission as the subcarrier
Show the answer
✓ A. Two or more signals are arranged to share discrete time slots of a data transmission
Time division multiplexing gives each signal its own time slot within a shared data stream.
2.What determines the PEP-to-average power ratio of an unprocessed single-sideband phone signal?
- AThe degree of carrier suppression
- BThe frequency of the modulating signal
- CAmplifier gain
- DSpeech characteristics
Show the answer
✓ D. Speech characteristics
In SSB the output follows the voice, so the PEP-to-average ratio is set by the speech itself: its peaks, pauses and energy.
3.What is the deviation ratio of an FM phone signal having a maximum frequency swing of plus or minus 5 kHz if the highest modulation frequency is 3 kHz?
- A0.167
- B6
- C0.6
- D1.67
Show the answer
✓ D. 1.67
Deviation ratio is maximum deviation divided by highest modulating frequency: 5 kHz divided by 3 kHz is about 1.67.
4.What is deviation ratio?
- AThe ratio of the highest audio modulating frequency to the average audio modulating frequency
- BThe ratio of the maximum carrier frequency deviation to the highest audio modulating frequency
- CThe ratio of the carrier center frequency to the audio modulating frequency
- DThe ratio of the audio modulating frequency to the center carrier frequency
Show the answer
✓ B. The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency
Deviation ratio is the maximum carrier deviation divided by the highest audio modulating frequency.