This page is part of the N0NJY General Class self-study course for Technician operators upgrading to General.
This module covers the technical details of how signals are generated, modulated, and transmitted. You need to understand emission types, modulation modes, digital mode characteristics, bandwidth, and what causes unwanted spurious emissions.
AM (Amplitude Modulation): The carrier amplitude varies with the audio signal. The carrier is always present, even when no audio is transmitted. Double-sideband AM occupies twice the bandwidth of SSB. AM is used in the AM broadcast band and in some aviation communications.
SSB (Single Sideband): The carrier and one sideband are removed. All transmitter power goes into the information-bearing sideband. Bandwidth is approximately half that of AM. SSB is the standard for HF voice operation.
FM (Frequency Modulation): The carrier frequency varies with the audio amplitude. Carrier amplitude is constant. FM has excellent noise rejection (capture effect) and is standard for VHF/UHF voice. FM requires more bandwidth than SSB but provides very high audio quality when the signal is adequate.
RTTY: FSK (frequency-shift keying). Carrier shifts between mark and space tones. Standard amateur RTTY: 45.45 baud, 170 Hz shift, approximately 250–300 Hz occupied bandwidth.
PSK31: Phase-shift keying at 31.25 baud. Occupied bandwidth: approximately 31 Hz. Excellent weak-signal performance. Popular for keyboard QSOs.
FT8: 8-tone FSK (8-FSK), 15-second transmit/receive cycles, 50 Hz bandwidth. Decodes signals 20+ dB below the noise floor. Has become the dominant weak-signal HF mode worldwide. Not conversational — structured exchanges only.
APRS / Packet: AX.25 protocol. 1200 baud on VHF using AFSK (audio frequency-shift keying). Integrates with GPS for real-time position tracking. Used in APRS (Automatic Packet Reporting System).
| Mode | Approximate Bandwidth |
|---|---|
| CW | 50–150 Hz |
| PSK31 | 31 Hz |
| FT8 | 50 Hz |
| RTTY (170 Hz shift) | ~250 Hz |
| SSB voice | 2.4 kHz |
| AM voice | 6 kHz |
| VHF FM voice | 10–15 kHz |
FCC rules require that emissions be no wider than necessary for the information being transmitted.
Harmonics: Integer multiples of the transmit frequency produced in the final amplifier. A 14 MHz transmitter may produce harmonics at 28, 42, and 56 MHz. A low-pass filter at the transmitter output suppresses them effectively.
Intermodulation Distortion (IMD): When two or more signals mix in a non-linear device, they produce products at various frequency combinations. An overdriven SSB transmitter creates splatter — unwanted sidebands that spread across the band. IMD is a common cause of interference complaints.
Parasitic Oscillations: Unintended RF oscillation caused by stray capacitance and inductance in amplifier circuits. Parasitic suppressors (small resistors or ferrite beads in the amplifier) prevent them.
Q1 (G8A01) — How is an FSK signal generated?
Q2 (G8B01) — What is the definition of symbol rate in a digital transmission?
Q3 (G8C01) — Which digital mode is designed to make it easier for stations to find each other across an entire HF band?
Q4 (G8A05) — What type of modulation varies only the frequency of the RF signal?
Q5 (G8B05) — What is the approximate bandwidth of a VHF repeater FM phone transmission?
Q6 (G8C05) — Which APRS feature enables real-time digital communication of GPS position data?
Q7 (G8A11) — What is an advantage of CDMA (code division multiple access)?
Q8 (G8A03) — What is the term for unwanted signals at frequencies outside of the transmitted bandwidth?