G8A: Modulation and Carriers – Ham Radio General License Study Guide
G8A covers how information is added to a radio carrier — the fundamental process of modulation — and how different methods create distinct signal characteristics. The fourteen questions in this group span analog modulation (AM, FM, phase modulation), digital modulation (FSK, QPSK, QPSK31), problems with overmodulation, the FT8 digital mode, and the engineering concepts of link budgets and link margins.
Topics include how direct binary FSK is generated, what process changes the phase angle of an RF signal, what process changes instantaneous frequency, what a reactance modulator produces, what type of modulation varies instantaneous power level, what characterizes QPSK31, which phone mode has the narrowest bandwidth, what overmodulation causes, what modulation FT8 uses, what flat-topping means, what the modulation envelope is, what QPSK modulation does, what a link budget is, and what link margin means.
Modulation Types
A carrier wave has three properties that can be varied to carry information: its amplitude, its frequency, and its phase. Each gives a different modulation type:
| Type | What Is Varied | Key Characteristic |
|---|---|---|
| Amplitude Modulation (AM) | Instantaneous power level (amplitude) | Signal strength rises and falls with audio; both sidebands present with carrier |
| Frequency Modulation (FM) | Instantaneous frequency of the RF wave | Carrier frequency deviates above/below center in proportion to audio amplitude |
| Phase Modulation (PM) | Phase angle of the RF signal | Closely related to FM; produced by a reactance modulator at the RF amplifier stage |
| Single Sideband (SSB) | Amplitude (like AM, but carrier and one sideband suppressed) | Narrowest bandwidth of any phone emission mode |
A reactance modulator connected to a transmitter RF amplifier stage produces phase modulation. The reactance modulator changes the effective reactance in the tuned circuit, which shifts the phase of the oscillator and creates a phase-modulated signal. Because the instantaneous frequency is the derivative of phase, phase modulation and frequency modulation produce similar-sounding audio on the air.
Digital Modulation: FSK and QPSK
Frequency Shift Keying (FSK)
Direct binary FSK modulation is generated by changing an oscillator's frequency directly with a digital control signal. The oscillator shifts between two frequencies — one representing binary 0 and the other representing binary 1. This direct method gives fast, clean frequency switching without the need for an audio tone or audio path.
FT8 uses 8-tone frequency shift keying — eight discrete frequency steps rather than just two. The eight tones allow three bits to be encoded per symbol, which helps achieve high data efficiency at very low signal-to-noise ratios. FT8 is a weak-signal mode designed for marginal propagation conditions.
QPSK and QPSK31
QPSK (Quadrature Phase Shift Keying) encodes digital data by using four phase states: 0°, 90°, 180°, and 270° phase shifts. Each symbol represents a pair of bits (2 bits per symbol), doubling the information density compared to BPSK, which uses only 0° and 180°.
QPSK31 applies QPSK at 31.25 baud. It has three characteristics that all appear on the exam:
- It is sideband sensitive — the received phase relationships reverse when the wrong sideband is used, making it essential to choose the correct sideband
- Its encoding provides error correction
- Its bandwidth is approximately the same as BPSK31 — about 31 Hz wide
The exam answer for QPSK31 characteristics is all these choices are correct.
Overmodulation and the Modulation Envelope
The modulation envelope of an AM signal is the waveform created by connecting the peak values of the modulated signal. Looking at an AM waveform on an oscilloscope, the outline traced along the tips of the RF peaks forms the envelope. The envelope has the same shape as the original audio waveform — which is why viewing the envelope tells you what the audio is doing to the carrier.
Overmodulation occurs when the audio signal drives the modulator beyond 100% modulation depth. The primary effect is excessive bandwidth — the signal generates significant energy outside the normal channel width, causing interference to adjacent channels.
Flat-topping is a specific symptom of overmodulation visible on an oscilloscope: instead of following the smooth peaks of the audio waveform, the RF envelope is clipped flat at the top and/or bottom. This indicates signal distortion caused by excessive drive or speech levels. It can also cause splatter — spurious emissions spreading into adjacent frequencies — which is identifiable on a waterfall or spectrum display as energy beyond the expected signal bandwidth.
Link Budget and Link Margin
A link budget is an accounting of all the gains and losses in a radio communication path. It is calculated as:
This includes the transmit antenna gain, receive antenna gain, path loss (free-space or calculated from the propagation model), cable and connector losses, and any other gain or loss in the system.
The link margin is the difference between the received signal power level and the minimum required signal level (sensitivity threshold) at the input to the receiver. A larger link margin means the system can tolerate more fading, interference, or component degradation before the link fails. It is the safety margin built into the design:
G8A Practice Questions
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G8B: Bandwidth and Frequency Control →
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