G1 · Published 29 August 2026 · By The Ham Desk

Sideband Conventions, Digital Modes and Receiver Controls

Once you leave FM behind, three practical questions decide whether a contact happens: which sideband the other station expects, how much spectrum your chosen mode occupies, and which receiver control to reach for when the band is noisy.

Band 20 m
Study time 16
Level Core
questions 10

Waveform and antenna diagram for Signals, Modulation and Digital Modes — Signals, Modulation and Digital Modes — waveform overlay and radiation pattern

Pre-flight briefing

Sideband is a convention, not a law

Single sideband suppresses the carrier and one sideband, keeping only the information-carrying half of an AM signal. It occupies roughly 2.4 to 3 kHz instead of AM's 6 kHz, and all the transmitter's power goes into the sideband instead of being split between the carrier, which carries nothing, and two mirror-image sidebands. By long convention, USB is used on bands above 10 MHz and LSB below, so 20 metres is USB and 40 and 80 metres are LSB.

Because SSB has no carrier, tuning is relative: you must zero beat the other station rather than find a fixed carrier. Digital mode software handles that automatically, which is one reason FT8 took over the weak-signal world.

The digital modes and their footprints

The classic narrow modes are CW at about 150 Hz, PSK31 with its 31.25 baud signal in roughly 31 Hz, and RTTY at 45.45 baud with a 170 Hz shift, occupying about 250 Hz. FT8 uses 15 second transmit and receive periods, 8-tone FSK and about 50 Hz per signal, which is why dozens of contacts fit inside a 3 kHz passband at once.

Packet radio and APRS are wider: 1200 baud AFSK packet needs about 12 kHz and APRS in North America lives at 144.390 MHz. Duty cycle matters as much as bandwidth - a digital mode that transmits continuously puts far more average power into the final amplifier than SSB voice does, so power is usually reduced accordingly.

Reading the receiver

An S-meter is a relative indicator: S9 is defined as a 50 microvolt signal on HF and each S unit is about 6 dB, so S9+20 dB means roughly 500 microvolts. Meter readings differ between radios and are not a measurement of the other station's power.

The controls repay knowing: ALC holds the transmitter's gain at the point where distortion is still low, and driving it hard causes splatter across the band; a noise blanker chops out short impulse noise such as ignition, while noise reduction works on the random hiss and must be used gently; a notch filter removes a steady heterodyne without touching the signal you are copying.

Terms you must be able to define
Term	What the examiner wants to hear
USB / LSB	Upper and lower sideband; the convention is USB above 10 MHz and LSB below.
PSK31	A 31.25 baud phase shift keyed mode whose signal is about 31 Hz wide.
RTTY	Frequency shift keyed teleprinter, 45.45 baud with a 170 Hz shift.
S unit	One S unit is about 6 dB; S9 corresponds to a 50 microvolt HF signal.
ALC	Automatic level control: limits drive to the final amplifier to keep the signal clean.

Question deck 10 questions

G5A01 Recall

Which sideband is conventionally used for SSB voice on the 20 metre band?

G5A02 Recall

What bandwidth does a typical SSB voice signal occupy?

G5A03 Core

About how much spectrum does a PSK31 signal occupy?

G5A04 Core

What is the standard frequency shift used by amateur RTTY?

G5A05 Core

How long is one transmit or receive period in an FT8 contact?

G5A06 Recall

What is the standard APRS frequency in North America?

G5A07 Core

What is the function of the ALC circuit in a transmitter?

G5A08 Core

What does an S-meter reading of S9 indicate on the HF bands?

G5A09 Stretch

Which receiver control is intended specifically for short bursts of impulse noise such as vehicle ignition?

G5A10 Stretch

Why should transmitter power usually be reduced when operating a continuous digital mode?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

G5A01 Explanation 1

Key C — Upper sideband

Above 10 MHz the convention is USB, so 20, 17, 15, 12 and 10 metres are USB; 160, 80, 60 and 40 metres are LSB. Nothing in Part 97 requires it, but calling CQ on the wrong sideband means nobody answers.

Rule 47 CFR §97.305 - emission types by band segment

G5A02 Explanation 2

Key B — About 2.4 to 3 kHz

SSB needs only one sideband, which spans the audio range up to about 3 kHz. AM needs both sidebands and the carrier, about 6 kHz; CW needs about 150 Hz; wideband FM needs about 16 kHz.

Rule 47 CFR §97.307(f) - emission bandwidth standards

G5A03 Explanation 3

Key A — About 31 Hz

The name states the baud rate - 31.25 symbols per second - and the bandwidth is roughly equal to it, about 31 Hz. That narrow footprint is why PSK31 conversations can sit side by side in a 3 kHz passband.

Rule 47 CFR §97.307(f) - data emission standards

G5A04 Explanation 4

Key D — 170 Hz

Amateur RTTY uses a 170 Hz shift at 45.45 baud, occupying about 250 Hz. The 850 Hz shift belongs to older commercial and military practice.

Rule 47 CFR §97.307(f)(2) - RTTY bandwidth

G5A05 Explanation 5

Key C — 15 seconds

FT8 uses 15 second periods with the transmission occupying about 12.6 seconds, leaving time for decoding and for the next slot. Both stations must be synchronised to the same clock, which is why FT8 software relies on accurate computer time.

Rule 47 CFR §97.309 - digital codes

G5A06 Explanation 6

Key B — 144.390 MHz

APRS uses 144.390 MHz in North America, with 1200 baud AFSK packet. 146.520 MHz is the 2 metre simplex calling frequency, 144.200 MHz is the SSB calling frequency, and 145.800 MHz is used by the ISS downlink.

Rule 47 CFR §97.305 - band plan convention for automatic packet

G5A07 Explanation 7

Key A — It limits drive to the final amplifier to prevent excessive distortion

ALC protects the final stage by reducing gain when the signal would otherwise be overdriven. Heavy ALC action is a warning sign: it usually means excessive microphone gain or drive, and the result is a splattering signal that interferes with nearby stations.

Rule 47 CFR §97.307(b) - spurious emissions

G5A08 Explanation 8

Key D — A received signal of about 50 microvolts at the antenna terminals

S9 is defined as a 50 microvolt HF signal, with each S unit worth about 6 dB. Meter calibration varies widely between radios, so treat S-meter readings as a convenience, not a measurement.

Rule Fundamentals - receiver indication

G5A09 Explanation 9

Key C — The noise blanker

A blanker recognises a short spike and mutes the receiver for its duration, which suits ignition and power line noise. Noise reduction works on continuous random noise, a notch removes a steady tone, and AGC controls receiver gain across fading signals.

Rule Fundamentals - receiver design

G5A10 Explanation 10

Key C — Because a mode that transmits continuously has a much higher duty cycle, and the final amplifier heats more

A 100 W SSB signal averages perhaps 25 to 30 W of dissipation because speech is intermittent, while a continuous carrier mode dissipates the full 100 W for the whole transmission. RTTY, FT8 and packet operators therefore run lower power into the same amplifier, and often lower duty cycle or thermally rated finals.

Rule 47 CFR §97.313 - transmitter power standards

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.