4. Signals and Modulation on The HamStudy Amateur Extra Class HamBook (2024 - 2028)

4.1 Spectra, Mixing, and SSB

Mon, 01 Jan 0001 00:00:00 +0000

Section 4.1: Spectra, Mixing, and SSB #

Many radios and digital-mode programs can draw the band as a spectrum or waterfall display, with frequency running across the screen. Each signal shows up with some width, and an SSB voice is wider than a CW signal. An oscilloscope shows a signal another way, as voltage changing over time. Comparing the two views shows how a waveform’s shape decides which frequencies it occupies.

4.2 Frequency and Phase Modulation

Mon, 01 Jan 0001 00:00:00 +0000

Section 4.2: Frequency and Phase Modulation #

FM is the everyday voice mode of VHF and UHF repeaters, usually set for about 5 kHz of deviation. That swing can come from changing an oscillator’s frequency directly or from shifting the carrier’s phase, which is phase modulation, or PM. The two methods treat high and low pitches differently.

Frequency and Phase Changes #

Direct FM starts at the oscillator, whose resonant circuit includes a capacitance that changes with the audio voltage, such as a varactor diode (Section 1.1). As that capacitance changes, so does the frequency. In an RF amplifier stage instead, the same changing reactance shifts the phase of the passing signal.

4.3 Digital Symbols and Codes

Mon, 01 Jan 0001 00:00:00 +0000

Section 4.3: Digital Symbols and Codes #

Each tone an RTTY signal sends stands for one bit, because RTTY chooses between only two. FT8 picks from eight, so each of its tones stands for three bits. Counting tones sent and counting bits are different measures, and the gap between them lets a signal carry more data without changing any faster. Codes then turn text into bits and catch noise’s mistakes.

4.4 Digital Signal Bandwidth

Mon, 01 Jan 0001 00:00:00 +0000

Section 4.4: Digital Signal Bandwidth #

A CW signal can sound clean on its own frequency and still click in a receiver tuned a kilohertz off, in time with its dots and dashes. Keying creates sidebands around the narrow carrier.

The Shape of a Dot #

Keying speed sets how often the signal changes, and every change spreads some energy away from the carrier. Rise and fall times set how abruptly each change happens. For keying, those times make up the shape factor, not to be confused with a filter’s shape factor in the previous chapter.

4.5 Sharing Signal Bandwidth

Mon, 01 Jan 0001 00:00:00 +0000

Sharing a repeater usually means taking turns. A DMR repeater lets two conversations run at once on one 12.5 kHz channel by giving each its own time slot. A system can also split a channel by frequency, and either way the receiver has to pull the streams apart again.

Dividing Frequency or Time #

Split by frequency, a channel might carry one data stream in a lower subchannel and another in a higher one. They travel at the same time, and receiver filters separate them.

4.6 Image and Television Signals

Mon, 01 Jan 0001 00:00:00 +0000

Section 4.6: Image and Television Signals #

Slow-scan television, or SSTV, sends a still picture through an ordinary voice channel, taking from a few seconds to a couple of minutes per image. Hams even use it to receive pictures from the International Space Station. The picture travels one row at a time, left to right, and the receiver must put each piece back in place.

A Picture Made from Tones #

In common analog modes, a 1,500 Hz tone means black and 2,300 Hz means white, with grays in between. Turning up the volume doesn’t brighten the picture; amplitude carries no brightness information.

4. Signals and Modulation on The HamStudy Amateur Extra Class HamBook (2024 - 2028)

4.1 Spectra, Mixing, and SSB

Section 4.1: Spectra, Mixing, and SSB #

4.2 Frequency and Phase Modulation

Section 4.2: Frequency and Phase Modulation #

Frequency and Phase Changes #

4.3 Digital Symbols and Codes

Section 4.3: Digital Symbols and Codes #

4.4 Digital Signal Bandwidth

Section 4.4: Digital Signal Bandwidth #

The Shape of a Dot #

4.5 Sharing Signal Bandwidth

Section 4.5: Sharing Signal Bandwidth #

Dividing Frequency or Time #

4.6 Image and Television Signals

Section 4.6: Image and Television Signals #

A Picture Made from Tones #