Step-by-step—Class 13

Last updated on 2025-11-18 at 16:46 EST (UTC-05:00)

From Sparks to Sidebands: Learning Modulation All Over Again

By Doug Nix, VE3— (future call sign pending!)

When I started my electronics career at GFC Hammond Electronics in Guelph back in 1985, I spent my days testing and repairing linear and switch-mode power supplies. At the time, “modulation” meant watching ripple currents on the Tektronix scope or chasing a dead short upstream from an LM723 regulator. It wasn’t until I recently delved deeper into amateur radio that I began to appreciate how the same fundamental principles—mixing, switching, filtering, duty cycles, and harmonics—show up everywhere in RF.

Last night’s RAC Basic lecture from Al Penney was another reminder that nearly every radio mode we use today is built on deceptively simple physical principles: change some characteristic of a carrier, and you can transport information across the planet. What changes is the cleverness with which we do it.

This post is my attempt to capture what I learned, in my own words, with enough technical depth to keep it interesting.

---

Continuous Wave (CW): Where Radio Began

The earliest radio systems didn’t carry voice—they simply turned a carrier on and off. That’s continuous wave (CW), and it’s astonishingly robust, even in 2025.

CW works because the information rate is slow and the bandwidth is tiny. That allows the receiver to use extremely narrow filters—sometimes only a few hundred hertz—which dramatically raises the signal-to-noise ratio. A CW signal can hide below the noise floor and still be copyable by a trained ear.

One technical detail I found particularly interesting: the bandwidth of a keyed carrier is governed by the abruptness of the transitions. “Abruptness” refers to the slew rate of the switching waveform. More abrupt changes require a faster slew rate, which in turn creates harmonics. Slowing the corners on the waveform “softens” the transitions, thereby reducing or eliminating the harmonics.

Hard keying produces broadband splatter due to the harmonics caused by the corners on the waveform—what old-timers call key clicks. So modern rigs shape those edges digitally to keep the spectrum clean.

AM: The First Voice Mode—and the Inefficient One

Amplitude Modulation was the first way we got voice on the air. It is also, by every modern measure, terribly inefficient.

Why?

Because an AM signal consists of:

• A carrier (which carries no information)
• A lower sideband
• An upper sideband

If your audio contains frequencies up to 6 kHz, the RF bandwidth becomes ±6 kHz on each side of the carrier—12 kHz total. Only the sidebands contain information; the carrier exists solely to help a simple diode demodulator recover the audio.

This is why:

• AM transmitters waste power on the carrier
• AM takes up twice the necessary bandwidth
• AM is inherently vulnerable to noise (almost all noise sources are amplitude-based)

PEP (peak envelope power) becomes especially important in AM. A 100 W PEP transceiver can legally deliver only ~25 W of AM carrier because 100% modulation requires the envelope to swing to 4× the carrier power.

As someone who spent years debugging over-stressed power supplies, I now appreciate how much engineering went into keeping broadcast transmitters from exceeding that 100% limit. Overmodulation introduces phase reversals and spurious emissions extending theoretically to infinity—an EMC/Spectrum nightmare.

Mixers: The Beating Heart of RF

Al’s explanation of mixing echoed something I first learned at Hammond while troubleshooting transformers: non-linear systems create new frequencies.

A mixer multiplies two signals:

• Carrier frequency f₁
• Modulating or local oscillator frequency f₂

The output contains:

• f₁ + f₂
• f₁ – f₂
• …plus other products if the device is imperfect, which it always is.

This simple principle is what makes the superheterodyne receiver possible, and it’s how we shift an audio waveform up into the RF spectrum. Every mode—AM, FM, SSB, digital—depends on this idea.

Sidebands and SSB: Why We Don’t Run AM Anymore

In amplitude modulation, all the information is in the sidebands. Once you realize that both sidebands contain the same information, it becomes obvious: send only one.

Enter Single Sideband (SSB)—a refinement of AM where:

• One sideband is removed
• The carrier is suppressed
• Bandwidth is cut in half
• Transmitted power is concentrated entirely in the useful spectrum

That’s why SSB is the standard for HF voice. With a typical 3 kHz voice channel:

• AM occupies ~6 kHz
• SSB occupies ~2.7–3.0 kHz
• FM occupies 10–15 kHz
• And you can fit 2× as many SSB QSOs into a band as AM

SSB also gives far better “punch” for the watt. A 100 W PEP SSB signal delivers voice peaks that punch through marginal conditions, and your average power is typically 10–20% of that unless you engage heavy speech processing [1].

FM: Quieting, Capture, and Why VHF Sounds So Clean

Frequency Modulation is a different animal. Instead of changing amplitude, we change the instantaneous frequency of the carrier.

Two things make FM special:

1. It rejects amplitude noise

Because the information is in the frequency deviation, not amplitude, the receiver can use a limiter to discard all amplitude variation. That’s why FM broadcast radio sounds clean even with multipath, ignition noise, and urban interference.

2. The Capture Effect

If two FM signals are present on the same frequency, the stronger one “captures” the receiver. This is both a blessing (for repeaters) and a curse (for aviation, which uses AM, allowing overlapping transmissions to still be heard).

FM also supports wideband (e.g., ±75 kHz for broadcast) and narrowband (typically ±2.5–5 kHz for amateur VHF/UHF).

Carson’s Rule gives a quick estimate of FM bandwidth:

BW ≈ 2 (Δf + f_m)
where Δf is peak deviation and f_m is highest audio frequency.

So a typical VHF ham radio:

• ±3 kHz deviation
• 3 kHz audio
• BW ≈ 12 kHz

No wonder we must be careful about over-deviating—your excitement literally widens your signal.

Digital Modes: RTTY, Packet, and the March Toward Software

Watching Al trace the evolution from Baudot, to AFSK, to modern sound-card decoding reminded me how far we’ve come from electromechanical teleprinters banging away in newsrooms.

RTTY uses frequency shift keying (FSK):

• One frequency = mark
• Another = space
• Typically ~170 Hz shift on HF

Packet radio took the next step by wrapping data into structured frames with error detection (FCS) and acknowledgments—essentially, an early version of TCP/IP over RF.

Today, software like fldigi, WSJT-X, and Direwolf replaces dozens of kilograms of mechanical equipment with a laptop and a USB cable.

It’s astonishing to think that in my Hammond days, many of these digital modes were still hardware-based. Today, everything from modulation to filtering to demodulation is done numerically.

Reflections as a New(er) Ham

Studying modulation theory now—at this stage of my life—is a strange kind of homecoming. It’s refreshing, frankly. I’m reconnecting with the core physics and mathematics that first drew me into electronics before life led me into safety engineering, risk assessment, standards development, and consulting work.

What strikes me most is that every mode is a tradeoff:

• AM: simple but inefficient
• SSB: efficient but harder to generate and tune
• FM: noise-resistant but spectrally wide
• CW: narrow and penetrating but slow
• Digital: flexible but computationally heavy

Each choice reflects a balance between bandwidth, power, complexity, and robustness—the same constraints I deal with every day in engineering safety systems.

And that’s the joy of amateur radio: it’s the perfect intersection of physics, engineering, history, and pure hands-on experimentation.

Can’t wait for the next chapter!

---

References

[1] A. Penney, Chapter 13 – Modulation and Transmitters. RAC Basic Qualification Course, Radio Amateurs of Canada. Ch13-Modulation-and-Transmitters

[2] W. Sabin and E. Schoenike, HF Radio Systems & Circuits. New York, NY, USA: McGraw-Hill, 1998.

[3] J. R. Carson, “Notes on the Theory of Modulation,” Proceedings of the IRE, vol. 10, no. 1, pp. 57–64, Feb. 1922.

[4] E. H. Armstrong, “A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation,” U.S. Patent 1,941,066, filed Dec. 26, 1933, and issued Dec. 26, 1933.

[5] Federal Communications Commission (FCC), Title 47 CFR Part 97 – Amateur Radio Service, U.S. Government Publishing Office, 2024.

[6] A. B. Carlson, Communication Systems: An Introduction to Signals and Noise in Electrical Communication, 4th ed. New York, NY, USA: McGraw-Hill, 2002.

[7] J. D. Kraus, Electromagnetics, 4th ed. New York, NY, USA: McGraw-Hill, 1992.

[8] G. L. Tarkington and R. L. Ferrel, “The Single-Sideband Story,” QST, vol. 41, no. 7, pp. 17–23, July 1957.

[9] M. Schwartz, W. R. Bennett, and S. Stein, Communication Systems and Techniques. New York, NY, USA: McGraw-Hill, 1966.

[10] H. T. Friis and C. B. Feldman, “A Multiple-Unit Steerable Antenna for Short-Wave Radio Transmission,” Bell System Technical Journal, vol. 17, no. 2, pp. 337–364, Apr. 1938.

[11] K. R. Hardis, “Understanding Peak Envelope Power,” QEX – Communications Quarterly, no. 169, pp. 3–10, Nov./Dec. 1995.

[12] ITU Radiocommunication Bureau, ITU-R SM.328-12: Spectra and Bandwidth of Emissions, International Telecommunication Union, Geneva, 2015.

[13] ITU Radiocommunication Bureau, ITU-R F.1110 – Frequency-Shift Keying and Keying Characteristics, International Telecommunication Union, Geneva, 2019.

[14] L. E. Kahn, “Single-Sideband Transmission by Envelope Elimination and Restoration,” Proceedings of the IRE, vol. 40, no. 7, pp. 803–806, July 1952.

[15] D. H. Johnson and J. E. Johnson, Modern Communications Systems: Principles and Applications. Boston, MA, USA: Pearson, 2010.