14.2 Single-Sideband Modulation

14.2 Single-Sideband Modulation

Quick Answer: SSB keeps one sideband, suppresses the carrier (and the other sideband). BW ≈ highest AF (~2.4–3 kHz voice). More power- and spectrum-efficient than full-carrier AM. Generate with balanced modulator + filter; receive with a product detector + local carrier. Rate and measure power as PEP.

If full-carrier AM is “send the whole sandwich,” single-sideband (SSB) is “send only the slice that carries the recipe.” Topic 3-H and Element 1 marine MF/HF practice both revolve around J3E—single-sideband suppressed-carrier telephony—the emission you select for 2182 kHz distress/calling voice and ordinary HF radiotelephone.

From DSB AM to SSB

Recall double-sideband full-carrier AM for a single tone: spectrum lines at fc − fm, fc, and fc + fm. Both sidebands contain the same intelligence; the carrier itself contains no message information—it is a phase/frequency reference and a power consumer.

SSB removes:

1. The carrier (or reduces it to a controlled residual), and
2. One of the two sidebands (USB or LSB).

What remains is one sideband whose spectral shape is the translated audio spectrum. That single sideband is the signal.

USB vs LSB

• Upper sideband (USB): frequencies above the suppressed carrier reference (fc + audio spectrum).
• Lower sideband (LSB): frequencies below that reference (fc − audio spectrum).

Convention in many HF bands: LSB on lower HF voice segments, USB on higher HF—but Element 3 cares that you know what USB/LSB are and that commercial J3E selects one deliberate sideband. Wrong sideband selection makes speech unintelligible Donald-Duck audio even when the RF is strong—use the clarifier/RIT only after the correct sideband mode is set.

Carrier suppression and residual carrier

Suppressed-carrier SSB means the transmitter does not radiate a strong continuous carrier with the sideband. Advantages:

• No continuous carrier power heating the PA and antenna when you are not speaking (speech-gated average power drops).
• No strong carrier to beat with other stations and create heterodyne whistles on crowded channels.
• All available peak power budget can go into the information-bearing sideband.

A tiny residual carrier may remain if the balanced modulator is imperfect; service work aims for deep carrier suppression. Some older/special systems use reduced or full carrier SSB variants, but modern marine/aviation commercial voice on MF/HF is J3E suppressed-carrier class.

Bandwidth — the first big advantage

For voice limited to about 2.4–3 kHz of audio (communications quality):

[ BW_{SSB} \approx f_{\mathrm{audio,max}} \approx 2.4\text{–}3,\mathrm{kHz} ]

Compare with DSB AM at the same AF: BW ≈ 2 × fmax ≈ 4.8–6 kHz. SSB therefore:

• Occupies roughly half the spectrum of comparable DSB AM,
• Allows narrower IF filters (~2.1–2.4 kHz crystal filters in receivers),
• Improves SNR in the receiver because less noise power is admitted in a narrower bandwidth (noise power scales with bandwidth for white noise).

Those numbers tie Topic 3-H to the receiver IF selectivity figures you already studied in Chapter 12.

Power efficiency — the second big advantage

Element 3 and your practice bank both emphasize: SSB uses power and bandwidth more efficiently than conventional AM.

Why power?

• Full-carrier AM spends a large fraction of average power in the carrier (no information).
• The second sideband duplicates the first sideband’s information while consuming sideband power.
• SSB can put essentially all radiated RF power into the single information sideband.

For a given peak envelope power rating and a given power-supply/PA size, SSB delivers more useful talk power at a distance than full-carrier AM. For a given communications range, the SSB transmitter can run lower average power and cause less interference density per Hertz of spectrum.

Exam phrasing to recognize: advantage of SSB over conventional AM → more efficient use of power and bandwidth (not “better hi-fi,” not “simpler transmitter,” not “lower frequency”).

How SSB is generated (filter method sketch)

Commercial SSB exciters commonly use the filter method:

1. Audio processor — limit AF bandwidth (~300 Hz–3 kHz), compress/clip carefully.
2. Balanced modulator — multiplies AF × RF carrier to produce double-sideband suppressed-carrier (DSB-SC); the carrier is canceled by balance.
3. Sideband filter — crystal/mechanical/DSP filter passes USB or LSB and rejects the unwanted sideband (and residual carrier).
4. Frequency conversion / mixers — translate the IF SSB signal to the final HF frequency.
5. Linear amplifier chain — Class A/AB (or linear solid-state) PA stages preserve the envelope; Class C is not linear enough for SSB.

Purpose of a balanced modulator in an SSB transmitter (pool): suppress the carrier while producing both sidebands. The following filter—not the balanced modulator alone—selects which sideband survives.

Phasing (Hilbert) methods generate SSB by canceling one sideband with quadrature networks; DSP radios implement equivalent math digitally. Concept for the exam still maps to: remove carrier, remove one sideband, amplify linearly.

How SSB is received

An envelope (diode) detector expects a strong carrier to “copy” the envelope. Pure J3E has no usable carrier, so receivers use a product detector: mix the IF SSB signal with a locally generated carrier (BFO / carrier insertion oscillator). When the local carrier is on-frequency, speech is natural; slightly off → pitch shift (Donald Duck). That is why HF sets have a clarifier/RIT—fine-tune receive pitch without necessarily moving the transmitted frequency.

Peak Envelope Power (PEP) vs average power

SSB voice has a varying envelope. Regulatory ratings, wattmeter peaks, and amplifier capability are stated as Peak Envelope Power (PEP):

PEP = average power supplied to the transmission line during one RF cycle at the crest of the modulation envelope.

Measurement practice

• A peak-reading PEP wattmeter (or scope + calculation into known Z) is the right tool for SSB peaks.
• An ordinary average-responding meter under-reads SSB talk power if you treat its average like a CW carrier reading.
• On a quiet mic, a proper J3E transmitter shows little average power—if you see full-scale continuous power with silence, you may have carrier leak, wrong mode (AM/CW), or a fault—not “healthy SSB.”

Worked comparison (conceptual numbers)

Suppose a PA can deliver 100 W PEP:

• On SSB, peaks hit 100 W; average might be tens of watts depending on voice and compression.
• On full-carrier AM limited by the same peak envelope hardware, the unmodulated carrier must sit lower so that 100% modulation peaks still fit under 100 W PEP—less sideband talk power for the same PA ceiling.

That is the hardware intuition behind “SSB is more power-efficient.”

Linearity and distortion (SSB-specific failure modes)

Because information is in the envelope shape, any amplifier nonlinearity creates intermodulation distortion (IMD)—splatter that widens the SSB signal beyond ~3 kHz. Service implications:

• Keep ALC within design range; avoid “flat-topping.”
• Use linear finals; do not drive Class C amplifiers with SSB.
• Two-tone tests measure IMD; a clean SSB signal stays within its assigned channel.

Overdrive symptoms on SSB look like wide, raspy, adjacent-channel interference—stop transmitting and reduce drive, just as Element 1 requires for overmodulating gear.

SSB exam-day checklist

1. USB/LSB = one sideband above/below the suppressed carrier reference.
2. J3E = SSB suppressed-carrier telephony (commercial MF/HF voice).
3. BW ≈ highest AF (~2.4–3 kHz voice)—about half DSB AM.
4. Advantage vs AM = power and bandwidth efficiency.
5. Balanced modulator → carrier off, both sidebands out; filter picks USB or LSB.
6. Product detector + local carrier at the receiver.
7. PEP at envelope crest ≠ long-term average of speech.
8. Linear amplification required; IMD/splatter if overdriven.

SSB is the efficiency engine of HF maritime and aeronautical voice. Next, pulse modulation concepts and the FCC emission designators that label AM, FM, SSB, and digital modes on licenses and equipment documents.

The two-tone test: why the tones must be non-harmonically related

Sub-topic 3-H-058 asks the two-tone test from three angles, and one of them turns on a detail candidates routinely miss.

Two conditions, and both are testable:

1. Non-harmonically related. If the second tone were an exact harmonic of the first — say 1000 Hz and 2000 Hz — then any distortion products the transmitter generates would land on top of the tones already present, and you could not tell a clean transmitter from a distorting one. Choosing tones with no integer relationship (the classic pair is 700 Hz and 1900 Hz) puts every intermodulation product on a frequency where nothing else sits, so distortion shows up unmistakably.
2. Within the modulation band pass. Both tones must fall inside the transmitter's audio passband — typically about 300–2700 Hz for SSB voice — or the transmitter's own filtering, not its linearity, dominates what you see.

On the oscilloscope, two equal non-harmonic tones through a linear SSB transmitter produce the characteristic interlaced sine-wave envelope with a clean crossover at the zero line. Flattened peaks mean the final is being driven into compression; crossover distortion at the centre points to a Class B biasing fault (section 13.1). This is the standard bench check before certifying an SSB installation, and it is why a two-tone generator sits in every marine radio shop.

Contrast the two production methods the same sub-topic asks about: an SSB signal comes from a balanced modulator plus sideband filter (above), whereas a double-sideband phone signal can be produced by modulating the supply voltage to a Class C amplifier — plate or collector modulation, which is full-carrier AM, not SSB.