13.3 SSB Transmitters & Modern Transmitter Technology

13.3 SSB Transmitters & Modern Transmitter Technology

Quick Answer: SSB filter method: balanced modulator (cancel carrier → DSB-SC) → sideband filter → mix to final frequency → linear amp chain. Cancel carrier and filter unwanted sideband. Voice PEP:average ≈ 2.5:1 (speech characteristics). ALC protects linearity. Scope: 200 V p-p / 50 Ω → 100 W PEP; 1200 W / 50 Ω → ~245 V. Tech: isolators vs IMD, neutralization vs parasitics, push-pull vs even harmonics, spread spectrum hop/multi-carrier sequences; SDR still needs clean linear RF.

Section 13.1 chose the linear final; §13.2 defined the balanced modulator and matching networks. This section is the complete SSB path (key topic 055) plus transmitter technology items (key topic 056) that keep commercial HF/VHF radiotelephone legal and reliable.

Why SSB exists in GROL services

Advantage of SSB over conventional AM: more efficient use of power and bandwidth. Full-carrier AM spends roughly two-thirds of its power in a carrier that carries no intelligence and duplicates audio in two sidebands. J3E (SSB suppressed-carrier telephony) puts power into one information sideband and roughly halves RF bandwidth—critical on crowded HF marine and aeronautical bands GROL techs maintain.

Filter-method SSB block diagram

Pool generation answer: balanced modulator followed by a filter. Expand that into the service-manual chain:

Carrier and sideband removal wording

To produce a single-sideband suppressed-carrier transmission it is necessary to cancel the carrier and to filter the unwanted sideband. Pool fill-in: Cancel, filter (not “filter, filter” or “cancel, cancel”).

• Cancel → balanced modulator symmetry nulls the carrier.
• Filter → crystal/mechanical/DSP sideband filter removes the unwanted sideband.

Phasing (Hartley) method cancels the unwanted sideband with quadrature networks instead of a sharp filter; Element 3’s explicit generation answer still points at balanced modulator + filter for the classic path.

Mix to final frequency

Generating clean SSB at a fixed IF is easier than building a tunable sharp filter at every HF channel. A mixer combines IF SSB with a VFO or synthesizer LO so the difference (or sum) lands on the assigned transmit frequency. Then only linear stages may follow—any Class C brick reintroduces the distortion problem from §13.1.

Linear amplifier chain rules

1. Every stage after the sideband filter that handles the SSB envelope must be linear (Class A/AB family).
2. Drive level must stay below flat-topping; ALC and microphone gain are operational controls for that limit.
3. Two-tone tests and spectrum checks after PA work verify IMD products stay inside emission masks.
4. Non-linear amplifier + SSB phone → distortion (pool 3-55G1).

Marine/aviation HF failures often look like “radio is loud but dirty”: neighbors report splatter while the operator’s monitor speaker still sounds OK. Suspect overdrive, failed ALC, biased-into-C finals after a “power mod,” or a non-linear external amplifier.

Speech characteristics, PEP, and average power

What sets PEP-to-average?

In a single-sideband phone signal, what determines the PEP-to-average power ratio? The speech characteristics (not carrier suppression alone, not amplifier power rating as the defining answer).

Approximate PEP-to-average ratio during normal voice modulation peaks: 2.5 to 1.

That means a transmitter running 100 W PEP on normal speech may show only ~40 W average on a slow power meter—yet finals, power supplies, and FCC power rules care about PEP. Average-reading meters under-report voice peaks; peak-reading wattmeters or scope methods are the honest tools.

Speech processing

Speech processing (compression/clipping with filtering) raises the average power closer to PEP so the channel sounds stronger without necessarily raising legal PEP. Done correctly, intelligibility in noise improves. Done aggressively, processors create distortion products that the linear PA dutifully amplifies as splatter. GROL operating practice: increase processing only until average rises modestly; watch ALC and adjacent-channel reports.

ALC — automatic level control

ALC samples RF output (or PA drive) and reduces audio or IF gain when peaks approach the distortion limit. It is the transmit cousin of receiver AGC:

After PA transistor replacement, always verify ALC threshold and foldback before returning a set to an aircraft or ship.

PEP math from oscilloscope and dummy loads

Scope method (pool)

Output peak envelope power measured on an oscilloscope showing 200 volts peak-to-peak across a 50-ohm load:

[ V_p = \frac{200}{2} = 100,\mathrm{V},\quad P_{\mathrm{PEP}} = \frac{V_p^2}{2R} = \frac{100^2}{2\times 50} = \frac{10000}{100} = 100,\mathrm{W} ]

(Equivalently (V_{\mathrm{rms}}=V_p/\sqrt{2}), then (P=V_{\mathrm{rms}}^2/R).) Pool answer: 100 watts.

Load voltage at a known power

Voltage across a 50-ohm dummy load dissipating 1,200 watts:

[ V = \sqrt{P R} = \sqrt{1200\times 50} = \sqrt{60000} \approx 245,\mathrm{V} ]

Pool answer: 245 volts (RMS for CW/carrier-like dissipation; use consistent RMS assumptions as the pool does).

Keep units straight on exam day: p-p vs peak vs RMS is the usual trap.

Technology cluster (3-56) — clean modern transmitters

Intermodulation between co-sited transmitters

Name of the condition: when signals of two transmitters in close proximity mix in one or both final amplifiers and create sum and difference frequencies → intermodulation interference.

How often reduced or eliminated: install a terminated circulator or ferrite isolator in the feed line to the transmitter and duplexer. The isolator presents a matched load to reverse energy so the final sees fewer external signals to mix with. Band-pass cavities and physical separation help too; the pool’s highlighted fix is the circulator/isolator path.

Parasitics and even harmonics

Neutralization cancels the internal feedback path that supports VHF/UHF parasites on HF finals. Push-pull symmetry cancels even harmonics in the combined output; odd harmonics still need the pi-L/LPF chain from §13.2.

Spread spectrum (definition level)

Wide-bandwidth communications system in which the RF carrier varies according to some predetermined sequence: spread-spectrum communication.

Modulation type that can be frequency hopping of one carrier or multiple simultaneous carriers: spread spectrum.

That is exam-depth: recognize hoppers and multi-carrier spreading as spread spectrum—not SSB, not ordinary FM, not SITOR/AMTOR labels.

Digital / software-defined transmitter notes (exam level)

Modern commercial and test-bench transmitters increasingly:

1. Generate the complex baseband (SSB, FM, digital voice/data) in DSP/FPGA software.
2. Convert with a DAC and I/Q modulator (or direct digital RF synthesis) up to the operating band.
3. Still rely on a linear RF power chain, ALC-like peak control, and analog filters/matching to meet spectral masks.

Software does not repeal Class A/AB vs Class C physics. An SDR exciter feeding a non-linear PA still splatters on SSB. Isolators, harmonic filters, and neutralization (or modern unconditional stability design) remain technician concerns. PLL/VCO synthesizers (from earlier topics) provide the agile LO for both classic mix-up SSB and SDR architectures.

Shop scenarios for Topic 3-G closeout

1. SSB sounds wide on a monitor scope, ALC pegged — reduce mic gain/processing; verify ALC detector and PA bias still linear.
2. Two base stations create mystery carriers at sum/difference offsets — co-sited intermodulation; add isolators, tune cavities, fix shielding.
3. HF PA “takes off” at VHF when drive removed — parasitics; neutralize / suppress, check layout and parasitic chokes.
4. Even harmonics strong, odds normal — suspect single-ended imbalance or failed push-pull device; restore symmetry and filtering.
5. PEP math on bench — measure V p-p on 50 Ω dummy load; use (P=V_p^2/(2R)).

Exam-day checklist (055–056)

1. SSB generation → balanced modulator + filter; cancel carrier, filter unwanted sideband.
2. After filter → mix to final frequency → linear driver/PA only.
3. Nonlinear SSB amp → distortion; PEP:average → speech characteristics, ~2.5:1 normal voice.
4. Scope 200 V p-p / 50 Ω → 100 W PEP; 1200 W / 50 Ω → ~245 V.
5. Co-sited mixing → intermodulation interference; fix with circulator/ferrite isolator.
6. Parasitics → neutralization; even harmonics → push-pull; hop/multi-carrier predetermined sequence → spread spectrum.

Master amplifier classes, modulator/matching blocks, and the SSB linear chain and you have Topic 3-G Transmitters for Element 3—ready for Topic 3-H modulation details that expand AM/FM/SSB emission theory.