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2.7 Modulation Techniques: AM, FM, and SSB
ACMA Amateur Radio Standard Recognition Certificate - Theory Exam
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1 Chapter 1: Electrical Principles and Components
1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour
2 Chapter 2: Transmitters and Receivers
2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth
3 Chapter 3: Transmission Lines and Antennas
3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses
4 Chapter 4: Radio Wave Propagation
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection
5 Chapter 5: Electrical and RF Measurements
5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions
6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)
6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints
7 Chapter 7: Safety and Environmental Hazards
7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety
2.7 Modulation Techniques: AM, FM, and SSB
Key Takeaways Explain This Topic AI
Amplitude Modulation (AM) varies carrier amplitude; total bandwidth is 2 * f_audio(max), and sidebands carry only 33.3% of total power at 100% modulation.
Overmodulation (m > 1.0) in AM clips wave peaks, causing severe harmonic splatter across adjacent channel frequencies.
Single Sideband (SSB) suppresses the unmodulated carrier and one sideband using a balanced modulator and sharp crystal filter, reducing bandwidth to ~2.4 kHz.
Frequency Modulation (FM) varies carrier frequency while keeping amplitude constant; bandwidth is calculated using Carson's Rule: B_FM = 2 * (delta_f + f_audio).
By long-standing amateur convention (IARU and WIA band plans, not an ACMA rule), LSB is used below 10 MHz (160m, 80m, 40m) and USB above 10 MHz (20m, 15m, 10m and all VHF/UHF bands).
Last updated: July 2026
2.7 Modulation Techniques: AM, FM, and SSB
Modulation is the process of superimposing information-bearing intelligence (such as human speech, telegraphy, or computer data) onto an RF carrier wave. Unmodulated RF carriers contain no information; altering the carrier's amplitude, frequency, or phase creates a modulated signal that can be transmitted through space and decoded at a distant receiver.
1. Amplitude Modulation (AM) Principles and Calculations
In Amplitude Modulation (AM) , the instantaneous amplitude of the RF carrier wave is varied in direct proportion to the instantaneous amplitude of the modulating audio signal, while the carrier frequency remains strictly constant.
Mathematical Expression and Modulation Index ( m m m )
The mathematical equation for an AM waveform is: ≠ ( t ) = A c [ 1 + m ⋅ m a ( t ) ] cos ( 2 π f c t ) \ne(t) = A_c \left[ 1 + m \cdot m_a(t) \right] \cos(2\pi f_c t) = ( t ) = A c [ 1 + m ⋅ m a ( t ) ] cos ( 2 π f c t )
Where Modulation Index ( m m m ) is defined as the ratio of peak modulating audio voltage ( V audio V_{\text{audio}} V audio ) to unmodulated RF carrier voltage ( V carrier V_{\text{carrier}} V carrier ): m = V audio V carrier m = \frac{V_{\text{audio}}}{V_{\text{carrier}}} m = V carrier V audio
Expressed as a percentage: Modulation Percentage = m × 100 % \text{Modulation Percentage} = m \times 100\% Modulation Percentage = m × 100%
Under-modulation ( m < 1.0 m < 1.0 m < 1.0 ): Safe operation, but transmitted RF power is underutilised, resulting in weaker recovered audio.
Full Modulation ( m = 1.0 m = 1.0 m = 1.0 ): 100 % 100\% 100% modulation. Audio peak voltage equals carrier voltage. Maximum linear audio power transmission.
Over-modulation ( m > 1.0 m > 1.0 m > 1.0 ): Audio voltage exceeds carrier voltage. During negative audio troughs, the RF carrier is cut off entirely to zero amplitude for finite intervals. This causes envelope clipping , creating severe odd-harmonic distortion and generating wideband interference known as splatter across adjacent frequencies.
AM Spectrum and Bandwidth
When a carrier at frequency f c f_c f c is amplitude-modulated by a single audio tone at frequency f a f_a f a , three distinct RF spectral components emerge:
- Carrier Frequency ( f c f_c f c )
- Upper Sideband (USB): f USB = f c + f a f_{\text{USB}} = f_c + f_a f USB = f c + f a
- Lower Sideband (LSB): f LSB = f c − f a f_{\text{LSB}} = f_c - f_a f LSB = f c − f a
The total bandwidth occupied by an AM signal is twice the highest modulating audio frequency: B AM = 2 ⋅ f a ( max ) B_{\text{AM}} = 2 \cdot f_{a(\text{max})} B AM = 2 ⋅ f a ( max )
ACMA Calculation: If voice audio is filtered to a maximum frequency of f a ( max ) = 3 kHz f_{a(\text{max})} = 3\text{ kHz} f a ( max ) = 3 kHz , the total occupied AM bandwidth is: B AM = 2 × 3 kHz = 6 kHz B_{\text{AM}} = 2 \times 3\text{ kHz} = 6\text{ kHz} B AM = 2 × 3 kHz = 6 kHz
AM Power Distribution
The total power ( P total P_{\text{total}} P total ) contained in a 100 % 100\% 100% modulated AM wave is distributed between the carrier and the two sidebands: P total = P carrier ( 1 + m 2 2 ) P_{\text{total}} = P_{\text{carrier}} \left( 1 + \frac{m^2}{2} \right) P total = P carrier ( 1 + 2 m 2 )
At 100 % 100\% 100% modulation ( m = 1.0 m = 1.0 m = 1.0 ): P total = P carrier ( 1 + 1 2 ) = 1.5 ⋅ P carrier P_{\text{total}} = P_{\text{carrier}} \left( 1 + \frac{1}{2} \right) = 1.5 \cdot P_{\text{carrier}} P total = P carrier ( 1 + 2 1 ) = 1.5 ⋅ P carrier
Carrier Power: Represents 1.0 1.5 = 66.7 % \frac{1.0}{1.5} = 66.7\% 1.5 1.0 = 66.7% of total transmitted power.
Sideband Power (Total): Represents 0.5 1.5 = 33.3 % \frac{0.5}{1.5} = 33.3\% 1.5 0.5 = 33.3% of total power.
Each Individual Sideband: Carries only 0.25 1.5 = 16.7 % \frac{0.25}{1.5} = 16.7\% 1.5 0.25 = 16.7% of total power.
Notice that two-thirds ( 66.7 % 66.7\% 66.7% ) of the total RF power is expended on the unmodulated carrier, which carries zero information! Furthermore, the two sidebands carry identical, redundant intelligence.
2. Double Sideband & Single Sideband (SSB) Generation
To overcome the extreme power and bandwidth inefficiency of standard AM, amateur radio relies heavily on Single Sideband (SSB) transmission.
The Filter Method of SSB Generation
Balanced Modulator: Audio speech and an RF carrier ( f c f_c f c ) enter a balanced modulator (ring diode bridge or Gilbert cell). The balanced circuit cancels out the carrier signal by 40 dB 40\text{ dB} 40 dB to 50 dB 50\text{ dB} 50 dB , producing a Double Sideband Suppressed Carrier (DSB-SC) signal containing only the upper and lower sidebands.
Sideband Filter: The DSB-SC signal passes through an ultra-sharp quartz crystal ladder filter. The filter selectively passes one sideband (e.g., Upper Sideband) while attenuating the unwanted sideband (Lower Sideband) by 50 dB 50\text{ dB} 50 dB or more. Audio + Carrier ⟶ Balanced Modulator ⟶ DSB-SC ⟶ Crystal Filter ⟶ SSB \text{Audio} + \text{Carrier} \longrightarrow \boxed{\text{Balanced Modulator}} \longrightarrow \text{DSB-SC} \longrightarrow \boxed{\text{Crystal Filter}} \longrightarrow \text{SSB} Audio + Carrier ⟶ Balanced Modulator ⟶ DSB-SC ⟶ Crystal Filter ⟶ SSB
ACMA Standard Sideband Band Conventions
Amateur radio operators adhere strictly to national and international sideband band plan conventions:
Lower Sideband (LSB): Used on all amateur bands below 10 MHz (specifically 160 m 160\text{m} 160 m , 80 m 80\text{m} 80 m , and 40 m 40\text{m} 40 m bands).
Upper Sideband (USB): Used on all amateur bands above 10 MHz (specifically 20 m 20\text{m} 20 m , 17 m 17\text{m} 17 m , 15 m 15\text{m} 15 m , 12 m 12\text{m} 12 m , 10 m 10\text{m} 10 m , and all VHF/UHF/Microwave bands).
Note: The 60 m 60\text{m} 60 m band ( 5.3 MHz 5.3\text{ MHz} 5.3 MHz ) band (5.3 MHz) is a USB exception overseas. Australian amateurs have no 60 m allocation - the ACMA declined amateur access to 5351.5-5366.5 kHz - so it does not appear in the Australian band plan.
Bandwidth Savings of SSB
Because an SSB signal consists of only a single sideband, its occupied RF bandwidth equals the highest audio frequency transmitted: B SSB = f a ( max ) ≈ 2.4 kHz B_{\text{SSB}} = f_{a(\text{max})} \approx 2.4\text{ kHz} B SSB = f a ( max ) ≈ 2.4 kHz
SSB uses less than half the spectrum space of AM ( 2.4 kHz 2.4\text{ kHz} 2.4 kHz vs 6.0 kHz 6.0\text{ kHz} 6.0 kHz ) and concentrates 100 % 100\% 100% of transmitter output power into intelligence-bearing sideband energy, offering a communications advantage of over 9 dB 9\text{ dB} 9 dB ( 8 × 8\times 8 × effective power improvement) over AM.
3. Frequency Modulation (FM) and Phase Modulation (PM)
In Frequency Modulation (FM) , the amplitude of the RF carrier remains strictly constant, while its instantaneous frequency is varied above and below the centre frequency in direct proportion to the modulating audio signal amplitude.
Key FM Terminology
Peak Frequency Deviation ( Δ f \Delta f Δ f ): The maximum excursion of the carrier frequency away from its unmodulated centre frequency. In Amateur Narrowband FM (NFM), standard peak deviation is Δ f = ± 5.0 kHz \Delta f = \pm 5.0\text{ kHz} Δ f = ± 5.0 kHz (or ± 2.5 kHz \pm 2.5\text{ kHz} ± 2.5 kHz on narrow channels).
Modulation Index ( β \beta β ): Ratio of peak frequency deviation to highest modulating audio frequency: β = Δ f f a \beta = \frac{\Delta f}{f_a} β = f a Δ f
Carson's Rule for FM Bandwidth
Unlike AM, an FM signal theoretically generates an infinite number of sideband pairs spaced at multiples of f a f_a f a ( f c ± f a , f c ± 2 f a , f c ± 3 f a , … f_c \pm f_a, f_c \pm 2f_a, f_c \pm 3f_a, \dots f c ± f a , f c ± 2 f a , f c ± 3 f a , … ), described mathematically by Bessel functions.
However, Carson's Rule establishes that 98 % 98\% 98% of total FM signal power is contained within a practical bandwidth defined as: B FM = 2 ⋅ ( Δ f + f a ( max ) ) B_{\text{FM}} = 2 \cdot (\Delta f + f_{a(\text{max})}) B FM = 2 ⋅ ( Δ f + f a ( max ) )
ACMA Calculation Example: Calculate the total bandwidth of an Australian 2-metre FM amateur transmission using peak deviation Δ f = 5 kHz \Delta f = 5\text{ kHz} Δ f = 5 kHz and maximum speech audio frequency f a ( max ) = 3 kHz f_{a(\text{max})} = 3\text{ kHz} f a ( max ) = 3 kHz .
B FM = 2 × ( 5 kHz + 3 kHz ) = 2 × 8 kHz = 16 kHz B_{\text{FM}} = 2 \times (5\text{ kHz} + 3\text{ kHz}) = 2 \times 8\text{ kHz} = 16\text{ kHz} B FM = 2 × ( 5 kHz + 3 kHz ) = 2 × 8 kHz = 16 kHz
This explains why standard VHF/UHF FM repeater channels require 25 kHz 25\text{ kHz} 25 kHz or 12.5 kHz 12.5\text{ kHz} 12.5 kHz channel spacing.
Phase Modulation (PM)
In Phase Modulation (PM) , the instantaneous phase of the carrier is shifted by speech. Because phase change is mathematically the time derivative of frequency, PM is functionally equivalent to FM with a + 6 dB/octave +6\text{ dB/octave} + 6 dB/octave audio pre-emphasis filter applied. Most commercial "FM" VHF hand-held transceivers actually generate Phase Modulation using reactance modulators.