Study guide
2.3 Receiver Performance: Sensitivity, Selectivity, and AGC
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
2.3 Receiver Performance: Sensitivity, Selectivity, and AGC
Key Takeaways Explain This Topic AI
Sensitivity defines the minimum RF input signal required to achieve a specified Signal-to-Noise Ratio (SNR), limited primarily by thermal noise floor.
Thermal noise floor in a given bandwidth at 290 K is calculated via P_N = -174 dBm + 10 log10(BW_Hz).
Selectivity describes a receiver's ability to isolate desired signals and reject adjacent interfering stations, established by IF filter bandwidth and shape factor.
Filter shape factor is the ratio of -60 dB bandwidth to -6 dB bandwidth (BW_-60dB / BW_-6dB); values closer to 1.0 indicate steeper, ideal filter skirts.
Automatic Gain Control (AGC) maintains constant audio output despite incoming signal strength variations; Slow AGC is used for SSB voice and Fast AGC for CW/digital modes.
Last updated: July 2026
2.3 Receiver Performance: Sensitivity, Selectivity, and AGC
Beyond Standard level: Third-order intercept point, numeric shape factors and noise-figure cascading are Advanced-level material. At Standard you need to recognise the terms sensitivity, selectivity, dynamic range and blocking, and describe what each one means.
Evaluating the quality of an amateur radio receiver requires examining specific performance metrics that determine how well the radio can pick up extremely weak signals in distant communications (DXing) and cope with extremely strong adjacent signals during high-density contests. The three critical benchmarks in receiver design are sensitivity , selectivity , and dynamic range .
1. Receiver Sensitivity and Thermal Noise Floor
Sensitivity is the measure of a receiver's ability to pick up weak radio signals. It is defined quantitatively as the minimum RF input signal voltage (expressed in microvolts, μ V \mu\text{V} μ V ) or power (in dBm \text{dBm} dBm ) applied to the antenna terminal that produces a specified Signal-to-Noise Ratio (SNR) or SINAD (Signal-plus-Noise-and-Distortion to Noise-and-Distortion ratio) at the audio output.
Standard SSB/CW Benchmark: 0.15 μ V 0.15\, \mu\text{V} 0.15 μ V to 0.25 μ V 0.25\, \mu\text{V} 0.25 μ V for a 10 dB ( S + N ) / N 10\text{ dB } (S+N)/N 10 dB ( S + N ) / N ratio.
Standard FM Benchmark: 0.25 μ V 0.25\, \mu\text{V} 0.25 μ V to 0.50 μ V 0.50\, \mu\text{V} 0.50 μ V for a 12 dB SINAD 12\text{ dB SINAD} 12 dB SINAD ratio.
Thermal Noise Floor (Johnson-Nyquist Noise)
The ultimate physical limit to receiver sensitivity is thermal noise generated by random electron motion in resistive components. The thermal noise power available in a given bandwidth is calculated using the thermal noise formula: P N = k ⋅ T ⋅ B P_N = k \cdot T \cdot B P N = k ⋅ T ⋅ B
Where:
- k = 1.38 × 10 − 23 J/K k = 1.38 \times 10^{-23}\text{ J/K} k = 1.38 × 1 0 − 23 J/K (Boltzmann's constant)
- T = 290 K T = 290\text{ K} T = 290 K (Standard room temperature in Kelvin)
B = Receiver noise bandwidth in Hertz (Hz) B = \text{Receiver noise bandwidth in Hertz (Hz)} B = Receiver noise bandwidth in Hertz (Hz)
Expressed in decibels relative to one milliwatt ( dBm \text{dBm} dBm ) at room temperature ( 290 K 290\text{ K} 290 K ): P N ( dBm ) = − 174 dBm + 10 log 10 ( B ) P_N (\text{dBm}) = -174\text{ dBm} + 10 \log_{10}(B) P N ( dBm ) = − 174 dBm + 10 lo g 10 ( B )
Noise Floor Calculations for Standard Bandwidths
CW Bandwidth ( 500 Hz 500\text{ Hz} 500 Hz ): P N = − 174 + 10 log 10 ( 500 ) = − 174 + 27 = − 147 dBm P_N = -174 + 10 \log_{10}(500) = -174 + 27 = -147\text{ dBm} P N = − 174 + 10 lo g 10 ( 500 ) = − 174 + 27 = − 147 dBm
SSB Bandwidth ( 2400 Hz 2400\text{ Hz} 2400 Hz ): P N = − 174 + 10 log 10 ( 2400 ) = − 174 + 33.8 = − 140.2 dBm P_N = -174 + 10 \log_{10}(2400) = -174 + 33.8 = -140.2\text{ dBm} P N = − 174 + 10 lo g 10 ( 2400 ) = − 174 + 33.8 = − 140.2 dBm
Notice that narrowing the receiver IF bandwidth reduces total noise power, significantly improving weak-signal readability!
Noise Figure (NF) and Noise Factor ( F F F )
No real-world receiver is perfectly noiseless. Active components (transistors, mixers) add internal noise to the signal as it passes through the stages. Noise Factor ( F F F ) is the ratio of input SNR to output SNR: F = SNR in SNR out F = \frac{\text{SNR}_{\text{in}}}{\text{SNR}_{\text{out}}} F = SNR out SNR in
Noise Figure (NF) is simply the noise factor expressed in decibels: NF (dB) = 10 log 10 ( F ) \text{NF (dB)} = 10 \log_{10}(F) NF (dB) = 10 lo g 10 ( F )
According to Friis' Formula for Noise , the noise figure of the very first stage (the RF preamplifier) dominates the overall noise figure of the entire receiver system. Adding a low-noise preamplifier at the antenna terminals improves overall receiver sensitivity on VHF/UHF bands where thermal noise dominates.
2. Receiver Selectivity and Bandwidth Standards
Selectivity is the receiver's ability to differentiate between a desired signal and unwanted interfering signals operating on closely adjacent frequencies. Selectivity is established primarily by the bandpass filters located in the Intermediate Frequency (IF) amplifier section.
Standard Bandwidth Requirements by Operating Mode
Operating Mode Nominal Receiver IF Bandwidth Filter Type / Implementation
Morse Code (CW) 250 Hz − 500 Hz 250\text{ Hz} - 500\text{ Hz} 250 Hz − 500 Hz Narrow Quartz Crystal Ladder / DSP Digital Bandpass
Single Sideband (SSB) 2.1 kHz − 2.8 kHz 2.1\text{ kHz} - 2.8\text{ kHz} 2.1 kHz − 2.8 kHz (Standard: 2.4 kHz 2.4\text{ kHz} 2.4 kHz ) Crystal Filter / Mechanical Filter / DSP IF Filter
Amplitude Modulation (AM) 6.0 kHz − 9.0 kHz 6.0\text{ kHz} - 9.0\text{ kHz} 6.0 kHz − 9.0 kHz Ceramic / LC Filter
Narrowband FM (NFM) 12.0 kHz − 15.0 kHz 12.0\text{ kHz} - 15.0\text{ kHz} 12.0 kHz − 15.0 kHz Dual Ceramic / DSP Filter
Filter Skirt Selectivity and Shape Factor
An ideal filter would possess a perfectly rectangular passband ("brick-wall filter"), passing all frequencies inside the band with zero attenuation and instantly blocking all frequencies outside. Real-world physical filters have sloped sides known as filter skirts .
The steepness of a filter's skirts is defined by its Shape Factor , calculated as the ratio of the bandwidth at − 60 dB -60\text{ dB} − 60 dB attenuation to the bandwidth at − 6 dB -6\text{ dB} − 6 dB attenuation: Shape Factor = Bandwidth at − 60 dB Bandwidth at − 6 dB \text{Shape Factor} = \frac{\text{Bandwidth at }-60\text{ dB}}{\text{Bandwidth at }-6\text{ dB}} Shape Factor = Bandwidth at − 6 dB Bandwidth at − 60 dB
An ideal theoretical filter has a Shape Factor of 1.0 .
High-grade commercial crystal filters achieve shape factors between 1.5 and 2.0 .
Modern Digital Signal Processing (DSP) sharp linear-phase filters can achieve shape factors approaching 1.1 to 1.2 , offering extraordinary adjacent-channel rejection.
3. Dynamic Range, Intermodulation, and Desensitisation
While sensitivity handles weak signals and selectivity handles nearby signals, Dynamic Range determines how well a receiver can process extremely weak signals in the presence of extremely strong signals elsewhere on the band without suffering distortion.
Intermodulation Distortion (IMD)
When two or more strong, unwanted signals ( f 1 f_1 f 1 and f 2 f_2 f 2 ) pass into a non-linear receiver front-end or mixer stage, they mix together to produce Intermodulation Distortion (IMD) products. The most troublesome are the 3rd-Order IMD products because they fall extremely close to the original frequencies: f IMD3 = 2 f 1 − f 2 and 2 f 2 − f 1 f_{\text{IMD3}} = 2f_1 - f_2 \quad \text{and} \quad 2f_2 - f_1 f IMD3 = 2 f 1 − f 2 and 2 f 2 − f 1
If f 1 = 7.100 MHz f_1 = 7.100\text{ MHz} f 1 = 7.100 MHz and f 2 = 7.105 MHz f_2 = 7.105\text{ MHz} f 2 = 7.105 MHz , the 3rd-order product lands at 2 ( 7.100 ) − 7.105 = 7.095 MHz 2(7.100) - 7.105 = 7.095\text{ MHz} 2 ( 7.100 ) − 7.105 = 7.095 MHz . If a weak desired station is operating on 7.095 MHz, it will be completely obliterated by this phantom signal generated inside the receiver!
Third-Order Intercept Point (IP3)
The Third-Order Intercept Point (IP3 or TOI) is a theoretical figure of merit used to quantify strong-signal handling. It represents the hypothetical input power level (in dBm \text{dBm} dBm ) at which the power of the 3rd-order IMD product would equal the fundamental signal power. Higher positive IP3 values (+15 dBm to +30 dBm) indicate superior resistance to intermodulation distortion.
Desensitisation (Blocking)
Desensitisation (or receiver blocking) occurs when an exceptionally strong off-frequency signal (such as a nearby commercial broadcasting station or local ham transmitter) enters the receiver front-end. Even if the strong signal is outside the IF passband, its high power level overloads active transistors, driving them out of their linear region into saturation. This drastically reduces the gain for all signals, causing weak desired signals to instantly disappear or drop in volume.
4. Automatic Gain Control (AGC) Operation
Signals arriving over ionospheric skywave paths constantly fluctuate in amplitude due to fading ( QSB ). If receiver gain remained fixed, audio volume would jump wildly between whisper-soft and ear-splitting levels.
Automatic Gain Control (AGC) solves this by automatically adjusting the gain of the IF and RF amplifier stages in inverse proportion to incoming signal strength.
AGC Feedback Mechanism
A portion of the amplified IF signal is tapped off after the crystal filter and fed to an AGC detector diode.
The AGC detector rectifies the AC signal into a negative DC control voltage proportional to RF signal level.
This DC control voltage is fed back to the bias networks of the RF and IF amplifier stages.
As incoming signal strength increases, the DC voltage reduces active device gain, holding the audio output level virtually constant.
AGC Time Constant Settings
Receivers feature selectable AGC decay time constants tailored to different operating modes:
FAST AGC (Decay time ~20 to 100 ms): Quick recovery time. Ideal for Morse code (CW) or fast data digital modes (FT8), allowing the receiver to recover sensitivity in the spaces between code elements.
SLOW AGC (Decay time ~1 to 3 seconds): Slow recovery time. Essential for Single Sideband (SSB) voice operation. Prevents the receiver gain from surging upward during brief pauses between words, which would cause loud background atmospheric noise "pumping."
OFF / MANUAL AGC: Disables automatic control. The operator manually adjusts the RF Gain control. Used in heavy contest environments or weak-signal EME (Earth-Moon-Earth) work to prevent strong nearby stations from pumping the AGC circuit.