Interference and Suppression

Chapter 8: Interference and Suppression

Radio Frequency Interference — receiver overload, harmonics, spurious emissions, and filter types

Receiver Overload — condition where the early stages of a receiver are overwhelmed by a strong nearby transmitter
Receiver Overload Diagnostic — interference is the same regardless of what transmit frequency is used

Receiver Overload vs Harmonics — any transmit frequency causes the same interference pattern, harmonics only specific frequencies cause interference
Diagnostic Rule — no matter what band equals overload, specific band equals harmonics

Intermodulation — special case of overload where two or more strong signals mix in a non-linear device
Intermodulation Symptoms — loss of sensitivity, squeals, and noises covering the intended signal

Desensitization — receiver loses sensitivity when a nearby transmitter operates on the same or adjacent frequency
Intermodulation Patterns — signals from different sources combine to produce interference

High-Pass Filter — on TV receiver blocks HF signals below 30 MHz while passing TV signals above 54 MHz
Low-Pass Filter — at transmitter blocks harmonics above 30 MHz from leaving the antenna

Audio Rectification — semiconductor junction in audio equipment behaves like a diode detector, retrieving audio from a radio signal
Audio Rectification Symptom — interference heard regardless of volume setting, indicating RF entering audio circuitry directly

Ferrite Core — winding wires around a ferrite core creates an inductor that opposes high-frequency AC while passing audio frequencies
Ferrite Core Applications — coils on ferrite cores for speaker leads, telephone wires, and intercom cables

Telephone RFI — cabling acts as an antenna, install a modular plug-in RFI filter close to the telephone device
PA System RFI — long speaker leads act as antennas, picking up SSB as distorted speech and CW as on-and-off clicking or humming

Common-Mode Choke — device installed on the transmission line that impedes the flow of stray RF current on the outside of the coaxial shield
Common-Mode Choke Symptom — distorted audio and noise from an outboard amplified speaker when transmitting on SSB with an OCF antenna

Spurious Emissions — unwanted signals radiated at a frequency other than the operating frequency
Spurious Emission Types — harmonics at integer multiples and parasitic oscillations above or below the fundamental

Key Clicks Type 1 — too-sharp rise and decay times of the keyed carrier cause clicks heard several kilohertz away at distant receivers
Key Clicks Type 2 — sparks from the making and breaking of the circuit at the Morse key cause local RF interference

Harmonic Radiation — unwanted signals at frequencies that are integer multiples of the fundamental operating frequency
Harmonic Diagnostic — interference to one or two specific TV channels from a specific band, not all channels from all bands
Low-Pass Filter — connecting a low-pass filter at the transmitter output suppresses harmonic radiation

Splatter — unwanted radio energy transmitted just outside the normal necessary bandwidth of the signal
Overmodulation — excessive microphone gain or speech processing forces the linear power amplifier into non-linear operation

Low-Pass Filter — passes frequencies below the cutoff and attenuates frequencies above the cutoff
High-Pass Filter — passes frequencies above the cutoff and attenuates frequencies below the cutoff
Band-Pass Filter — passes one specific range of frequencies and attenuates frequencies above and below that range
Notch Filter — attenuates one specific narrow frequency while passing all other frequencies
Band-Reject Filter — attenuates a range of frequencies while passing all frequencies outside that range

Overload, Harmonics, Intermodulation — overload affects any frequency, harmonics on precise multiples, intermodulation from signal mixing
Audio Rectification and RFI — semiconductor junctions act as detectors, ferrite cores on leads block RF, common-mode choke on feedline
Spurious Emissions and Key Clicks — unwanted signals at other frequencies, key clicks from sharp waveform or sparking at key contacts
Harmonic Radiation and Splatter — energy at integer multiples, low-pass filter suppresses harmonics, splatter from overmodulation
Filter Types — low-pass, high-pass, band-pass, notch, and band-reject filters each with specific pass and stop characteristics