E4E

Noise, interference and grounding

Identify noise sources by their signature, choose blankers, notch filters and chokes, and place surge protectors on a single point ground.

14 pool questions1 on the exam

Syllabus: Noise and interference: external RF interference; electrical and computer noise; line noise; DSP filtering and noise reduction; common-mode current; surge protectors; single point ground panel

Each noise source has a signature

Learn each source by its signature: bursts, a comb of carriers, fixed spurs, or mixing products.

Symptom	Cause
Intermittent loud roaring or buzzing on the AC line	all of these: arcing thermostat contacts, a defective doorbell or its transformer, a malfunctioning illuminated sign
Carriers at regular intervals across a wide range	switch-mode power supplies
Unstable modulated or unmodulated signals at specific frequencies	computer network equipment
Local AM broadcast signals turning up as spurious signals on MF/HF	corroded metal connections mixing and reradiating them

RememberMatch the sound or the spectrum to the culprit.

Blankers cut pulses. Notches and noise reduction work on steady signals.

The blanker cuts the receiver off during sharp pulses. Great on impulse noise. A very strong signal can fool it.

Tool	Removes	Catch
Noise blanker	impulse noise	strong signals may be distorted and appear to cause spurious emissions
Digital noise reduction	broadband white noise, ignition noise and power line noise: all of them
Automatic notch filter	interfering carriers	on CW it can remove the CW signal too

RememberBlanker: impulses. DSP noise reduction: white, ignition and line noise. ANF: also eats CW.

Common-mode current makes cables radiate. Choke it.

Common-mode current is the same on every conductor. It turns a cable into an antenna. A ferrite choke blocks it and passes the wanted current.

Common-mode current flows equally on all conductors of an unshielded multiconductor cable. On shielded cable, common-mode currents on the shield and conductors are what make it radiate or receive interference.

Source	Cure
Automobile charging system (alternator)	ferrite chokes on the charging system leads
Line-driven AC motor	brute-force AC-line filter in series with the motor's power leads

RememberSame direction on everything = antenna. Ferrite chokes block it.

One ground panel, with the surge protector on it

Everything that enters the station passes through one panel, so every protector shares the same ground at the same moment.

A single point ground panel ensures all lightning protectors activate at the same time: they share one ground, so no one cable is left at a different potential. Install the station AC surge protector on that panel.

RememberOne panel, one ground, every protector on it.

Loading lesson…

Go deeper

Compendium articles on the ideas in this lesson, beyond what the exam asks.

Check what stuck

Practise the 14 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

Practise E4E →