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Amateur Radio Info & Exams - Practices 3 - Noise suppression

Noise blankers

A noise-blanker, often marked "NB", is a function in many radios, even 27 MHz CBs, used to reduce impulse noise, most notably, vehicle ignition noise. Such noise is wide-band, and I have certainly heard it from passing vehicles on 2 metres SSB while pulled over beside the road on a ridge before heading down the hill to get home. These have traditionally been analogue circuits, but DSP can perform this task.

The cousin of ignition noise is the noise from the strong electrical pulses required to drive high pressure injectors in modern diesel engines.

Power lines, especially of higher voltages, often generate 50 or 60 Hz modulated RF interference, causing a loud buzz.


Digital Signal Processing can do a range of tasks, most notably reducing interference on received radio signals, but can also be used to process outdoing signals.

The FT-8757D, the D indicating included DSP, for example, includes wideband noise reduction (with adjustable aggressiveness), a bandpass filter with adjustable lower and upper frequencies, and automatic notch filters to remove unwanted carriers and other tones.

They do however need to be set up correctly, as they can also reduce desired signals, especially CW. They may also do this to sustained notes in music in broadcast signals.

Relevant Questions

These are the actual questions from the Extra licence exam pool, as published by the NCVEC.

What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals?
A. Removal of the CW signal as well as the interfering carrier
B. Any nearby signal passing through the DSP system will overwhelm the desired signal
C. Received CW signals will appear to be modulated at the DSP clock frequency
D. Ringing in the DSP filter will completely remove the spaces between the CW characters

This function, often part of a DSP system, can filter out the desired CW signal, answer A.

Which of the following types of receiver noise can often be reduced with a digital signal processing noise filter?
A. Broadband white noise
B. Ignition noise
C. Power line noise
D. All of these choices are correct

DSP reduces white noise, ignition noise, and garbage from power lines, answer D.

Which of the following signals might a receiver noise blanker be able to remove from desired signals?
A. Signals which are constant at all IF levels
B. Signals which appear across a wide bandwidth
C. Signals which appear at one IF but not another
D. Signals which have a sharply peaked frequency distribution

These are effective against interfering signals which appear across a wide bandwidth, answer B.

How can conducted and radiated noise caused by an automobile alternator be suppressed?
A. By installing filter capacitors in series with the DC power lead and a blocking capacitor in the field lead
B. By installing a noise suppression resistor and a blocking capacitor in both leads
C. By installing a high-pass filter in series with the radio's power lead and a low-pass filter in parallel with the field lead
D. By connecting the radio's power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads

Connect the radio direct to the battery, and install a "coaxial" capacitor in the alternator leads, answer D.

These are a capacitor which is inside a metal tube with a grounding tab which is used to mount the device. While there are terminals or wires on both ends, there is a high current DC path through the devices, and the capacitance is to ground.

How can radio frequency interference from an AC motor be suppressed?
A. By installing a high pass filter in series with the motor's power leads
B. By installing a brute-force AC-line filter in series with the motor leads
C. By installing a bypass capacitor in series with the motor leads
D. By using a ground-fault current interrupter in the circuit used to power the motor

The solution is to install a "brute-force" AC-line filter in series with the motor, answer B.

These typically consist of a parallel capacitor, inductors in series with the current carrying lines, and another parallel capacitor. It is possible to add further stages, and to perhaps MOVs.

What is one type of electrical interference that might be caused by a nearby personal computer?
A. A loud AC hum in the audio output of your station receiver
B. A clicking noise at intervals of a few seconds
C. The appearance of unstable modulated or unmodulated signals at specific frequencies
D. A whining type noise that continually pulses off and on

PCs can cause unstable modulated or unmodulated signals at various frequencies, answer C.

These may vary with keyboard input, scrolling, disk access, etc.

Which of the following can cause shielded cables to radiate or receive interference?
A. Low inductance ground connections at both ends of the shield
B. Common mode currents on the shield and conductors
C. Use of braided shielding material
D. Tying all ground connections to a common point resulting in differential mode currents in the shield

Most consumer electronics is interconnected using shielded, unbalanced cables, and these can pick up or radiate common-mode interference, answer B.

Interconnects between radios and soundcards or modems are also typically unbalanced cables, and these occasionally pick up RF. Professional audio equipment typically uses balanced cables, which tend to reject common-mode currents.

What current flows equally on all conductors of an unshielded multi-conductor cable?
A. Differential-mode current
B. Common-mode current
C. Reactive current only
D. Return current

These are common-mode currents, answer B.

What undesirable effect can occur when using an IF noise blanker?
A. Received audio in the speech range might have an echo effect
B. The audio frequency bandwidth of the received signal might be compressed
C. Nearby signals may appear to be excessively wide even if they meet emission standards
D. FM signals can no longer be demodulated

High level, nearby signals may appear to be excessively wide, answer C.

What might be the cause of a loud roaring or buzzing AC line interference that comes and goes at intervals?
A. Arcing contacts in a thermostatically controlled device
B. A defective doorbell or doorbell transformer inside a nearby residence
C. A malfunctioning illuminated advertising display
D. All of these choices are correct

All these arcy-sparky dodgy devices will cause this problem, answer D.

What could cause local AM broadcast band signals to combine to generate spurious signals in the MF or HF bands?
A. One or more of the broadcast stations is transmitting an over-modulated signal
B. Nearby corroded metal joints are mixing and re-radiating the broadcast signals
C. You are receiving skywave signals from a distant station
D. Your station receiver IF amplifier stage is defective

This is intermodulation, caused by an unintentional diode formed in a corroded joint mixing and re-radiating the signals, answer B.

On to: Electrical Principles 1 - Resonance, Impedance, and Phase Angles

You can find links to lots more on the Learning Material page.

Written by Julian Sortland, VK2YJS & AG6LE, May 2022.

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