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Subelement E4
AMATEUR PRACTICES
Section E4E
Noise suppression: system noise; electrical appliance noise; line noise; locating noise sources; DSP noise reduction; noise blankers; grounding for signals
Which of the following types of receiver noise can often be reduced by use of a receiver noise blanker?
  • Ignition noise
  • Broadband white noise
  • Heterodyne interference
  • All of these choices are correct

A noise blanker is used to combat "impulse" type noise; such as automobile ignition noise. Noise blankers usually have an adjustable threshold setting and narrow noise spikes whose amplitudes exceed this threshold cause a brief interruption in the receiver's internal signal path to eliminate the spike. Noise blankers are usually present in the IF circuitry of the receiver before the high-selectivity IF filters.

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Which of the following types of receiver noise can often be reduced with a DSP noise filter?
  • Broadband white noise
  • Ignition noise
  • Power line noise
  • All of these choices are correct

DSP noise reduction operates by examining a characteristic of signals and noise called correlation, and dynamically filtering out the undesired noise. In most DSP noise blanker implementations the amount of noise reduction varies according to the correlation characteristics of the noise. Correlation is a measure of the “regularity” of a signal. Random noise such as white noise or static is uncorrelated. Speech and ignition noise is moderately correlated. Heterodynes and pure tones are highly correlated.

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

Answer: Signals which appear across a wide bandwidth

Think of lightning. It is a signal which covers a wide bandwidth as it can be heard in AM or SSB modes on receivers across many frequencies/bands. Noise Blanker helps to eliminate this.

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How can conducted and radiated noise caused by an automobile alternator be suppressed?
  • By installing filter capacitors in series with the DC power lead and a blocking capacitor in the field lead
  • By installing a noise suppression resistor and a blocking capacitor in both leads
  • 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
  • By connecting the radio's power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads

Connecting leads directly to the battery bypasses many sources of noise currents that flow through the car. The battery also absorbs voltage spikes and some noise from the alternator. If this is not sufficient, then adding additional capacitors across the alternator, or a "coaxial capacitor" in line with its leads, will absorb even higher frequency noise before it gets to the battery.

The other three answers all mention putting a capacitor or high-pass filter in series with the radio's power leads. Capacitors block DC from flowing and thus the radio would not power on. Note that coaxial capacitors are different; they are inline but they are not in series.

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How can noise from an electric motor be suppressed?
  • By installing a high pass filter in series with the motor's power leads
  • By installing a brute-force AC-line filter in series with the motor leads
  • By installing a bypass capacitor in series with the motor leads
  • By using a ground-fault current interrupter in the circuit used to power the motor

A Brute Force filter is a ham "slang" term. The term refers to any large, well shielded filter. The answer to this question is easy to remember just remember "Brute Force".

An example of the kind of filter this question is referring to would be: http://www.digikey.com/product-detail/en/20DRGG5/603-1147-ND/1718592

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What is a major cause of atmospheric static?
  • Solar radio frequency emissions
  • Thunderstorms
  • Geomagnetic storms
  • Meteor showers

Atmospheric static is radio noise caused by natural processes, such as lightning discharges in thunderstorms. At VHF and above they are at lower levels and thermal noise in the receiver is the limiting factor.

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How can you determine if line noise interference is being generated within your home?
  • By checking the power line voltage with a time domain reflectometer
  • By observing the AC power line waveform with an oscilloscope
  • By turning off the AC power line main circuit breaker and listening on a battery operated radio
  • By observing the AC power line voltage with a spectrum analyzer

Naturally, turning off the power means that anything connected to the mains power won't be running anymore, and thus won't be making any noise.

You don't need any additional instruments when the radio is already the one detecting the problem in the first place!

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What type of signal is picked up by electrical wiring near a radio antenna?
  • A common-mode signal at the frequency of the radio transmitter
  • An electrical-sparking signal
  • A differential-mode signal at the AC power line frequency
  • Harmonics of the AC power line frequency

Electrical mains wiring can act as an effective antenna, conducting transmitted signals into connected electrical appliances. These unintended signals are referred to as common-mode noise, which refers to the presence of unintended signals on all AC lines: hot, neutral and ground.

Electrical spark generated signals are caused by gaps in high voltage transmission lines, across which we can have arcing, or by certain types of motors. These are not mentioned in the stem so that answer is incorrect.

AC power line related factors may cause interference on the receiving antenna, but the question is asking about interference being received on the mains wiring, therefore those two choices can be eliminated.

The clue here is a signal is picked up in the electrical wiring, meaning we are discussing a transmission related event. Therefore the common-mode signal generated by radio transmission is the correct choice. If the radio antenna were merely receiving, then we would not expect adjacent wiring to have any induced current.

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

When the receiver's noise blanker is turned on and/or set to maximum, strong signals on nearby frequencies may be heard on your frequency as noise or chatter. The audible effect is similar to "splatter," which is sometimes heard when a station operates an improperly adjusted transmitter and generates a signal with excessive bandwidth.

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What is a common characteristic of interference caused by a touch controlled electrical device?
  • The interfering signal sounds like AC hum on an AM receiver or a carrier modulated by 60 Hz hum on a SSB or CW receiver
  • The interfering signal may drift slowly across the HF spectrum
  • The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band
  • All of these choices are correct

Many touch controlled devices contain a crude oscillator with an output rich in harmonics. They operate by sensing changes in oscillator frequency as a hand is brought close to them.

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Which is the most likely cause if you are hearing combinations of local AM broadcast signals within one or more of the MF or HF ham bands?
  • The broadcast station is transmitting an over-modulated signal
  • Nearby corroded metal joints are mixing and re-radiating the broadcast signals
  • You are receiving sky wave signals from a distant station
  • Your station receiver IF amplifier stage is defective

Strong AM signals such as those from local broadcast stations can be detected or mixed by any kind of non-linear junction, from a diode to even a rusty bolt, much like the oldschool cat's whisker detector.

Broadcast stations are regularly monitored and would notice any kind of abnormal transmission, so a mistake on their part is highly unlikely.

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What is one disadvantage of using some types of automatic DSP notch-filters when attempting to copy CW signals?
  • A DSP filter can remove the desired signal at the same time as it removes interfering signals
  • Any nearby signal passing through the DSP system will overwhelm the desired signal
  • Received CW signals will appear to be modulated at the DSP clock frequency
  • Ringing in the DSP filter will completely remove the spaces between the CW characters

Notch filters are very narrow filters designed to remove very narrow tones caused by spurious interference, inconsiderate amateur operators "tuning up", and other sources of narrow-band interference. An auto-notch filter looks automatically for any such tones and removes them; however it cannot easily distinguish between intentional CW modulation and undesired interference. As a result, desired CW signals will be mistaken for interference and be suppressed.

Auto-notch filters work best for wider signals like SSB phone that do not feature any strong, narrow tones.

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

To eliminate this would require some detective work. Regularity and the times that the problem occurs would provide clues. If it was frequent enough the old trick of switching off your main house breaker would tell you if it is on your property.

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

A computer is full of digital signals. From an RF point of view these are square waves rich in harmonics, hence the large numbers of specific frequencies.

Hint or method to remember the answer: Some hams refer to this noise as "birdies" on the band, which can describe the bird-like sound generated when you spin across the band and go past the frequency where this computer (man-made) noise is being generated.

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

Good currents flow from a source out to a load and back through the same cable to return to the source. This is called "differential-mode current" -- the current going out and coming back is in balance and contained safely within the shield and can neither radiate out, nor be affected by noise coming in.

Common-mode current flows on the outside of the shield. Since the shield is a single conductor, something else is needed complete a circuit -- usually through your safety earth connection or somewhere else undesirable. This unintended circuit occupies a big area which makes a nice loop antenna to radiate and absorb noise.

The other three options are all good practice to prevent ground loops and other noise problems, and prevent (not cause) radiated interference.

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What current flows equally on all conductors of an unshielded multi-conductor cable?
  • Differential-mode current
  • Common-mode current
  • Reactive current only
  • Return current

Common-mode current flows equally on all conductors of an unshielded multi-conductor cable.


In the question, "equally" in this case means "same direction" more than "same amount." Ideally the same current that flows to your antenna in one conductor returns from your antenna in the other. This is known as differential-mode current.

If some of the current that flows to your antenna in one conductor also flows to your antenna in the other, it is known as common-mode current.

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