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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DSP noise reduction operates by examining a characteristic of signals and noise called correlation and dynamically filtering out the undesired noise. Correlation is a measure of the “regularity” of a signal. In most DSP noise blanker implementations, the amount of noise reduction varies according to the correlation characteristics of the noise. 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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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.

Other wide bandwidth signals include automobile spark plugs, electric motors, generators, or alternators with armatures, electric switches, or anything else that produce sparks.

Memory tip: read "blanker," think blanket. Blankets are wide.

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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.

Memory Tip: the correct answer is the only one with the word "alternator" in it.

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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

Mind Tickler: Only one answer has the term AC in it.

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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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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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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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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.

Cheat: IF. Only the correct answer has the word "if" in it.

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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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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 old-school 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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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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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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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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TIP: To radiate, there has to be current conducting somewhere. All answers have shield, but only the correct mentions currents and conductors.

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 to 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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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. -- qubit

Common-mode current is often caused by strong signals being picked up by antenna, television, telephone, or power wiring. These signals can be significantly reduced using a common mode choke by winding several turns of the wire around a ferrite toroid core with the ferrite type being selected to match the frequency of the common-mode current.

Hint: Common is only in the correct answer.

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