Automatic notch filters work by identifying carriers near the receive frequency and applying a notch filter to cancel them. They can sometimes misidentify a desired CW signal as being undesirable and notch it out as well.

Memory Aid: CW and interfering carriers are mentioned in both the question and the correct answer.

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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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The name "noise blanker" makes sense if you understand what it does. They are used for removing regular (periodic) spikes (impulses) of noise from things like a car's spark plugs. A spark from a spark plug has many properties:

1. It's very broadband in RF, so it's hard to eliminate with a frequency-based filter.
2. it's very short in time. This is what "impulse" means; all of the energy is packed into a very short time. Mathematically, impulse is often described as zero time.
3. It's very regular or periodic.

A noise blanker watches for periodic spikes of energy (impulses), estimates their frequency, and then simply turns off your audio (blanks it) when those noise spikes happen.

Turning off the audio regularly won't help with white noise or hum and buzz.

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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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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.

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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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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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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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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.

Silly Hint: AM (music) - Look for answer with Metal (band).... aka Metallica...

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Switched-mode power supplies work by rectifying mains power into a very high voltage DC power source, which is then turned on and off very quickly by a transistor (usually a FET). With a capacitor on the output, the voltage is then dependent on the duty cycle of that FET (how long it is on versus the entire cycle time). This is how computer power supplies produce 3.3, 5, and 12 volts, because the current output voltage can be measured and the duty cycle can be adjusted accordingly.

Interference is generated because the frequency of that transistor turning on/off is often hundreds of kilohertz. That produces a square wave, which radiates a large amount of EMI as a result of the large current the transistor switches.

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Radars have a vastly different signal profile. Traditional radar such as those for aircraft use much higher frequencies, and even military over-the-horizon radar is usually confined to a narrow frequency bandwidth.

Wireless security cameras usually use signals in ISM bands, such as 2.4GHz, similar to how Wi-Fi or other communications protocols work.

Electric fences work by using very high voltages with very low currents to give living beings that are near/touching the fence a shock. They only trigger every few seconds, and usually don't radiate significant amounts of energy. Even so, the EMI they produce is very wideband - think of it as a spark gap, because that's essentially what it is. Thus they do not fit the characteristic described in the question.

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There could be several connected grounding rods outside so the best location for the surge protector is at the panel where they terminate before coming into the shack.

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