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Subel. E4
14 questions
Subelement E4

AMATEUR PRACTICES

Section E4E

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

What problem can occur when using an automatic notch filter (ANF) to remove interfering carriers while receiving CW signals?

  • Correct Answer
    Removal of the CW signal as well as the interfering carrier
  • Any nearby signal passing through the DSP system will overwhelm the desired signal
  • Excessive ringing
  • All these choices are correct

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.

Last edited by scoopgracie. Register to edit

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Which of the following types of noise can often be reduced by a digital noise reduction?

  • Broadband white noise
  • Ignition noise
  • Power line noise
  • Correct Answer
    All 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. 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.

Last edited by marvsherman419. Register to edit

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Which of the following types of noise are removed by a noise blanker?

  • Broadband white noise
  • Correct Answer
    Impulse noise
  • Hum and buzz
  • All these choices are correct
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How can conducted noise from an automobile battery charging system be suppressed?

  • By installing filter capacitors in series with the alternator leads
  • By installing a noise suppression resistor and a blocking capacitor at the battery
  • By installing a high-pass filter in series with the radio’s power lead and a low-pass filter in parallel with the antenna feed line
  • Correct Answer
    By installing ferrite chokes on the charging system leads
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What is used to suppress radio frequency interference from a line-driven AC motor?

  • A high-pass filter in series with the motor’s power leads
  • Correct Answer
    A brute-force AC-line filter in series with the motor’s power leads
  • A bypass capacitor in series with the motor’s field winding
  • A bypass choke in parallel with the motor’s field winding

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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What type of electrical interference can be caused by computer network equipment?

  • A loud AC hum in the audio output of your station’s receiver
  • A clicking noise at intervals of a few seconds
  • Correct Answer
    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
  • Correct Answer
    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

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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What current flows equally on all conductors of an unshielded multiconductor cable?

  • Differential-mode current
  • Correct Answer
    Common-mode current
  • Reactive current only
  • Magnetically-coupled current only

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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What undesirable effect can occur when using a noise blanker?

  • Received audio in the speech range might have an echo effect
  • The audio frequency bandwidth of the received signal might be compressed
  • Correct Answer
    Strong signals may be distorted and appear to cause spurious emissions
  • 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.

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

Last edited by dwage@dwage.com. Register to edit

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Which of the following can create intermittent loud roaring or buzzing AC line interference?

  • Arcing contacts in a thermostatically controlled device
  • A defective doorbell or doorbell transformer inside a nearby residence
  • A malfunctioning illuminated advertising display
  • Correct Answer
    All 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.

Last edited by marvsherman419. Register to edit

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What could be the cause of local AM broadcast band signals combining to generate spurious signals on the MF or HF bands?

  • One or more of the broadcast stations is transmitting an over-modulated signal
  • Correct Answer
    Nearby corroded metal connections are mixing and reradiating the broadcast signals
  • You are receiving skywave signals from a distant station
  • Your station receiver IF amplifier stage is overloaded

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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What causes interference received as a series of carriers at regular intervals across a wide frequency range?

  • Correct Answer
    Switch-mode power supplies
  • Radar transmitters
  • Wireless security camera transmitters
  • Electric fences
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Where should a station AC surge protector be installed?

  • At the AC service panel
  • At an AC outlet
  • Correct Answer
    On the single point ground panel
  • On a ground rod outside the station
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What is the purpose of a single point ground panel?

  • Remove AC power in case of a short-circuit
  • Prevent common-mode transients in multi-wire systems
  • Eliminate air gaps between protected and non-protected circuits
  • Correct Answer
    Ensure all lightning protectors activate at the same time
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