Extra Exam-Amateur Practices e4

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Extra Exam-Amateur Practices e4
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Amateur Radio Extra Exam - E4 Question Set - Amateur Practices
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  1. E4A01 How does a spectrum analyzer differ from an oscilloscope?
    A. A spectrum analyzer measures ionospheric reflection; an oscilloscope displays electrical signals
    B. A spectrum analyzer displays the peak amplitude of signals; an oscilloscope displays the average amplitude of signals
    C. A spectrum analyzer displays signals in the frequency domain; an oscilloscope displays signals in the time domain
    D. A spectrum analyzer displays radio frequencies; an oscilloscope displays audio frequencies
    • (C)
    • Use a spectrum analyzer to view signals in the frequency domain (amplitude vs frequency) and an oscilloscope to view them in the time domain (amplitude vs time). For both instruments the vertical axis is signal amplitude. The difference is that the spectrum analyzer displays frequency along the horizontal axis and the oscilloscope displays time. The diagram shows how the complex signal at A can be viewed in the time domain by an oscilloscope or in the frequency domain by a spectrum analyzer. The oscilloscope would show the sum of the two signals as at B. The spectrum analyzer would show two separate components at different frequencies as at C. This type of display is usually more useful in analyzing complex signals.
  2. E4A02 Which of the following parameters would a spectrum analyzer display on the horizontal axis?
    A. SWR
    B. Q
    C. Time
    D. Frequency
    • (D)
    • Use a spectrum analyzer to view signals in the frequency domain (amplitude vs frequency) and an oscilloscope to view them in the time domain (amplitude vs time). For both instruments the vertical axis is signal amplitude. The difference is that the spectrum analyzer displays frequency along the horizontal axis and the oscilloscope displays time.
  3. E4A03 (A)Which of the following parameters would a spectrum analyzer display on the vertical axis?
    A. Amplitude
    B. Duration
    C. SWR
    D. Q
    • (A)
    • Use a spectrum analyzer to view signals in the frequency domain (amplitude vs frequency) and an oscilloscope to view them in the time domain (amplitude vs time). For both instruments the vertical axis is signal amplitude. The difference is that the spectrum analyzer displays frequency along the horizontal axis and the oscilloscope displays time.
  4. E4A04 Which of the following test instruments is used to display spurious signals from a radio transmitter?
    A. A spectrum analyzer
    B. A wattmeter
    C. A logic analyzer
    D. A time-domain reflectometer
    • (A)
    • A spectrum analyzer is the best instrument for displaying spurious signals because they can be measured by frequency as well as amplitude. The other instruments listed here do not display signal frequency and so are not suited for this purpose.
  5. E4A05 Which of the following test instruments is used to display intermodulation distortion products in an SSB transmission?
    A. A wattmeter
    B. A spectrum analyzer
    C. A logic analyzer
    D. A time-domain reflectometer
    • (B)
    • A spectrum analyzer is the best instrument for displaying intermodulation products because they can be measured by frequency as well as amplitude. The other instruments listed here do not display signal frequency and so are not suited for this purpose.
  6. E4A06 Which of the following could be determined with a spectrum analyzer?
    A. The degree of isolation between the input and output ports of a 2 meter duplexer
    B. Whether a crystal is operating on its fundamental or overtone frequency
    C. The spectral output of a transmitter
    D. All of these choices are correct
    • (D)
    • All of the choices are measurements of amplitude (attenuation between isolator ports is an amplitude measurement) versus frequency, so all can be measured with a spectrum analyzer.
  7. E4A07 Which of the following is an advantage of using an antenna analyzer compared to an SWR bridge to measure antenna SWR?
    A. Antenna analyzers automatically tune your antenna for resonance
    B. Antenna analyzers do not need an external RF source
    C. Antenna analyzers display a time-varying representation of the modulation envelope
    D. All of these choices are correct
    • (B)
    • An antenna analyzer has a variable frequency source built-in so keying a transmitter is not required. Antenna analyzers also provide information about the antenna’s feed point impedance that is unavailable from an SWR bridge.
  8. E4A08 Which of the following instruments would be best for measuring the SWR of a beam antenna?
    A. A spectrum analyzer
    B. A Q meter
    C. An ohmmeter
    D. An antenna analyzer
    • (D)
    • An antenna analyzer has a variable frequency source built-in so keying a transmitter is not required. Antenna analyzers also provide information about the antenna’s feed point impedance that is unavailable from an SWR bridge.
  9. E4A09 Which of the following describes a good method for measuring the intermodulation distortion of your own PSK signal?
    A. Transmit into a dummy load, receive the signal on a second receiver, and feed the audio into the sound card of a computer running an appropriate PSK program
    B. Multiply the ALC level on the transmitter during a normal transmission by the average power output
    C. Use an RF voltmeter coupled to the transmitter output using appropriate isolation to prevent damage to the meter
    D. All of these choices are correct
    • (A)
    • If the level of the audio PSK31 signal into the transmitter is too high, it will overdrive the modulation and power amplifier stages, producing splatter. Since internal monitoring circuits do not provide information about the spectrum of the transmitter’s output signal, it is necessary to use a separate receiver to monitor and adjust the input audio levels.
  10. E4A10 Which of the following tests establishes that a silicon NPN junction transistor is biased on?
    A. Measure base-to-emitter resistance with an ohmmeter; it should be approximately6 to 7 ohms
    B. Measure base-to-emitter resistance with an ohmmeter; it should be approximately 0.6 to 0.7 ohms
    C. Measure base-to-emitter voltage with a voltmeter; it should be approximately 6 to 7 volts
    D. Measure base-to-emitter voltage with a voltmeter; it should be approximately0.6 to 0.7 volts
    • (D)
    • When an NPN transistor is biased on, base-to-emitter voltage (VBE) should measure 0.6 to 0.75 V from base to emitter with the positive voltmeter lead connected to the base.
  11. E4A11 Which of these instruments could be used for detailed analysis of digital signals?
    A. Dip meter
    B. Oscilloscope
    C. Ohmmeter
    D. Q meter
    • (B)
    • The oscilloscope can display waveforms that are changing rapidly, such as those of a digital signal.
  12. E4A12 Which of the following procedures is an important precaution to follow when connecting a spectrum analyzer to a transmitter output?
    A. Use high quality double shielded coaxial cables to reduce signal losses
    B. Attenuate the transmitter output going to the spectrum analyzer
    C. Match the antenna to the load
    D. All of these choices are correct
    • (B)
    • Most spectrum analyzer inputs are at risk of damage from input signals stronger than 1 watt. An attenuator should be used to reduce the signal amplitude coming from a transmitter before it is applied to the spectrum analyzer input.
  13. E4B01 Which of the following factors most affects the accuracy of a frequency counter?
    A. Input attenuator accuracy
    B. Time base accuracy
    C. Decade divider accuracy
    D. Temperature coefficient of the logic
    • (B)
    • A frequency counter counts the number of pulses applied to its input during a period of time and displays the results. The time base determines the period for counting pulses, so the accuracy of the time base determines the accuracy of the counter and the stability of the time base determines the stability of the frequency counter. The block diagram of the basic parts of a frequency counter. The time base is based on the output of a highly stable and accurate crystal oscillator.
  14. E4B02 What is an advantage of using a bridge circuit to measure impedance?
    A. It provides an excellent match under all conditions
    B. It is relatively immune to drift in the signal generator source
    C. The measurement is based on obtaining a signal null, which can be done very precisely
    D. It can display results directly in Smith chart format
    • (C)
    • A bridge circuit's null is very sharp so that it is easy to adjust the bridge precisely to the deepest point of the null. That means the unknown impedance value is also measured precisely. The figure shows a Wheatstone bridge circuit. A bridge circuit is actually a pair of voltage dividers (A) with R1 = R2. One of the dividers is adjustable as in (B). When the unknown impedance is attached at RX and the bridge adjusted so that the voltages E1 and E2 are equal, the voltmeter V indicates a null or zero voltage. The value of RS is then equal to RX.
  15. E4B03 If a frequency counter with a specified accuracy of +/- 1.0 ppm reads 146,520,000 Hz, what is the most the actual frequency being measured could differ from the reading?
    A. 165.2 Hz
    B. 14.652 kHz
    C. 146.52 Hz
    D. 1.4652 MHz
    (C)This is a series of questions for which the answer is computed by a process based on the formula Error = Frequency x Accuracy

    • In this case, when you substitute the numbers, you get It makes the math a bit easier to realize that if you write the frequency in MHz then it will cancel with the accuracy in ppm.
  16. E4B04 If a frequency counter with a specified accuracy of +/- 0.1 ppm reads 146,520,000 Hz, what is the most the actual frequency being measured could differ from the reading?
    A. 14.652 Hz
    B. 0.1 MHz
    C. 1.4652 Hz
    D. 1.4652 kHz
    • (A)
    • This is a series of questions for which the answer is computed by a process based on the formula Error = Frequency x Accuracy In this case, when you substitute the numbers, you get Error (in Hz) = Frequency (in MHz) × Accuracy (in ppm) = 146.52 × ±0.1 = ±14.652 Hz It makes the math a bit easier to realize that if you write the frequency in MHz then it will cancel with the accuracy in ppm.
  17. E4B05 If a frequency counter with a specified accuracy of +/- 10 ppm reads 146,520,000 Hz, what is the most the actual frequency being measured could differ from the reading?
    A. 146.52 Hz
    B. 10 Hz
    C. 146.52 kHz
    D. 1465.20 Hz
    • (D)
    • This is a series of questions for which the answer is computed by a process based on the formula Error = Frequency x Accuracy In this case, when you substitute the numbers, you get Error = Frequency (in MHz) × Accuracy (in ppm) = 146.52 MHz × ±10 ppm = ±1465.20 Hz It makes the math a bit easier to realize that if you write the frequency in MHz then it will cancel with the accuracy in ppm.
  18. E4B06 How much power is being absorbed by the load when a directional power meter connected between a transmitter and a terminating load reads 100 watts forward power and 25 watts reflected power?
    A. 100 watts
    B. 125 watts
    C. 25 watts
    D. 75 watts
    • (D)
    • A directional wattmeter reads the total amount of power in a feed line traveling in each direction. The actual amount of power being absorbed by the load is the difference between the forward and reflected power readings, Forward power – Reflected power = 100 – 25 = 75 watts.
  19. E4B07 Which of the following is good practice when using an oscilloscope probe?
    A. Keep the signal ground connection of the probe as short as possible
    B. Never use a high impedance probe to measure a low impedance circuit
    C. Never use a DC-coupled probe to measure an AC circuit
    D. All of these choices are correct
    • (A)
    • High-frequency signals being measured with an oscilloscope probe must flow through the probe tip, to the ‘scope’s input circuit through a short coaxial cable, and back to the circuit through the ground connection lead. The probe tip connection and coaxial cable have very good high-frequency characteristics, but an excessively long ground connection introduces inductance into the signal path that can cause distortion or high-frequency rolloff of the signal.
  20. E4B08 Which of the following is a characteristic of a good DC voltmeter?
    A. High reluctance input
    B. Low reluctance input
    C. High impedance input
    D. Low impedance input
    • (C)
    • A good voltmeter, either ac or dc, measures voltage while drawing as little current as possible from the circuit being measured. This requires the meter to have a high impedance input circuit.
  21. E4B09 What is indicated if the current reading on an RF ammeter placed in series with the antenna feed line of a transmitter increases as the transmitter is tuned to resonance?
    A. There is possibly a short to ground in the feed line
    B. The transmitter is not properly neutralized
    C. There is an impedance mismatch between the antenna and feed line
    D. There is more power going into the antenna
    • (D)
    • Higher feed line current means that more power is flowing to the antenna.
  22. E4B10 Which of the following describes a method to measure intermodulation distortion in an SSB transmitter?
    A. Modulate the transmitter with two non-harmonically related radio frequencies and observe the RF output with a spectrum analyzer
    B. Modulate the transmitter with two non-harmonically related audio frequencies and observe the RF output with a spectrum analyzer
    C. Modulate the transmitter with two harmonically related audio frequencies and observe the RF
    output with a peak reading wattmeter
    D. Modulate the transmitter with two harmonically related audio frequencies and observe the RF output with a logic analyzer
    • (B)
    • A spectrum analyzer is the best instrument to display and measure spurious emissions because it displays the signal frequency as well as amplitude. The test tones into the transmitter should not be harmonically related (integer multiples of the same frequency) so that any spurious signals do not occur at the same frequency. Spurious outputs from non-harmonically related signals will appear as independent signal components.
  23. E4B11 How should a portable antenna analyzer be connected when measuring antenna resonance and feed point impedance?
    A. Loosely couple the analyzer near the antenna base
    B. Connect the analyzer via a high-impedance transformer to the antenna
    C. Connect the antenna and a dummy load to the analyzer
    D. Connect the antenna feed line directly to the analyzer's connector
    • (D)
    • The analyzer is actually a variable-frequency low-power transmitter with a built-in impedance bridge. So SWR is measured just as with standalone transmitters and SWR meters — the feed line is connected directly to the analyzer’s output.
  24. E4B12 What is the significance of voltmeter sensitivity expressed in ohms per volt?
    A. The full scale reading of the voltmeter multiplied by its ohms per volt rating will provide the input impedance of the voltmeter
    B. When used as a galvanometer, the reading in volts multiplied by the ohms/volt will determine the power drawn by the device under test
    C. When used as an ohmmeter, the reading in ohms divided by the ohms/volt will determine the voltage applied to the circuit
    D. When used as an ammeter, the full scale reading in amps divided by ohms/volt will determine the size of shunt needed
    • (A)
    • Ohms/volt is a measure of how sensitive the meter is because it is the reciprocal of the current (volts divided by ohms) required for a full-scale reading. Higher values of ohms/volt mean lower input currents for an equivalent voltage measurement and less loading of the circuit being tested.
  25. E4B13 How is the compensation of an oscilloscope probe typically adjusted?
    A. A square wave is displayed and the probe is adjusted until the horizontal portions of the displayed wave are as nearly flat as possible
    B. A high frequency sine wave is displayed and the probe is adjusted for maximum amplitude
    C. A frequency standard is displayed and the probe is adjusted until the deflection time is accurate
    D. A DC voltage standard is displayed and the probe is adjusted until the displayed voltage is accurate
    • (A)
    • Most oscilloscopes include a circuit that outputs a square wave calibrator signal of known voltage and frequency. The probe can be connected to the calibrator output and adjusted until the flat portions of the square wave are parallel to the zero-voltage horizontal axis, not tilted or rounded in any way. This means the probe’s frequency response has been adjusted properly and input signals will not be distorted.
  26. E4B14 What happens if a dip meter is too tightly coupled to a tuned circuit being checked?
    A. Harmonics are generated
    B. A less accurate reading results
    C. Cross modulation occurs
    D. Intermodulation distortion occurs
    • (B)
    • When a dip meter it is too tightly coupled with the tuned circuit being checked, a less accurate reading results. Whenever two circuits are coupled, no matter how loosely, each circuit affects the other to some extent. Coupling that is too tight will almost certainly result in an inaccurate reading on the dip meter.
  27. E4B15 Which of the following can be used as a relative measurement of the Q for a series-tuned circuit?
    A. The inductance to capacitance ratio
    B. The frequency shift
    C. The bandwidth of the circuit's frequency response
    D. The resonant frequency of the circuit
    • (C)
    • The Q of a resonant circuit equals resonant frequency (f0) divided by bandwidth (BW). By measuring the circuit’s frequency response around its resonant frequency, both f0 and BW can be determined and Q calculated.
  28. E4C01 What is an effect of excessive phase noise in the local oscillator section of a receiver?
    A. It limits the receiver’s ability to receive strong signals
    B. It reduces receiver sensitivity
    C. It decreases receiver third-order intermodulation distortion dynamic range
    D. It can cause strong signals on nearby frequencies to interfere with reception of weak signals
    • (D)
    • One result of receiver phase noise is that as you tune closer to a strong signal, the receiver noise floor appears to increase. In other words, you hear an increasing amount of noise in an otherwise quiet receiver as you tune toward the strong signal. This reciprocal mixing means that strong signals may interfere with the reception of a nearby weak signal.
  29. E4C02 Which of the following portions of a receiver can be effective in eliminating image signal interference?
    A. A front-end filter or pre-selector
    B. A narrow IF filter
    C. A notch filter
    D. A properly adjusted product detector
    • (A)
    • Front-end filters and pre-selectors attenuate out-of-band signals on image frequencies so that images in the receiver are greatly reduced or eliminated. An IF or notch filter cannot prevent images because the images are produced before the IF stages. A product detector operates on whatever signal the IF stages produce, whether a real or image signal.
  30. E4C03 What is the term for the blocking of one FM phone signal by another, stronger FM phone signal?
    A. Desensitization
    B. Cross-modulation interference
    C. Capture effect
    D. Frequency discrimination
    • (C)
    • The capture effect in FM receivers results in the loudest signal received being the only signal demodulated, even if it is only two or three times (3 to 5 dB) stronger than other signals on the same frequency. This can be an advantage if you want to suppress interference to the stronger signal. However, the capture effect can prevent you from hearing a weaker signal in the presence of a stronger one.
  31. E4C04 What is the definition of the noise figure of a receiver?
    A. The ratio of atmospheric noise to phase noise
    B. The noise bandwidth in Hertz compared to the theoretical bandwidth of a resistive network
    C. The ratio of thermal noise to atmospheric noise
    D. The ratio in dB of the noise generated by the receiver compared to the theoretical minimum noise
    • (D)
    • The noise figure of a receiver measures the noise contributed by the receiver circuits. The lower a receiver’s noise figure, the lower its minimum detectible signal (MDS). Noise figure is measured in dB.
  32. E4C05 What does a value of -174 dBm/Hz represent with regard to the noise floor of a receiver?
    A. The minimum detectable signal as a function of receive frequency
    B. The theoretical noise at the input of a perfect receiver at room temperature
    C. The noise figure of a 1 Hz bandwidth receiver
    D. The galactic noise contribution to minimum detectable signal
    • (B)
    • Noise in a receiver is primarily caused by temperature-related movement of electrons in the receiver’s input circuits. The amount of noise power also depends on the receiver’s bandwidth, with wider bandwidths allowing more noise power to be received. The theoretical noise floor limit is a level of –174 dBm in a bandwidth of 1 Hz.
  33. E4C06 A CW receiver with the AGC off has an equivalent input noise power density of -174 dBm/Hz.  What would be the level of an unmodulated carrier input to this receiver that would yield an audio output SNR of 0 dB in a 400 Hz noise bandwidth?
    A. 174 dBm
    B. -164 dBm
    C. -155 dBm
    D. -148 dBm
    • (D)Since noise power is directly proportional to bandwidth, the ratio of filter bandwidths also determines the amount of additional noise power received according to the following formula:
    • Using the numbers in the question, MDS = –174 dBm + 10 log (400) = –174 dBm + 26 dB = –148 dBm
  34. E4C07 What does the MDS of a receiver represent?
    A. The meter display sensitivity
    B. The minimum discernible signal
    C. The multiplex distortion stability
    D. The maximum detectable spectrum
    • (B)
    • The MDS or minimum discernable signal (also minimum detectable signal) is the signal level that is equal to the receiver noise floor in a specified bandwidth.
  35. E4C08 How might lowering the noise figure affect receiver performance?
    A. It would reduce the signal to noise ratio
    B. It would improve weak signal sensitivity
    C. It would reduce bandwidth
    D. It would increase bandwidth
    • (B)
    • Noise figure is a measure of how much noise is generated in the receiver. If noise figure is reduced, the receiver’s noise contribution is reduced. That improves (increases) the signal-to-noise ratio.
  36. E4C09 Which of the following choices is a good reason for selecting a high frequency for the design of the IF in a conventional HF or VHF communications receiver?
    A. Fewer components in the receiver
    B. Reduced drift
    C. Easier for front-end circuitry to eliminate image responses
    D. Improved receiver noise figure
    • (C)
    • Since image frequencies are separated from the intended receive frequency by the IF, a higher IF results in the image frequencies being farther from the intended receive frequency. That makes it easier to filter out signals at the image frequencies without affecting the desired signals.
  37. E4C10 Which of the following is a desirable amount of selectivity for an amateur RTTY HF receiver?
    A. 100 Hz
    B. 300 Hz
    C. 6000 Hz
    D. 2400 Hz
    • (B)
    • The filter bandwidth should be wide enough to pass the mark and space frequencies along with their respective sidebands. These are spaced less than 200 Hz apart in the usual RTTY signal. You’ll want a little bit extra bandwidth to allow for tuning error. Don’t make the filter too wide or it will pass additional noise and interference.
  38. E4C11 Which of the following is a desirable amount of selectivity for an amateur SSB phone receiver?
    A. 1 kHz
    B. 2.4 kHz
    C. 4.2 kHz
    D. 4.8 kHz
    • (B)
    • Intelligibility of a voice signal is mostly contained in the range of 300 Hz to 2700 Hz. While wider filters may increase fidelity to some degree depending on the transmitted signal, they will pass more noise and interference. Under crowded band conditions, filters with a bandwidth as low as 1.5 kHz can be useful but at the cost of greatly reduced fidelity.
  39. E4C12 What is an undesirable effect of using too wide a filter bandwidth in the IF section of a receiver?
    A. Output-offset overshoot
    B. Filter ringing
    C. Thermal-noise distortion
    D. Undesired signals may be heard
    • (D)
    • Some operators like to use wide filters when tuning or monitoring a band that is quiet in terms of noise and in terms of the number of transmitting stations. Few want wider filters when the band is active and many stations are transmitting. A filter that is wider than necessary allows extra signals (and noise) to pass through the IF to the detector and on to the audio output.
  40. E4C13 How does a narrow-band roofing filter affect receiver performance?
    A. It improves sensitivity by reducing front end noise
    B. It improves intelligibility by using low Q circuitry to reduce ringing
    C. It improves dynamic range by attenuating strong signals near the receive frequency
    D. All of these choices are correct
    • (C)
    • A roofing filter is applied to the signal path before the final filters that have bandwidths closer to that of the desired signal. The function of a roofing filter is to remove strong nearby signals that may overload the receiver circuits before being rejected by the narrower single-signal filters.
  41. E4C14 On which of the following frequencies might a signal be transmitting which is generating a spurious image signal in a receiver tuned to 14.300 MHz and which uses a 455 kHz IF frequency?
    A. 13.845 MHz
    B. 14.755 MHz
    C. 14.445 MHz
    D. 15.210 MHz
    • (D)
    • Image frequencies are located twice the IF from the desired signal. In this case, the images are located at:

    LO = 14.300 MHz ± 2 x 455 kHz = 15.210 MHz and 13.390 MHz
  42. E4C15 What is the primary source of noise that can be heard from an HF receiver with an antenna connected?
    A. Detector noise
    B. Induction motor noise
    C. Receiver front-end noise
    D. Atmospheric noise
    • (D)
    • Below 20 MHz, the largest contributor of noise is external to the receiver is atmospheric noise from lightning and other forms of electrical activity.
  43. E4D01 What is meant by the blocking dynamic range of a receiver?
    A. The difference in dB between the noise floor and the level of an incoming signal which will cause 1 dB of gain compression
    B. The minimum difference in dB between the levels of two FM signals which will cause one signal to block the other
    C. The difference in dB between the noise floor and the third order intercept point
    D. The minimum difference in dB between two signals which produce third order intermodulation products greater than the noise floor
    • (A)
    • Blocking dynamic range (BDR) refers to the ability of a receiver to respond linearly to strong signals. The definition of BDR is the difference between MDS and the input signal level at which receiver gain drops (gain compression) by 1 dB. This graph shows how the output power of the receiver for the desired signal and the output power for the second and third-order distortion products vary with changes of the input signal power. The input consists of two equal-power sine-wave signals. Higher intercept points represent better receiver IMD performance.
  44. E4D02 Which of the following describes two problems caused by poor dynamic range in a communications receiver?
    A. Cross-modulation of the desired signal and desensitization from strong adjacent signals
    B. Oscillator instability requiring frequent retuning and loss of ability to recover the opposite sideband
    C. Cross-modulation of the desired signal and insufficient audio power to operate the speaker
    D. Oscillator instability and severe audio distortion of all but the strongest received signals
    • (A)
    • A receiver with poor IMD dynamic range will exhibit cross modulation of the desired signal by strong adjacent signals. One with poor blocking dynamic range will suffer from desensitization.
  45. E4D03 How can intermodulation interference between two repeaters occur?
    A. When the repeaters are in close proximity and the signals cause feedback in the final amplifier of one or both transmitters
    B. When the repeaters are in close proximity and the signals mix in the final amplifier of one or both transmitters
    C. When the signals from the transmitters are reflected out of phase from airplanes passing overhead
    D. When the signals from the transmitters are reflected in phase from airplanes passing overhead
    • (B)
    • Intermodulation can be a problem in transmitters as well as receivers. This can happen when two transmitters are in close proximity and the signals mix in one or both of their final amplifiers or in a non-linear device or junction near the transmitters. This can result in severe interference.
  46. E4D04 Which of the following may reduce or eliminate intermodulation interference in a repeater caused by another transmitter operating in close proximity?
    A. A band-pass filter in the feed line between the transmitter and receiver
    B. A properly terminated circulator at the output of the transmitter
    C. A Class C final amplifier
    D. A Class D final amplifier
    • (B)
    • Circulators and isolators are usually highly effective in eliminating intermodulation between two transmitters. They work like one-way valves, allowing energy to flow from the transmitter to the antenna while greatly reducing energy flow in the opposite direction. You might think that installing some type of filter would cure the problem but that’s not true in this case because the offending transmitter will have a very strong signal in the filter’s passband.
  47. E4D05 What transmitter frequencies would cause an intermodulation-product signal in a receiver tuned to 146.70 MHz when a nearby station transmits on 146.52 MHz? 
    A. 146.34 MHz and 146.61 MHz
    B. 146.88 MHz and 146.34 MHz
    C. 146.10 MHz and 147.30 MHz
    D. 173.35 MHz and 139.40 MHz
    • (A)
  48. E4D06 What is the term for unwanted signals generated by the mixing of two or more signals?
    A. Amplifier desensitization
    B. Neutralization
    C. Adjacent channel interference
    D. Intermodulation interference
    • (D)
    • Intermodulation interference occurs when the signals of two transmitters mix together in one or both of their final amplifiers and unwanted signals at the sum and difference frequencies of the original signals are generated.
  49. E4D07 (D)Which of the following describes the most significant effect of an off-frequency signal when it is causing cross-modulation interference to a desired signal?
    A. A large increase in background noise
    B. A reduction in apparent signal strength
    C. The desired signal can no longer be heard
    D. The off-frequency unwanted signal is heard in addition to the desired signal
    • (D)
    • The term cross-modulation is used when modulation from an unwanted signal is heard in addition to the desired signal.
  50. E4D08 What causes intermodulation in an electronic circuit?
    A. Too little gain
    B. Lack of neutralization
    C. Nonlinear circuits or devices
    D. Positive feedback
    • (C)
    • In a linear circuit, the output is a faithful representation of the input. Nonlinearities in either circuits or devices cause distortion. This nonlinearity is the cause of intermodulation in an electronic circuit.
  51. E4D09 What is the purpose of the preselector in a communications receiver?
    A. To store often-used frequencies
    B. To provide a range of AGC time constants
    C. To increase rejection of unwanted signals
    D. To allow selection of the optimum RF amplifier device
    • (C)
    • Communications receivers are often operated in an environment where very strong out-of-band signals are present, such as from commercial or military stations or shortwave broadcast stations. These signals can be so strong that they overload the receiver input circuits, causing desensitization and numerous spurious signals. A preselector is a relatively broad filter that attenuates these signals, helping the receiver filter them out without being overloaded.
  52. E4D10 What does a third-order intercept level of 40 dBm mean with respect to receiver performance?
    A. Signals less than 40 dBm will not generate audible third-order intermodulation products
    B. The receiver can tolerate signals up to 40 dB above the noise floor without producing third-order intermodulation products
    C. A pair of 40 dBm signals will theoretically generate a third-order intermodulation product with the same level as the input signals
    D. A pair of 1 mW input signals will produce a third-order intermodulation product which is 40 dB stronger than the input signal
    • (C)
    • The third-order intercept point of a receiver is that input level where third-order IMD products equal the desired (first-order) output level. The desired output increases 1 dB for a 1-dB increase of the input. The second-order IMD increases 2 dB, and third order goes up 3 dB for each 1-dB increase in input level. A third-order intercept point of 40 dBm means that it would take a pair of signals with levels of 40 dBm (10 watts!) to create intermodulation products with the same strength as the receiver output on the desired frequency. Real-world receivers will experience blocking or gain compression before the input rises to that level and for that reason you have to compute the intercept point. It can’t be measured directly.
  53. E4D11 Why are third-order intermodulation products created within a receiver of particular interest compared to other products?
    A. The third-order product of two signals which are in the band of interest is also likely to be within the band
    B. The third-order intercept is much higher than other orders
    C. Third-order products are an indication of poor image rejection
    D. Third-order intermodulation produces three products for every input signal within the band of interest
    • (A)
    • The frequencies of the strongest IMD components come from the equations
    • These frequencies are close to the frequencies of the signals generating the IMD and are likely to be in the same band as the desired signal. So third-order IMD products are the most likely to cause interference.
  54. E4D12 What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
    A. Desensitization
    B. Quieting
    C. Cross-modulation interference
    D. Squelch gain rollback
    • (A)
    • Desensitization or “desense” is caused by a reduction in gain or gain compression that causes the receiver output to drop as if it was less sensitive.
  55. E4D13 Which of the following can cause receiver desensitization?
    A. Audio gain adjusted too low
    B. Strong adjacent-channel signals
    C. Audio bias adjusted too high
    D. Squelch gain misadjusted
    • (B)
    • Desensitization from a strong signal near the desired signal causes the receiver circuits to be overloaded such that they can no longer amplify the desired signal by the correct amount.
  56. E4D14 Which of the following is a way to reduce the likelihood of receiver desensitization?
    A. Decrease the RF bandwidth of the receiver
    B. Raise the receiver IF frequency
    C. Increase the receiver front end gain
    D. Switch from fast AGC to slow AGC
    • (A)
    • Removing or attenuating the strong nearby signals is the best way of eliminating receiver desensitization. This is the function of roofing filters that remove strong signals near the desired signal before a single-signal filter determines the receiver’s final selectivity.
  57. E4E01 Which of the following types of receiver noise can often be reduced by use of a receiver noise blanker?
    A. Ignition noise
    B. Broadband white noise
    C. Heterodyne interference
    D. All of these choices are correct
    • (A)
    • Noise blankers work best on impulse noise created by sharp, short pulses. Of the noise types listed, ignition noise from the firing of a vehicle’s spark plugs is the best candidate for removal by a noise blanker.
  58. E4E02 Which of the following types of receiver noise can often be reduced with a DSP noise filter?
    A. Broadband white noise
    B. Ignition noise
    C. Power line noise
    D. All of these choices are correct
    • (D)
    • DSP noise filters can handle a wider range of noise characteristics than an analog filter. All three noise types listed here are can be removed by DSP filters.
  59. E4E03 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
    • (B)
    • Noise blankers work by detecting signals that simultaneously appear across a wide bandwidth, a characteristic of impulse noise pulses.
  60. E4E04 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 by installing 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
    • (D)
    • Conducted and radiated noise caused by an automobile alternator can be suppressed by connecting the radio’s power leads directly to the battery which supplies the “cleanest” power and by installing coaxial capacitors in the alternator leads to filter out noise from that source.
  61. E4E05 How can noise from an electric 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
    • (B)
    • A brute-force, high-pass ac line filter in series with the motor leads can suppress noise from an ac electric motor.
  62. E4E06 What is a major cause of atmospheric static?
    A. Solar radio frequency emissions
    B. Thunderstorms
    C. Geomagnetic storms
    D. Meteor showers
    • (B)
    • Lightning generated in thunderstorms is a major cause of atmospheric static which can propagate over long distances. The other sources presented here do not produce large amounts of atmospheric static.
  63. E4E07 How can you determine if line noise interference is being generated within your home?
    A. By checking the power line voltage with a time domain reflectometer
    B. By observing the AC power line waveform with an oscilloscope
    C. By turning off the AC power line main circuit breaker and listening on a battery operated radio
    D. By observing the AC power line voltage with a spectrum analyzer
    • (C)
    • If the line noise disappears when the ac power to the home is removed, then the noise source is supplied by the circuits in the home.
  64. E4E08 What type of signal is picked up by electrical wiring near a radio antenna?
    A. A common-mode signal at the frequency of the radio transmitter
    B. An electrical-sparking signal
    C. A differential-mode signal at the AC power line frequency
    D. Harmonics of the AC power line frequency
    • (A)
    • “Common-mode” means that the electrical wiring is working like an antenna and the signal is being picked up by all of the conductors “in common.”
  65. E4E09 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
    • (C)
    • Because noise blankers work by detecting strong signals across a wide bandwidth, strong local signals can cause the receiver blanking circuit to activate by mistake. This causes the strong signal to sound distorted in the receiver and also causes distortion of nearby signals, making them appear to have an excessively wide bandwidth. If a strong signal seems to have excessive bandwidth, turn off the receiver noise blanker to see if that might be the problem.
  66. E4E10 What is a common characteristic of interference caused by a touch controlled electrical device?
    A. 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
    B. The interfering signal may drift slowly across the HF spectrum
    C. The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band
    D. All of these choices are correct
    • (D)
    • Touch-controlled devices operate by detecting a shift in an oscillator’s frequency when the device is touched. With poor ac line filtering (or none at all), these oscillators often generate interference at harmonics of their operating frequency that may be modulated by the ac power 60 Hz frequency. The oscillators are also rather unstable and so the interference often drifts in frequency.
  67. E4E11 Which of the following 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?
    A. The broadcast station is transmitting an over-modulated signal
    B. Nearby corroded metal joints are mixing and re-radiating the broadcast signals
    C. You are receiving sky wave signals from a distant station
    D. Your station receiver IF amplifier stage is defective
    • (B)
    • Local AM broadcast stations generate very strong signals that can cause current to flow in any metal conductor, such as fences, gutters, metal roofs and siding, pipes, and so forth. Because metal joints exposed to the weather are often corroded and nonlinear, they act as mixers, creating mixing products at various combinations of the broadcast station frequencies.
  68. E4E12 What is one disadvantage of using some types of automatic DSP notch-filters when attempting to copy CW signals?
    A. The DSP filter can remove the desired signal at the same time as it removes interfering signals
    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
    • (A)
    • A CW signal can appear much like an interfering carrier to a DSP notch filter. The result is that the filter will automatically remove the CW signal along with the properly-removed tone. For that reason, using the auto-notch filters doesn’t usually work well on CW.
  69. E4E13 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
    • (D)
    • Power line noise, especially if the source is nearby, can be very loud and contain many harmonics of the 60 Hz line frequency. The resulting signal has the roaring, buzzing quality mentioned in the question. Along with sources such as defective power line insulators, line noise can also be generated by any kind of electrical contacts, switches, or lighting equipment. The arcing associated with this type of noise can be a fire hazard in a business or home.
  70. E4E14 What is one type of electrical interference that might be caused by the operation of 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
    • (C)
    • Interference from a computer (or any microprocessor-controlled device) usually consists of signals created by the sharp rise and fall of the myriad digital signals in the computer. These signals are unmodulated, so they are heard as tones. The signal frequencies change as the digital signal changes and as the computer’s internal clock frequencies change with temperature.

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