General Exam-Signals and Emissions

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rledwith
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General Exam-Signals and Emissions
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2013-07-02 23:17:57
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Amateur Radio General Exam - G8 Question Set - Signals and Emissions
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  1. G8A01 What is the name of the process that changes the envelope of an RF wave to carry information?
    A. Phase modulation
    B. Frequency modulation
    C. Spread spectrum modulation
    D. Amplitude modulation
    • (D)
    • There are several ways to change or modulate an RF wave for the purpose of conveying information. CW simply turns the signal on and off to convey information. Amplitude modulation changes the strength or amplitude of the wave while frequency modulation changes its frequency. Phase modulation conveys information by changing the phase of the wave.
  2. G8A02 What is the name of the process that changes the phase angle of an RF wave to convey information?
    A. Phase convolution
    B. Phase modulation
    C. Angle convolution
    D. Radian inversion
    • (B)
    • There are several ways to change or modulate an RF wave for the purpose of conveying information. CW simply turns the signal on and off to convey information. Amplitude modulation changes the strength or amplitude of the wave while frequency modulation changes its frequency. Phase modulation conveys information by changing the phase of the wave.
  3. G8A03 What is the name of the process which changes the frequency of an RF wave to convey information?
    A. Frequency convolution
    B. Frequency transformation
    C. Frequency conversion
    D. Frequency modulation
    • (D)
    • There are several ways to change or modulate an RF wave for the purpose of conveying information. CW simply turns the signal on and off to convey information. Amplitude modulation changes the strength or amplitude of the wave while frequency modulation changes its frequency. Phase modulation conveys information by changing the phase of the wave.
  4. G8A04 What emission is produced by a reactance modulator connected to an RF power amplifier?
    A. Multiplex modulation
    B. Phase modulation
    C. Amplitude modulation
    D. Pulse modulation
    • (B)
    • Reactance modulators are the most common method of generating (phase modulation) PM signals. Phase modulators cause the output signal phase to vary with both the modulating signal’s amplitude and frequency.
  5. G8A05 What type of modulation varies the instantaneous power level of the RF signal?
    A. Frequency shift keying
    B. Pulse position modulation
    C. Frequency modulation
    D. Amplitude modulation
    • (D)
    • In an AM transmission, at any given instant the amplitude or envelope of the RF signal changes according to the modulating audio signal.
  6. G8A06 What is one advantage of carrier suppression in a single-sideband phone transmission?
    A. Audio fidelity is improved
    B. Greater modulation percentage is obtainable with lower distortion
    C. The available transmitter power can be used more effectively
    D. Simpler receiving equipment can be used
    (C)

    In a single-sideband, suppressed-carrier, amplitude-modulated transmitter, the RF carrier signal is reduced or suppressed. The less energy that is used by the carrier the more power that can be put into the sidebands. This increases the efficiency of the system.
  7. G8A07 Which of the following phone emissions uses the narrowest frequency bandwidth?
    A. Single sideband
    B. Double sideband
    C. Phase modulation
    D. Frequency modulation
    • (A)
    • In a single-sideband (SSB) amplitude-modulated transmitter, the RF carrier signal is reduced or suppressed, and one of the sidebands is removed. Suppression of the carrier allows more power to be put into the sideband. Because the carrier is suppressed and one sideband is filtered out, only enough frequency bandwidth is required to transmit a single sideband. This has the narrowest bandwidth of all the popular phone emissions. The bandwidth of an SSB signal is between about 2 and 3 kHz, the bandwidth of a double-sideband AM signal is about 6 kHz and the bandwidth of frequency and phase modulated phone signals is about 16 kHz.
  8. G8A08 Which of the following is an effect of over-modulation?
    A. Insufficient audio
    B. Insufficient bandwidth
    C. Frequency drift
    D. Excessive bandwidth
    • (D)
    • When an SSB signal is overmodulated the output waveform of the signal is distorted, which causes spurious emissions outside the normal bandwidth of the signal. When an FM or PM signal is overmodulated, the deviation of the signal becomes too high and again, spurious emissions appear outside the normal bandwidth of the signal. In both cases, the spurious emissions cause interference to other stations.
  9. G8A09 What control is typically adjusted for proper ALC setting on an amateur single sideband transceiver?
    A. The RF clipping level
    B. Transmit audio or microphone gain
    C. Antenna inductance or capacitance
    D. Attenuator level
    • (B)
    • To reduce overmodulation, the Automatic Level Control (ALC) circuit reduces microphone gain when it detects excessive audio levels. For proper adjustment on most transmitters, the microphone gain control should be adjusted so that there is a slight movement of the ALC meter on modulation peaks.
  10. G8A10 What is meant by flat-topping of a single-sideband phone transmission?
    A. Signal distortion caused by insufficient collector current
    B. The transmitter's automatic level control is properly adjusted
    C. Signal distortion caused by excessive drive
    D. The transmitter's carrier is properly suppressed
    • (C)
    • The figure shows an overmodulated signal (B) as seen on an oscilloscope with flattening at the maximum levels of the envelope. This is referred to as flat-topping.
  11. G8A11 What happens to the RF carrier signal when a modulating audio signal is applied to an FM transmitter?
    A. The carrier frequency changes proportionally to the instantaneous amplitude of the modulating signal
    B. The carrier frequency changes proportionally to the amplitude and frequency of the modulating signal
    C. The carrier amplitude changes proportionally to the instantaneous frequency of the modulating signal
    D. The carrier phase changes proportionally to the instantaneous amplitude of the modulating signal
    • (A)
    • Frequency Modulation (FM) results when the modulating signal causes the frequency of an oscillator to change in proportion to the modulating signal’s amplitude. If the oscillator’s frequency changes in proportion to both the modulating signal’s amplitude and frequency, the result is phase modulation (PM).
  12. G8A12 What signal(s) would be found at the output of a properly adjusted balanced modulator?
    A. Both upper and lower sidebands
    B. Either upper or lower sideband, but not both
    C. Both upper and lower sidebands and the carrier
    D. The modulating signal and the unmodulated carrier
    • (A)
    • The process of mixing two ac signals creates two new signals. One of the new signals has a frequency equal to the sum of the two input signal frequencies and the other new signal has a frequency equal to the difference between the two input signal frequencies. The output signal from most mixers will also include some signal energy at the original input signals. A mixer can generate a double-sideband signal if the input signals are the carrier and the modulating audio.A balanced modulator is a mixer that includes some additional circuitry that can be adjusted to balance out or cancel the unwanted signals that remain at the carrier and modulating audiofrequencies. When a balanced modulator is property adjusted, neither of the original inputsignals reach the modulator output, only the sum and difference frequency signals. These signals are the upper (sum) and lower (difference) sidebands of an AM signal. A balanced modulator is used to produce the modulated RF signal for a single-sideband transmitter. The unwanted sideband is removed by a filter to produce the SSB signal.
  13. G8B01 What receiver stage combines a 14.250 MHz input signal with a 13.795 MHz oscillator signal to produce a 455 kHz intermediate frequency (IF) signal?
    A. Mixer
    B. BFO
    C. VFO
    D. Discriminator
    • (A)
    • The mixer stage in a receiver combines the input signal with an oscillator signal to produce the sum and the difference of the two signals. Filters or mixer design will give one desired new signal, known as the intermediate frequency (IF) signal. For example, if a 13.795 MHz variable frequency oscillator (VFO) signal is mixed with a 14.25 MHz RF signal, it will produce new signals at 28.045 MHz and 0.455 MHz, or 455 kHz. A filter is used to remove the 28.045 MHz signal and pass the 455 kHz IF signal.
  14. G8B02 If a receiver mixes a 13.800 MHz VFO with a 14.255 MHz received signal to produce a 455 kHz intermediate frequency (IF) signal, what type of interference will a 13.345 MHz signal produce in the receiver?
    A. Quadrature noise
    B. Image response
    C. Mixer interference
    D. Intermediate interference
    • (B)
    • The mixer stage in a receiver combines the input signal with an oscillator signal to produce the sum and the difference of the two signals. Filters or mixer design will select the desired new signal. known as the intermediate frequency (IF) signal. In this case the input signal is 14.255 MHz and the oscillator signal is 13.800 MHz. The sum of these two signals is 28.055 MHz andthe difference, which is used as the intermediate frequency, is 0.455 MHz or 455 kHz. These signals are then passed through a filter to eliminate the unwanted frequencies leaving only the desired intermediate frequency of 455 kHz.A signal at 13.455 MHz, when subtracted from the 13.800 MHz oscillator signal will also produce the 455 kHz intermediate frequency. (See Figure G8-7) When an undesired input signal also produces
    • a signal at the intermediate frequency, the resulting interference is called image response interference. One way to reduce image response interference is to use an input filter that allows signals in the desired receive frequency range to pass through, but blocks signals outside of that range.
  15. G8B03 What is another term for the mixing of two RF signals?
    A. Heterodyning
    B. Synthesizing
    C. Cancellation
    D. Phase inverting
    • (A)
    • The process of mixing signals is known as heterodyning. A receiver that mixes a local oscillator (LO) signal with a received RF signal to produce a signal at some intermediate frequency (IF) that is higher in frequency than the baseband audio signal is called a superheterodyne receiver, or a superhet. The IF signal is processed further by filtering and amplifying it, and then converting the signal to baseband audio. The audio signal is further amplified and fed to a speaker or is connected to headphones so you can hear the received signal.
  16. G8B04 What is the name of the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency?
    A. Mixer
    B. Reactance modulator
    C. Pre-emphasis network
    D. Multiplier
    • (D)
    • An FM transmitter often makes use of a device called a frequency multiplier. In a VHF FM transmitter, a reactance modulator operates on a radio-frequency oscillator that is normally operating in the high-frequency (HF) range. A frequency multiplier doubles or triples the frequency of the modulated signal, usually by generating harmonics of the modulated HF signal and selecting one for output to the next stage in the transmitter. Sometimes several multiplier stages are required to produce a signal at the desired output frequency.
  17. G8B05 Why isn't frequency modulated (FM) phone used below 29.5 MHz?
    A. The transmitter efficiency for this mode is low
    B. Harmonics could not be attenuated to practical levels
    C. The wide bandwidth is prohibited by FCC rules
    D. The frequency stability would not be adequate
    • (C)
    • FM phone signals have a bandwidth of about 16 kHz. At frequencies below 29.5 MHz, the maximum FCC allowable bandwidths are narrower than 16 kHz. The HF bands are relatively narrow, and would not be able to accommodate many wide bandwidth FM signals.
  18. G8B06 What is the total bandwidth of an FM-phone transmission having a 5 kHz deviation and a 3 kHz modulating frequency?
    A. 3 kHz
    B. 5 kHz
    C. 8 kHz
    D. 16 kHz
    • (D)
    • To determine bandwidth of an FM-phone transmission, use the following formula:

    BW = 2 x (D + M)

    • BW = bandwidth
    • D = frequency deviation (the instantaneous change in frequency for a given signal)
    • M = maximum modulating audio frequency
    • The total bandwidth of an FM-phone transmission having a 5 kHz deviation and a 3 kHz modulating frequency would be:

    2 x (5kHz÷3kHz)= 16kHz
  19. G8B07 What is the frequency deviation for a 12.21-MHz reactance-modulated oscillator in a 5-kHz deviation, 146.52-MHz FM-phone transmitter?
    A. 101.75 Hz
    B. 416.7 Hz
    C. 5 kHz
    D. 60 kHz
    • (B)
  20. G8B08 Why is it important to know the duty cycle of the data mode you are using when transmitting?
    A. To aid in tuning your transmitter
    B. Some modes have high duty cycles which could exceed the transmitter's average power rating.
    C. To allow time for the other station to break in during a transmission
    D. All of these choices are correct
    • (B)
  21. G8B09 Why is it good to match receiver bandwidth to the bandwidth of the operating mode?
    A. It is required by FCC rules
    B. It minimizes power consumption in the receiver
    C. It improves impedance matching of the antenna
    D. It results in the best signal to noise ratio
    • (D)
    • By matching the receiver bandwidth and the signal bandwidth, noise outside the signal's bandwidth is rejected and no necessary signal energy is discarded. Both result in improvement of the signal-to-noise ratio (SNR).
  22. G8B10 What does the number 31 represent in PSK31?
    A. The approximate transmitted symbol rate
    B. The version of the PSK protocol
    C. The year in which PSK31 was invented
    D. The number of characters that can be represented by PSK31
    • (A)
    • PSK31 is a digital mode that transmits symbols at a rate of 31.25 baud.
  23. G8B11 How does forward error correction allow the receiver to correct errors in received data packets?
    A. By controlling transmitter output power for optimum signal strength
    B. By using the varicode character set
    C. By transmitting redundant information with the data
    D. By using a parity bit with each character
    • (C)
    • Forward error correction (FEC) is the practice of sending redundant data in the transmitted packet that allows the receiver to correct some types of errors that may be caused by noise, fading, or interference. There are a number of FEC methods involving special codes.
  24. G8B12 What is the relationship between transmitted symbol rate and bandwidth?
    A. Symbol rate and bandwidth are not related
    B. Higher symbol rates require higher bandwidth
    C. Lower symbol rates require higher bandwidth
    D. Bandwidth is constant for data mode signals
    • (B)
    • Advanced modulation techniques can pack multiple bits of data into each transmitted symbol, but it is the symbol rate that sets a minimum limit on bandwidth. Increasing the rate at which symbols are transmitted requires more signal bandwidth in order to maintain a minimum signal-to-noise ratio.

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