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Subelement D
Circuits
Section 31
Phase Locked Loops (PLLs); Voltage Controlled Oscillators (VCOs); Mixers
What frequency synthesizer circuit uses a phase comparator, look-up table, digital-to-analog converter, and a low-pass antialias filter?
  • Correct Answer
    A direct digital synthesizer.
  • Phase-locked-loop synthesizer.
  • A diode-switching matrix synthesizer.
  • A hybrid synthesizer.
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A circuit that compares the output of a voltage-controlled oscillator (VCO) to a frequency standard and produces an error voltage that is then used to adjust the capacitance of a varactor diode used to control frequency in that same VCO is called what?
  • Doubly balanced mixer.
  • Correct Answer
    Phase-locked loop.
  • Differential voltage amplifier.
  • Variable frequency oscillator.
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RF input to a mixer is 200 MHz and the local oscillator frequency is 150 MHz. What output would you expect to see at the IF output prior to any filtering?
  • Correct Answer
    50, 150, 200 and 350 MHz.
  • 50 MHz.
  • 350 MHz.
  • 50 and 350 MHz.

The output includes:

  • The difference (50 MHz)
  • The two inputs (150 and 200 MHz)
  • The sum (350 MHz)

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What spectral impurity components might be generated by a phase-locked-loop synthesizer?
  • Spurs at discrete frequencies.
  • Random spurs which gradually drift up in frequency.
  • Correct Answer
    Broadband noise.
  • Digital conversion noise.
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In a direct digital synthesizer, what are the unwanted components on its output?
  • Broadband noise.
  • Correct Answer
    Spurs at discrete frequencies.
  • Digital conversion noise.
  • Nyquist limit noise pulses.
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What is the definition of a phase-locked loop (PLL) circuit?
  • A servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator.
  • A circuit also known as a monostable multivibrator.
  • A circuit consisting of a precision push-pull amplifier with a differential input.
  • Correct Answer
    A servo loop consisting of a phase detector, a low-pass filter and voltage-controlled oscillator.
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