Key Topic 31: Phase Locked Loops (PLLs); Voltage Controlled Oscillators (VCOs); Mixers

Question 1

3-31D1: What frequency synthesizer circuit uses a phase comparator, look-up table, digital-to-analog converter, and a low-pass antialias filter?

A. A direct digital synthesizer.
B. Phase-locked-loop synthesizer.
C. A diode-switching matrix synthesizer.
D. A hybrid synthesizer.

Answer 1

A. A direct digital synthesizer.

Question 2

3-31D2: 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?

A. Doubly balanced mixer.
B. Phase-locked loop.
C. Differential voltage amplifier.
D. Variable frequency oscillator.

Answer 2

B. Phase-locked loop.

Question 3

3-31D3: RF input to a mixer is 200 MHz and the local oscillator frequency is 150 MHz. What output would you expect to see if the IF output prior to any filtering?

A. 50, 150, 200, and 350 MHz.
B. 50 MHz.
C. 350 MHz.
D. 50 and 350 MHz.

Answer 3

A. 50, 150, 200, and 350 MHz.

Question 4

3-31D4: What spectral impurity components might be generated by a phase-locked-loop synthesizer?

A. Spurs at discrete frequencies.
B. Random spurs which gradually drift up in frequency.
C. Broadband noise.
D. Digital conversion noise.

Answer 4

C. Broadband noise.

Question 5

3-31D5: In a direct digital synthesizer, what are the unwanted components on its output?

A. Broadband noise.
B. Spurs at discrete frequencies.
C. Digital conversion noise.
D. Nyquist limit noise pulses.

Answer 5

B. Spurs at discrete frequencies.

Question 6

3-31D6: What is the definition of a phase-locked loop (PLL) circuit?

A. A servo loop consisting of a ratio detector, reactance modulator, and voltage-controlled oscillator.
B. A circuit also known as a monostable multivibrator.
C. A circuit consisting of a precision push-pull amplifier with a differential input.
D. A servo loop consisting of a phase detector, a low-pass filter, and voltage-controlled oscillator.

Answer 6

D. A servo loop consisting of a phase detector, a low-pass filter, and voltage-controlled oscillator.