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

A direct digital synthesizer.

From kb6nu's site, article titled Extra Class question of the day: frequency synthesizers

"A direct digital synthesizer is the type of frequency synthesizer circuit that uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter. The phase accumulator is a principal component of a direct digital synthesizer (DDS).

The information is contained in the lookup table of a direct digital frequency synthesizer is the amplitude values that represent a sine-wave output."

It is worth noting that the question referenced was later removed from the Amateur Radio question pool.

For more information, please see Analog site for the article All About Direct Digital Synthesis by Eva Murphy and Colm Slattery

See Wikipedia's article on Frequency synthesizer

Also, see the Electronics Notes site for the article Digital PLL Frequency Synthesizer and their YouTube channel's video called Digital PLL Frequency Synthesizers: what they are, how they work 6:03 minutes

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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?

Phase-locked loop.

The varactor diode is a variable capacitor, helping various circuits to be quickly tuned through the control of voltage application. The varactor diode resembles a p-n diode, having nonlinear reactance.

For more on vacator diode:

Wikipedia's article Varicap
Electronics Notes article Varactor Diode or Varicap Diode
Circuit Globe article Varactor Diode

For more information, please see the TutorialsPoint site for the article Phase Locked Loop IC

Also, see the Analog site for the article Phase-Locked Loop (PLL) Fundamentals by Ian Collins

And, see the Electronics Notes site for the article PLL Phase Locked Loop Tutorial & Primer

See Wikipedia's article Phase-locked loop

Lastly, see the All About Circuits site for the article What Exactly Is a Phase-Locked Loop, Anyways? by Robert Keim

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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?

50, 150, 200 and 350 MHz.

From wp2ahg:

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?

Broadband noise.

For more information, please see the TutorialsPoint site for the article Phase Locked Loop IC

Also, see the Analog site for the article Phase-Locked Loop (PLL) Fundamentals by Ian Collins

And, see the Electronics Notes site for the article PLL Phase Locked Loop Tutorial & Primer

See Wikipedia's article Phase-locked loop

Lastly, see the All About Circuits site for the article What Exactly Is a Phase-Locked Loop, Anyways? by Robert Keim

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In a direct digital synthesizer, what are the unwanted components on its output?

Spurs at discrete frequencies.

From kb6nu's site, article titled Extra Class question of the day: frequency synthesizers
(this question for Extra class was retired)

"A direct digital synthesizer is the type of frequency synthesizer circuit that uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter. The phase accumulator is a principal component of a direct digital synthesizer (DDS).

The information is contained in the lookup table of a direct digital frequency synthesizer is the amplitude values that represent a sine-wave output."

For more information, please see Analog site for the article All About Direct Digital Synthesis by Eva Murphy and Colm Slattery

See Wikipedia's article on Frequency synthesizer

Also, see the Electronics Notes site for the article Digital PLL Frequency Synthesizer and their YouTube channel's video called Digital PLL Frequency Synthesizers: what they are, how they work 6:03 minutes

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What is the definition of a phase-locked loop (PLL) circuit?

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

It is a control, using oscillator frequency, providing feedback, adjusting locally the phase signal equal the phase of the input.

Please see the Electronics Notes site for the article PLL Phase Locked Loop Tutorial & Primer

Also, see the Analog site for the article Phase-Locked Loop (PLL) Fundamentals by Ian Collins

And, see the Tech Target site for the article phase-locked loop (PLL) by Rahul Awati and John Burke, Nemertes Research

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