Question E7H03:
What is a phase-locked loop?  

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

Correct Answer is  C: An electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator  

A phase-locked loop (PLL) is a feedback control system that forces a voltage-controlled oscillator (VCO) to match the phase (and thus frequency) of a stable reference.

Why the correct answer is correct (Choice C)
- A PLL contains:
  - Phase detector (or phase comparator): compares the phase of the VCO signal (often after division) to a stable reference and outputs an error voltage proportional to phase difference.
  - Low-pass filter (loop filter): smooths the phase detector output to a DC control voltage.
  - Voltage-controlled oscillator (VCO): its frequency shifts with control voltage.
  - Stable reference oscillator: provides the frequency/phase reference.
- In lock, the loop drives the VCO so the phase error is constant, making the VCO track the reference.

Useful formulas and relationships
- Phase detector (idealized):  
  $$V_e = K_d\,(\phi_{\text{ref}} - \phi_{\text{vco}})$$
- VCO frequency sensitivity:  
  $$f_{\text{out}} = f_{\text{free}} + K_v\,V_c$$
- Locked condition (with integer dividers N in feedback and M on the reference path):  
  $$\frac{f_{\text{out}}}{N} = \frac{f_{\text{ref}}}{M} \;\Rightarrow\; f_{\text{out}} = \frac{N}{M}\,f_{\text{ref}}$$
- Rules of thumb:
  - Lock range > capture range; both set by loop filter and loop gain.
  - PLLs are used for frequency synthesis, carrier recovery, and FM/FSK demodulation.

Why the other choices are incorrect
- Choice A: Mentions a ratio detector and reactance modulator. A ratio detector is an FM demodulator; a reactance modulator is used to frequency-modulate an oscillator. That’s not the standard PLL block set and omits the explicit phase detector/loop filter structure tied to a reference.
- Choice B: A monostable multivibrator is a one-shot pulse generator, not a feedback loop that locks phase/frequency.
- Choice D: A push-pull amplifier with differential phase input is an amplifier topology, not a feedback control system that includes a VCO, phase detector, and loop filter.

Key idea: A PLL is a servo loop that minimizes phase error to lock a VCO to a reference, enabling precise frequency/phase control.