---
title: "PLL components and AM demodulation using PLL"
subject: "Communication Systems"
chapter: 2
key: "semester_6/communication_systems:pll-components-am-demod"
source: https://www.bctengineer.com/q/semester_6/communication_systems/pll-components-am-demod
site: BCT Engineering
---

# PLL components and AM demodulation using PLL

Communication Systems · Chapter 2: Amplitude Modulation

- Asked in: 2074 Bhadra (CS I), 2+6 marks; 2072 Asoj (CS I), 5 marks; 2071 Bhadra (CS I), 6 marks (asked 3 times)
- Video: [S-V 3.8 Application of PLL as a FM and AM Demodulator| Electronics | Niteen Mohod](https://www.youtube.com/watch?v=CUmDbpoDtKI) — Dr. Niteen Mohod
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/communication_systems/pll-components-am-demod
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-2

## Question

What are the essential components that constitute PLL? How can PLL be used to demodulate AM signal?

## Short answer

A PLL consists of a phase detector, loop filter, and VCO. For AM demodulation, the PLL locks to the carrier; a 90° phase shift yields an in-phase carrier for a product detector, which recovers the message after low-pass filtering.

## Answer

A **phase-locked loop (PLL)** is a negative-feedback system that makes the phase and frequency of a local oscillator follow those of an input signal.

#### Essential components of a PLL

```text
 vin ->[Phase    ]-> ve ->[Loop  ]-> vc --+
       [detector ]        [filter]        |
           ^                              |
           |                              v
           +-------------[  VCO  ]<-------+
                  vo (output)
```

1. **Phase detector (multiplier):** compares the input $v_{in}$ with the VCO output $v_o$ and produces an error voltage $v_e$ proportional to their phase difference, e.g. $v_e\propto\sin(\theta_i-\theta_o)$.
2. **Loop filter (low-pass):** removes the high-frequency (sum) terms of the phase detector output and sets the loop dynamics: lock range, capture range and noise bandwidth.
3. **Voltage-controlled oscillator (VCO):** its frequency changes in proportion to the control voltage: $f_o=f_{free}+K_vv_c$. It is pulled until its frequency equals the input frequency.
4. (Often) a **loop amplifier** to increase loop gain.

When locked, the VCO has the same frequency as the input, with a small fixed phase difference (90° for a multiplier phase detector at zero error).

#### PLL used to demodulate AM
Coherent (synchronous) AM detection needs a local carrier exactly matching the received carrier. The PLL extracts it.

```text
 AM in --+-->[ PLL ]-->[ 90 deg ]--+  c(t)=cos(wc t)
         |   (locks to carrier)    |
         |                         v
         +----------------------->(X)-->[ LPF ]--> m(t)
                              product detector
```

**Operation:**
1. The received AM signal is $s(t)=A_c[1+k_am(t)]\cos\omega_ct$. It has a strong carrier component at $f_c$.
2. The PLL locks to this carrier. Because the multiplier phase detector settles at $90°$ phase difference, the VCO output is $\sin\omega_ct$ (quadrature).
3. A $90°$ phase shifter converts the VCO output to $\cos\omega_ct$, a clean carrier **in phase** with the received carrier.
4. A product detector multiplies the AM signal by this carrier:

$$
\begin{aligned}
v(t)&=A_c[1+k_am(t)]\cos\omega_ct\cdot\cos\omega_ct\\
&=\frac{A_c}{2}[1+k_am(t)]+\frac{A_c}{2}[1+k_am(t)]\cos2\omega_ct
\end{aligned}
$$

5. The low-pass filter removes the $2\omega_c$ term; a DC-blocking capacitor removes $A_c/2$, leaving $\frac{A_ck_a}{2}m(t)$, the message.

**Advantages over envelope detection:** works for modulation index above 100% and for weak, noisy signals, gives less distortion, and the PLL's narrow loop bandwidth rejects noise. The PLL automatically tracks carrier drift. For DSB-SC (no carrier), a Costas loop or squaring loop is used instead.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which set lists the essential blocks of a PLL?
   A. Mixer, antenna, speaker
   B. Phase detector, loop filter, VCO
   C. Envelope detector, AGC, LO
   D. Modulator, BPF, amplifier
   Answer: B. A PLL consists of a phase detector, a low-pass loop filter and a VCO in a feedback loop. (Key point: PLL components)

2. What does the loop filter do?
   A. Removes high-frequency sum terms and sets loop dynamics
   B. Generates the VCO output
   C. Increases the input frequency
   D. Measures message power
   Answer: A. It is a low-pass filter that removes the sum-frequency terms from the phase detector and determines lock/capture range and noise bandwidth. (Key point: Loop filter role)

3. With a multiplier phase detector, when the PLL is locked the VCO is how far out of phase with the input?
   A. 0 degrees
   B. 90 degrees
   C. 180 degrees
   D. 45 degrees
   Answer: B. A multiplier detector's error is zero at 90 degrees, so the VCO output is in quadrature, which is why a 90 degree shifter is needed. (Key point: Locked phase)

4. How is the PLL used in AM demodulation?
   A. It locks to the carrier; after a 90 degree shift the carrier multiplies the AM signal and an LPF recovers m(t)
   B. It rectifies the AM wave directly
   C. It filters out the carrier only
   D. It amplifies the sidebands
   Answer: A. The recovered in-phase carrier is multiplied with the AM wave; the LPF removes the 2wc term, leaving the message after DC blocking. (Key point: AM demodulation)

## Related in Communication Systems

- [Phase error in coherent DSB-SC detection; AM demodulation by PLL](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aphase-error-dsbsc-am-pll) — Communication Systems, chapter 2
- [FM demodulation using PLL](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Afm-demod-pll) — Communication Systems, chapter 3, asked 2×
- [PLL lock and capture range; FM demodulation](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apll-lock-capture-fm-demod) — Communication Systems, chapter 3
- [Amplitude limiter; FM demodulation using PLL](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Alimiter-discriminator-pll-fm) — Communication Systems, chapter 3, asked 2×
- [Envelope detector; can it demodulate DSB-SC?](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aenvelope-detector-dsbsc) — Communication Systems, chapter 2, asked 2×
- [Demodulation of DSB-FC AM](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Adsbfc-demodulation-method) — Communication Systems, chapter 2

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