AM Detectors: Envelope vs Synchronous

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Questions and Answers

What is the purpose of the local oscillator (LO) in a superheterodyne receiver?

To filter out unwanted frequencies

To demodulate the incoming signal

To amplify the incoming signal

To convert the incoming signal to a fixed intermediate frequency (IF) (correct)

Which type of detector is commonly used for demodulating frequency modulated (FM) signals?

Synchronous detector

Ratio detector (correct)

Envelope detector

Slope detector

What is the purpose of ganged tuning in a superheterodyne receiver?

To demodulate the incoming signal

To filter out unwanted frequencies

To ensure a fixed intermediate frequency (IF) by changing the variable capacitors together (correct)

To adjust the sensitivity of the receiver

Which type of detector is commonly used for demodulating amplitude modulated (AM) signals?

Envelope detector Signup and view all the answers

What is the advantage of using a superheterodyne receiver over a tuned radio-frequency (TRF) receiver?

Better selectivity at higher frequencies Signup and view all the answers

In a synchronous detector, what is the purpose of synchronizing the local oscillator with the carrier frequency?

To demodulate the incoming signal by coherent detection Signup and view all the answers

What are the two main types of AM detectors described in the text?

Envelope detector and synchronous detector Signup and view all the answers

What is the main advantage of the envelope detector over the synchronous detector?

It requires less complex circuitry Signup and view all the answers

Which type of modulation signal is the synchronous detector typically used for?

DSB-SC or SSB-SC Signup and view all the answers

What is the key requirement for the synchronous detector to operate correctly?

Regeneration of the carrier frequency Signup and view all the answers

What is the purpose of the capacitor C1 in the envelope detector circuit?

To remove the DC voltage component Signup and view all the answers

What is the relationship between the RC time constant and the modulating signal frequency in the envelope detector?

The RC time constant should be much larger than the modulating signal period Signup and view all the answers

In the given text, what is the purpose of the diode arrangement?

To produce a switching waveform that modulates the carrier wave Signup and view all the answers

Which of the following statements is true about the output of the ring modulator?

It contains the audio signal and two sideband frequencies Signup and view all the answers

What is the primary function of a radio receiver mentioned in the text?

To select, amplify, and demodulate the desired signal Signup and view all the answers

Which type of modulation is described in the given text?

Frequency Modulation (FM) Signup and view all the answers

What is the purpose of the carrier wave in the modulation process described in the text?

To transmit the audio signal over long distances Signup and view all the answers

What is the major difference between different types of radio receivers?

The way they demodulate the received signal Signup and view all the answers

Study Notes

Receiver Types

There are two main types of receivers: Tuned Radio-Frequency (TRF) Receiver and Superhetrodyne Receiver.

TRF Receiver

Used in old radio receivers
Very simple and high sensitivity receiver
Difficulties arise at higher frequencies, in the Tuning amplifiers
Quality factor (Q) affects the bandwidth and selectivity of the receiver
Example: If Rx is used with a bandwidth of 10 KHz at Tuning frequency 535 KHz, Q = 53.5, but for practical purposes, Q = 120, which increases the bandwidth to 13.7 KHz and decreases the selectivity of the receiver.

Superhetrodyne Receiver

Signal voltage is combined with Local Oscillator (LO) voltage and converted into a signal of a lower, generally fixed frequency (Intermediate Frequency, IF)
The signal at this frequency contains the same modulation as the original carrier and is amplified and detected to reproduce the original signal
Ganged Tuning is used to ensure fixed IF by changing the variable capacitors together in the LO, mixer, and RF

Receiver Parameters

Sensitivity

Ability of the receiver to amplify weak signals
Two types of AM detectors: non-coherent (Envelope detector) and coherent (Synchronous detector)

Envelope Detector

Used for speech, music, and video
Diode is used to detect the envelope of the AM wave
Diode operates as a half-wave rectifier and the capacitor charge time constant is equal to (RC)
Output is proportional to the input
Condition for RC: RC ≤ 1 - m² / (mW)

Synchronous Detector

Used for DSB-SC or SSB-SC signals
Multiplicative mixer where two signals multiply in a non-linear device
Regenerates the carrier
Output contains audio signal and two sideband frequencies, but no carrier

Ring Modulator

Carrier is applied to the midpoint of the input and output transformer
Rectifiers construct a ring shape across the network
Output contains audio signal and two sideband frequencies, but no carrier

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Description

Learn about the two types of AM detectors: the non-coherent Envelope detector used for speech, music & video, and the coherent Synchronous detector used in DSB-SC or SSB-SC signals. Understand the differences in their operation and applications.