The figure shows the block diagram of frequency discriminator. Wh
The figure shows the block diagram of frequency discriminator. What does the second block represent?
A. Envelope detector
B. Low-pass filter
C. Ratio detector
D. Band-reject filter
Please scroll down to see the correct answer and solution guide.
Right Answer is: A
SOLUTION
Frequency discriminator is a demodulation process for a frequency modulated (FM) signal.
The Block diagram of a frequency discriminator is as shown:
s(t) = Ac cos (2π fct + 2π kf ∫ m(τ) dτ)
After passing through differentiator, the FM signal is converted into a combination of FM and AM signals.
\(\frac{{ds\left( t \right)}}{{dt}} = \underbrace {\left( {2\pi {A_c}{f_c} + 2\pi {A_c}{k_f}m\left( t \right)} \right)}_{\begin{array}{*{20}{c}} {Amplitude\;of\;demodulated\;}\\ {signal\;i.e.\;\;message\;signal}\\ {with\;same\;dc\;term} \end{array}}in(2\pi {f_c}t + 2\pi {k_f}\smallint m\left( \tau \right)d\tau - 180^\circ )\)
Envelope detector detect the demodulated signal and block the dc term and generate the message signal
∴ Option (1) is correct.
Important Points:
The circuit diagram of an Envelope Detector is as shown:
For better detection, the following condition must be satisfied:
\(\frac{1}{{{f_c}}} < RC < \frac{1}{{{f_m}}}\)
Where, fc = carrier frequency and
fm = message frequency
