# Analog Communications Questions and Answers – Envelope Detector

This set of Analog Communications Multiple Choice Questions & Answers (MCQs) focuses on “Envelope Detector”.

1. Envelope Detector is a/an ________
a) Coherent detector
b) Asynchronous Detector
c) Synchronous Detector
d) Product Demodulator

Explanation: An envelope detector is used to demodulate a previously modulated signal by removing all high frequency components of the signal. The capacitor and resistor form a low-pass filter to filter out the carrier frequency. Envelope detectors are asynchronous in nature. The advantage of asynchronous over synchronous is that it is simple, cheap and setup is faster.

2. Effective radiated power (ERP) is equal to ________
a) Pt×Gt×Pd
b) Pt×Pd
c) Gt×Pd
d) Pt×Gt

Explanation: Effective Radiated Power (ERP) measures the combination of the power emitted by the transmitter and the ability of the antenna to direct that power in a given direction. Thus, Effective Radiated Power is equal to the product of antenna gain (Gt) and power transmitted (Pt).

3. Power density from an isotropic antenna is equal to ________
a) $$\frac{P_t}{4 \pi R^2}$$
b) $$\frac{P_c}{4 \pi R^2}$$
c) $$\frac{P_t}{2 \pi R^2}$$
d) $$\frac{P_t}{4 \pi R}$$

Explanation: Power of a transmitter that is radiated from an isotropic antenna will have a uniform power density in all direction. An isotropic antenna is an ideal antenna that radiates its power uniformly in all directions. There is no actual physical isotropic antenna. However, an isotropic antenna is often used as a reference antenna for the antenna gain. The power density at any distance ‘R’ for an isotropic antenna is equal to $$\frac{P_t}{4 \pi R^2}$$.

4. Compact discs mainly use ________
a) optical recording
b) magnetic recording
c) magnetic retrieval
d) both optical and magnetic recording

Explanation: Compact disk (CD) is mainly used for storage of data. It was developed only for store and play only audio but was later adapted for storage of data also. It mainly used optical recording and retrieval.

5. If the distance between antenna and source doubles, the received signal power decreases by 14.
a) True
b) False

Explanation: It is due to the R2 term in formula, . Thus, the received signal power is inversely proportional to the distance between antenna and source.
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6. Power density follows inverse square law.
a) True
b) False

Explanation: From the given formula, With increase in distance, power density decreases. So, we can also say that power density is inversely related to distance. Thus, Power density follows inverse square law.

7. There is no requirement of IF amplifier stages for a video monitor.
a) True
b) False

Explanation: IF amplifiers are used to change the frequency levels in circuits that are too selective, difficult to tune, and unstable. However, Video amplifier is used to amplify video signals before passing them to a Cathode ray tube. And there is no need of IF amplifier stages for a video monitor.

8. Helical antenna is circularly polarized.
a) True
b) False

Explanation: A helical antenna is an antenna consisting of one or more conducting wires (monofilar, bifilar, or quadrifilar with 1, 2, or 4 wires respectively) wound in the form of a helix. In helical antenna, polarization is equally divided between vertical and horizontal components and thus it is circularly polarized.

9. The intermediate frequency of a superheterodyne receiver is 500KHz. What is the image frequency at 1200KHz?
a) 600KHz
b) 500KHz
c) 700KHz
d) 200KHz

Explanation: The image frequency is an undesired input frequency which is demodulated by superheterodyne receivers along with the desired incoming signal. This results in two stations being received at the same time, thus producing interference. In the given problem, Image frequency is equal to (1200 – 500) which is equal to 700KHz.

10. Which of these amplifiers is used for impedance matching?
a) Common Base
b) Common Emitter
c) Common Collector
d) Common Base & Emitter

Explanation: An amplifier is an electronic circuit that is used to amplify the voltage signal or a current signal. Output impedance of common collector amplifier is low and thus it is suitable for impedance matching.

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