21. Which of the following are two methods for generating FM signal?
Coherent method, noncoherent method
Product detector, envelope detector
Direct method, indirect method
Slope detector, Zero crossing detector
Answer.3. Direct method, indirect method
Explanation
FM signals can be generated either by using direct frequency modulation, which is achieved by inputting a message directly into a voltage-controlled oscillator, or by using indirect frequency modulation, which is achieved by integrating a message signal to generate a phase-modulated signal, which is then used to modulate a crystal-controlled oscillator, the result of which is transmitted through a frequency multiplier to produce an FM signal.
22. In _______ narrowband FM signal is generated using a balanced modulator.
Direct Method
Indirect Method
Both 1 and 2
None of the above
Answer.2. Indirect Method
Explanation
In the indirect method, a narrowband FM signal is generated using a balanced modulator, and frequency multiplication is used to increase both the frequency deviation and the carrier frequency to the required level.
23. Which of the following is used to vary the frequency of the carrier frequency in accordance with the baseband signal amplitude variations in direct method of FM generation?
Integrator
Envelope detector
Multivibrator
Voltage-controlled oscillators
Answer.4. Voltage-controlled oscillators
Explanation
A voltage-controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. In direct method, VCOs are used to vary the frequency of the carrier signal in accordance with the baseband signal amplitude variations. These oscillators use devices with reactance that can be varied by the application of a voltage.
24. Which of the following is not a technique for FM demodulation?
Slope detection
Zero crossing detection
Product detector
Phase-locked discriminator
Answer.3. Product detector
Explanation
There are different types of FM demodulators including Slope Detector, Foster-Seeley Discriminator, Ratio Detector, Zero crossing detection, Phase-locked discriminator
Slope detection is the very simplest form of frequency modulation, FM demodulation. An FM slope detector consists of a tuned circuit where the center frequency is tuned to a frequency slightly offset from the carrier of the signal.
A zero-crossing detector or ZCD is one type of voltage comparator, used to detect a sine waveform transition from positive and negative, that coincides when the i/p crosses the zero voltage condition.
Phase-locked loops, PLLs are ideal for demodulating frequency modulation, FM being easy to incorporate in ICs and providing a linear output.
25. Which of the following FM demodulator is sometimes known as pulse averaging discriminator?
Slope detection
Zero crossing detection
Quadrature detection
Phase-locked discriminator
Answer.2. Zero crossing detection
Explanation
The zero-crossing rate (ZCR) is the rate at which a signal changes from positive to zero to negative or from negative to zero to positive.
Zero crossing detection FM demodulator is sometimes known as pulse averaging discriminator
The zero-crossing properties of the frequency-modulated carrier signal can be used to design FM detectors. The rationale behind this technique is to use the output of the zero-crossing detector to generate a pulse train with an average value that is proportional to the frequency of the input signal.
The most commonly used zero-crossing detectors assume that the message signal is varying very slowly. The FM signal is approximated by a sinusoid, locally, and the frequency is estimated thus leading to the demodulation of the FM signal.
26. PLL in FM detection stands for ______
Phase-locked loop
Programmable logic loop
Phase-locked logic
Programmable locked loop
Answer.1. Phase-locked loop
Explanation
PLL in FM detection stands for a phase-locked loop. The phase-locked loop (PLL) block is a feedback control system that automatically adjusts the phase of a locally generated signal to match the phase of an input signal.
The present-day AM/FM receivers almost invariably use the PLL for detection. Due to its feedback structure, a PLL-based FM detector has a considerably improved performance in noise over the slope detector.
PLL demodulators are easier to align and implement using inexpensive ICs.
27. What is the maximum frequency of modulating signal in FM system, if it has 8 significant sidebands, and the maximum bandwidth is 32 kHz?
4 kHz
8 kHz
16 kHz
2 kHz
Answer.4. 2 kHz
Explanation
BW = 32 KHz
Given FM system has 8 sidebands (significant) i.e. 8 sidebands on the positive side of frequency & 8 sidebands on the negative side of the frequency
So, It has sidebands up to order ‘8’.
According to carson rule:
Significant sideband upto order of (β + 1) has B.W
BW = (β + 1) 2fm
So,
32 = 8 × 2 fm
fm = 2 kHz
28. The four basic elements in a PLL are loop filter, loop amplifier, VCO and
Upconverter
Downconverter
Phase detector
Frequency multiplier
Answer.3. Phase detector
Explanation
A phase-locked loop is a control system that generates an output signal whose phase is related to the phase of an input signal.
The circuit can track an input frequency or it can generate a frequency that is a multiple of the input frequency.
The basic elements of a PLL circuit are a phase comparator/detector, a loop filter, voltage-controlled oscillator (VCO).
29. In angle modulation, signal to noise ratio before detection is a function of ______
Modulation index
Input signal to noise ratio
Maximum frequency of the message
IF filter bandwidth
Answer.4. IF filter bandwidth
Explanation
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of the desired signal to the level of background noise.
In angle modulation systems, the signal-to-noise ratio before detection is the function of the receiver IF filter bandwidth received carrier power, and received interference.
However, signal to noise ratio after detection is a function of the maximum frequency of the message, input signal-to-noise ratio, and modulation index.
30. An increase in modulation index leads to an increase in bandwidth in case of
AM
FM
SSB
None
Answer.2. FM
Explanation
For AM, the bandwidth is given by:
B.W. = 2 fm
For an SSB signal with one sideband suppressed, the bandwidth is given by:
B.W. = fm
fm = maximum frequency present in the modulating signal.
The bandwidth of FM is given by:
BW = 2(β + 1)fm
β = Modulation index
31. Phase modulation is generally not used for transmission because
It needs higher power
It needs large Antenna Sizes
It is complex for decoding
It does not carry much information
Answer.3. It is complex for decoding
Explanation
Phase modulation (PM) is a modulation pattern that encodes information as variations in the instantaneous phase of a carrier wave
Phase modulation is widely used for transmitting radio waves and is an integral part of many digital transmission coding schemes that underlie a wide range of technologies like Wi-Fi, GSM, and satellite television.
Phase Modulation is generally not used for transmission because it is complex for decoding.
32. FM can improve receiver performance through adjustment of _______
Power Index
Modulation Index
Transmitter
None of the above
Answer.2. Modulation Index
Explanation
FM can improve receiver performance through adjustment of the modulation index at the transmitter, and not the transmitted power. This is not the case in AM since linear modulation techniques do not trade bandwidth for SNR.
33. In a frequency modulation system, the maximum frequency deviation allowed is 1000, and modulating frequency is 1 kHz. Determine modulation index β.