Question Bank
Unit 4
1.
What is FM threshold effect?(AU: MAY 2011)
As
the carrier to noise ratio is reduced, clicks are heard in the receiver output.
As the carrier to noise ratio reduces further, crackling, or sputtering sound
appears at the receiver output. Near the breaking point, the theoretically
calculated output signal to noise ratio becomes large, but its actual value is
very small. This phenomenon is called threshold effect.
2.
What is capture effect in FM? (AU: MAY12, DEC 10)
When
the noise interference is stronger than FM signal, then FM receiver locks to
Interference.
This suppresses FM signal. When the noise interference as well as FM signal are
of equal strength, then the FM receiver locking fluctuates between them. This
phenomenon is called capture effect.
3.
What is meant by figure of merit of a receiver?(AU:DEC10)
The
ratio of output signals to noise signal to channel signal to noise ratio is
called figure of merit. It is the measurement of signal power in the channel.
It is used much in communication system for identifying noise performance.
4.
What is the Purpose of re-emphasis and de-emphasis in FM? (AU: DEC13, DEC10)
The
PSD of noise at the output of FM receiver sally increases rapidly at high
frequencies but the PSD of message signal falls off at higher frequencies. This
means the message signal doesn’t utilize the frequency band in efficient
manner. Such more efficient use of frequency band and improved noise
performance can be obtained with the help of re-emphasis and de-emphasis.
5.
What are extended threshold demodulators?
Threshold
extension s also called threshold reduction. It is achieved with the help of
FMFB demodulator. In the local oscillator is replaced by voltage controlled
oscillator (VCO).The VC frequency changes as per low frequency variations of
demodulated signal. Thus the receiver responds only to narrow band of
noise centered around instantaneous carrier frequency. This reduces the
threshold of FMFB receiver.
6. What is
threshold effect with respect to noise?
When the carrier
to noise ratio reduces below certain value, the message information is lost.
The performance of the envelope detector deteriorates rapidly and it has no
proportion with carrier to noise ratio. This is called threshold effect.
7. Define
pre-emphasis and de-emphasis.
Pre-emphasis: It
artificially emphasizes the high frequency components before modulation. This
equalizes the low frequency and high frequency portions of the PSD and complete
band is occupied.
De-emphasis:
This circuit attenuates the high frequency components. The attenuation
characteristic is exactly opposite to that of pre-emphasis circuit. De-emphasis
restores the power distribution of the original signal. The signal to noise
ratio is improved because of pre-emphasis and de-emphasis circuits.
8.
State the reasons for higher noise in mixers.
1.
Conversion trans-conductance of mixers is much lower than the
Trans-conductance
of amplifiers
2.
If image frequency rejection is inadequate, the noise associated with the image
frequency also gets accepted.
9.
Define signal to noise ratio.
Signal to noise
ratio is the ratio of signal power to the noise power at the same point in a
system. Signal-to-noise ratio
(often abbreviated SNR or S/N) is a measure used in science and
engineering that compares the level of a desired signal to
the level of background noise. It
is defined as the ratio of signal power to the noise power, often expressed in decibels. A
ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise
10.
Define noise figure.
Noise
figure is decibel representation of Noise factor. i.e., Noise figure = (Noise
factor) dB
Noise figure (NF)
and noise factor (F) are
measures of degradation of the signal-to-noise ratio (SNR), caused by components in a radio frequency (RF)
signal chain. It is a number by which the performance of an amplifier or a
radio receiver can be specified, with lower values indicating better
performance.
Part
– B
1.
Explain various sources and types of noise in detail (AU: MAY 13, 14) (16)
2.
a) Thermal noise from a resistor is 4x10^-17 Watts for given bandwidth and at
temperature 20 degree Celsius. What is the noise power when T=50
And T=70°K (AU:
DEC 11) (8)
b) Two resistors 20K and 50K at room
temperature. Calculate for a bandwidth of 100 KHz, thermal noise for i)each
resistor ii) in series iii) in parallel. (AU:
DEC 11) (2 marks)
c) Define noise factor and noise figure (AU: MAY 11,
DEC 12) (2 marks)
d) Define Friis’s Formula (AU:
DEC 8) (2 Marks)
e) Define noise temperature (AU: MAY
13) (2 marks)
3. a) An
amplifier1 has noise figure of 9DB and power gain of 15DB. It is connected in
cascaded to another amplifier2 with noise figure 20DB. Calculate overall Noise
figure.(AU: DEC10) (8)
b) Consider two amplifiers in cascade. The
first stage has power gain of 40DB and noise figure of 2DB, the second stage
has noise figure of 3DB. Calculate overall noise figure.
(AU: MAY
11) (4)
c) A receiver has a noise figure of 12 DB;
it is fed by low noise amplifier with gain 50 DB and a noise temperature of
90K. Calculate the noise temperature of receiver and overall noise temperature
at room temperature 290K (4
marks)
4. a)
Define white noise and calculate power spectral density and autocorrelation of
the white noise.
(AU: MAY 12) (4 marks)
b) Explain in detail about Narrowband noise
with envelope and phase components. (AU: MAY 11) (12 marks)
5.
a) Describe in detail the noise performance of the AM receiver (AU:MAY 13) (8 marks)
b) Explain the noise performance of FM
receiver in detail. (AU: MAY 13) (8
marks)
****************************ALL
THE BEST **********************************/