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Chapter: Communication Theory - Noise Characterisation

Important Short Questions and Answers: Noise Characterisation

Communication Theory - Noise Characterisation - Important Short Questions and Answers: Noise Characterisation

NOISE CHARACTERISATION

 

1. Define  noise.

 

Noise is defined as any unwanted form of energy, which tends to interfere with proper reception and reproduction of wanted signal.

 

2. Give the classification of noise.

 

Noise is broadly classified into two types. They are

(i)External noise

(ii)Internal noise.

 

3.     What are the types of External noise? External noise can be classified into

1.     Atmospheric noiseq

2.     Extraterrestrial noises

3.     Man –made noises or industrial noises

 

4.     What are types of internal noise?

Internal noise can be classified into

1.     Thermal noise

2.     Shot noise

3.     Transit time noise

4.     Miscellaneous internal noise

 

5.     What are the types of extraterrestrial noise and write their origin?

 

The two type of extraterrestrial noise are solar noise and cosmic noise Solar noise is the electrical noise emanating from the sun. Cosmic noise is the noise received from the center part of our galaxy, other distant galaxies and other virtual point sources.

 

6. Define transit time of a transistor.

 

Transit time is defined as the time taken by the electron to travel from emitter to the collector.

 

7. Define flicker noise.

 

Flicker noise is the one appearing in transistors operating at low audio frequencies. Flicker noise is proportional to the emitter current and junction temperature and inversely proportional to the frequency.

 

8.     State the reasons for higher noise in mixers.

1.     Conversion transconductance of mixers is much lower than the transconductance 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.

 

10. Define thermal noise. Give the expression for the thermal noise voltage across a resistor.

 

The electrons in a conductor possess varying amounts of energy. A small fluctuation in this energy produces small noise voltages in the conductor. These random fluctuations produced by thermal agitation of the electrons is called thermal noise.

 

11. Define noise temperature. (In terms of hypothetical temperature)

 

The available noise power is directly proportional to temperature and it is independent of value of resistance. This power specified in terms of temperature is called as noise temperature. It is denoted by Te . It is given as,

 

Te  = (F −1 )T

 

12. What is shot noise?

 

When current flows in electronic device, the fluctuations number of electrons or holes generates the noise. It is called shot noise. Shot noise also depends upon operating conditions of the device.

 

13. Give the expression for noise voltage in a resistor.

 

The Mean –Square value of thermal noise voltage is given by,

Vn 2 = 4 k TBR

K – Boltz man constant,

R – Resistance

T – Obsolute temperature, B Bandwidth

 

 14. What is White Noise?

 

Many types of noise sources are Gaussian and have flat spectral density over a wide frequency range. Such spectrum has all frequency components in equal portion, and is therefore called white noise. The power spectral density of white noise is independent of the operating frequency. The Power spectral density of White Noise is given as,


15. What is narrowband noise?

 

The receiver of a communication system usually includes some provision for preprocessing the received signal. The preprocessing may take the form of a narrowband filter whose bandwidth is large enough to pass modulated component of the received signal essentially undistorted but not so large as to admit excessive noise through the receiver. The noise process appearing at the output of such filter is called narrow band noise.

 

16. Define noise equivalent bandwidth.

 

The noise equivalent bandwidth of the filter is defined as the bandwidth of an ideal filter at which the noise power passed by real filter and ideal filter is same.

 

17. Define noise factor.

 

Noise factor (F) is defined as the ratio of signal to noise power ratio at the input to signal to noise power ratio at the output

 

18.   Give the characteristics of shot noise.

 

(i)    Shot noise is generated due to fluctuations in the number of electrons or holes. (ii) Shot noise has uniform spectral density.

 

(iii) Mean square noise current depends upon direct component of current. (iv) Shot noise depends upon operating conditions of the device.

 

19.  What is FM threshold effect?

 

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.

 

20.  What is capture effect in FM?

 

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.

 

21. What is meant by figure of merit of a receiver?

 

The ratio of output signals to noise ratio to channel signal to noise ratio is called figure of

merit.,

 

22.  What is the Purpose of re-emphasis and de-emphasis in FM?

 

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.

 

 23.  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.

 

24.  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.

 

 

25.  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.

 

26.  Write the expression of the SNR for a synchronous detector.

 DSB-SC receiver uses synchronous detection. Its signal to noise ratio is given as,



27. Define superheterodyne principle.

 

It can be defined as the process of operation of modulated waves to obtain similarly modulated waves of different frequency. This process uses a locally generated carrier wave, which determines the change of frequency.


28. 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.

 29. What is threshold effect in an envelope detector? Explain.


When a noise is large compared to the signal at the input of the envelope detector, the detected output has a message signal completely mingled with noise. It means that if the input SNR is below a certain level, called threshold level, the noise dominates over the message signal, threshold is defined as value of the input signal to noise ratio (So/No) below which the output signal to noise ratio (Si/Ni) deteriorates much more rapidly than the input signal to noise ratio. The threshold effect in an envelope detector whenever the carrier power-to-noise power ratio approaches unity or less.



GLOSSARY TERMS:

 

1.     Correlated noise, is a form of internal noise which is correlated to the signal and is absent in the absence of signal.

 

2.     Noise, unwanted introduction of energy which interferes with the proper reception and reproduction of transmitted signals.

 

3.     Coherent detection, carrier wave at the transmitted side is phase locked within the carrier wave at the receiver side.

 

4.     Non Coherent detection, carrier wave at the transmitted side need not phase locked within the carrier wave at the receiver side.

 

5.     Demodulation, which is used to recover the original message signals.

 

6.     Mixer, non linear device which combines RF frequency and local oscillator frequency.

 

 

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