Unit IV Qanda

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  SRI VENKATESWARA COLLEGE OF ENGINEERINGDepartment of Electronics and Commnication En!ineerin!EC # Commnication T$eor%&NIT ' IV(ART ' A 1. W$at is t$e fnction of a recei)er*Ans+  The function of a receiver is to recover the srcinal signal. There are no. of signals floating in the receiver should be able to select the signal of the desired frequency. It should then be able to demodulate the received signal to recover the srcinal signal. It should be noted that the signal received is low level ie., a few  picowatts which needs to be amplified.2. W$at are t$e t%pes of A, Recei)ers*Ans . Two types of AM receivers are popular and are of commercial importance.They are1. Turned radio frequency T! # receivers, and2. $uperhetrodyne receivers.%. Define sper $eterod%ne principle+ It can be defined as the process of operation of modulated waves to obtain similarly modulated waves of different frequency. This process uses a locally generated carrier wave, which determines the change of frequency.And also we can define that a device performs the frequency translation of a modulated signal is &nown as a frequency mi'er. the operation is often called frequency mi'ing, frequency conversion, or heterodyning.(. Enlist ad)anta!es of sper$etrod%ne recei)ers+Ans+ The superhetrodyne receiver has the following advantages on T! receivers, i# Improved selectivity in terms of ad)acent channels.ii# More uniform selectivity in terms of ad)acent channels.iii# Improved receiver stability.iv# *igher gain per stage because I amplifiers are operated at a lower frequency.v# +niform band width because of fi'ed intermediate frequency.These advantages ma&e them suitable for most of the radio receiver applicationssuch as AM, M, $$, communications, T- and radar receivers.. Gi)e t%pes of mi-in! met$ods+Ans+  There are two mi'ing methods 1. Additive mi'ing2. Multiplicative mi'ing./. W$at is ima!e fre.enc% re/ection ratio and 0$at is its importance*Ans+  The Image frequency re)ection ratio can be defined as a ratio of the gain at the signal frequency to the gain at the image frequency. This gives the degree of image frequency re)ection.0. W$% is local oscillator fre.enc% al0a%s $i!$er t$an t$e si!nal fre.enc%*Ans+  The frequency of local oscillator is always &ept higher than the signal frequencyto get I. .  ( &* 3.4. frequency minus signal frequency.5. W$at is do1le spottin!*  Ans+  6hen a receiver pic&s up the same short wave station at two nearby pointson the receiver dial, the phenomenon is &nown as double spotting.7. Define Selecti)it%+Ans+  $electivity8 is the characteristic of a receiver to select a particular frequencyand re)ect unwanted frequencies.19. Define Sensiti)it%+Ans+  This is the ability of a radio receiver to amplify the wea&est signal received.11. E-plain 2Fidelit%3+Ans+   idelity is a quality of a receiver to reproduce all frequency components presentin the input signal correctly.12.A 19 &* signal modulates a 1199 &* carrier. ind the frequencyto be generated by the local oscillator 3.4.# of the receiver to demodulate the signal. Soltion . 6e &now 3 .  ( &* requency of 3.4.  I. . : carrier frequency  ( : 1199  1/ &* Ans1%.In a super hetrodyne receiver with no ! section, the loaded ; of the antenna circuit is 112. <alculate 8Image frequency re)ection1(. ind image frequency re)ection of an AM amplifier if ; of the tan& is 12 and receiver is tuned to 1299 &*.1.A superheterodyne receiver is tuned at 1999 &*. If the I is (9&*, what is the image frequency=  1/.The ; of a tuned circuit is 29 at the resonant frequency of 94&*. 6hat is the  bandwidth=10. Define Noise45ietin! effect*Ans+  >oise power of M detector varies inversely with carrier power. The decrease innoise power with an increase in carrier power is called noise?quieting effect.15. W$at is t$res$old effect in A, recei)ers*Ans+  The loss of the message signal ' t# in an envelope detector due to the presence of the large noise is &nown as the Threshold effect.The threshold effect is also defined as 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 i@p signal to noise ratio$ i @> i # is  below a certain level called threshold level.17. Define captre effect in F,* Ans+ 6hen the interference is stronger then it will suppress the desired M input. 6hen the interference signal and M input are of equal strength, the receiver fluctuates bac& and forth between them .This phenomenon is &nown as the capture effect.6e may also define as in low noise case the distortion produced by the noise at the o@p of M detector is negligible in comparison to the desired modulating signal.And noise almost suppressed by the signal. This phenomenon is called as capture effect.29. Define F, t$res$old effect*Ans+ As the input noise power is increased the carrier to noise ratio is decreased the receiver brea&s and as the carrier to noise ratio is reduced further crac&ling sound is heard and the output $>! cannot be predicted by the equation. This phenomenon is &nown as threshold effect.It is also defined as when the $>! becomes even slightly less than unity, an impulse of noise is generated. This noise impulse appears at the output of the detector in the form of clic& sound. If the $>! ratio is further decreased so that the ratio is moderately less than unity, the impulses are generated rapidly and clic&s merge in to spluttering sound. This phenomenon is &nown as Bthreshold effectC.21. 6o0 is t$res$old redction ac$ie)ed in F, s%stems* Ans+ Threshold reduction is achieved in M system by using an M demodulator with negative feedbac& or by using a phase loc&ed loop demodulator. $uch devices are referred to as e'tended?threshold demodulators.22. Define (re4emp$asis and de4emp$asis . Ans+  >oise at the higher?modulating signal frequencies is inherently greater in amplitude than noise at the lower frequencies. i.e the higher ? modulating?signal frequencies have a lower signal?to?noise ration than the lower frequencies. To compensate for this, the high? frequency modulating signals are emphasied or  boosted in amplitude in the transmitter. To compensate for this boost, the high?frequency signals are attenuated or deemphasied in the receiver after demodulation has been performed.2%. W$at is t$e fi!re of merit of a A, s%stem 0it$ 788 percent modlation*  Ans+ The figure of merit of an AM system with 199 percent modulation is 1@%.This means that other factors being equal an AM system must transmit three times as muchaverage power as a suppressed system in order to achieve the same quality of noise  performance.2(. Compare t$e performance of A, and F, s%stems+S+No+Amplitde modlationFre.enc% modlation 1.Amplitude of the carrier is varied according to amplitude of modulating signal requency of the carrier is varies according to amplitude of the modulating signal2.Am has poor fidelity due to narrow band width$ince the band width is large, fidelity is  better %.Most of the power is in carrier hence less efficientAll the transmitted power is useful.(. >oise interference is more>oise interference is minimum..Ad)acent channel interference is presentAd)acent channel interference is avoided due to guard bands./.AM broad cast operates in M and * range. M broadcast operates in -* and +* range.0.In AM only carrier and two side bands are present.Infinite number of sidebands are present.5.The transmission equipment is simple.The transmission equipment is comple'.7.Transmitted power varies according to modulation inde'.Transmitted power remains constant irrespective of modulation inde'.19.Depth of modulation has limitations. It can be increased above 1.Depth of modulation has no limitation. It can be increased by increasing frequency deviation.2. Compare t$e noise performance of an A, and F, s%stem*Ans+  The figure of merit of AM system is 1@% when the modulation is 199 percent andthat of M is %@2# m f 2 .The use of M offers improved noise performance over AM when %@2#m f 2 E 1@%. 6here m f   Fmodulation inde' in M.  
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