MAHALAKSHMI ENGINEERING COLLEGE-TRICHY

QUESTION BANK

SATELLITE COMMUNICATION DEPT./SEM.:ECE/VIII

UNIT IV

PART-A

1. What are the advantages of the super heterodyne receiver over TRF receiver?

(AUC MAY 2004)

The superhetrodyne receiver has the following advantages on TRF receivers, (i) Improved selectivity in terms of adjacent channels. (ii) More uniform selectivity in terms of adjacent channels. (iii) Improved receiver stability. (iv) Higher gain per stage because IF amplifiers are operated at a lower frequency. (v) Uniform band width because of fixed intermediate frequency. These advantages make them suitable for most of the radio receiver applications such as AM, FM, SSB, communications, TV and radar receivers

2. What is FM threshold effect? (AUC DEC 2011)(AUC MAY 2010)(AUC MAY 2004)

As the input noise power is increased the carrier to noise ratio is decreased the receiver breaks and as the carrier to noise ratio is reduced further crackling sound is heard and the output SNR cannot be predicted by the equation. This phenomenon is known as threshold effect. It is also defined as when the SNR 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 click sound. If the SNR ratio is further decreased so that the ratio is moderately less than unity, the impulses are generated rapidly and clicks merge in to spluttering sound. This phenomenon is known as ―threshold effect‖.

3. Draw the pre emphasis circuit

4. What is meant by figure of merit of a receiver? (AUC NOV 2006) Figure of merit =

(SNR)o/(SNR)c

5. What is the purpose of pre-emphasis and de-emphasis in FM?

(AUC NOV 2006, AUC MAY 2007, AUC NOV 2007)

Noise 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 emphasized or boosted in amplitude in the transmitter. To compensate for this boost, the high-frequency signals are attenuated or deemphasized in the receiver after demodulation has been performed.

6. What is threshold effect with respect to noise? (AUC MAY 2007)

The loss of the message signal x (t) in an envelope detector due to the presence of the large noise is known 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(Si/Ni) is below a certain level called threshold level.

7. What do you understand by ‗capture effect‘ in FM? (AUC MAY 2012)(AUC MAY 2008)

When the interference is stronger then it will suppress the desired FM input. When the interference signal and FM input are of equal strength, the receiver fluctuates back and forth between them .This phenomenon is known as the capture effect. We may also define as in low noise case; the distortion produced by the noise at the o/p of FM detector is negligible in comparison to the desired modulating signal. And noise almost suppressed by the signal. This phenomenon is called as capture effect.

8. Compare the noise performance of AM and FM systems.(AUC MAY 2009)(AUC MAY

2008)

The figure of merit of AM system is 1/3 when the modulation is 100 percent and that of

FM is (3/2) mf2.The use of FM offers improved noise performance over AM when

(3/2)mf2 > 1/3.mf â€―modulation index in FM.

9. The figure of merit of AM system is 1/3 when the modulation is 100 percent and that of

FM is (3/2) mf2.The use of FM offers improved noise performance over AM when

(3/2)mf2 > 1/3. Where mf –modulation index in FM.

10. The figure of merit of AM system is 1/3 when the modulation is 100 percent and that of

FM is (3/2) mf2.The use of FM offers improved noise performance over AM when

(3/2)mf2 > 1/3. Where mf –modulation index in FM. What are the characteristics of

super heterodyne receivers? (AUC MAY 2010)

1. Greater gain thus better sensitivity

2. Improved image frequency rejection

3. Better signal to noise ration

4. Better selectivity

11. What do you understand by FM stereo multiplexing?(AUC MAY 2009)

12. What are the methods to improve FM threshold reduction?(AUC MAY 2010)

1. Preemphasis and deemphasis

2. FMFB(Frequency modulation with feedback)

13. Define pre-emphasis and de-emphasis.(AUC DEC 2010)

Noise 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 emphasized or boosted in amplitude in the transmitter. To compensate for this boost, the high-frequency signals are attenuated or deemphasized in the receiver after demodulation has been performed.

14. Compare the noise performance of DSBSC receiver using coherent detection with AM (AUC MAY 2011)

Parameter Am receiver using envelope DSBSC reciver using

Detection coherent detection

Noise performance Poor Good noise performance

then AM

Bandwidth 2Fm 2Fm

Theresholf AM exhibit a threshold No threshold

effects for low (Si/Ni)

(SNR)o& (SNR)c (SNR)o= (SNR)o= (SNR)c

ma2/2+ ma

2 (SNR)c

15. What are the methods to improve FM threshold reduction? (AUC MAY 2011)

Threshold reduction is achieved in FM system by using an FM demodulator with

negative feedback or by using a phase locked loop demodulator. Such devices are

referred to as extended-threshold demodulators. 16. Define threshold effect in AM receiver.(AUC DEC 2010) (AUC DEC 2011)

The loss of the message signal x (t) in an envelope detector due to the presence of the large noise is known 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(Si/Ni) is below a certain level called threshold level.

17. Determine the range of tuning of a local oscillator of a super Heterodyne receiver when

fLO>fc. The broadcast frequency range is 540Hz to 1600 Hz. Assume Fif=445kHz

(AUC MAY 2012)

Fs=1000kHz Fo=545kHz

Fi=Fs-Fo =455kHz

18. In a super hetrodyne receiver with no RF section, the loaded Q of

the antenna circuit is 112. Calculate :

Image frequency rejection

19. Find image frequency rejection of an AM amplifier if Q of the tank

is 125 and receiver is tuned to 1200 kHz.

20. A superheterodyne receiver is tuned at 1000 kHz. If the IF is 450kHz, what is the image

frequency?

21. The Q of a tuned circuit is 250 at the resonant frequency of 50OkHz. What is the

bandwidth?

22. Enlist advantages of superhetrodyne receivers. Ans. The superhetrodyne receiver has the following advantages on TRF receivers, (i) Improved selectivity in terms of adjacent channels. (ii) More uniform selectivity in terms of adjacent channels. (iii) Improved receiver stability. (iv) Higher gain per stage because IF amplifiers are operated at a lower frequency. (v) Uniform band width because of fixed intermediate frequency. These advantages make them suitable for most of the radio receiver applications such as AM, FM, SSB, communications, TV and radar receivers.

PART-B

1. What are pre-emphasis and de-emphasis in FM? Draw suitable circuits and explain.Explain

pre-emphasis and De-emphasis in detail. (ii)What is the necessity of pre-emphasis and de-

emphasis used in FM System? Explain pre-emphasis and de-emphasis with neat

circuit.(6)(AUC DEC 2011)(AUC MAY 2007, AUC MAY 2004,AUC NOV 2005, AUC

NOV2006, AUC MAY 2008) )(AUC DEC 2011) (AUC MAY 2010)

2. (i) Explain the capture effect and FM threshold effect.

FM Threshold Effect: The output signal to noise ratio of FM receiver is valid only if the carrier to noise ratio is measured at the discriminator input is high compared to unity. It is observed that as the input noise is increased so that the carrier to noise ratio

decreased, the FM receiver breaks. At first individual clicks are heard in the receiveroutputand as the carrier to noise ratio decreases still further, the clicks rapidly mergein to a crackling or sputtering sound.Near the break point eqn8.50 begins to fail

predicting values of output SNR largerthan the actual ones. This phenomenonis known as the threshold effect.The threshold effect is defined as the minimum carrier to noise ratio that givest he output SNR not less than the value predicted by the usual signal to noise formula assuming a small noise power. For a qualitative discussion of the FM threshold effect, Consider, when there is no signal present, so that the carrier is unmodulated. Thenthe composite signal at the frequency discriminator input is x(t) = [Ac +nI(t)] cos 2_fct – nQ(t) sin 2_fct Where nI(t) and nQ(t) are inphase and quadrature component of the narrow band noise n(t) with respect tocarrier wave Accos 2_fct. The phasor diagram of fig8.17 below shows the phase relations b/n the various components of x(t) As the amplitudes and phases of nI(t) and nQ(t) change randomly with time the point P wanders around the point Q. When the carrier to noise ratio is large nI(t) and nQ(t) are small compared to Ac, so that point P always around Q. Thus the angle _(t) small and within a multiple of 2_ radians.

(ii) Compare the noise performance of AM and FM systems.

(AUC NOV2005, AUC MAY2007)

3. (i) Derive the output SNR for envelope detection. Derive the expression for figure of merit of

an AM Receiver using envelope

Detection. What do you infer from the expression? (16)(AUC DEC 2010)

4. Explain the FM receiver with block diagram. (AUC NOV 2007) Define and explain FM

Threshold effect. With suitable diagram, explain

threshold reduction by FMFB demodulator. (16)(AUC DEC 2010)

5. State central limit theorem. (AUC NOV 2008)

6.Derive the expression for the Power spectral density at the output of a FM discriminator when

the received signal is corrupted by additive white Gaussian Noise.(AUC MAY 2009) 7.Derive an expression for SNR at input (SNRc) and output of (SNRo) of a coherent detector.

(16)(AUC MAY 2010) (ii) Compare the performances of AM and FM systems. (6)(AUC MAY 2010)

8. Sketch the block diagram of DSB-SC/AM system and derive the figure of merits Sketch the

block diagram of DSB-SC/AM system and derive the figure of merit.(AUC NOV 2008) (AUC

MAY 2012) Obtain the expression for figure of merit of DSB-SC receiver using coherent

detection. (AUCDEC2011)

(ii)Compare the performances of AM and FM systems(AUC MAY 2011)

9. Using super heterodyne principle, draw the block diagram of AM radio(AUC MAY 2006)

(ii)Using super heterodyne principle, draw the block diagram of AM radio receiver and

briefly explain it (AUC MAY 2011) Explain the working of super heterodyne receiver.

(AUC MAY 2006)

An AM receiver processes amplitude-modulated signals received by its antenna. It

delivers an outpu tthat is a reproduction of the signal that originally modulated the rf

carrier at the transmitter. The signal can then be applied to some reproducing device,

such as a loudspeaker, or to a terminal device, such as ateletypewriter. Actual AM

receivers vary widely in complexity. Some are very simple; others contain a large

number of complex circuits.

FUNCTIONS

Selection

Selection is the ability of the receiver to select a particular frequency of a station from

all other station frequencies appearing at the antenna of the receiver.

Detection

Detection is the action of separating the low (audio) frequency intelligence from the high

(radio) frequency carrier. A detector circuit is used to accomplish this action.

Reproduction

Reproduction is the action of converting the electrical signals to sound waves, which can

then be interpreted by your ear as speech, music, and the like. An example of this might

be the stereo speakers in your car.

RECEIVER CHARACTERISTICS

Sensitivity, noise, selectivity, and fidelity are important receiver characteristics. These

characteristics will be useful to you when performing receiver tests. They can help you to

determine whether a receiver is working or not or in comparing one receiver to another.

Sensitivity

The ability of a receiver to reproduce weak signals is a function of the sensitivity of a

receiver. The weaker a signal that can be applied to a receiver and still produce a certain

value of signal output, the better the sensitivity rating. Sensitivity of a receiver is

measured under standardized conditions. It is expressed in terms of the signal voltage,

usually in the micro volts that must be applied to the antenna input terminals to give an

established level of the output. The output may be an ac or dc voltage measured at the

detector output or a power measurement (measured in decibels or watts) at the

loudspeaker or headphone terminals.

Noise

All receivers generate a certain amount of noise, which you must take into account

when measuring sensitivity. Receiver noise may originate from the atmosphere

(lightning) or from internal components (transistors, tubes). Noise is the limiting factor of

sensitivity. You will find sensitivity is the value of input carrier voltage (in microvolts) that

must be applied from the signal generator to the receiver input to develop a specified

output power.

Selectivity

Selectivity is the degree of distinction made by the receiver between the desired signal

and unwanted signals. You will find the better the ability of the receiver to reject

unwanted signals, the better its selectivity. The degree of selection is determined by the

sharpness of resonance to which the frequency determining circuits have been

engineered and tuned. You usually measure selectivity by taking a series of sensitivity

readings. As you take the readings, you step the input signal along a band of

frequencies above and below the circuit resonance of the receiver; for example, 100

kilohertz below to 100 kilohertz2-11 above the tuned frequency. As the tuned

frequency, the input level required to maintain a given output level will fall. As you pass

the tuned frequency, the required input level will rise. Input voltage levels are then

compared with frequency. They can be plotted on paper or you might view them on an

oscilloscope. They would appear in the form of a response curve. The steepness of the

response curve at the tuned frequency indicates the selectivity of the receiver.

Fidelity

The fidelity of a receiver is its ability to accurately reproduce, in its output, the signal that

appears at its input. You will usually find the broader the band passed by frequency

selection circuits, the greater your fidelity. You may measure fidelity by modulating

an input frequency with a series of audio frequencies; you then plot the output

measurements at each step against the audio input frequencies. The

resulting curve will show the limits of reproduction. You should remember that good

selectivity requires that a receiver pass a narrow frequency band.

Good fidelity requires that the receiver pass a broader band to amplify the

outermost frequencies of the sidebands. Receivers you find in general use are a

compromise between good selectivity and high fidelity.

SUPERHETERODYNE RECEIVER

The super heterodyne is the type receiver most familiar to you. You probably see

one daily in your home in the form of an AM and/or fm radio. We will discuss the

basic workings of both AM and fm types and their differences.

Amplitude Modulation Receiver developed to overcome the disadvantages of earlier type

receivers. Let‘s assume you are tuning the receiver. When doing this you are actually

changing the frequency to which the rf amplifier is tuned. The rf carrier comes in from the

antenna and is applied to the rf amplifier. The output of the amplifier is an amplified

carrier and is sent to the mixer. The mixer also receives an input from the local oscillator.

These two signals are beat together to obtain the IF through the process of

heterodyning. this time you should note the dotted lines connecting the local oscillator, rf

amplifier, and the mixer. This is used on block diagrams and schematics to indicate

GANGED TUNING.

Ganged tuning is the process used to tune two or more circuits with a single control. In

our example, change the frequency of the receiver all three stages change by the same

amount. There is a fixed difference in frequency between the local oscillator and the rf

amplifier at all times. This difference in frequency is the IF. This fixed difference and

ganged tuning ensures a constant IF over the frequency range of the receiver.

Figure .—AM super heterodyne receiver and waveforms.

The IF carrier is applied to the IF amplifier. The amplified IF carrier is then sent to the

detector. The output of the detector is the audio component of the input signal. This

audio component is then passed through an audio frequency amplifier. The amplified

audio component is sent to a speaker for reproduction. This allows you to hear the

signal. Super heterodyne receiver may have more than one frequency-converting stage

and as many amplifiers as needed to obtain the desired power output.

10. Discuss the effects of noise on the carrier in FM receiver with suitable mathematical

derivation. (AUC MAY 2012) Discuss about the noise performance of an FM receiver to

evaluate its figure of merit. (AUC NOV 2008) (ii) Discuss about the noise performance of

an FM receiver to evaluate its figure of merit.(AUC NOV 2008) (i)Derive the figure of

merit for FM Receiver.(10)

11. Sketch the block diagram of SSB/AM system and derive the figure of merit.