search
person
Why should local oscillator frequency be greater than signal frequency in AM receiver? Also explain why IF has constant value?

1 Answer

Reason for LO frequency to be greater than signal frequency

The local oscillator frequency (f0) is made greater than signal frequency (Fs) in radio receiver:

Local oscillator frequency range is 995 KHz to 2105 KHz for MW band.

Fmax/Fmin = 2105/995 = 2.2

If local oscillator has been designed to be below signal frequency, the range would be 85 to 1195 KHz and frequency ratio is,

Fmax/Fmin = 1195/85 =14.0

The normal tunable capacitance ratio is, Cmax/Cmin = 10

So this capacitance ratio easily gives the frequency ratio of 2.2:1.

Hence, the 2.2:1 ratio required for the local oscillator operating above signal frequency is well within range whereas the other system has a frequency ratio of 14:1 whose capacitance are not practically available.

Why IF has constant value?

*The problem in the TRF receiver are solved in this receiver by converting every selected RF signal to a fixed lower frequency called as the Intermediate frequency(IF)

*This frequency contains the same modulation as the original carrier. The IF signal is then amplified and detected to get back the modulating signal.

*As the IF is lower than the lowest RF signal frequency the possibility of oscillations and instability is minimized.

*Also the required value of Q for constant BW does not depend on the frequency of desired signal because the IF is constant and same for all the incoming RF signal.
thumb_up_alt 0 like thumb_down_alt 0 dislike

Related questions

Description : A super heterodyne AM receiver is tuned to a station operating at 1200 KHz .Find local oscillator frequency and image frequency.

Answer : A super heterodyne AM receiver is tuned to a station operating at 1200 KHz Intermediate frequency is 455KHz. IF frequency=f0-fs Local oscillator frequency is f0=IF +fs=455K+1200K=1655kHz The image frequency which gives the same IF is f0+2*IF=2110KHz

Description : A superheterodyne radio receiver with an IF of 455KHZ is turned to 1000KHZ. Find: (i) Image frequency (ii) Local oscillator frequency

Answer : Given Intermediate Frequency fi=455KHz Signal frequency =fs=1000KHz Local oscillator frequency fo=fs+fi  Fo=1000KHz+455KHz  =1455KHz Image frequency is the input frequency which produces the same intermediate frequency fsi=fs+2fi  =1000KHz+2*455KHz  =1910KHz 

Description : Describe the functions of mixer and local oscillator in radio receiver

Answer : Frequency Mixer:- The function of frequency mixer is to heterodyne signal frequency fs and local oscillator frequency fo. At the output, it produces the difference frequency known as ... local oscillator frequency fo is then mixed with incoming frequency to give intermediate frequency. 

Description : A telephone cable has following primary constants per loop kilometer ,R=30Ω, L=20mH,C=0.06µF,G=0.If the applied signal has an angular frequency of 5000 rad/sec.., Determine (i) Characteristics impedence (ii) Attenuation constant

Answer : A telephone cable has following primary constants per loop kilometer ,R=30Ω, L=20mH,C=0.06µF,G=0.If the applied signal has an angular frequency of 5000 rad/sec.., Determine (i) Characteristics impedence (ii) Attenuation constant

Description : Explain the following characteristics of AM radio receiver: (i) Sensitivity (ii) Selectivity.

Answer : Sensitivity:- The ability to amplify the weak signals is called sensitivity. It is the function of overall receiver gain. Sensitivity of radio receiver is decided by the gain of the RF IF ... perfectly the receiver is able to select the desired carrier frequency and reject other frequencies. 

Description : Show that AM wave consist of two side bands and carrier. Also prove the bandwidth of AM is double of the modulating frequency.

Description : Explain the demodulation of AM signal using diode detector.

Answer : Simple diode detector:- Explanation:- This is essentially just a half wave rectifier which charges a capacitor to a voltage nearly to the peak voltage of the incoming AM waveform s(t). ... detector output is proportional to the signal strength. Stronger Am signal higher is the dc AGC voltage.

Description : State the IF frequency and bandwidth of FM receiver.

Answer : IF for FM receiver: 10.7 MHz. Bandwidth of FM receiver:88MHz to 108 MHz

Description : Define selectivity and sensitivity of AM Receiver. 

Answer : Selectivity:-It needs to accept signals of the wanted frequency and reject other unwanted frequencies. Sensitivity:-The sensitivity of a radio receiver is its ability to amplify weak signals. It is ... the receiver input terminals to give a standard output power, measured at the output terminals. 

Description : Define modulation? State the desirable value of modulation index of AM.

Answer : Modulation: It is the process in which any of these Parameters (Amplitude, Frequency, Phase) of high frequency signal (carrier signal) are varied in accordance with low frequency signal(modulating signal) For an AM, modulation index range is 0 to 1.

Description : Explain the working of amplitude limiter in FM receiver with circuit diagram.

Answer : Amplitude limiter: The function of amplitude limiter is to remove all amplitude variation of FM carrier voltage that may occur due to atmospheric disturbances. Use of amplitude limiter makes the system less noisy  Circuit Diagram:

Description : Draw practical set-up and explain the procedure to measure selectivity of radio receiver.

Answer : Procedure to measure selectivity of radio receiver: Throughout the measurement the receiver is kept tuned to desired frequency 950 Khz. Now the generator output frequency is deviated below ... the generator output voltage is adjusted to get a standard 50 miliwatt receiver output power.

Description : Draw block diagram of FM receiver and explain the use of limiter circuit.

Answer : Diagram Explanation Amplitude limiter: The function of amplitude limiter is to remove all amplitude variation of FM carrier voltage that may occur due to atmospheric disturbances. Use of amplitude limiter makes the system less noisy.

Description : Draw the TRF receiver block diagram and explain its working. 

Answer : Explanation:- 1. TRF receivers are simple and having high sensitivity. The AM transmission takes place in MW band and SW band. MW frequency range is 540KHz to 1640KHz. 2. ... power level to drive the loudspeaker. 7. The loudspeaker converts electrical signal into original sound information.

Description : Explain effect of 'm' on AM with neat waveforms.

Answer : i)m< 1  If m < 1 or if the percentage of modulation is less than 100% the this type of modulation is known as under modulation The amplitude of modulating signal less than carrier ... greater amplitude part of its information is lost in the process of modulation which is undesirable.

Description : Draw the block diagram of AM transmitter, explain its operation.

Answer : AM transmitter is of two types i)High Level AM Transmitter ii)Low Level AM Transmitter i) RF crystal oscillator: It is a source of carrier signal of desired frequency. The circuit is ... AM signal is then applied to the transmitting antenna which radiates this AM signal into air or space. 

Description : Why electromagnetic waves are said to be transverse wave ? Explain.

Answer : The electromagnetic waves are oscillations which propagate through free space. In electromagnetic waves the direction of electric field, magnetic field & propagation are mutually perpendicular. Hence electromagnetic waves are called as transverse wave.

Description : If Ram knows that y is an integer greater than 2 and less than 7 and Hari knows that y is an integer greater than 5 and less than 10, then they may correctly conclude that (A) y can be exactly ... two values (C) y may be any of three values (D) there is no value of y satisfying these conditions

Answer : (A) y can be exactly determined

Description : State two disadvantages of TRF receiver over superheterodyne receiver.

Answer : Disadvantages of TRF Receiver: 1. Instability due to oscillatory nature of RF amplifier. 2. Variation in bandwidth over tuning range. 3. Insufficient selectivity at high frequencies 4. Poor adjacent channel rejection capability 

Description : Describe the block diagram of FM superhetrodyne receiver

Answer : RF amplifier:- There are two important functions of RF amplifier: 1) To increase the strength of weak RF signal. 2) To reject image frequency signal. In FM broadcast the ... FM broadcast, the maximum modulating frequency is 15 kHz. Hence the audio amplifier must have large bandwidth.

Description : Write any two drawbacks of TRF radio receiver.

Answer : Drawbacks of TRF Receiver: 1. Instability due to oscillatory nature of RF amplifier. 2. Variation in bandwidth over tuning range. 3. Insufficient selectivity at high frequencies 4. Poor adjacent channel rejection capability. 

Description : Draw the block diagram of FM super heterodyne radio receiver with waveforms.

Answer : block diagram of FM super heterodyne radio receiver

Description : 1. Who has the authority to remove the Vice-President from his office before the expiry of his term? 2. Which is the most malleable metal? 3. For internal financing of Five Year Plans, the ... is received by the earth's surface? 20. Who was the famous Indo-Greek king who embraced Buddhism?

Answer : Answer : 1. Parliament 2. Gold 3. Taxation only 4. Atmospheric refraction 5. English 6. Vijaynagar Empire 7. Gymnosperm 8. 14 9. Johannes Kepler 10. Varuna 11. Cavendish 12. Warm, dry and ... Torrid Zone 15. Varuna 16. Leprosy 17. Comptroller and Auditor General 18. Oxidation 19. 47% 20. Menander

Description : Define modulation index in AM and give its formula.

Answer : In AM wave, the modulation index (m) is defined as the ratio of the amplitude of the modulating signal (Vm ) to the amplitude of carrier signal (Vc). m = Vm / Vc

Description : Explain any four different frequency bands and give their two applications of each.

Answer : Explain any four different frequency bands and give their two applications of each.

Description : Give the expression for modulation index for AM and FM.

Answer : 1.Modulation index for AM ma=Vm / Vc Where Vm is modulating voltage Vc is the carrier voltage 2.Modulation index for FM mf=δ /fm where δ is the deviation fm is the modulating frequency

Description : Differentiate between AM and FM on the basis of: (i) Definition (ii) Bandwidth (iii) Modulation Index (iv) Application

Answer : Compare AM and FM on the basis of i)Definition ii)Bandwidth iii) Wave propogation iv)Number of sidebands

Description : For AM, fc =500kHz , fm = 5 kHz Determine: (i) Upper and lower sideband frequencies (ii) Bandwidth

Answer : Given data fc = 500KHz , fm = 5KHz USB =fc+fm USB = 500+5  =505 KHz. LSB =fc – fm LSB = 500 – 5  =495 KHz Bandwidth = 2fm .  = 2* 5 =10 KHz

Description : Explain with a neat diagram of ratio detector. Why limiter stage is not used before ratio detector.

Answer : With diode D2 reversed , O is positive with respect to b, so that Vab is a sum voltage rather than the difference it was in the discriminator it is now possible to connect a large capacitor ... of limiter stage due to large capacitor C5 .Hence, limiter stage is not used in ratio detector.  

Description : Define: Base band signal with one example.

Answer : The electrical equivalent of the original information signal is known as the Baseband signal. Example:-The information or the input signal to a communication system can be analog i.e., sound, picture or it can be digital e.g. the computer data.

Description : Sketch AM signal in (1)Time domain (2)Frequency domain.

Answer : AM in Time domain AM in frequency domain

Description : The student marks should not be greater than 100. This is (A) Integrity constraint (B) Referential constraint (C) Over-defined constraint (D) Feasible constraint

Answer : (A) Integrity constraint

Description : In a radio transmitter, the frequency of the crystal oscillator will be stable for a long time if the quality factor of the crystal resonator is  (a) > 100 (b) > 500 (c) > 1000 (d) > 20000

Answer : In a radio transmitter, the frequency of the crystal oscillator will be stable for a long time if the quality factor of the crystal resonator is  > 1000

Description : Why the emf of a cell is always greater than its terminal voltage? Explain the same.

Answer : Every practical source offers some opposition to the current due to its internal parts or components. Such a resistance of internal parts of source is called internal resistance of source. When source delivers ... less than emf induced. So, emf of a cell is always greater than terminal voltage.

Description : In a FM system, the maximum deviation is 75KHz. Find bandwidth for modulating frequency i. fm=500Hz ii. fm=5KHz iii. fm=10KHz Draw conclusion for bandwidth of FM from answer.

Answer : Given deviation∆=75kHz i) fm=500Hz bandwidth B.W=2(∆+fm)  =2(75k+500)=151kHz  ii). fm=5KHz bandwidth B.W=2(∆+fm)  =2(75k+5k)=160KHz iii) fm=10KHz bandwidth B.W=2(∆+fm)  =2(75k+10k)=170KHz As the modulating frequency increases bandwidth also increases.

Description : Draw amplitude modulated waveform in time domain and frequency domain with proper labeling.

Answer : AM in Time domain  AM in frequency domain

Description : Define modulation index in FM. What is maximum value of deviation ratio.

Answer : Modulation Index of FM: It is defined as the ratio of Frequency Deviation (δ) to the modulating signal frequency (fm). (OR)  

Description : Define critical frequency w.r. to wave propagation.

Answer : Define critical frequency w.r. to wave propagation.

Description : Define: 1)Frequency 2)Bandwidth 3)Wavelength 4)Time period

Answer : Frequency: Frequency is the number of cycles per second. Bandwidth: A range of frequencies within a given band, in particular that used for transmitting a signal. Wavelength: Wavelength can be defined as ... for one complete cycle of vibration to pass a given point.  OR Time period=1/frequency

Description : State the frequency range for audio frequency and voice frequency.

Answer : The generally accepted standard range of audible frequencies is 20 to 20,000 Hz, The usable voice frequency band ranges from approximately 300 Hz to 3400 Hz.

Description : Compare ground wave and space wave propagation on the basis of: (i) Frequency range (ii) Method of propagation.

Answer : Compare ground wave and space wave propagation on the basis of: (i) Frequency range (ii) Method of propagation.

Description : Write down different frequencies for following(frequency ranges). (1) Voice frequency (2) High frequency (3) IR frequency (4) Visible frequency(light)

Answer : Sr. No Frequency Range 1 Voice Frequency 300 Hz to 3kHz 2 High Frequency  3 MHz to 30 MHz 3 IR Frequency 30 THz to 430 THz 4 Visible Spectrum (light) 375 THz to 750 THz

Description : Define: (i) Image frequency and (ii) Double spotting

Answer : i)Image frequency:- Image Frequency is defined as the signal frequency plus twice the intermediate frequency. It is denoted as fsi = fs+2fi Where, fs = Signal Frequency fi =intermediate frequency ... It is due to the poor front end selectivity i.e., inadequate image frequency rejection.

Description : Define the terms: 1) Maximum usable frequency 2) Fading

Answer : 1)Maximum usable frequency:- Maximum usable frequency is defined as the limiting frequency ,when the angle of incidence is other than the normal . OR The highest frequency that can be used for sky ... which left the same source but arrived at the destination by different paths is known as fading.

Description : A pupil should definitely be encouraged to go to college if Options: A) He has better than the average intelligence B) He has financial backing to see him through C) He has met high-school graduation requirements D) All the above are true and he wishes to go

Answer : D) All the above are true and he wishes to go 

Answer : A receiver has poor selectivity. It will also have poor blocking.

Description : Women’s hair is about half the diameter of men’s hair. While it might sound strange, it shouldn’t come as too much of a surprise that men’s hair should be coarser than that of women. Hair diameter also varies on average between races, making hair plugs on some men look especially obvious.

Description : The relationship between kinetic energy and the potential energy of a swinging pendulum bob is one of the following. Is it: w) kinetic energy is greater than potential energy x) kinetic ... kinetic energy is equal to potential energy z) kinetic energy plus potential energy equals a constant  

Answer : ANSWER: Z -- KINETIC ENERGY PLUS POTENTIAL ENERGY EQUALS A CONSTANT 

← Prev Question Next Question →
editChoose topic
...