Chapter 3 Sr Question Mar No k. 1 Write any two drawbacks of TRF radio receiver 1. Instability due to oscillatory nature of RF amplifier.. Variation in bandwidth over tuning range. 3. Insufficient selectivity at high frequencies. Poor adjacent channel rejection capability What is the purpose of keeping RF section before mixer stage? A radio receiver always has a RF section before the mixer stage because: 1. It selects the wanted frequency and rejects the unwanted frequencies.. Amplifier improves quality of receiver output and removes noise from received signal. 3. Better coupling of receiver to the antenna.. Prevention in the re radiation of the local oscillator through the antenna of the receiver 3 State the IF frequency and bandwidth of FM receiver IF for FM receiver: 10.7 MHz. Bandwidth of FM receiver:88mhz to 108 MHz What is the tuning range and IF value of (i) MW band AM and (ii) FM radio receiver? 5 Tuning Range of: MW AM band: 50 to 160 KHz FM Receiver: 88 to 108 MHz Intermediate Frequency (IF) of: MW AM band: 55 KHz. FM Receiver: 10.7 MHz What is tracking? List its types. Tracking is a process in which the local oscillator frequency follows or tracks the signal frequency to have a correct frequency difference. Types of Tracking: 1. Two Point Tracking-1. Padder Tracking.Trimmer Tracking. Three Point Tracking
6 What are the different types of FM Detectors 1) Single slope detector ) Balanced slope detector 3) Phase discriminator ) Ratio detector 7 State advantages & disadvantages of balanced slope detector Advantages :- i) It is more efficient than simple slope detector. ii) It is linear. Disadvantages: - i) Amplitude limiting is not provided. ii)difficult to tune 3 tuned circuits to 3 different frequency 8 State the function of limiter ckt. Used in FM Receiver. 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: 9 State merits of delayed AGC as compared with simple AGC. Merits of Delayed AGC: 1. No reduction in gain for weak signals.. Reduction in gain only for strong signals. 3. Delayed AGC is adjustable 10 Draw the block diagram of AM super heterodyne radio receiver and state the function of each block. Diagram-
Explanation- Receiving antenna- AM receiver operates in the frequency range of 50 KHz to 160 KHz. RF stage- Selects wanted signal and rejects all other signals and thus reduces the effect of noise. Mixer- Receives signal from RF stage Fs and the local oscillator Fo, and are mixed to produce intermediate frequency signal IF which is given as: IF=Fo-Fs Ganged Tuning- To maintain a constant difference between the local oscillator and RF signal frequency, gang capacitors are used. IF stage- The IF signal is amplified by the IF amplifier with enough gain. Detector-Amplified signal is detected by the detector to get original modulating signal. The detector also provides control signals to control the gain of IF and RF stage called as AGC. AGC- Automatic gain control controls the gain of RF and IF amplifiers to maintain a constant output level at the speaker even though the signal strength at the antenna varies. 11 State the functions of RF section used in AM radio receiver. State any four advantages of RF A radio receiver always has a RF section because 1. It is a tunable circuit connected to the antenna terminals.. It selects the wanted frequency and rejects the unwanted frequencies. 3. Amplifier improves quality of receiver output.. Better coupling of receiver to the antenna. 5. Prevention in the reradiation of the local oscillator through the antenna of the receiver.
Advantages: advantages- 1. Greater gain i.e. better sensitivity. Improved image frequency rejection 3. Improved signal to noise ratio. Better selectivity 1 Define intermediate frequency (IF).why local oscillator frequency (f0) is made greater than signal frequency (Fs) in radio receiver? The intermediate frequency (IF) of a receiving system is usually a compromise, since there are reasons why it should be neither low, nor high, nor in a certain range between these two. The local oscillator frequency (f0) is made greater than signal frequency (Fs) in radio receiver: Local oscillator frequency range is 995 KHz to 105 KHz for MW band. Fmax/Fmin = 105/995 =. 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 =1.0 The normal tunable capacitance ratio is, Cmax/Cmin = 10 So this capacitance ratio easily gives the frequency ratio of.:1. Hence, the.:1 ratio required for the local oscillator operating above signal frequency is well within range whereas the other system has a frequency ratio of 1:1 whose capacitance are not practically available. 13 Explain the function of mixer in AM receiver with neat diagram. Explaination: The mixer receives signals from the Rf amplifies at frequency fs and from the local oscillator at frequency for such that fo > fs. The mixer will mix these signal to produce signals having frequencies fs,fo, (fo + fs) and (fo - fs). out of these the difference of frequency component i.e. (fo - fs) is selected and all other are rejected. 1 What is the need of AGC? Explain simple AGC with its characteristics Graph
The need of AGC circuit is to maintain the output voltage level (volume) of radio receiver constant over a wide range of RF input signal level. AGC also helps to smooth out the rapid fading which may occur with long distance short wave reception & prevents overloading of the last IF amplifier which might otherwise have occurred. Simple AGC: Simple AGC is a system by means of which overall gain of a radio receiver is varied, automatically with the changing strength of the receiver signal to keep the output substantially constant. Hence the receiver gain is automatically reduced as the input signal becomes more & more strong. 15 Draw practical AM diode detector circuit. Sketch its input and output waveforms. Diagram:- Explanation- The circuit operates in the following manner- The diode has been reversed so that now the negative envelope is demodulated. Due to this negative envelope AGC voltage will be developed.r1 and R ensures that there is a series DC path to ground for diode. waveform R1 and C1 is the low pass filter used to remove RF ripple that is present in the detector o/p. C is coupling capacitor that prevents the diode DC o/p from reaching the volume control R. R3 and C3 is a low pass filter which removes AF components and helps to produce AGC voltage. The DC AGC voltage is proportional to the amplitude of AM signal.
16 Draw & explain the balanced slope detector. Working Principle: The difficulties arising in simple slope detector circuit are overcome balanced slope detector. The circuit uses two slope detectors, connected back to back to the opposite ends of center tapped transformer and hence fed 180o out of phase. The circuit is divided in to three tuned circuits. Primary side tuned circuit is tuned to center frequency fc. Secondary side top of tuned circuit in tuned above If i.e. (fc + f) and bottom of tuned circuit is below IF i.e. (fc- f). Each tuned circuit is connected to diode detector and RC load. R1C1 and RC are filtered to remove RF ripple. Final output voltage v0 is v0 = v01- v0 Circuit Operation: The circuit depends on range of frequencies For fin = fc : Voltage at T1 = voltage at T Input voltage at D1 = Input voltage at D V01 = V0 V0 = 0 fc < fin <(fc + f): Voltage induced in T1 > voltage induced in T Input voltage at D1 > Input voltage at D Output voltage V01 is positive as frequency increases towards (fc = f) The positive output voltage increases as shown in figure. (fc - f) < fin < fc: Voltage induced in T> voltage induced in T1 Input voltage to D > Input voltage to D1 V0 is negative. V0> V01 The negative output voltage increases towards (fc - f) as shown in figure.
17 Define sensitivity and selectivity. Draw the graph of sensitivity and selectivity for radio receiver. Sensitivity-The ability to amplify weak signals is called sensitivity. The sensitivity is expressed in millivolt Selectivity: The ability of radio receiver to reject the unwanted signals. 18 Draw practical set-up and explain the procedure to measure selectivity of radio Receiver.
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 and above the 950 KHz in suitable steps. Every time the generator output voltage is adjusted to get a standard 50 miliwatt receiver output power. The attenuation is calculated and plotted as shown in figure 19 Draw a neat circuit diagram of two stage IF amplifier & explain its working The IF amplifier is a fixed frequency amplifier. It is supposed to select desired signal and rejects adjacent unwanted frequencies. The above diagram is two stage amplifiers with single tuned IF transformers. The IF amplifier are class A amplifiers provide gain in the range 10 to 30 db. Ferrite core transformers are used for coupling between the stages 0 Draw the block diagram of FM receiver & explain the function of any three blocks.
RF amplifier: There are two important functions of RF amplifier: To increase the strength of weak RF signal. ) To reject image frequency signal. In FM broadcast the channel bandwidth is large as compared to AM broadcast. Hence the RF amplifier must be design to handle large bandwidth. 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 intermediate frequency fi. The intermediate frequency used in FM receiver is higher than that in AM receiver. Its value is 1MHz (practical value of IF is 10.7MHz). Local oscillator: Since FM broadcast operates in VHF and UHF band, a separate local oscillator is used in FM receiver The local oscillator frequency fo is kept smaller than the signal frequency fs by an amount equal to the intermediate frequency fi (fi = fs-fo). IF amplifier: Two or more stages of IF amplifier are used to provide large gain to the receiver. This increases the sensitivity of a receiver. If amplifier should be designed to handle large bandwidth. Amplitude limiter: The function of amplitude limiter is to remove all amplitude variation of FM carrier voltage that may occur due to atmospheric noise. Use of amplitude limiter makes the system less noisy. FM Discriminator or detector: It separates modulating signal from frequency modulated carrier signal. Thus it produces audio signal at its output. Audio frequency voltage and power amplifier: Audio amplifier increases voltage and power level of audio signal to a suitable level.in FM broadcast, the maximum modulating frequency is 15 khz. Hence the audio amplifier must have large bandwidth. 1 Draw the neat circuit diagram of FET amplitude limiter used in FM receiver.
In order to make full use of the advantages offered by FM, a demodulator must be preceded by an amplitude limiter, unwanted amplitude changes in the signal fed to the FM demodulator are spurious. They must therefore be removed if distortion is to be avoided. The point is significant, since most FM demodulator react to amplitude changes as well as frequency changes. The limiter is a form of clipping device. When input signal voltage rises, current flows in the Rg - Cg bias circuit & a negative voltage is developed across the capacitor. It is seen that the bias on the FET is increased in proportion to the size of the input vo As a result the gain of the amplifier is lowered, & the output voltage tends to remains constant Draw the circuit diagram & explain the working of phase discriminator. This discriminator is also known as the center tuned discriminator or the FosterSeeley discriminator after its inventors. It is possible to obtain the same S- Shape response curve from a circuit in which the primary & the secondary winding are both tuned to the center frequency of the incoming signal. This is desirable because it greatly simplifies alignment & also because the process yields far better linearity. Thus. Although the individual component voltage will be the same at the diode input at all frequencies, the vector sums will differ with the phase difference between primary & secondary windings. The result will be that the individual output voltage will be equal only at fc. At all other frequencies the output of one diode will be greater than that of the other. Which diode has the larger output will depend entirely on whether fm is above or below fc. As for the output arrangements, it will be positive or negative according to the input frequency. As required the magnitude of the output will depend on the deviation of the input frequency from fc. 3 Explain the working of FM demodulator using phase lock loop with the help of circuit diagram
Explanation:- FM signal which is to be demodulated is applied to input of PLL.VCO output must be identical to input signal if PLL is to remain locked. As PLL is locked, VCO starts tracking the instantaneous frequency in the FM input signal The error voltage produced at the output of the amplifier is proportional to the deviation of the input frequency from the centre frequency FM. Thus AC component of the error voltage represents the modulating signal. Thus at the error amplifier output we get demodulated FM output. Draw circuit of Ratio detector circuit. Why Limiter stage is not used before Ratio detector? The additional feature of the ratio detector is the amplitude limiting action which is incorporated due to the large capacitor C5,due to this the amplitude limiter is not required prior to the ratio detector. 5 Define Image Frequency. The RF, local oscillator frequency, IF frequency for AM Receiver is 800KHz, 155KHz & 55KHz respectively. Determine image frequency Given fs= 800kHz fo=155khz. IF=55KHz.
Image frequency(fsi)=fs+if Fsi=800+*55 Fsi=1710KHz. Image frequency(fsi) if unwanted station signal frequency(fsi)manges to produce desired IF at the output of mixer then it amplified by IF amplifier along with desired signal fs.this will create interference because carrier of both the station tuned at the same position.this unwanted signal at the frequency fsi is known as image frequency. Fsi=fs+IF 6 In broadcast super heterodyne receiver having loaded Q of antenna coupling of 100, if intermediate frequency of 55 khz, calculate image frequency and its rejection ratio at 1000 Given:-Q=100 Intermediate frequency = IF= 55 KHz Incoming signal Frequency Fs=100 KHz 1) fsi is given as- fsi= Fs+ IF = 1000 x 103+ (55 x 103) = 1910 KHz ) Rejection ratio is given by α = Where Q is the loaded Q of tuned circuit or antenna coupling ρ = - = - = 1.386 Therefore, α = = = 138.6 7 The desired signal frequency is 93 MHz and the intermediate frequency is 10.7 MHz calculate the local oscillator frequency and image frequency Given fs= 800kHz fo=155khz. IF=55KHz. Image frequency(fsi)=fs+if Fsi=800+*55 Fsi=1710KHz. 8 A super heterodyne AM receiver is tuned to a station operating at 100 KHz. Find local oscillator frequency and image frequency. fs= 100kHz receiver is super heterodyne so consider IF=55KHz. Image frequency(fsi)=fs+if Fsi=100+*55 Fsi=110KHz. Image frequency=110khz. Fo=fs+IF Fo=100+55 Fo=1655KHz.
Local oscillator frequency=1655khz.