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CH.3 Radio Receiver (Marks ) Visit for more Learning Resources Introduction Functions of Receiver: Transmitted signals receive at receiving antenna. Select the desired station signal and reject all other unwanted signals. Amplify selected
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01
CH.3 Radio Receiver
(Marks ) Visit for more Learning Resources<br>
(Marks ) Visit for more Learning Resources<br>
02
Introduction Functions of Receiver:
Transmitted signals receive at receiving antenna.
Select the desired station signal and reject all other unwanted signals.
Amplify selected signal.
Demodulate the amplified signal.
Original modulating signal is power amplified.
Power amplified signal drives the loudspeaker.
Loudspeaker converts electrical signal into original sound information<br>
Transmitted signals receive at receiving antenna.
Select the desired station signal and reject all other unwanted signals.
Amplify selected signal.
Demodulate the amplified signal.
Original modulating signal is power amplified.
Power amplified signal drives the loudspeaker.
Loudspeaker converts electrical signal into original sound information<br>
03
Types of Radio Receiver There are two types of radio receiver.
Radio Receiver
TRF Radio Receiver Superhetrodyne
(Tuned Radio Frequency) Radio receiver
TRF receiver is oldest but simple. But it has many drawbacks.
Now-a-days, superehetrodyne receivers are in use and most popular because it has advantages over TRF.<br>
Radio Receiver
TRF Radio Receiver Superhetrodyne
(Tuned Radio Frequency) Radio receiver
TRF receiver is oldest but simple. But it has many drawbacks.
Now-a-days, superehetrodyne receivers are in use and most popular because it has advantages over TRF.<br>
04
Tuned Radio Frequency (TRF) Receiver<br>
05
Drawbacks in TRF Receiver Even though TRF receiver are simple in operation, but has some problems:
(i) Instability:
Due to oscillatory nature of RF amplifiers.
(ii) Variation in bandwidth over tuning range:
Variation due to variation in Quality factor ‘Q’.
If BW of receiver increases, it will pick-up the adjacent channel along with the desired one.
(iii) Insufficient selectivity:
i.e. Insufficient adjacent frequency rejection. Due to increased bandwidth at higher frequencies, the ability of the TRF receiver to select the desired signal and reject all other signals is affected.<br>
(i) Instability:
Due to oscillatory nature of RF amplifiers.
(ii) Variation in bandwidth over tuning range:
Variation due to variation in Quality factor ‘Q’.
If BW of receiver increases, it will pick-up the adjacent channel along with the desired one.
(iii) Insufficient selectivity:
i.e. Insufficient adjacent frequency rejection. Due to increased bandwidth at higher frequencies, the ability of the TRF receiver to select the desired signal and reject all other signals is affected.<br>
06
AM Superheterodyne Receiver The problems in TRF radio receiver are solved in superheterodyne receiver.
Superheterodyne Principle
In superheterodyne receivers the incoming signal is mixed with output of local oscillator and converted into a signal with lower fixed frequency called intermediate frequency (IF). This is known as superheterodyne principle.
Superheterodyne means mixing of two frequencies.
The mixing process is multiplication of incoming signal (fs) and output of local oscillator signal (fo). The product gives rise to two signals with fs + fo and fo fs. The difference frequency produced (fo fs) is taken as If.<br>
Superheterodyne Principle
In superheterodyne receivers the incoming signal is mixed with output of local oscillator and converted into a signal with lower fixed frequency called intermediate frequency (IF). This is known as superheterodyne principle.
Superheterodyne means mixing of two frequencies.
The mixing process is multiplication of incoming signal (fs) and output of local oscillator signal (fo). The product gives rise to two signals with fs + fo and fo fs. The difference frequency produced (fo fs) is taken as If.<br>
07
Continued…….<br>
08
Continued…. We know that AM radio receiver operates in MW and SW band frequencies. MW band frequency range 540 kHz to 1640 kHz.
IF for MW band 455 kHz
Example - Pune station operates at 791 kHz frequency.<br>
IF for MW band 455 kHz
Example - Pune station operates at 791 kHz frequency.<br>
09
1 RF Section A radio receiver always has RF section, which is tunable circuit connected to the antenna terminals.
It selects the wanted frequency and reject unwanted frequencies.
Such a receiver need not have an RF amplifier.
In the domestic radio receivers RF amplifier is not used for economic reason, however RF amplifier improves quality of receiver output.
Reasons for Use of RF Amplifier:
The receiver having an RF stage is superior in performance.
On other hand RF amplifier is uneconomical.<br>
It selects the wanted frequency and reject unwanted frequencies.
Such a receiver need not have an RF amplifier.
In the domestic radio receivers RF amplifier is not used for economic reason, however RF amplifier improves quality of receiver output.
Reasons for Use of RF Amplifier:
The receiver having an RF stage is superior in performance.
On other hand RF amplifier is uneconomical.<br>
10
Continued----- Advantages of RF Amplifier(Characteristics)
Greater gain i.e. better sensitivity.
Improved image frequency rejection.
Improved signal to noise ratio.
Better selectivity
Improves quality of receiver output.
Better coupling of receiver to antenna.
prevention in reradiation of local oscillator.<br>
Greater gain i.e. better sensitivity.
Improved image frequency rejection.
Improved signal to noise ratio.
Better selectivity
Improves quality of receiver output.
Better coupling of receiver to antenna.
prevention in reradiation of local oscillator.<br>
11
Mixer or Frequency Changing The mixer or frequency changer is nothing but a non-linear resistance.
It has two inputs at frequencies fs and fo.
Output
Frequency changing in radio receiver changes the signal frequency (fs) into Intermediate frequency IF = fo – fs.<br>
It has two inputs at frequencies fs and fo.
Output
Frequency changing in radio receiver changes the signal frequency (fs) into Intermediate frequency IF = fo – fs.<br>
12
Local Oscillator In the receivers operating upto the limit of shortwave broadcasting, that is 36 MHz.
Most commonly colpitts and clap oscillators are used for higher operating frequencies.
Where the frequency stability of the local oscillator must be high, AFC may be used.<br>
Most commonly colpitts and clap oscillators are used for higher operating frequencies.
Where the frequency stability of the local oscillator must be high, AFC may be used.<br>
13
Continued…. Local oscillator frequency range is 995 kHz to 2105 kHz for MW band. It gives frequency ratio.
fmax/fmin=2105/995 = 2.2 (i.e. 2.2 : 1)
If the 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 (i.e. 14 : 1)
The normal tunable capacitance ratio is,
Cmax/Cmin= 10 (i.e. 10: 1)
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.<br>
fmax/fmin=2105/995 = 2.2 (i.e. 2.2 : 1)
If the 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 (i.e. 14 : 1)
The normal tunable capacitance ratio is,
Cmax/Cmin= 10 (i.e. 10: 1)
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.<br>
14
Tracking Definition:
Tracking is a process in which the local oscillator frequency follows or tracks the signal frequency to have a correct frequency difference.
Due to tracking errors stations will appear away from their correct position.
There are two types of tracking.
Tracking
Two Point Tracking Three Point Tracking
(i) Padder Tracking
(ii) Trimmer Tracking<br>
Tracking is a process in which the local oscillator frequency follows or tracks the signal frequency to have a correct frequency difference.
Due to tracking errors stations will appear away from their correct position.
There are two types of tracking.
Tracking
Two Point Tracking Three Point Tracking
(i) Padder Tracking
(ii) Trimmer Tracking<br>
15
Padder tracking Fig.Padder tracking arrangement<br>
16
Continued…. Cp is small variable capacitor known as padder capacitor connected in series with oscillator coil.
• Cp and Cosc are connected in series so that effective capacitance will be less than Cosc alone.
Ceff = Cp.Cosc/ Cp+Cosc
Fig. Error in Padder tracking<br>
• Cp and Cosc are connected in series so that effective capacitance will be less than Cosc alone.
Ceff = Cp.Cosc/ Cp+Cosc
Fig. Error in Padder tracking<br>
17
Trimmer Tracking Fig. arrangement of Trimmer tracking
CTr is a small variable capacitor known as trimmer capacitor.
• CTr and Cosc are connected in parallel so that effective capacitance will be greater than Cosc alone.
Ceff. = CTr + Cosc<br>
CTr is a small variable capacitor known as trimmer capacitor.
• CTr and Cosc are connected in parallel so that effective capacitance will be greater than Cosc alone.
Ceff. = CTr + Cosc<br>
18
This will decrease the oscillator frequency making the tracking error negative shown in Fig. below
Fig. Error in Trimmer Tracking<br>
Fig. Error in Trimmer Tracking<br>
19
Three Point Tracking Fig. Three Point Tracking
It is a combination of padder and trimmer tracking.
Therefore, both positive and negative error in tracking exists.<br>
It is a combination of padder and trimmer tracking.
Therefore, both positive and negative error in tracking exists.<br>
20
Here three frequencies are of correct tracking shown in Fig. below.
Fig. Error in Three Point Tracking
Role of Padder capacitor Cp is same as explained in Padder tracin
• But due to combination of Cp and CTr the positive tracking error frequency range is less<br>
Fig. Error in Three Point Tracking
Role of Padder capacitor Cp is same as explained in Padder tracin
• But due to combination of Cp and CTr the positive tracking error frequency range is less<br>
21
Intermediate Frequency Choice of IF:
• 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 choice of IF depends on the factors:
1.If the intermediate frequency is too high, results in poor selectivity and poor adjacent channel rejection results.
2.High value of IF increases tracking difficulties.
3.As the IF is lowered, image-frequency rejection becomes poorer.
4.A very low IF can make the selectivity too sharp, cutting-off the sidebands.
5.If the IF is very low, the frequency stability of the local oscillator must be made corresponding higher.
6.The IF must not fall within the tuning range of the receiver, else instability will occur and heterodyne whistles will be heard, making it impossible to tune the frequency band immediately adjacent to the IF.<br>
• 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 choice of IF depends on the factors:
1.If the intermediate frequency is too high, results in poor selectivity and poor adjacent channel rejection results.
2.High value of IF increases tracking difficulties.
3.As the IF is lowered, image-frequency rejection becomes poorer.
4.A very low IF can make the selectivity too sharp, cutting-off the sidebands.
5.If the IF is very low, the frequency stability of the local oscillator must be made corresponding higher.
6.The IF must not fall within the tuning range of the receiver, else instability will occur and heterodyne whistles will be heard, making it impossible to tune the frequency band immediately adjacent to the IF.<br>
22
Continued…. IF frequencies used:<br>
23
Continued…. Why IF has constant value?
(a) If the IF is too high, poor selectivity and poor adjacent channel rejection results.
(b) High value of IF increases tracking difficulties.
(c) As the IF is lowered, image-frequency rejection becomes poor.
(d) A very low IF can make the selectivity too sharp, cutting of the sidebands.
(e) It must not fall within the tuning range of the receiver, else instability occur. This IF has constant value.<br>
(a) If the IF is too high, poor selectivity and poor adjacent channel rejection results.
(b) High value of IF increases tracking difficulties.
(c) As the IF is lowered, image-frequency rejection becomes poor.
(d) A very low IF can make the selectivity too sharp, cutting of the sidebands.
(e) It must not fall within the tuning range of the receiver, else instability occur. This IF has constant value.<br>
24
IF Amplifier The IF amplifier is a fixed frequency amplifier, with very important function of rejecting adjacent unwanted frequencies.
i.e. it decides sensitivity and selectivity of receiver.
•It provides maximum gain and selectivity in the receiver. mW AM receiver has IF value of 455 kHz.
•Its frequency response should be steep skirts.
•The two-stage IF amplifier as shown in Fig. is used to get a higher gain.
All IF transformers (IFT) are single tuned.
Note that neutralization may be used (capacitor Cn) in the transistor, IF amplifier depending on the frequency and type of transistor used.<br>
i.e. it decides sensitivity and selectivity of receiver.
•It provides maximum gain and selectivity in the receiver. mW AM receiver has IF value of 455 kHz.
•Its frequency response should be steep skirts.
•The two-stage IF amplifier as shown in Fig. is used to get a higher gain.
All IF transformers (IFT) are single tuned.
Note that neutralization may be used (capacitor Cn) in the transistor, IF amplifier depending on the frequency and type of transistor used.<br>
25
Continued…. Fig. Two Stage IF Amplifier<br>
26
Image Frequency Rejection In the broadcast AM receives the local oscillator frequency is higher than the incoming by intermediate frequency i.e.
fo = fs + IF
or IF = (fo fs)
•Assume that the local oscillator frequency is set to 'fo' and an unwanted signal at frequency fsi = (fo + IF) manages to reach at the input of the mixer. Then the mixer output consists of the four frequency components of
fo, (fo + IF), (2fo + IF) and IF<br>
fo = fs + IF
or IF = (fo fs)
•Assume that the local oscillator frequency is set to 'fo' and an unwanted signal at frequency fsi = (fo + IF) manages to reach at the input of the mixer. Then the mixer output consists of the four frequency components of
fo, (fo + IF), (2fo + IF) and IF<br>
27
Continued…. Where the last component at IF is the difference between fsi and fo [i.e. IF = fsi fo]. This component will also be amplified by the IF amplifier alongwith the desired signal at frequency fs. This will create interference because both the stations corresponding to carrier frequencies fs and fsi will be tuned at the same position.
• This unwanted signal at frequency fsi is known as Image frequency and it is said to be the image of the signal fs. The relation between fs and fsi is
Image frequency= fsi= fs +2IF<br>
• This unwanted signal at frequency fsi is known as Image frequency and it is said to be the image of the signal fs. The relation between fs and fsi is
Image frequency= fsi= fs +2IF<br>
28
Double Spotting The phenomenon related to image problem is double spotting.
This is usually biggest problem for receivers with a low value of IF.
This means that the image frequency is near to the signal frequency and image rejection is not as good as it could be.
When the receiver is tuned across the band, a strong signal appears to be at two different frequencies, once at the desired frequency and again when the receiver is tuned to two times IF (i.e. 2IF) below the desired frequency.
In this second case, the signal becomes the image, reduced in strength by the image rejection, thus, it appears the same signal nearby (i.e. same station) that is located at two frequencies in the band.
So that, two station programs will appear at a time through loudspeaker which can not be understandable and irritating to the hears.
Better to avoid double spotting.<br>
This is usually biggest problem for receivers with a low value of IF.
This means that the image frequency is near to the signal frequency and image rejection is not as good as it could be.
When the receiver is tuned across the band, a strong signal appears to be at two different frequencies, once at the desired frequency and again when the receiver is tuned to two times IF (i.e. 2IF) below the desired frequency.
In this second case, the signal becomes the image, reduced in strength by the image rejection, thus, it appears the same signal nearby (i.e. same station) that is located at two frequencies in the band.
So that, two station programs will appear at a time through loudspeaker which can not be understandable and irritating to the hears.
Better to avoid double spotting.<br>
29
Characteristics of AM Radio Receiver The performance of radio receiver is determined by its characteristics/ parameters.
• These are of three types.
Characteristics of Radio Receiver
Sensitivity Selectivity Fidelity<br>
• These are of three types.
Characteristics of Radio Receiver
Sensitivity Selectivity Fidelity<br>
30
Sensitivity The ability to amplify the weak signals is called sensitivity.
It is the function of the overall receiver gain.
Sensitivity of radio receiver is decided by the gain of the RF and IF amplifiers.
Practically, it is defined as the carrier voltage, which must be applied to the receiver input terminals to get standard output power at output terminals.
The loudspeaker is replaced by load resistance of equal value of speaker.
The sensitivity is expressed in m volt or millivolt.
It may be measured at various frequencies in the radioband.
Improvement in Sensitivity:
The high gain IF amplifiers provides better sensitivity. Hence, smaller input signal is required to produce desired level of output.<br>
It is the function of the overall receiver gain.
Sensitivity of radio receiver is decided by the gain of the RF and IF amplifiers.
Practically, it is defined as the carrier voltage, which must be applied to the receiver input terminals to get standard output power at output terminals.
The loudspeaker is replaced by load resistance of equal value of speaker.
The sensitivity is expressed in m volt or millivolt.
It may be measured at various frequencies in the radioband.
Improvement in Sensitivity:
The high gain IF amplifiers provides better sensitivity. Hence, smaller input signal is required to produce desired level of output.<br>
31
Procedure to Measure Sensitivity: fig.Set-up to plot sensutivity curve<br>
32
Continued….. Adjust the output of AM generator to 30% modulation index, with modulating signal frequency 400 Hz. Observe and note this AM wave on CRO.
Connect the external AM generator output at the antenna terminal.
Adjust carrier frequency of AM input at 540 kHz. Then adjust the output voltage of the signal generator to get a standard output of 50 mW across Req Measure the corresponding input voltage.
Repeat Step – 3 for various values of carrier frequency from 540 kHz to 1640 kHz.
Plot the graph of carrier frequency on X-axis versus receiver input on Y-axis. This is the sensitivity curve shown in Fig. 3.7.<br>
Connect the external AM generator output at the antenna terminal.
Adjust carrier frequency of AM input at 540 kHz. Then adjust the output voltage of the signal generator to get a standard output of 50 mW across Req Measure the corresponding input voltage.
Repeat Step – 3 for various values of carrier frequency from 540 kHz to 1640 kHz.
Plot the graph of carrier frequency on X-axis versus receiver input on Y-axis. This is the sensitivity curve shown in Fig. 3.7.<br>
33
Continued…..<br>
34
Continued…. Fig. sensitivity Curve<br>
35
Selectivity Selectivity is the ability of radio receiver to reject the unwanted signals.
Selectivity depends on IF amplifier. Higher the ‘Q’ of the tuned circuit better is the selectivity.
It is used to distinguish between two adjacent carrier frequencies.
It shows how perfectly the receiver is able to select the desired carrier frequency and reject other frequencies<br>
Selectivity depends on IF amplifier. Higher the ‘Q’ of the tuned circuit better is the selectivity.
It is used to distinguish between two adjacent carrier frequencies.
It shows how perfectly the receiver is able to select the desired carrier frequency and reject other frequencies<br>
36
Continued….<br>
37
Fidelity Fidelity is the ability of the radio receiver to reproduce all the modulating frequencies equally.
Fidelity depends on the frequency response of the audio frequency amplifier.
The fidelity curve is shown in Fig.<br>
Fidelity depends on the frequency response of the audio frequency amplifier.
The fidelity curve is shown in Fig.<br>
38
Continued…..<br>
39
Demodulation of AM Signal Definition:
The process in which modulated signal is converted back into original modulating signal is called demodulation.
Demodulation of AM signal is done by diode detector circuit.<br>
The process in which modulated signal is converted back into original modulating signal is called demodulation.
Demodulation of AM signal is done by diode detector circuit.<br>
40
Simple Diode Detector Fig.a) simple diode detector b) Input/output waveform<br>
41
Distortions in Simple Diode Detector Two types of distortions appear at output in simple diode detector.
Distortions in Diode Detector
Diagonal Clipping Negative Peak Clipping
One is caused by (1) AC and DC diode load impedances being unequal and other by the fact that (2) the AC load impedance acquires a reactive component at highest audio frequencies.<br>
Distortions in Diode Detector
Diagonal Clipping Negative Peak Clipping
One is caused by (1) AC and DC diode load impedances being unequal and other by the fact that (2) the AC load impedance acquires a reactive component at highest audio frequencies.<br>
42
Diagonal Clipping This type of distortion occurs when the time constant RC of load circuit is very large.
Due to this the RC circuit cannot follow fast changes in modulating envelope at detector output, such type of distortion is called diagonal clipping and shown in Fig.
Fig. : Diagonal Clipping
Diagonal clipping does not occur when percentage modulation is below about 60% so that it is possible to design a diode detector that is free from this type of distortion.<br>
Due to this the RC circuit cannot follow fast changes in modulating envelope at detector output, such type of distortion is called diagonal clipping and shown in Fig.
Fig. : Diagonal Clipping
Diagonal clipping does not occur when percentage modulation is below about 60% so that it is possible to design a diode detector that is free from this type of distortion.<br>
43
Negative Peak Clipping This type of distortion occurs when the modulation index in the demodulated wave is higher than it was in modulated wave applied to detector input.
Small Transmitted Modulation Index: No Clipping
(b) Large Transmitted Modulation Index: Negative Peak Clipping
Fig. 3.14: Diode Detector Output<br>
Small Transmitted Modulation Index: No Clipping
(b) Large Transmitted Modulation Index: Negative Peak Clipping
Fig. 3.14: Diode Detector Output<br>
44
Practical Diode Detector<br>
45
Automatic Gain Control (AGC) The overall gain of receiver is decided by the weakest signal to be received. If a stronger signal is received it will result in higher output levels. Inability to handle these levels by receiver circuits can lead to distortions in the received signal.
The solution to this problem is to provide gain control in the receiver. We can connect a potentiometer to control gains of RF and IF amplifier, so that larger signals can be taken care of it.
A more logical and effective solution would be to adjust the gain as and when there is change in the received signal level. Large signal levels will cause gain of the receiver to be reduced whereas a weak signals will have higher gain.
The dynamic range of receiver is the measure of receiver ability to receive both very strong and very weak signals, without having distortions. It is expressed in dB. The use of AGC increases dynamic range of receiver.<br>
The solution to this problem is to provide gain control in the receiver. We can connect a potentiometer to control gains of RF and IF amplifier, so that larger signals can be taken care of it.
A more logical and effective solution would be to adjust the gain as and when there is change in the received signal level. Large signal levels will cause gain of the receiver to be reduced whereas a weak signals will have higher gain.
The dynamic range of receiver is the measure of receiver ability to receive both very strong and very weak signals, without having distortions. It is expressed in dB. The use of AGC increases dynamic range of receiver.<br>
46
Continued….. Need of agc
AGC mean automatic gain control.
At receiver many station signals are collected at receiving antenna.
All these received signals are of different signal strengths (i.e. same are weak signals and some strong signals).
Even though signal strength at input of receiver is fluctuating, it is necessary to keep the receiver output constant. This work is done by Automatic Gain Control (AGC).
AGC is used to adjust the receiver gain automatically.
AGC increases the dynamic range of receiver.<br>
AGC mean automatic gain control.
At receiver many station signals are collected at receiving antenna.
All these received signals are of different signal strengths (i.e. same are weak signals and some strong signals).
Even though signal strength at input of receiver is fluctuating, it is necessary to keep the receiver output constant. This work is done by Automatic Gain Control (AGC).
AGC is used to adjust the receiver gain automatically.
AGC increases the dynamic range of receiver.<br>
47
Continued…. Types of AGC:
There are two types of AGC.
AGC
Simple AGC Delayed AGC<br>
There are two types of AGC.
AGC
Simple AGC Delayed AGC<br>
48
Simple AGC Simple AGC is a system by means of which the overall gain of a radio receiver is varied automatically.
This is done to keep the receiver output constant even when the signal strength at the input of the receiver is changing.
The AGC bias (DC voltage) derived from the detector depends on signal level.
Higher signal level produces more negative voltage.
This negative voltage can be used to control dc bias of IF/RF amplifiers.
The AGC bias is proportional to the strength of received signal.
The AGC bias is applied to selected number of RF, IF amplifiers and mixer stage.<br>
This is done to keep the receiver output constant even when the signal strength at the input of the receiver is changing.
The AGC bias (DC voltage) derived from the detector depends on signal level.
Higher signal level produces more negative voltage.
This negative voltage can be used to control dc bias of IF/RF amplifiers.
The AGC bias is proportional to the strength of received signal.
The AGC bias is applied to selected number of RF, IF amplifiers and mixer stage.<br>
49
Continued….. Fig. AGC Characteristics Input signal strength<br>
50
Continued…. Advantages of Simple AGC:
1. Simple.
2. Low cost.
3. Improvement over No AGC.
Disadvantages:
Reduction in gain of the receiver will take place even for the weak signals.
Use:
Used in domestic radio receivers.<br>
1. Simple.
2. Low cost.
3. Improvement over No AGC.
Disadvantages:
Reduction in gain of the receiver will take place even for the weak signals.
Use:
Used in domestic radio receivers.<br>
51
Delayed AGC In this technique AGC bias is applied only after signal strength has reached a particular level.
From Fig. above, we can say that the AGC bias is not applied until the input signal strength reaches a predetermined level, so called delayed AGC.
After predetermined level, AGC bias is applied like simple AGC but more strongly.
The problem of reducing the receiver gain for weak signals is thus avoided.
The method of implementing delayed AGC technique is shown in Fig. below.<br>
From Fig. above, we can say that the AGC bias is not applied until the input signal strength reaches a predetermined level, so called delayed AGC.
After predetermined level, AGC bias is applied like simple AGC but more strongly.
The problem of reducing the receiver gain for weak signals is thus avoided.
The method of implementing delayed AGC technique is shown in Fig. below.<br>
52
Continued….. Fig. Delayed AGC circuit<br>
53
Continued…. Advantages of Delayed AGC:
1. No reduction in gain for weak signals.
2. Reduction in gain only for strong signals.
Use:
Delayed AGC is used in high quality receivers like communication receiver.<br>
1. No reduction in gain for weak signals.
2. Reduction in gain only for strong signals.
Use:
Delayed AGC is used in high quality receivers like communication receiver.<br>
54
Comparison between Simple and Delayed AGC<br>
55
FM Receiver The FM receiver is also superheterodyne receiver. It differs from AM receiver as:
Operating frequencies in FM are higher.
Need of amplitude limiter and de-emphasis circuit.
Totally different methods of demodulation.
Different methods of obtaining AGC.<br>
Operating frequencies in FM are higher.
Need of amplitude limiter and de-emphasis circuit.
Totally different methods of demodulation.
Different methods of obtaining AGC.<br>
56
FM Superhetrodyne Radio Receiver<br>
57
Limiter Circuit (a) Circuit (b) Limiter action
(a) Circuit (b) Limiter action
Fig. 3.20: Single Stage Tuned Limiter<br>
(a) Circuit (b) Limiter action
Fig. 3.20: Single Stage Tuned Limiter<br>
58
FM Detector The function of a frequency-to-amplitude changer or FM detector (or demodulator) is to change the frequency deviation of the incoming carrier into an AF amplitude variation.
The detector or demodulator circuit should be:
Insensitive to amplitude changes.
Not be too critical in its adjustment and operation.
Converts frequency variations into amplitude.
There are different types of FM detectors.
FM Detectors Types
Simple Slope Detector
Balanced Slope Detector
Phase Discriminator (Foster seely Discriminator)
Ratio Detector
(v) PLL Detector<br>
The detector or demodulator circuit should be:
Insensitive to amplitude changes.
Not be too critical in its adjustment and operation.
Converts frequency variations into amplitude.
There are different types of FM detectors.
FM Detectors Types
Simple Slope Detector
Balanced Slope Detector
Phase Discriminator (Foster seely Discriminator)
Ratio Detector
(v) PLL Detector<br>
59
Simple Slope Detector Fig. : Simple Slope Detector<br>
60
Continued…. Fig. : Slope Detector Characteristic Curve<br>
61
Continued… Disadvantages of Simple Slope Detector:
The simple slope detector does not satisfy any conditions suitable to FM detection as:
1. It is inefficient.
2. It is linear only along very limited frequency range.
3. It is quite difficult to adjust, since the primary and secondary windings of transformer are tuned to slightly differing frequencies<br>
The simple slope detector does not satisfy any conditions suitable to FM detection as:
1. It is inefficient.
2. It is linear only along very limited frequency range.
3. It is quite difficult to adjust, since the primary and secondary windings of transformer are tuned to slightly differing frequencies<br>
62
Balanced Slope Detector The difficulties arising in simple slope detector circuit are overcome in balanced slope detector.
Fig. : (a) Balanced Slope Detector<br>
Fig. : (a) Balanced Slope Detector<br>
63
Continued…. Final output voltage V0 is
Vo=Vo1-Vo2
Circuit Operation:
The circuit operation depends on range of frequencies.
(i) For fin = fc:
Voltage at T1 = Voltage at T2
Input voltage at D1=Input voltage at D2
V01 = V02
Vo = 0
(<br>
Vo=Vo1-Vo2
Circuit Operation:
The circuit operation depends on range of frequencies.
(i) For fin = fc:
Voltage at T1 = Voltage at T2
Input voltage at D1=Input voltage at D2
V01 = V02
Vo = 0
(<br>
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Continued… (ii) fc < fin < (fc + f):
Voltage induced in T1 > Voltage induced in T2.
Input voltage at D1 > Input voltage at D2.
Vo1> Vo2
Output voltage V0 is positive as frequency increase towards (fc + f).
The positive output voltage increases as shown in Fig (b).
(iii) (fc − f) < fin < fc:
Voltage induced in T2 > Voltage induced in T1.
Input voltage to D2 > Input voltage to D1.
V0 is negative. V02 > V01.<br>
Voltage induced in T1 > Voltage induced in T2.
Input voltage at D1 > Input voltage at D2.
Vo1> Vo2
Output voltage V0 is positive as frequency increase towards (fc + f).
The positive output voltage increases as shown in Fig (b).
(iii) (fc − f) < fin < fc:
Voltage induced in T2 > Voltage induced in T1.
Input voltage to D2 > Input voltage to D1.
V0 is negative. V02 > V01.<br>
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Continued… The negative output voltage increases towards (fc − f) as shown in fig.b.
Fig. : (b) Balanced Slope Detector Characteristics<br>
Fig. : (b) Balanced Slope Detector Characteristics<br>
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Continued… Advantage:
It is more efficient and linear than simple slope detector.
Disadvantage:
1. Difficult to tune three tuned circuits to three different frequencies.
2. Amplitude limiting is not provided.<br>
It is more efficient and linear than simple slope detector.
Disadvantage:
1. Difficult to tune three tuned circuits to three different frequencies.
2. Amplitude limiting is not provided.<br>
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Phase Discriminator It is also known as Foster Seely Discriminator.
Fig. : Phase Discriminator<br>
Fig. : Phase Discriminator<br>
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Primary and secondary windings both are tuned to the center frequency ‘fc’ of the incoming signal.
• Although the individual component voltages will be the same at diode inputs at all frequencies, but the vector sum will differ with the phase difference between primary and secondary windings.<br>
• Although the individual component voltages will be the same at diode inputs at all frequencies, but the vector sum will differ with the phase difference between primary and secondary windings.<br>
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Continued… Circuit Operation:
(i) When fin = fc:
Primary and secondary voltages are exactly 90 out of phase.
As shown in vector diagram,
Input at D1 = Input at D2
V01 = V02
Vo = 0<br>
(i) When fin = fc:
Primary and secondary voltages are exactly 90 out of phase.
As shown in vector diagram,
Input at D1 = Input at D2
V01 = V02
Vo = 0<br>
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Continued… (ii) When fin > fc:
Primary and secondary voltages are less than 90 out of phase.
Input at D1 >Input at D2
V01 >V02
Vo is positive.
(iii) When fin < fc:
Primary and secondary voltages are more than 90 out of phase.
Input at D2 > Input at D1
V02 > V01
Vo2 is Positive.<br>
Primary and secondary voltages are less than 90 out of phase.
Input at D1 >Input at D2
V01 >V02
Vo is positive.
(iii) When fin < fc:
Primary and secondary voltages are more than 90 out of phase.
Input at D2 > Input at D1
V02 > V01
Vo2 is Positive.<br>
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Continued… Advantages:
It simplifies the alignment (tuning) as both the tuned circuits are tuned to same frequency.
Better linearity.
Disadvantage:
It does not provide amplitude limiting. So that produces error at output.<br>
It simplifies the alignment (tuning) as both the tuned circuits are tuned to same frequency.
Better linearity.
Disadvantage:
It does not provide amplitude limiting. So that produces error at output.<br>
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Ratio Detector Modification of phase discriminator by adding amplitude limiting facility is called as ratio detector.
Fig. Ratio Detector Circuit<br>
Fig. Ratio Detector Circuit<br>
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Continued… Circuit Operation:
With diode D2 reversed, O (alphabet O) is now positive with respect to b, so that Va is now sum voltage.
Large capacitor C5 is connected to keep this sum voltage constant.
Output voltage V0 is equal to half of the difference between the output voltages from the individual diodes.
Vo= (Vo1-Vo2)/2
Thus, output voltage is proportional to the difference between the individual output voltages.<br>
With diode D2 reversed, O (alphabet O) is now positive with respect to b, so that Va is now sum voltage.
Large capacitor C5 is connected to keep this sum voltage constant.
Output voltage V0 is equal to half of the difference between the output voltages from the individual diodes.
Vo= (Vo1-Vo2)/2
Thus, output voltage is proportional to the difference between the individual output voltages.<br>
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Continued… Fig. Ratio Detector Response<br>
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Continued…. Amplitude Limiting Action:
As FM input voltage tries to increase, the secondary voltage also increases. So that extra diode current flows through D1 and D2. Hence, load current increases.
But voltage across C5 will not change instantaneously.
Thus, load current has increased but load voltage is almost constant.
The ratio detector thus provides the amplitude limiting by the process called ‘Diode Variable Damping’.
Function of L3:
L3 is used to match the low impedance secondary to primary.
Also L3 gives a voltage step-down to prevent too-great damping of primary by the ratio detector action.<br>
As FM input voltage tries to increase, the secondary voltage also increases. So that extra diode current flows through D1 and D2. Hence, load current increases.
But voltage across C5 will not change instantaneously.
Thus, load current has increased but load voltage is almost constant.
The ratio detector thus provides the amplitude limiting by the process called ‘Diode Variable Damping’.
Function of L3:
L3 is used to match the low impedance secondary to primary.
Also L3 gives a voltage step-down to prevent too-great damping of primary by the ratio detector action.<br>
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Continued…. Advantages:
Easy alignment.
Good linearity.
Amplitude limiting is provided so that additional limiter is not required.
Disadvantages:
Complicated operation.
More components are required.<br>
Easy alignment.
Good linearity.
Amplitude limiting is provided so that additional limiter is not required.
Disadvantages:
Complicated operation.
More components are required.<br>
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Why limiter stage is not used before ratio detector? In ratio detector a large value capacitor is placed that functions as amplitude limiter.
Limiter Function:
If the input voltage fall, the diode current will fall, but the load voltage will not, at first, because of the presence of the large capacitor.
The effect is that of an increased diode load impedance, the diode current has fallen, but the load voltage remained constant.
So that, damping is reduced and the gain of the driving amplifier increases, this time counteracting an initial fall in the input voltage.
The ratio detector provides what is known as diode variable damping.
This maintains a constant output voltage desire changes in the amplitude of the input.
Thus, limiter stage is not used before ratio detector.<br>
Limiter Function:
If the input voltage fall, the diode current will fall, but the load voltage will not, at first, because of the presence of the large capacitor.
The effect is that of an increased diode load impedance, the diode current has fallen, but the load voltage remained constant.
So that, damping is reduced and the gain of the driving amplifier increases, this time counteracting an initial fall in the input voltage.
The ratio detector provides what is known as diode variable damping.
This maintains a constant output voltage desire changes in the amplitude of the input.
Thus, limiter stage is not used before ratio detector.<br>
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PLL as FM Demodulator Fig. : Phase Locked Loop FM Detector (Demodulator)<br>
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Comparison of FM Detectors<br>
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Frequencies used in Radio Receiver<br>
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