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LECTURE ON AM/FM TRANSMITTER

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Presentation on theme: "LECTURE ON AM/FM TRANSMITTER"— Presentation transcript:

1 LECTURE ON AM/FM TRANSMITTER
Subject: Communication Systems-I Class: 4th, ECE PRESENTED BY DIMPLE BADHWAR LECTURER, GPC BATHINDA. Mobile No.: Punjab Edusat Society

2 Block diagram of communication system
Punjab Edusat Society

3 TRANSMITTER Information or Message Transducer Transmitter Communication Channel or Medium It takes the information to be communicated in electrical form and convert it into an electronic signal compatible with the communication medium . Information in Electrical form Punjab Edusat Society

4 TRANSMITTER In this block diagram of communication system, the upper section is called the transmitting section. Punjab Edusat Society

5 TRANSMITTER The main parts of transmitter are explained as follows :
Microphone : It converts sounds into electrical signals in wires. It is the opposite of a loudspeaker. Modulator : The audio signal is modulated into the radio frequency carrier in this modulator stage. Punjab Edusat Society

6 TRANSMITTER Frequency generator : The frequency generation stage will decide the frequency on which the transmitter will operate. RF power amplifier : The power amplification of the radio signal is carried out in the final stage. It makes the signal stronger so that it can be transmitted through the channel over long distances. Punjab Edusat Society

7 TRANSMITTER An antenna is a transducer which converts electrical signals into electromagnetic waves. Punjab Edusat Society

8 BASIC BLOCKS OF TRANSMITTER
Modulator RF oscillator Power amplifier Punjab Edusat Society

9 BASIC FUNCTIONS OF TRANSMITTER
Modulation Carrier generation Amplification (Power) It is an electronic unit which accepts the information signal to be transmitted and converts it into an RF signal capable of being transmitted over long distances . Punjab Edusat Society

10 BASIC FUNCTIONS OF TRANSMITTER
Every transmitter has three basic functions as follows: The transmitter must generate a signal of correct frequency at a desired point in the spectrum. Secondly it must provide some form of modulation to modulate the carrier. Third it must provide sufficient power amplification in order to carry the modulated signal to a long distance. Punjab Edusat Society

11 CLASSIFICATION OF RADIO TRANSMITTERS
According to the type of modulation used. According to service involved. According to the frequency range involved. According to the power used. Punjab Edusat Society

12 CLASSIFICATION BASED ON TRANSMITTED FREQUENCY
Low frequency (LF) transmitters (30 KHZ- 300KHZ) Medium frequency (MF) transmitters ( 300 KHZ-3 MHZ) High frequency (HF) transmitters (3 MHZ- 30MHZ) Punjab Edusat Society

13 CLASSIFICATION BASED ON TRANSMITTED FREQUENCY
Very high frequency (VHF) transmitters (30MHZ-300 MHZ) Ultra high frequency (UHF) transmitters (300 MHZ- 3GHZ) Microwave transmitters (>3GHZ) Punjab Edusat Society

14 CLASSIFICATION BASED ON TYPE OF SERVICE INVOLVED
Radio broadcast transmitters. Radio telephony transmitters. Radio telegraph transmitters. Punjab Edusat Society

15 CLASSIFICATION BASED ON TYPE OF SERVICE INVOLVED
Television transmitters. Radar transmitters. Navigational transmitters. Punjab Edusat Society

16 CLASSIFICATION BASED ON TYPES OF MODULATION
CW Transmitters AM Transmitters FM Transmitters SSB Transmitters Punjab Edusat Society

17 CONTINIOUS WAVE (CW) TRANSMITTERS
The CW Transmitter is the simplest type of transmitter. It is a simple crystal oscillator circuit. This oscillator generates a carrier signal of the desired frequency. Punjab Edusat Society

18 CONTINIOUS WAVE (CW) TRANSMITTERS
Information to be transmitted is expressed in a special form of code using dots and dashes to represent letters of the alphabet and numbers. The information transmitted in this way is called as continuous wave (CW) transmission. Punjab Edusat Society

19 CONTINIOUS WAVE (CW) TRANSMITTERS
The key is a simple hand operated switch connected in emitter of the transistor. By closing the key we can turn on the crystal oscillator on and by opening the key the oscillator is turned off. Punjab Edusat Society

20 CONTINIOUS WAVE (CW) TRANSMITTERS
When the key is closed, the oscillator produces a sinusoidal signal at a frequency equal to the crystal frequency, whereas with the key open, the output of oscillator is zero. The key is opened and closed in order to produce zero output and dots or dashes. Punjab Edusat Society

21 CONTINIOUS WAVE (CW) TRANSMITTERS
Dots correspond to the short duration output whereas a dash corresponds to a long duration output. The required messages can be transmitted using different combinations of dots and dashes for different alphabets and letters. Punjab Edusat Society

22 ADVANTAGES OF CW TRANSMITTER
Simple to construct Compact and portable Can be operated on batteries Punjab Edusat Society

23 DISADVANTAGES A skilled operator is required to convert the message to be sent into a coded form of dots and dashes. Long distance communication is not possible. Voice or picture can not be sent. Punjab Edusat Society

24 AM TRANSMITTER Amplitude modulation technique is used in AM transmitters, here the amplitude of carrier is varied in proportion with the amplitude of the modulating signal, keeping its frequency and phase constant. Used in radio & TV broadcasting. Punjab Edusat Society

25 AM TRANSMITTER In AM Transmitter, AM signal is transmitted by a transmitter. The information is contained in its amplitude variation. Punjab Edusat Society

26 TYPES OF AM TRANSMITTERS
Low Level modulation transmitters. High Level modulation transmitters. Punjab Edusat Society

27 LOW LEVEL MODULATION TRANSMITTERS
The generation of AM wave takes place at a low power level. The generated AM signal is then amplified using a chain of linear amplifier ( A , AB or B). Low Level Modulator Power Amplifier (Linear) RF Carrier Oscillator Signal Source (Modulating Signal) Punjab Edusat Society

28 STABILISED RF OSCILLATOR
LOW LEVEL MODULATION TRANSMITTERS Antenna STABILISED RF OSCILLATOR Class A Buffer amplifier Linear Amplifiers Power Amplifiers Modulator AF modulating signal Audio processing & filtering Class A AF amplifier Punjab Edusat Society

29 LOW LEVEL MODULATION TRANSMITTERS
The RF oscillator produces the carrier signal. The RF oscillator is stabilized in order to maintain the frequency deviation within the prescribed limit. The carrier frequency is equal to the transmitter frequency. Usually the transmitter operates on assigned frequencies or channels. Crystal provides the best way to obtain the described frequency with good stability. Punjab Edusat Society

30 LOW LEVEL MODULATION TRANSMITTERS
We cannot use the LC oscillator because they have low frequency stability. The carrier signal from the crystal oscillator is applied to the modulator with a modulating signal. At the output of the modulator we get the AM wave. Punjab Edusat Society

31 LOW LEVEL MODULATION TRANSMITTERS
The modulating signal is obtained from a source such as a microphone and applied to a buffer processing unit. The buffer is a class A amplifier which isolates the AF source from the rest of high power circuit and amplifies it to an adequate level. Punjab Edusat Society

32 LOW LEVEL MODULATION TRANSMITTERS
The amplified modulating signal is applied to the modulator along with the carrier. At the output of the modulator we get the AM wave. The AM signal is then amplified using a chain of linear amplifier to raise the power level. Punjab Edusat Society

33 LOW LEVEL MODULATION TRANSMITTERS
The linear amplifier can be class A, AB or B type amplifiers. The linear amplifier are used in order to avoid the wave form distortion in AM wave. The amplitude modulated signal is then transmitted using transmitted antenna. Punjab Edusat Society

34 LOW LEVEL MODULATION TRANSMITTERS
The transistorized modulator circuits can be used for low level modulator due to the low power which is to be handled. The low level transmitter does not require a large AF modulator power so its design is simplified. Punjab Edusat Society

35 LOW LEVEL MODULATION TRANSMITTERS
Overall efficiency is much lower compared to high level modulation . This reduce to the use of less efficient linear amplifiers. Punjab Edusat Society

36 AUDIO PROCESSING The AF modulating signal is passed through an audio processing unit before applying it to the modulator. This block carries out some form of “speech processing” in the form of filtering and amplitude control. The weak signals amplified automatically with a higher gain and strong signals are amplified with smaller gain. This will bring all the signals to a sufficient level. Punjab Edusat Society

37 HIGH LEVEL MODULATION TRANSMITTERS
The generation of AM wave takes place at high power levels. HIGH LEVEL MODULATION TRANSMITTERS RF Carrier Oscillator Narrow Band Power Amplifier High Level Modulator Wide Band Power Amplifier Signal Source (Modulating Signal) Punjab Edusat Society

38 HIGH LEVEL MODULATION TRANSMITTERS
Highly efficient class C amplifier are used in high level modulation. Efficiency is more than low level modulation. Punjab Edusat Society

39 Stabilized RF crystal oscillator
HIGH LEVEL MODULATION TRANSMITTERS Antenna Stabilized RF crystal oscillator Class A RF amplifier Class C RF power amplifier High Level Modulator Audio processing & filtering Class A AF amplifier Class B AF power amplifier AF modulating signal Punjab Edusat Society

40 HIGH LEVEL MODULATION TRANSMITTERS
Many of the AM transmitters use the high level modulation technique. The crystal oscillator produces the required carrier signal. The class A amplifier following the oscillator acts as a buffer which isolates the oscillator from the high power circuit. Punjab Edusat Society

41 HIGH LEVEL MODULATION TRANSMITTERS
The output of this class A amplifier is applied to a class C power amplifier. It raises the power level of the carrier to an intermediately high value. The AF modulating signal is applied to the audio processing unit which processes this signal as discussed in the previous section. Punjab Edusat Society

42 COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL MODULATION
Power level : Modulation is carried out at low power level. Amplifier stages: Need lesser amplifier stages. Modulation is carried out at high power level. Need more amplifier stages. Punjab Edusat Society

43 COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL MODULATION
Power efficiency : After modulation linear amplifiers can only be used. This gives lower power efficiency. Power losses : Power losses in amplifiers is higher, the cooling problem is severe. Non linear amplifiers can also be used. This leads to higher power efficiency. Power losses is less, the cooling problem is not severe. Punjab Edusat Society

44 COMPARISION BETWEEN LOW-LEVEL AND HIGH-LEVEL MODULATION
Applications : Used as higher power broadcast transmitters. Used in TV transmitters. Punjab Edusat Society

45 FM TRANSMITTERS Frequency modulation technique is used.
In FM frequency of the carrier is varied in proportion with the amplitude of the modulating signal keeping its amplitude constant. Punjab Edusat Society

46 FM TRANSMITTERS Used in radio, TV sound broadcasting & police wireless transmission. In FM transmitter the FM signal is transmitted by a transmitter. The information is contained in its frequency variation. Punjab Edusat Society

47 FM TRANSMITTERS The FCC has assigned a band of 20 MHz to the commercial FM broad cast service. This band extends from 88 MHz to 108 MHz. Punjab Edusat Society

48 FM TRANSMITTERS This 20 MHz band is divided in 100 channels, each having a bandwidth of 200 KHz. For providing high quality reliable music the maximum frequency deviation allowed is 75 KHz, with a maximum modulating signal frequency of 15 KHz. Punjab Edusat Society

49 METHODS OF FM GENERATION
Indirect Methods Direct Methods Punjab Edusat Society

50 DIRECT FM In direct FM generation the frequency of the carrier is changed directly in proportion with the modulating signal amplitude. Types of Direct FM Reactance modulator Varactor diode modulator Punjab Edusat Society

51 DIRECT FM These methods use a varactor diode or a reactance transistor for presenting a variable reactance across the frequency determining circuit of an oscillator. Punjab Edusat Society

52 DIRECT FM Modulating Signal Variable reactance device Oscillator Tank
Circuit Variable reactance device Punjab Edusat Society

53 DIRECT FM When the variable reactance of the device varies with the modulating signal the oscillator generator the corresponding FM signal. Punjab Edusat Society

54 DIRECT MODULATORS Few other direct modulators are:
Varactor diode modulator. Reactance modulator. V.C.O. modulator. Stabilized reactance modulator. Cross by direct FM transmitters. Punjab Edusat Society

55 REACTANCE MODULATOR In the reactance modulator, a transistor or FET is operated as a variable reactance (inductive or capacitive) device. This device is connected across the tuned circuit of an oscillator. As the instantaneous value of modulating voltage changes, the reactance offered by the transistor or FET will change proportionally. Punjab Edusat Society

56 VARACTOR DIODE Here the voltage applied across the varactor diode varies in proportion with the modulating voltage. This will vary the junction capacitance of the varactor diode. The varactor diode appears in parallel with the oscillator tuned circuit. Hence the oscillator frequency will change with change in varactor diode capacitance and FM wave is produced. Punjab Edusat Society

57 ADVANTAGES OF DIRECT FM MODULATION
The main advantage of direct FM generation is the simplicity of the modulators and their low cost. Punjab Edusat Society

58 DISADVANTAGES OF DIRECT METHOD
In the direct method of FM generation we have to use the LC oscillator. The LC oscillator frequency is not stable. Therefore its not possible to use such oscillator for communication or broadcast purpose. Punjab Edusat Society

59 DIRECT FM Therefore we have to use a scheme in which we can use the crystal oscillator to control the carrier frequency. Therefore we have to use the automatic frequency control system. Punjab Edusat Society

60 INDIRECT METHOD ( ARMSTRONG METHOD)
Here FM is obtained through phase modulation. A Crystal oscillator is used and hence the frequency stability is very high. A narrow band PM signal is generated via DSB-SC signal and a 90° phase shifted sub carrier signal from a crystal oscillator. Punjab Edusat Society

61 INDIRECT METHOD ( ARMSTRONG METHOD)
The integration of the modulating signal makes the NBPM signal to NBFM signal. This NBFM signal is applied to a harmonic generator (frequency multiplier) which will increase both the carrier frequency and the maximum deviation to the required. Punjab Edusat Society

62 COMPARISION OF AM AND FM BROADCASTING
AM Broadcasting It requires smaller transmission bandwidth. It can be operated in low, medium and high frequency bands. It has wider coverage. FM Broadcasting It requires larger bandwidth. It needs to be operated in very high and frequency bands. Its range is restricted to 50 km. Punjab Edusat Society

63 COMPARISION OF AM AND FM BROADCASTING
The demodulation is simple. The stereophonic transmission is not possible. The system has poor noise performance. The process of demodulation is complex. In this, stereophonic transmission is possible. It has an improved noise performance. Punjab Edusat Society

64 REVISION What is Transmitter Classification of Transmitters
Based on Type of modulation Based on Transmitted frequency AM Transmitters Types of AM transmitters Low Level modulation transmitter High Level modulation transmitter FM Transmitters Types of FM generation Direct method of FM generation Indirect method of FM generation Punjab Edusat Society


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