A personal Fm transmitter is low- power Fm radio transmitter that broadcasts a signal from a portable audio device ( such as an MP3 player)  to a standard Fm radio . Most of these transmitters plug into the signal over an Fm broadcast band frequency 

  

CLASSIFICATION OF TRANSMITTERS ON THE BASIS OF MODULATION:


    1. Amplitude Modulation Transmitters:

    2. Frequency Modulation Transmitters

    3. Pulse Modulation Transmitter


CLASSIFICATION OF RADIO TRANSMITTERS ACCORDING TO THE TYPE OF SERVICE INVOLVED:


    1. Radio Broadcast Transmitters: 

  •     Used to transmit speech, talks, music, dramas etc. 
  •     May be either AM or FM based.
  •     AM operates on long waves, medium waves and short waves and radiate carrier power as low as about 1Kw and high as 100 Kw. 
  •     FM broadcasts on very short wave or on ultra waves and radiate power of the order of 100 Kw.

    2. Radio Telegraph Transmitters: 

  •     A radio telegraph transmitter transmits signals from one radio station to another radio station.
  •     It may use either amplitude modulation or frequency modulation. 
  •     When point-to-point radio communication is involved, the transmitting antennas are highly directive so that the electromagnetic energy is beamed into a narrow beam directed towards the receiving antenna at the receiving radio station.

    3. Television Transmitters: 

  •     Television broadcast requires two transmitters one for transmission of picture and the other for transmission of sound. 
  •     Both operate in very high frequency or in ultra high frequency range. 
  •      The picture transmitter is amplitude modulated by the picture signal occupying a band of about 5MHz Vestigial transmission is used i.e one full sideband and only a vestige or a part of the other sideband together with the carrier are radiated from the transmitting aerial. 
  •     The total bandwidth occupied by one television channel is about 7 MHz. The sound carrier is frequency modulated.

    4. Radar Transmitter: 

  •     Radar (abbreviation for Radio Detection and Ranging) may be of two types:

  1. Pulse Radar and
  2.  C.W. (Continuous Wave Radar).

  •     Pulse radar transmitter uses pulse modulation of carrier. 
  •      It uses high output power typically 100 kW peak and operates at microwave frequencies typically 3000 MHz  or 10,000 MHz. 
  •      The C.W. radar transmitter may use frequency modulation of the carrier voltage or may utilize Doppler Effect.

    5. Navigation Transmitters: 

  •     A number of navigational aids using special types of radio transmitters and receivers are used these days for sea and air navigation. 
  •     Also radio aids are used for blind landing of aircrafts. 
  •     Typical radio aids of landing are (a) I.L.S. (Instrumental Landing System) and (b) G.C.A. (Ground Controlled Approach).

    • CLASSIFICATION OF RADIO TRANSMITTERS ON THE BASES OF CARRIER FREQUENCY:

1) Long Wave Transmitters: 

  •     These transmitters operate on long waves i e on frequencies below 300 kHz.
  •     Such long wave radio transmitters are used for broadcast in temperate countries, where atmospheric disturbances on long waves are not severe.
  •     Since long wave radio signals travel along the surface of earth and are rapidly attenuated, the carrier power radiated from the transmitting aerial must be very large typically 100 kW or more. 

2) Medium Wave Transmitters: 

  •      These transmitters operate on frequencies in the range of 550 to 1650 kHz and are usually used for broadcast. Hence the band of frequency extending from 550 to 1650 kHz is commonly referred to as the Broadcast Band. 
  •      The carrier power may vary from as low as 5 kW to as high as 500 to 1000kW. 

3) Short Wave Transmitters: 

  •     These transmitters operate on frequencies in the short wave range of 3 to 30MHz. 
  •     Ionosphere propagation of electromagnetic waves takes place at such short waves. 
  •     The attenuation of radio waves travelling from the transmitting aerial to the distant receiving aerial though the ionosphere is small. Hence carrier power required to be radiated from the transmitting aerial is small

    

        Classification of AM Transmitter on the basis of Power Used:

Transmitters that transmit AM signals are known as AM transmitters. These transmitters are used in medium wave (MW) and short wave (SW) frequency bands for AM broadcast. The MW band has frequencies between 550 KHz and 1650 KHz, and the SW band has frequencies ranging from 3 MHz to 30 MHz. The two types of AM transmitters that are used based on their transmitting powers are: 

· High Level 


· Low Level 

High level transmitters use high level modulation, and low level transmitters use low level modulation. In broadcast transmitters, where the transmitting power may be of the order of kilowatts, high level modulation is employed. In low power transmitters, where only a few watts of transmitting power are required, low level modulation is used.

High-Level and Low-Level Transmitters 

Below figures show the block diagram of high-level and low-level transmitters. The basic difference between the two transmitters is the power amplification of the carrier and modulating signals. 


Figure (a) shows the block diagram of high-level AM transmitter

In high-level transmission, the powers of the carrier and modulating signals are amplified before applying them to the modulator stage, as shown in figure (a).

In low-level modulation, the powers of the two input signals of the modulator stage are not amplified. 

The required transmitting power is obtained from the last stage of the transmitter, the class C power amplifier. 

The various sections of the figure (a) are: 

· Carrier oscillator 

· Buffer amplifier 

· Frequency multiplier 

· Power amplifier 

· Audio chain 

· Modulated class C power amplifier


Carrier oscillator 

The carrier oscillator generates the carrier signal, which lies in the RF range. The frequency of the carrier is always very high. 

Buffer Amplifier 

The purpose of the buffer amplifier is to matches the output impedance of the carrier oscillator with the input impedance of the frequency multiplier 

Frequency Multiplier 

This stage is also known as harmonic generator. The frequency multiplier generates higher harmonics of carrier oscillator frequency. 

Power Amplifier 

The power of the carrier signal is then finally amplified in the power amplifier stage. This is the basic requirement of a high-level transmitter. A class C power amplifier is used here. 

Audio Chain 

The audio signal to be transmitted is obtained from the microphone, as shown in figure (a). The audio driver amplifier amplifies the voltage of this signal. This amplification is necessary to drive the audio power amplifier. Next, class A or  class B power amplifier amplifies the power of the audio signal.

Modulated Class C Amplifier 

This is the output stage of the transmitter. The modulating audio signal and the carrier signal, after power amplification, are applied to this modulating stage. The modulation takes place at this stage. The class C amplifier also amplifies the power of the AM signal to the required transmitting power. This signal is finally passed to the antenna, which radiates the signal into space of transmission. 


 Block diagram of a low-level AM transmitter.

The low-level AM transmitter shown in the figure (b) is similar to a high-level transmitter, except that the powers of the carrier and audio signals are not amplified. These two signals are directly applied to the modulated class C power amplifier. 


FM transmitter: 

FM systems are operated usually at a frequency above 40 MHz.  FM signal can be generated by two methods:

    (a) Direct Method of FM generation (Reactance FET Modulator)

    (b) Indirect Method of FM generation (Armstrong FM Transmitter)

Block diagram and working principles of reactance FET:

The FM transmitter is a single transistor or FET circuit. 

Generally, the FM transmitter uses VHF radio frequencies of 88 to 108 MHz to transmit & receive the FM signal. 

The performance and working of the wireless audio transmitter circuit depend on the induction coil & variable capacitor.

In direct method, the baseband or modulating signal directly modulates the carrier. The carrier signal is generated with the help of an oscillator circuit.

This oscillator circuit uses a parallel tuned L-C circuit. Thus the frequency of oscillation of the carrier generation is governed by the expression:                                    É·c = 1/(LC)1/2

The frequency of this oscillator is changed by changing the reactive components involved in the tuned circuit. If L or C of a tuned circuit of an oscillator is changed in accordance with the amplitude of modulating signal then FM can be obtained across the tuned circuit as shown in figure below.




A two or three terminal device is placed across the tuned circuit. The reactance of the device is varied proportional to modulating signal voltage. This will vary the frequency of the oscillator to produce FM. The devices used are FET, transistor or varactor diode.



ADVANTAGES 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 it’s not possible to use such oscillator for communication or broadcast purpose.
  •     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.

    ARMSTRONG FM TRANSMITTER:

  •      Armstrong method of FM generation is the indirect method because the modulating signal directly varies the phase of the carrier, which indirectly changes the frequency.
  •     The figure below shows the block diagram of wideband FM generation through Armstrong method.


    

    The source of carrier for the Armstrong transmitter is the crystal oscillator. A relatively low frequency sub-carrier (fc) is phase shifted by 90° and is fed to a balanced modulator, where it is mixed with the input modulating signal (fm).

A double sideband suppressed carrier wave is produced at the output from the balanced modulator, and this is combined with the original carrier in the combing network to generate a narrow band frequency modulated waveform.

A combination of multipliers and mixers are thus placed to develop the desired transmit carrier frequency with 75 kHz frequency deviation.

Armstrong suggested the method of reducing disturbances in radio signalling by this method of frequency modulation.

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.

    • The demodulation is simple.

    • The system has poor noise performance.

         FM Broadcasting

    • It requires larger bandwidth.

    • It needs to be operated in very high frequency bands.

    • Its range is restricted to 50 km.

    • The process of demodulation is complex.

    • It has an improved noise performance.


Radio receiver:    

Radio receiver is an electronic equipment which pick ups the desired signal, reject the unwanted signal and demodulate the carrier signal to get back the original modulating signal.

Functions: 

  •      Intercept  the incoming modulated signal
  •      Select desired signal and reject unwanted signals
  •      Amplify selected R.F signal
  •      Detect modulated signal to get back original modulating signal
  •      Amplify modulating frequency signal 

Classification of Radio Receivers

  •     AM Receiver
  •     Tuned Radio Frequency Receiver (TRF)
  •     Superheterodyne AM receiver
  •     FM Receiver
  •     Communication Receiver

Tuned Radio Frequency Receiver (TRF):

A tuned radio frequency receiver (or TRF receiver) is a type of radio receiver that is composed of one or more tuned radio frequency (RF) amplifier stages followed by a detector (demodulator) circuit to extract the audio signal and usually an audio frequency amplifier. This type of receiver was popular in the 1920s. These can be aligned at broadcast frequencies 535-1640 KHz.


The classic TRF receivers consisted of three sections:

  •      One or more tuned RF amplifier stages: 

                                                                           These selects and amplify the desired weak signal to a                                                                           level sufficient to drive the detector, while rejecting all                                                                          other signals picked up by the antenna.

  •     Detector or Demodulator:  

                                                    Actually demodulation is done here in which original message                                                                  signal  is separated from the carrier signal.

  •     Audio amplifier stages 

                                               which increase the power of the audio signal.


Advantages of TRF receivers:

High Sensitivity

Low cost

Disadvantages of TRF receivers:

Instable

Poor Selectivity

BW variations for different frequencies

Due to these drawbacks TRF are rarely used.


 Superheterodyne AM receiver:

The AM super heterodyne receiver takes the amplitude modulated wave as an input and produces the original audio signal as an output. To heterodyne means to mix two frequencies together so as to produce a beat frequency, namely the difference between the two.


RF Tuner Section

The amplitude modulated wave received by the antenna is first passed to the tuner circuit. The tuner circuit is nothing but a LC circuit. It selects the desired frequency. It also tunes the local oscillator and the RF filter at the same time

The resultant output is a mixture of two frequencies (f1+f2), (f1−f2) produced by the mixer, which is called as the Intermediate Frequency (IF).

IF Filter

Intermediate frequency filter is a band pass filter, which passes the desired frequency. It eliminates all other unwanted frequency components present in it. This is the advantage of IF filter, which allows only IF frequency

AM Demodulator

The received AM wave is now demodulated using AM demodulator. This demodulator uses the envelope detection process to receive the modulating signal.

Audio Amplifier

This is the power amplifier stage, which is used to amplify the detected audio signal. The processed signal is strengthened to be effective. This signal is passed on to the loudspeaker to get the original sound signal

Advantages of Superheterodyne Radio Receiver:

    • High Sensitivity

    • High Selectivity

    • High adjacent channel rejection ratio

    • BW remains constant over the entire range

Disadvantages of Superheterodyne Radio Receiver:

    • High cost due to the use mixer & local oscillator

Performance Characteristics of a Radio Receiver :

    i. Selectivity

    ii. Sensitivity

    iii. Fidelity

    iv. Image frequency and rejection ratio

    v. S/N Ratio

    vi. Double Spotting

Sensitivity: 

    • This is the ability of a radio receiver to amplify weak signals.

    • Broadcast receivers/ radio receivers should have reasonably high sensitivity so that it may have            good response to the desired signal. 

    • But should not have excessively high sensitivity otherwise it will pick up all undesired noise              signals.

    • Sensitivity of a receiver is expressed in microvolt of the received signal.

    • Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.

    • Sensitivity of the receiver depends on :

            ▪ Noise power present at the input to the receiver

            ▪ Receiver noise figure

            ▪ Bandwidth improvement factor of the receiver

The best way to improve the sensitivity is to reduce the noise level.

Fidelty:              

    • Fidelity is defined as – a measure of the ability of a communication system to produce an exact            replica of the original source information at the output of the receiver.

    • Any variations in the demodulated signal that are not in the original information signal are considered as distortion.

    • Radio receiver should have high fidelity or accuracy.

    • Example- In an A.M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5 KHz.


    

   IMAGE FREQUENCY AND REJECTION RATIO:

    • In radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces. 

    • Image frequency – any frequency other than the selected radio frequency carrier that will produce a cross-product frequency that is equal to the intermediate frequency if allowed to enter a receiver and mix with the local oscillator. 

    • It is given by signal frequency plus twice the intermediate frequency

                 fsi = fs + 2fIF                         

    • It is equivalent to a second radio frequency that will produce an IF that will interfere with the IF from the desired radio frequency.

        ◦ if the selected RF carrier and its image frequency enter a receiver at a same time, they both mix with the local oscillator frequency and produce different frequencies that are equal to the IF.

        ◦ The higher the IF, the farther away the image frequency is from the desired radio frequency. Therefore, for better image frequency rejection, a high IF is preferred. 

        


     S/N Ratio: 

Signal Power/Noise Power     =   Ps/Pn

S/N ratio can also be expressed in decibels:

S/N = 10 log10 (Ps/Pn) dB

Selection criteria for IF:


Type of Receiver                 Band Used                     IF Range

AM receiver                       540-1640 KHz                 455 KHz     

FM receiver                         88-108 MHz                   10.7 MHz

TV receiver                       VHF(54-225 MHz)           30-46 MHz

                                          UHF (450-940 MHz)

Automatic Gain Control (AGC): AVC

An AGC circuit compensate for minor variations in the received RF signal level. It automatically increases receiver gain for weak RF input signal. And automatically decrease the receiver gain when strong RF signal is received

    i. Simple AGC: 

    ii. Delayed AGC

Simple AGC:

 The AGC circuit monitors the received signal level & sends a signal back to the RF & IF amplifiers to adjust their gain automatically. The AGC circuit produces a voltage that adjusts the receiver gain and keeps the IF carrier power at the input to the detector constant.

Delayed AGC:

 With simple AGC, the AGC bias begins to increase as soon as the received signal level increases. But In delayed AGC, bias is not applied to the amplifiers until signal strength crosses a predetermined level. This type of system develops no AGC feedback until an established received signal strength is attained. For signals weaker than this value, no AGC is developed. For sufficiently strong signals, the delayed AGC circuit operates essentially the same as ordinary AGC    


FM Receiver:



COMMUNICATION RECEIVER:

    • Image rejection (HF-IF) & adjacent channel rejection (LF-IF) is achieved at high level through this receiver.

    • Although the basic idea of superheterodyne radio receiver works very well, but an extension of the principle known as Double conversion superheterodyne radio receiver may be used.

    • A Communication Receiver is one whose main function is the reception of signals used for communication rather than for entertainment.

    • It improves the performance in a number of areas including stability, image rejection and adjacent channel filter performance.


    

  Receiver converts the incoming signal down to a relatively high 1st IF. This enables the high levels of image rejection to be achieved. It is then passed through a 2nd mixer to convert it down to a lower IF where the narrow band filtering is accomplished so that the adjacent channel signals can be removed.

BFO: 

Communication receivers can also receive telegraphic signals that use Morse code. These codes are transmitted as dots, dashes and spaces. To detect a Morse code BFO is used. A beat frequency oscillator or BFO is an LC oscillator used to create an audio frequency signal from Morse code radiotelegraphy (CW) transmissions to make them audible. The 2nd IF signal and the o/p of the BFO together generate whistles that indicate the presence of a dot, a dash or a space. A switch is provided in the receiver to select the option of receiving an audio signal or a telegraph signal. This switch remains off if telegraph signals are not received.

Squelch Circuit: When the communication transmitter does not transmit any signal, the receiver receives only the noise present at its input. A good quality communication receiver can amplify this noise. A squelch or a muting circuit actually eliminates the noise here.