Analog Communication Part 2

8/2/2019 Analog Communication Part 2

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AM/Radio Receiver: A radio receiver is an electronic equipment which picks up the

desired signal and rejected the unwanted signal, amplifies the desired signal,

demodulated to get back the original modulation frequency signal. Radio receiver

design includes the electronic design of different components of a radio receiver

which processes the radio frequency signal from an antenna in order to produce

usable information such as audio frequency.

Main Function of an AM/Radio Receiver: The main functions of a radio receiver

are:

Interception the electromagnetic wave in receiving antenna to produce the

desired radio frequency modulated carrier.

Select the desired signal and reject the unwanted signal.

Amplify the radio frequency signal

Detect (Demodulate) the radio frequency carrier to get back the original

modulation frequency voltage.

Amplify the modulation frequency voltage.

Classification of AM/Radio Receiver: There are six types of radio receiver those

are:

AM Broadcast Receiver

FM Broadcast Receiver

TV Receiver

Communication Receiver

Code Receiver

Radar Receiver

Basic function of AM/Radio Receiver: A radio receiver in its most elementary form

performs the followingfour essential functions:

Reception: This consists in receiving or picking the electromagnetic wavesthrough receiving antenna.

Selection: In this section selecting the desired radio wave and reject the

unwanted signal.


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Detection/Demodulation: The desired signal in the form of a modulated

carrier voltage is detected in a detector to recover the original modulating

signal voltage.

Reproduction:This consists in feeding the detected signal to a loud speaker

to reproduce the sound waves giving the original program.

How the Tuned Radio Frequency (TFR) Receiver block diagram and

operation:

Tuned Radio Frequency Receiver:

Figure 1 Block Diagram of TRF Receiver

Receiving Antenna: Receiving Antenna received the electromagnetic waves in

order to produce a tiny voltage at its terminals that is applied to a receiver to be

amplified.

Tuned Radio Frequency Amplifier: it is a small tuned amplifier with tuned circuit

both input and output circuit. Both of the tuned circuits are tuned to desire carrier

frequency. Accordingly the tuned circuit selects the desired carrier frequency and

rejects the unwanted frequency including the image frequency.

Detector: The detectors are also used for the demodulation of the incoming signal.

The demodulation is completed by this stage and forwarded this signal to Radio

frequency amplifier.

Radio frequency Amplifier: Radio frequency amplifier amplify the original signal

and passed through audio amplifiers and then the resultant signal formed is put into

an amplifier or loudspeaker.

Limitation of Tuned Radio Frequency Receiver:

TRF receiver with one or two tuned RF amplifier stages has enough sensitivity

and selectivity but if too many stages are used the circuit become to selectiveand may lower the fidelity of the receiver.

The selectivity of the receiver varies considerably with the frequency of the

received signal, selectivity decreased as the carrier frequency increase.

Radio Frequency Amplifier: it is a small tuned amplifier with tuned circuit both

input and output circuit. Both of the tuned circuits are tuned to desire carrier


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frequency. Accordingly the tuned circuit selects the desired carrier frequency and

rejects the unwanted frequency including the image frequency.

Advantage of Radio Frequency Amplifier:

Generate gain, better sensitivity

Improve signal to noise ration

Improve rejection of adjacent unwanted signal. I.e. better selectivity

Improve image signal rejection

Additional Advantage of Radio Frequency Amplifier:

Prevention of re-radiation of the local oscillator voltage through antenna.

Improve coupling of the receiver to antenna

Figure 2 Circuit of Radio Frequency Amplifier Stage

The schematic diagram of a typical RF amplifier that is used in an AM radio receiver.In the input circuit is the antenna of the radio (L1-a coil) which forms part of an LCcircuit which is tuned to the desired signal by receiving through receiving antenna.Antenna tuning capacitor tuned the received desired signal and trimmed by theantenna trimmer and set the frequency assignment tuning by by biasing andsignaling decoupling network. L1 is wound on the same core as L2, which couples theinput signal through C2 to the transistor (T1). R1 is used to provide proper bias to T1from the base power supply (VBB). Rl provides proper bias to the emitter of T1, andCz is used to bypass Rl. The primary of T1 and RF tuning capacitor form a parallel LCcircuit which acts as the load for T1. This LC circuit is tuned by RF tuning capacitorand RF trimmer, which is ganged to the antenna and the LC circuit to be tunedtogether.

Constituent Stage of a super heterodyne Receiver:


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Figure 3 Diagram of Super Heterodyne Receiver

Antenna: Voltage induced in the antenna is communicated to the receiver input

circuit by means of a feeder wire or lead in wire.

R.F Amplifier: This stage is generally a tuned voltage amplifier tuned to the

frequency mixer which contributes large noise. Also amplify the input signal voltage

to suitable high level before feeding in to the frequency mixer.

Frequency Converter Stage: This stage consist a frequency mixer and a local

oscillator. To the frequency mixer are feed both the local oscillator voltage and as

well as signal voltage. The mixer produces at its output the various intermodulation

terms which is picked by tuned circuit in the output circuit of the mixer which known

as intermediate frequency (if).

I.F Amplifier: It consists of two or more stage of fixed frequency voltage amplifier.

This I.F amplifier provides most of the receiver amplification and selectivity.

Second Detector: Output of the I.F amplifier is fed to the second detector which is alinear diode detector. Output of this detector is the original modulation frequencyvoltage.

Audio Frequency Amplifier: The output of the second detector is fed into the audiofrequency amplifier which completes the additional amplification of this signalvoltage.

Audio Voltage Amplifier: This stage completes the audio voltage amplificationpart. And forward output portion.

Loud Speaker: Amplified audio output voltage of audio power amplifier is fed to loudspeaker through impedance machine transformer. The loud speaker reproduces theoriginal program.

Classification of Amplitude Modulation (AM) method: Method of amplitudemodulation can be divided into two parts:


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1. Linear Modulation Methoda. Linear shunt Plate modulationb. Linear series Plate Modulationc. Grid-bias Modulationd. Cathode Modulatione. Suppressor Modulation

f. Screen Grid Modulationg. Collector Modulation

2. Square Law Modulation Methoda. Square Law Diode Modulation

b. Van der Bijl Modulationc. Balanced Modulator

Linear shunt Plate modulation:

Figure 4 Basic Circuit of Linear Series modulated Class C Amplifier

The modulating voltage vm results in variation of instantaneous plate supply voltagevbb and hence the peak tank current Iim being proportional to vbb undergoes variation

in accordance with instantaneous modulation voltage vm..This result in linear plate

modulation.

Figure 5 Variation of plate tank current in linear plate modulation


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Advantage:i. Distortion is smallii. Plate circuit efficiency is high

iii. Adjustments are easy

Disadvantage: The main disadvantage of this modulation method is that a largeamount of modulating power is required. This necessitates heavy, bulky and costly

modulating equipment.

Grid Bias Modulation:

Figure 6 Basic Circuit of Grid bias Modulation

Figure 7 Variation of tank current in grid bias modulation

The combined low- and high-frequency voltages apply to the grid a wave whose form

is that of the r.f. swinging back and forth on an axis which, instead of being straight,

shifts back and forth at audio frequency. The resultant plate current is shown at the

right in Fig. 16 K. These pulses of current stimulate proportionately strong oscillations


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in the tank circuit LC which is tuned to the radio frequency. The efficiency of

operation of the grid-bias-modulation method is quite low as compared with that of

the plate-modulation scheme. On the other hand, the power output of the modulator

tube of the grid-bias method need only be one or two watts to operate a modulated

tube of considerable power. A battery might well be used, or a rectifier with regulated

output.

Some significant characteristics of Grid Bias Modulation:

1. The amount of power required from modulating amplifier is small as compared

with the plate modulation.

2. For 100% modulation the peak amplitude of modulated voltage is twice the

amplifier of the unmodulated carrier voltage.

3. The phase circuit efficiency at zero modulation equal about 34% this is only

about one half of the plate circuit efficiency of class c amplifier. At 100%

modulation the plate circuit efficiency is about 51%.

Suppressed Carrier Balanced Modulator:


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Figure 8 Basic circuit of suppressed carrier balanced modulator using transistor

The carrier voltage is supplied in parallel to the input of the two matched transistor T1

and T2 whereas modulating voltage is applied in push pull to the same transistors.The output voltage consists of the modulation frequency voltage and the two

sidebands. The carrier has been suppressed. The LC circuit is tuned to resonate at

frequency and it responds to a band of frequencies centered. Suppression of carrier

results in economy of power. Suppressed carrier balanced modulator is commonly

used in carrier current telephony in which one sideband is usefully filtered out to

reduce the width of the channel required for the transmission and the remaining

sideband is transmitted to the receiving end.

Analog Communication Part 2

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