PRESENTATION ON INDUSTRIAL SUMMER TRAINING

(JUNE-JULY 2014)

PRASAR BHARATISTAFF TRAINING INSTITUTE(TECHNICAL)

ALL INDIA RADIO & DOORDARSHAN

SUBMITTED BY : UTKARSH KATHURIA BRANCH : ECE- A

ENROLLMENT NO. : 13520802812

ABOUT THE ORGANISATION• Prasar Bharati is a statutory autonomous body established

under the Prasar Bharati Act and came into existence on 23.11.1997.

• It is the Public Service Broadcaster of the country.• The objectives of public service broadcasting are achieved in

terms of Prasar Bharati Act through All India Radio and Doordarshan.

• The Staff Training Institute (Technical) is the apex in-house training Institute of All India Radio & Doordarshan, primarily catering to the training needs of engineering personnel of All India Radio & Doordarshan. The Institute conducts about 120 courses every year and trains around 1,600 engineering personnel.

MODULATION• In the modulation process, some characteristic of a high-

frequency carrier signal (bandpass), is changed according to the instantaneous amplitude of the information (baseband) signal.

• WHY MODULATION ?– Suitable for signal transmission (distance…etc)– Multiple signals transmitted on the same channel– Stability and noise rejection

• APPLICATION EXAMPLES– broadcasting of both audio and video signals. – Mobile radio communications, such as cell phone.

• BASIC MODULATION TYPES– Amplitude Modulation: changes the amplitude.– Frequency Modulation: changes the frequency.

AMPLITUDE MODULATION

• The amplitude of high-carrier signal is varied according to the instantaneous amplitude of the modulating message signal m(t).

Carrier Signal: or

Modulating Message Signal: or

The AM Signal:

cos(2 ) cos( ) ( ) : cos(2 ) cos( )

( ) [ ( )]cos(2 )

c c

m m

AM c c

f t tm t f t t

s t A m t f t

MODULATION INDEX OF AM SIGNAL

• Modulation Index is defined as

m

c

AkA

Modulation index k is a measure of the extent to which a carrier voltage is varied by the modulating signal. When k=0 no modulation, when k=1 100% modulation, when k>1 over modulation.

AM GENERATIONLOW LEVEL MODULATOR• FET can be used as vltage variable resistance.• Op – amp is used in non-conducting configuration with gain 1+Rf/Ri.• Input Ri is provided by n-channel JFET used as a voltage variable resistor.• A modulating signal m(t) is applied at the gate of JFET such that

resistance Ri vary according to it.• Due to change in Ri, gain of amplifier changes and the carrier signal

applied to the op-amp is modulated in the output according to the message signal m(t).

DEMODULATION OF AM

• ENVELOPE DEMODULATION Envelope detector circuit uses a diode, a capacitor and a

resistor and it is just like a half wave rectifier followed by a low-pass filter. It is a linear detector which takes high frequency RF signal as input and gives an output which is the envelope of the input signal.

• CIRCUIT OPERATION Here the input signal is rectified by the series diode D. The

combination of capacitor C and resistor R behaves like a low-pass filter. The input signal contains both the original message and the carrier wave where the capacitor helps in filtering out the RF carrier waves. The capacitor gets charged during the rising edge and discharges through the resistor R in falling edge. Thus the capacitor helps in giving an envelope of the input as output.

FREQUENCY MODULATION

• If the frequency of the carrier is varied in accordance with the amplitude of the modulating signal (information), it is called frequency modulation.

• Equation of FM is Vfm = Vc sin(wc + mf sin wm)t

• Frequency modulation is widely used in VHF communication systems e.g. FM broadcasting, transmission of sound signal in TV, Satellite Communication etc.

Figure : Frequency Modulated wave The instantaneous frequency varies about the average frequency (carrier

frequency) at the rate of modulating frequency.

FREQUENCY DEVIATION (Δf)

• The amount by which the frequency varies away from the average frequency (carrier frequency) is called frequency deviation. It is proportional to the amplitude of modulating signal.

Δf = Kf Vm

Where, Kf = frequency deviation constant Vm = amplitude of modulating signal

MODULATION INDEX

• Modulation index(mf) = Δf / fm

• Thus for a given frequency deviation modulation index varies inversely as the modulating frequency fm

BANDWIDTH IN FM

• In FM, the BW is based on the number of significant sidebands, which depends upon modulation index mf. In practice, the number of significant sidebands is determined by acceptable distortion. These contain about 98% of the radiated power. By way of best approximation, the Carson’s Rule (rule of thumb) gives a simple formula for bandwidth as

BW = 2(1+mf)fm

= 2(f + fm)

GENERATION OF FM SIGNAL

There are two methods of FM signal Genration

• Direct Method of FM generation.

• Indirect Method of FM generation.

DIRECT METHOD OF FM GENERATION

• VARACTOR DIODE MODULATOR

The modulator makes use of a varactor diode (also known as varicap or capacitance diode). The capacitance of this diode varies with the applied bias voltage (DC voltage + modulating voltage).

The diode forms, at least partially the tuning capacitor of the tank circuit, that determines the frequency of the oscillator. The capacitance varies with the applied modulating voltage and so does the frequency.

• Vo - provides a suitable bias to the varactor diode.• Cb - blocking capacitor. It blocks the DC bias voltage of varactor diode so that operating point of transistor and bias voltage of varactor diode can be chosen independently.• X(t)- Modulating signal. • Cd - Diode function capacitance

at its operating voltage.• Ct - total capacitance of the tank circuit

• Ct - C + Cd

• RFC Radio frequency choke. It blocks the oscillation going to bias voltage Vo.

• The modulating signal added to DC voltage Vo through transformer T.

• Hence the capacitance of varactor diode is varied in accordance with the modulating signal.

• This capacitance becomes the part of the main tank circuit. • Accordingly, frequency of oscillation is controlled by the modulating signal.

• The FM output is taken from the collector of the transister through a buffer amplifier so that the load impedance on the oscillator is essentially constant.

INDIRECT METHOD OF FM GENERATION• ARMSTRONG METHOD In this method frequency modulation is obtained through a phase

modulator. The modulating signal is integrated prior to modulating the carrier, so that the output of the phase modulator becomes a frequency modulated signal. The required phase modulated signal is generated with the help of a double side band - SC modulator.

FM DEMODULATON• BALANCED SLOPE DETECTOR Circuit Description The circuit diagram, shown in Figure (a), has two slope

detectors marked slope detector 1 and slope detector 2. Both the slope detractors are called balanced because they have identical components as follows:

Slope detector 1: It consists of a detector diode D1, filter capacitor C1, load resistor R1, and variable capacitor CT1. The variable capacitor CT1 is called the tuning capacitor because it is adjusted to tune the upper winding of the secondary Winding of the input transformer T.

Slope detector 2: It consists of a detector diode D2, that is identical to D1. It also has filter capacitor C2, load resistor R2, and variable capacitor CT2. The tuning capacitor CT2 tunes the lower winding of the secondary winding of the input transformer T.

The primary winding of the input transformer T is tuned to the central frequency of the carrier signal fcby using the tuning. capacitor CT. The secondary windings are tuned to different frequencies so that the circuit is staggered tuned. The outputs of the detector diodes D1 and D2, are filtered by C1, R1 and C2, R2, respectively. The voltage V1. which is developed across R1, and voltage V2, which is developed across R2 are added together between the points E and F to get the final output voltage Vo, which is the modulating voltage em.

CIRCUIT OPERATION

(i) Fin=fc : When the input frequency(fin) is instantaneously equal to carrier frequency(fc), the induced voltage in the T1 winding of secondary is exactly equal to that induced in the winding T2. Thus the input voltages to both the diodes D1 and D2 will be the same. Therefore, their dc o/p voltages V01 and V02 will also be identical but they have opposite polarities. Hence the net o/p voltage V0 = 0.

(ii) Fc= fin < (fc+Δf) : The induced voltage in winding T1 is higher then the induced in T2.

(iii) Therefore the input to D1 is higher than D2. Hence the positive o/p V01 of D1 is higher than the negative o/p V02 of D2. Therefore the o/p voltage V0 is positive. As the input frequency increase towards (fc+ Δf), the positive o/p voltage increases as shown in curve.

MICROPHONES

• A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal, sometimes referred to as a mike or mic.

TYPES OF MICROPHONES

Dynamic or moving coil microphones Condenser, capacitor or electrostatic microphones Carbon microphones Piezoelectric microphones Fiber optical microphones Laser microphones MEMS microphones Wireless MicrophonesSpecial microphones

DYNAMIC MICROPHONES• A lightweight diaphragm, usually made of plastic film, is

attached to a very small coil of wire suspended in the field of a permanent magnet. When a sound causes the diaphragm to vibrate, the whole assembly works as a miniature electricity generator, and a minute electric current is produced. Because the electrical output is so very small, it has to be amplified using a mic preamp before it is large enough to be useful.

CAPACITOR MICROPHONES• The diaphragm is mounted close to, but not touching, a rigid back plate.

(The plate may or may not have holes in it.) A battery is connected to both pieces of metal, which produces an electrical potential, or charge, between them. The amount of charge is determined by the voltage of the battery, the area of the diaphragm and back plate, and the distance between the two. This distance changes as the diaphragm moves in response to sound. When the distance between diaphragm and back plate changes, current flows in the wire as the battery maintains the correct charge. This additional current is the audio.

VISITS• DOORDARSHAN BHAWAN, MANDI HOUSE

• EARTH STATION,TODAPUR

CONCLUSION

• The training at PRASAR BHARATI was an enriching experience.

• It enlightens us with emerging trends and tools in technology.

• It also highlights the advancement in technology.

• It encourages youngsters to pursue a career in communications.

THANK YOU