RADIO TRANSMITTER

A transmitter is an electronic device used in telecommunications to produce radio waves in order to transmit or send data with the aid of an antenna. OR Transmitters are devices that are used to send out data as radio waves in a specific band of the electromagnetic spectrum in order to fulfil a specific communication need, be it for voice or for general data.

A transmitter is composed of:

   Power supply the energy source used to power the device and create the energy for broadcasting.
   Electronic oscillator generates a wave called the carrier wave wheredata is imposed and carried through the air.
   Modulator adds the actual data into the carrier wave by varying some aspect of the carrier wave.
   RF amplifier Increases the power of the signal in order to increase the range where the waves can reach.
   Antenna tuner or impedance matching circuit Matches the impedance of the transmitter to that of the antenna
    in order for the transfer of power to the antenna to be efficient and prevent a condition called standing waves,
    where power is reflected from the antenna back to the transmitter resulting in power wastage or damage to the antenna.

FUNCTION OF RADIO TRANSMITTER

   Able to generate a radio frequency alternating current that is then applied to the antenna.
   Able to mix generated frequency and information before transmission.
   Able to take energy from the power source and transforms this into a radio frequency alternating current.

OPERATIONS OF TRANSMITTER

   A radio transmitter is an electronic circuit which transforms electric power from a    battery or electrical mains into a radio frequency alternating current, which reverses    direction millions to billions of times per second. The energy in such a rapidly reversing    current can radiate off a conductor (the antenna) as electromagnetic waves (radio waves).    The transmitter also impresses information such as an audio or video signal onto the radio    frequency current to be carried by the radio waves. When they strike the antenna of a radio    receiver, the waves excite similar (but less powerful) radio frequency currents in it. The    radio receiver extracts the information from the received waves.

STAGES IN RADIO TRANSMITTER

Carrier oscillator

The carrier oscillator generates the carrier signal, which lies in the RF range. The frequency of the carrier is always very high. Because it is very difficult to generate high frequencies with good frequency stability, the carrier oscillator generates a sub multiple with the required carrier frequency. This sub multiple frequency is multiplied by the frequency multiplier stage to get the required carrier frequency. Further, a crystal oscillator can be used in this stage to generate a low frequency carrier with the best frequency stability. The frequency multiplier stage then increases the frequency of the carrier to its required value.

Buffer Amplifier

The purpose of the buffer amplifier is twofold. It first matches the output impedance of the carrier oscillator with the input impedance of the frequency multiplier, the next stage of the carrier oscillator. It then isolates the carrier oscillator and frequency multiplier. This is required so that the multiplier does not draw a large current from the carrier oscillator. If this occurs, the frequency of the carrier oscillator will not remain stable.

Frequency Multiplier

The sub-multiple frequency of the carrier signal, generated by the carrier oscillator, is now applied to the frequency multiplier through the buffer amplifier. This stage is also known as harmonic generator. The frequency multiplier generates higher harmonics of carrier oscillator frequency. The frequency multiplier is a tuned circuit that can be tuned to the requisite carrier frequency that is to be transmitted.

Power Amplifier

The power of the carrier signal is then amplified in the power amplifier stage. This is the basic requirement of a high-level transmitter. A class C power amplifier gives high power current pulses of the carrier signal at its output.

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, a class A or a 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 reacquired transmitting power. This signal is finally passed to the antenna, which radiates the signal into space of transmission.

RADIO RECEIVERS

A radio receiver is an electronic device that takes a transmitted signal, extracts the original signal from it and amplifies that signal. The process of extracting the signal is called demodulation. A radio station, for example, will broadcast a signal which is then detected by a receiver. The receiver, in turn, will separate that signal from many others and then play it through its speakers. There are several different types of signals that the receiver can be designed to demodulate and decode including sounds, pictures and digital data, to name a few. Some radio receiver types are much simpler than others, whereas some have higher levels of performance and are not confined by space as much. In view of the huge difference in requirements and performance levels needed, many different types of radio can be seen these days.

Radio receiver types

Many of the different radio receiver types have been around for many years. The component technology, and in particular semiconductor technology has surged forwards enabling much higher levels of performance to be achieved in a much smaller space.
There are number of different types of radio:
Tuned radio frequency, TRF : This type of radio receiver was one of the first that was used. The very first radio receivers of this type simply consisted of a tuned circuit and a detector. Crystal sets were early forms of TRF radios. Later amplifiers were added to boost the signal level, both at the radio frequencies and audio frequencies. There were several problems with this form of receiver. The main one was the lack of selectivity. Gain and sensitivity were also an issue.
Regenerative receiver: The regenerative radio receiver significantly improved the levels of gain and selectivity obtainable. It used positive feedback and ran at the point just before oscillation occurred. In this way a significant multiplication in the level of "Q" of the tuned circuit was gained.
Super regenerative receiver: The super regenerative radio receiver takes the concept of regeneration a stage further by using a second lower frequency oscillation within the same stage, this second oscillation quenches or interrupts the oscillation of the main regeneration typically at frequencies of around 25 kHz or so above the audio range. In this way the main regeneration can be run so that the stage is effectively in oscillation where it provides very much higher levels of gain. Using the second quench oscillation, the effects of running the stage in oscillation are not apparent to the listener, although it does emit spurious signals which can cause interference locally.
Superheterodyne receiver: The superheterodyne form of radio receiver was developed to provide additional levels of selectivity. It uses the heterodyne or mixing process to convert signals done to a fixed intermediate frequency. Direct conversion receiver: This type of radio format converts the signal directly down to the baseband frequency. Initially it was used for AM, Morse (CW) and SSB transmissions, but now it is widely used for digital communications where IQ demodulators are used to take advantage of the variety of phase shift keying, PSK, and quadrature amplitude modulation, QAM signals. Many of these different types of radio receiver are in widespread use today. Each type of radio has its own characteristics that lend its use to particular applications.

BLOCK DIAGRAM OF A RADIO RECEIVER

RF Amplification: In terms of amplification, the level is carefully chosen so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved. The amplifier must also be a low noise design. Any noise introduced in this block will be amplified later in the receiver.
Mixer / frequency translator block: The tuned and amplified signal then enters one port of the mixer. The local oscillator signal enters the other port. The performance of the mixer is crucial to many elements of the overall receiver performance. It should be as linear as possible. If not, then spurious signals will be generated and these may appear as ' phantom' received signals.
Local oscillator: The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor. Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy.
Intermediate frequency amplifier, IF block: Once the signals leave the mixer they enter the IF stages. These stages contain most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on the next. Filters may consist simply of LC tuned transformers providing inter-stage coupling, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required.
Detector/demodulator stage: Once the signals have passed through the IF stages of the super-heterodyne receiver, they need to be demodulated. Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered.

RESONANCE CIRCUIT

Resonance in AC circuit implies a special frequency determined by the values of the resistance, capacitor, and inductance. For understanding resonance circuit completely, one must start from the begining, which is studying similarities between Resistor, capacitor and inductor.

Resistor: it works very well both in DC and AC circuit. It limits AC and DC which is its desigined function in circuits.

Capacitor: it works very well in AC circuit, it also can limit the flow of AC just like a resistor. But it does not work well in DC circuit because it blocks DC. Therefore, it allows AC to flow even though it limits it. But it completely block DC. It works like open circuit in DC circuit.

Inductor: this component also does resistive work in AC circuit, that is it limit the flow of current. But it works as an ordinary wire in DC circuit (short circuit), therefore it cannot be used in DC circuit.

Therefore, resistor, capacitor and inductor all function as a resistor in an AC circuit, but not in DC circuit. And there is reason while they are not all called resistors, it is because they react differently while carrying out there resistive functions.

Resistance (R): this is the term used in describing the opposition of electron flow caused by resistors the S.I unit for resistance of resistor is in Ohm.

Reactance (XC): this is the term used in describing the opposition of electron flow caused by capacitor the S.I unit for reactance of capacitor is in Ohm.
Xc = 1/2πfc ,unit is in Ω(ohms)

Reactance (XL): this is the term used in describing the opposition of electron flow caused by inductor the S.I unit for reactance of capacitor is in Ohm. Xc =2πfL,unit is in Ω(ohms)

CONVERTING CAPACITANCE TO OHM

In Summary, if you are given the capacitance of capacitor (S.I. unit in farad) using that information you can find the reactance (just like resistance). Even though capacitor is measure in farad, if you go to market you can't see even 1farad capacitor because it will be as large as a tank. Therefore, capacitors are measured in small values like micro-farad (10-6), nano-farad (10-9) and pico-farad (-12). But during calculations one must alway convert it back to farads before performing arithmetic with the value. This conversion below will assist you.

1,000,000 µf = 1farad
1 µf = 1 X 10-6 farad = 0.000001farad

1,000,000,000 nf = 1farad
1 nf = 1 X 10-9 nano-farad = 0.000000001farad

1,000,000,000,000 pf = 1farad
1 pf = 1 X 10-12 farad = 0.000000000001farad

Question 1. A capacitor whose capacitance is 2 µf, calculate the reactance of the capacitor assume the ac system operates in 50hz.

SOLUTION:
using the reactance of capacitor formular
Xc =  1   ;
  2πfC   
where f = 50Hz, C = 2 X 10-6(in f) converting microfarad to farad.

Xc =     1      
  2 x 22/7 x 50 x 2 x 10-6
applying the law of indices, both 7 and 10-6 can go up (note sign change)
Xc =   7 x 106    
  2 x 22 x 50 x 2

Xc = 1591Ω


Question 2. calculate the reactance of capacitor of 10uf, in a circuit of 60hz and applied voltage of 220volts.

SOLUTION:
using the reactance of capacitor formular
Xc =  1   ;
  2πfC   
where f = 60Hz, C = 10 X 10-6(in f) converting microfarad to farad.

Xc =     1      
  2 x 22/7 x 60 x 10 x 10-6
applying the law of indices, both 7 and 106 goes up (not sign change)
Xc =   7 x 106    
  2 x 22 x 60 x 10

Xc = 265.2Ω


Question 3. calculate the reactance of capacitor of 22pf, in a circuit of 50hz and applied voltage of 220volts.

SOLUTION:
using the reactance of capacitor formular
Xc =  1   ;
  2πfC   
where f = 50Hz, C = 22 X 10-12(in f) converting picofarad to farad.

Xc =     1      
  2 x 22/7 x 50 x 22 x 10-12

Xc =   7 x 1012    
  2 x 22 x 50 x 22

Xc = 144,600,000 Ω
Xc = 144.6MΩ


CONVERTING INDUCTANCE TO OHM

The values of Inductors in ohms can also be calculated if you know the inductance of the inductor. Just like resistor, inductors opposes free flow of electrons due to self-inducing magnetic force. Inductors are measure in henry, smaller quantity of inductors are measure in mini-henry.