In the previous sections, we have seen how modulating information is modulated and transmitted with the help of modulator and transmitter. The transmitter transmits the modulated carrier through the transmitting antenna to the air. the modulating signal thus propagated and collected by receiving antenna.

The primary requirement of any communication receiver is that it should have the ability to select the desired signal from among thousands of others present and to provide sufficient  amplification to recover the modulating signal. To provide this primary requirement receiver has to carryout different functions given below:

1. Collect the electromagnetic waves transmitted by the transmitter.
2. Select the desired signal and reject others.
3. Amplify the selected modulated  carrier signal.
4. Detect the modulating signal from the modulated RF signal and
5. Amplify the modulating signal to operate the loud speaker.

TYPES OF AM RADIO RECEIVER

               radio receivers are classified as

(a) Tuned radio frequency (TRF) receiver

(b) Super heterodyne receiver.

The TRF (Tuned Radio Frequency) receiver and super heterodyne receiver are the two main configurations of the am receivers; those have real practical or commercial significance. Most of the present day receivers use super heterodyne configuration but the TRF receivers are simple and easily understood.

we will publish detailed info regarding TRF & SHR (radio receivers) soon , keep reading.

Multiplexing is a method of transmitting more than one quantity over the same transmission channel. In practice, we are often required to transmit many quantities from the transmitter to the receiver. if a separate channel is used for each quantity, the cost of installation, maintenance and periodic replacement becomes economically prohibitive. the use of multiplexing reduces the overall cost because a single transmission channel is shared by many quantities.

There are two main types of multiplex systems which are:

1. Time Division Multiplexing(TDM)
2. Frequency Division Multiplexing (FDM)

• TIME DIVISION MULTIPLEXING (TDM)

in this system, the information from different measuring points is transmitted serially one after the other over the same communication channel. When all the measurements have been sampled, the cycle is repeated. The time required to complete the transmission of all the measurement may be sufficiently short so that one measurement cannot change excessively between samplings.

• FREQUENCY DIVISION MULTIPLEXING(FDM)

In this system, several information can be simultaneously transmitted using different carrier frequencies and employing modulation methods (AM, FM OR PM) discussed earlier. By proper selection of the magnitude of different carrier frequencies, several data can simultaneously be transmitted over a communication link. these can be recovered separately at the receiving end by using demodulators and filters.

In order that a modulated wave is audible, it is necessary to change the nature of modulated wave. This is accomplished by a circuit called detector. A detector circuit performs the following two functions.

(1) It rectifies the modulated wave i.e. negative half of the modulated wave is eliminated,. a modulated wave has positive and negative halves exactly equal. Therefore, average current is zero and speaker can not respond. If the negative half of this modulated wave is eliminated, the average value of this wave will not be zero since the resultant pulses  are now all in one direction.

(2) It separates the audio signal from the carrier. The rectified  modulated wave contains the audio signal from the carrier. It is desired to recover the audio signal. this is achieved by a filter circuit which removes the carrier frequency and allows the audio signal to reach the load i.e. speaker.

The process of recovering the audio signal from the modulated wave is known as demodulation or detection.

At the broad casting station, modulation is done to transmit the audio signal over large distances to a receiver. when the modulated wave is picked up by the radio receiver, it is necessary to recover the audio signal from it.The process is accomplished in the radio receiver and is called demodulation.

NECESSITY OF DEMODULATION

It was noted previously that amplitude modulated wave consists of carrier and sideband frequencies. the audio signal is contained in the side band frequencies which are radio frequencies. If the modulated wave after amplifications is direct fed to the speaker as shown in the above figure, no sound will be heard. It is because of current tends to move it in the opposite direction i.e. diaphragm will not move at all. Consequently, no sound will be heard.

From the above discussion, it is concluded that audio signal must be separated from the carrier at a suitable stage in the receiver. The recovered audio signal is then amplified and fed to the speaker for conversion into sound.

In phase modulation, the amplitude of the modulating signal is used to change the carrier phase from the value it would have if un-modulated. The phase angle of the carrier wave is thus advanced or retarded by some angle, which is proportional to the signal frequency.