The receiver is a device that converts electrical signal into physical message. Here in analog communication the receiver accepts the amplitude modulated wave as an input and produces original wave as the output with the help of antenna. First let us discuss the requirements of a receiver.
AM receiver receives AM wave and demodulates it by using the envelope detector. Similarly, FM receiver receives FM wave and demodulates it by using the Frequency Discrimination method. Following are the requirements of both AM and FM receiver.
For the above requirements to be fulfilled, the tuner circuit and the mixer circuit should be very effective. The procedure of RF mixing is an interesting phenomenon.
The RF mixing unit generates an Intermediate Frequency (IF) where any received signal is converted in order to function the signal effectively.
RF Mixer is an important stage in the receiver phase. It is a non-linear electronic circuit which create new frequencies with the help of two signals. Two signals of different frequencies are taken where one signal level affects the level of the other signal, to produce the resultant mixed output. The input signals and the resultant mixer output is illustrated in the following figures.
You can see the above frequencies. Let the first and second signal frequencies beIf these two signals are applied as inputs of RF mixer, then it produces an output signal, with possible frequencies ofIf this is observed in the frequency domain, the pattern looks like the following figure.
Let the first and second signal frequencies be f1f1 and f2f2. If these two signals are applied as inputs of RF mixer, then it produces an output signal, having frequencies of
The AM super heterodyne receiver accepts the amplitude modulated wave as an input and generates the original audio signal as an output. In other words it has the ability of selecting a particular signal, while rejecting the others. Sensitivity is the capacity of detecting RF signal and demodulating it, even at the lowest power level.
Radio amateurs are the initial radio receivers. However, they have some drawbacks such as poor sensitivity and selectivity. To avoid these drawbacks, a receiver called super heterodyne receiver was invented. The below block diagram of AM receiver is shown in the following figure.
The tuner circuit is referred as radio receiver which is also called as resonant or tank circuit. It is also called as LC circuit. The amplitude modulated wave that is received by the antenna is first passed to the tuner circuit through a transformer. It selects the frequency, to receive the desired AM modulated wave. It also tunes the local oscillator and the RF filter at the same time.
The production of IF helps in the demodulating of any station signal that has any carrier frequency. Thus, all signals are translated to a fixed carrier frequency for moderate selectivity.
Intermediate frequency filter is a frequency that has band pass filter, which passes the desired frequency. It eliminates all other unwanted frequency components present in it. This is one of the advantage of IF filter, which allows only IF frequency.
The AM wave which was received is now demodulated using AM demodulator. This demodulator in order to receive the modulating signal uses the envelope detection process.
Audio Amplifier is the power amplifier stage, which is used to detect the amplified audio signal. The processed signal is strengthened so that it could be effective. This signal is then passed on to the loudspeaker to get the original audio signal.
The block diagram of FM receiver is shown in the following figure.
This block diagram of FM receiver is similar to the block diagram of AM receiver. The two blocks Amplitude limiter and De-emphasis network are inserted before and after FM demodulator. The operation performed by the remaining blocks is the same as that of AM receiver.
As we know that in FM modulation, the amplitude of FM wave remains constant. However, if some noise or any disturbance is added with FM wave in the channel, due to that disturbance the amplitude of FM wave may vary. To avoid this nuisance the amplitude limiter helps to maintain the amplitude of FM wave in constant by removing all the unwanted peaks of the noise signal.
In FM transmitter, we have observed the pre-emphasis network (High pass filter), which is included before FM modulator. This helps to improve the SNR of high frequency audio signal. De-emphasis is nothing but the reverse process of pre-emphasis. Thus, in this FM receiver, the de-emphasis network which is also known as (Low pass filter) is included after FM demodulator. To increase the power level this signal is passed to the audio amplifier. With this passing the signal on to the loud speaker we get the original sound signal.
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