Explain frequency modulation.

Modulation is the process by which information is encoded from a message source in order to optimise it for transmission. Frequency Modulation (FM) is the encoding of information in a carrier wave by changing the instantaneous frequency of the wave. FM technology is widely used in the fields of computing, telecommunications, and signal processing. There are several methods that can be used to generate either direct or indirect frequency-modulated signals. 1) A voltage-controlled oscillator or a Varactor diode oscillator :- A voltage-controlled oscillator can be used to form Direct FM modulation by directly feeding the message into the input of the oscillator. In the case of the varactor diode, we place this device within the tuned circuit of an oscillator circuit. 2) Crystal oscillator circuit :- Varactor diode can also be used within a crystal oscillator circuit wherein the signal needs to be multiplied in frequency, and only narrowband FM is attained. 3) Phase locked loop :- This is an excellent method to generate frequency modulation signals. However, the constraints within the loop should be checked carefully and once everything is stable it offers an excellent solution.

Explain the function of superheterodyne AM receiver with block diagram.

As you can see the block diagram has 11 different stages, each stage has a specific function which is explained below RF Filter: The first block is the ferrite rod antenna coil and variable capacitor combo, that serves two purposes - RF is induced into the coil and the parallel capacitor controls the resonant frequency of it, as ferrite antennas receive the best when the resonant frequency of the coil and capacitor is equal to the station's carrier frequency – this way it acts as an input filter of the receiver. Heterodyne Local Oscillator: The second block is to heterodyne, also known as the local oscillator (LO). The frequency of the local oscillator is set, so either the sum or the difference of the RF signal’s frequency and the LO’s frequency is equal to the IF used in the receiver (usually around 455 kHz). Mixer: The third block is the mixer, the RF signal and the LO signal is fed to the mixer to produce the desired IF. Mixers found in common AM receivers output the sum, the difference of the LO and RF’s frequencies and the LO and RF signals themselves. First IF Filter: The fourth block is the first IF filter. In most AM receivers, it is a resonant circuit placed in the collector of the mixer transistor with the resonant frequency equal to the IF frequency. Its purpose is to filter off all signals with a frequency different from the IF frequency because those signals are unwanted mixing products and don’t carry the audio signal of the station we want to listen to. First IF Amplifier: The fifth block is the first IF amplifier. Gains of 50 to 100 in each IF stages are common if the gain is too high, distortion can take place, and if the gain is too high, IF filters are too close to each other and not properly shielded, parasitic oscillation can take place. Second IF Filter: The sixth block is the second IF filter, just like the first one it is a resonant circuit placed in the collector of the transistor. It only lets signals of the IF frequency – improving selectivity. Second IF Amplifier: The seventh block is the second IF amplifier, it is practically the same as the first IF amp except it is not controlled by AGC, as having too many AGC controlled stages, increases distortion. Third IF Filter: the eighth block is the third IF filter, just like the first and the second one is a resonant circuit placed in the collector of the transistor. It only lets signals of the IF frequency – improving selectivity. It feeds the IF signal to the detector. Detector: The ninth block is the detector, usually in the form of a germanium diode or a diode-connected transistor. It demodulates AM by rectifying the IF. On its output, there is a strong IF ripple component that is filtered out by a resistor-capacitor low pass filter, so only AF component remains, it is fed to the audio amp. Audio Amplifier: The tenth block is the audio amplifier; it amplifies the audio signal and passes it onto the speaker. Between the detector and the audio amplifier, a volume control potentiometer is used. Speaker: The last block is the speaker (usually 8 ohms, 0.5W) that outputs audio to the user. The speaker is sometimes connected to the audio amplifier through a headphone jack that disconnects the speaker when headphones are plugged in.

Which type of communication is employed in mobile phones?

The space wave propagation is employed in mobile phones, then; Space wave propagation: This transmission is limited to the line of sight. Waves generally travel in a straight line from the transmitter to the receiver. It is used for propagation of waves of frequency greater than 40 MHz . The ground wave propagation is not used for low range, it is used in landlines. And in the case of sky wave propagation the frequency is more than instead of reflection, refraction takes place and the waves are not received by the receiver.