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Digital communication is a mode of communication where the information is encoded digitally as discrete signals and electronically transferred to the recipients.

Modulation is needed

(i) to reduce length of antenna

(ii) to increase operating range

(iii) for wireless communication and

(iv) to reduce interference.

FM is better for high sensitivity reception.

(i) Amplitude modulation.

(ii) Frequency modulation.

(iii) Phase modulation.

Single side band suppressed carrier.

Single side band transmitted carrier.

Frequency modulation.

In angle modulation, total angle φ = w_ct + θ of the carrier wave is varied. Different types of angle modulation are

(i) Frequency modulation.

(ii) Phase modulation.

(i) Power saving is 83.3%

(ii) Operating cost is less.

(iii) Reduced interference of noise.

(iv) Requires only small power supply.

Frequency deviation is the change in carrier frequency produced by the modulating signal from its centre frequency.

According to Carsons rule, bandwidth is twice the sun of maximum frequency deviation and highest modulating frequency.

75 kHz is the maximum permitted frequency deviation in FM.

The sideband transmission is important in AM because it contains the information signal.

Deviation ratio is the ratio of maximum allowed frequency deviation to the maximum frequency of modulating signal.

Bandwidth of FM wave is defined as width of the frequency spectrum that contains all side frequency components having amplitude equal to or greater than one percent of the amplitude of the carrier.

Modulation index of AM wave is the ratio of amplitude of modulating signal (V_m) to the amplitude of the carrier wave (V_m) It indicates the depth of modulation.

AM is used for broadcasting:

(i) Radio on long medium and short waves.

(ii) TV picture in ultra short waves.

(iii) Radio telephony on short waves.

Guard band is the range of frequencies added on either side of channel width to prevent interference between adjacent channels.

Carrier swing is the total variation in frequency from minimum to maximum value.

Significant side bands are the side bands having amplitude more than or equal to 1% of carrier amplitude.

Infinite side bands.

Transmission efficiency is the ratio of power contained in the side band to the total power.

Transmission efficiency,

\(\eta=\frac{m^2_a}{2+m^2_a}\)

The power in each side band in AM when m_a= 1 is one sixth of total power of AM wave.

As the side bands contain the required signal, their transmission is important.

\(m=\sqrt{m_1^2+m_2^2+m_3^2+.....M_n^2}\)

Transmission efficiency of an AM wave is the ratio of power contained in the side bands to the total power.

V_min = V_c – m_aV_m
= V_c  – V_m  (\(\therefore\) m = 1)
= 6 – 4
= 2V

Modulation index,

= m_a = \(\frac{V_m}{V_c}=\frac{4}{5}=0.8\)

Total modulation index.

(i) A lower value of IF improves selectivity’ but elimination of image frequency becomes difficult.

(ii) If IF is low, the selectivity becomes two sharp resulting in sideband cutting. Very high selectivity makes a receiver more difficult to tune oscillator circuits.

(iii) IF should not be too high because high IF leads to reduction in selectivity, gam sue adjacent channel rejection.

If the frequency of local oscillator is higher than the required signal frequency, then maximum to minimum capacitance ratio required by the variable capacitor is 4.4:1 But the tuning capacitors available commercially have a capacitance ratio of 10:1.

If the oscillator frequency is kept lower than the signal frequency, the maximum to minimum capacitance ratio required is 158:1. It is not practicable to make a tunable capacitance with this type of range. Hence the local oscillator frequency is made higher than the signal frequency in all AM receivers.

Selectivity is the ability of a radio receiver to distinguish between desired signal to which the receiver is tuned and the other signal frequencies.

De-emphasis is the process of attenuating the relative amplitude of high frequency components of the modulating signal.

Diode detector is also called envelop detector.

Demodulation is the process of recovering a modulating signal from the modulated wave.

The limiter keeps IF amplifier output voltage constant.

Transmission lines are the material structure used for transport of electrical signals or energy from one location to another far off destination.

Yagi is used to describe type of antenna and is credited to famous Japanese antenna experts Yagi and Uda.

They are used for transport of electrical signals from one location to another far off destination.

Yagi antenna is called so in memory of Japanese antenna experts Yagi and Uda. It is an array of parallel, straight antenna elements.

(i) Horn antennas are used as feeders for parabolic antennas.

(ii) They are active elements in dish antennas.

Loop antenna is designed to receive radio signals more efficiently than horn antenna. The loop is wound on a form, which may be either box or spiral wound.

Horn antenna has a metal waveguide shaped like a horn to direct radio waves into beam. They are used at frequencies above 300 MH_fc

Total power,

P_T = P_c\((1+\frac{m^2_a}{2})\)

For m_a = 1,

P_T = P_c\((1+ \frac{1^2}{2})\)

P_T = \(\frac{3}{2}\)P_c.

∴ Total power in AM is \(\frac{3}{2}\)

P_T = P_c + P_LSB + P_USB

2500 = P_c + 300 + 300

P_c = 2500 - 600

P_c = 1900W

P_T = P_c(1 + \(\frac{m_a^2}{2}\))

2500 = 1900[1 + \(\frac{m_a^2}{2}\)]

\(\frac{2500}{1900}\) = 1 + \(\frac{m_a^2}{2}\)

1.315 - 1 = \(\frac{m_a^2}{2}\)

\(m_a^2\) = 2(0.315) = 0.631

m_a = \(\sqrt{0.631}\)

m_a = 0.794

= 0.794 x 100

= 79.4%

m_a = 80%

= \(\frac{80}{100}\)

= 0.8

P_T = 2200 W

P_T = P_c(1 + \(\frac{m_a^2}{2}\))

2200 = P_c[1 + \(\frac{(0.8)^2}{2}\)]

2200 = P_c[1 + 0.32]

P_c = \(\frac{2200}{1.32}\)

P_c = 1666W

P_T = P_c + Power in the sidebands

Power in two sidebands = P_T - P_c

= 2200 - 1666W

= 534W

Power in each sideband = \(\frac{534}{2}\)

= 267W

f_m = 5kHz,

m = 0.5

m = \(\frac{\Delta f}{f_m}\)

0.5 = \(\frac{\Delta f}{5}\)

Δf = 2.5kHz

Δf = 25 kH_z, f_m = 5kH
m = \(\frac{\Delta f}{f_m}=\frac{25}{5}=5\)
Carrier swing, C_s = 2.Δf
= 2 x 25kH_z
= 50 kHz

Let the instantaneous voltage of modulating signal be given by the expression V_m = V_m Sin ωt.
Let the instantaneous voltage of high frequency carrier be given by v_c = V_c sinω_ct. In amplitude modulation, amplitude of carrier varies proportional to instantaneous amplitude of modulating signal.
Hence amplitude of modulated signal is
A = V_c + V_m
= V_c + V_m sin ω_mt
= Vc[1 + \(\frac{V_m}{V_c}\)sin ω_mt]
= Vc[1 + m_a Sin ω_mt ]
The instantaneous value of amplitude modulated signal is
V_Am = A Sin ω_ct
V_am - Vc[1 + m_a Sinω_mt] Sin ω_ct. where m_a is called modulation index.