Note this points which are regularly asked in all competitive exams :

• Accoustic channel is used for  person to person voice communications.
• At high frequency transit time noise is important.
• HF mixer is noiser than HF amplifier.
• Strongest noise interface occurs at 20 to 120 MHZ.
• Leak-type bias is used in a plate modulated class C amplifier to prevent excess grid current.
• Output stage of television transmitter is grid modulated class c amplifier.
• Advantage of base modulation over collector modulation in class c amplifier is lower modulating power.
• At 100% modulation in AM ratio of modulating power to total power is 1:3

Code for different kind of modulations

1. Vestigial sideband – C3F
2. DSB, full carrier – A3E
3. Independent Side Band – B8E
4. Single side band full carrier – H3E
5. SSB, suppressed carrier – J3E
6. SSB, reduced carrier – R3E

SSB transmitter

SSB transmitter contains class B amplifier. When modulation index and antenna current of AM is doubled then it is SSB,SC. To remove unwanted side band in SSB – filter system, phase shift method and third method is used . R3E modulation is used for frequency stabilization in receiver. B8E is used to provide two or more voice circuits with single carrier.

Difference between PM and FM

Difference between PM and FM is modulation index definition. In FM amplitude of side band depends on modulation index. An FM signal with modulation index m is passed through frquancy trippler then output have modulation index 3m. An FM signal with frequency deviation D is passed through mixer where frequency reduced by 1/5^th then deviation of out is D itself. By amplifying higher audio frequencies pre-emphasis provide noise immunity. In an FM stereo multiplex transmission difference signal modulates 38k carrier

Rank deciding Points – FM Radio :

• Squilsh circuit turn off audio amplifier when the carrier is absent.
• Controlled oscillator synthesis is relatively free of spurious frequency over direct method.
• FET can be used as first amplifier in the receiver so that negative clipping can be avoided.
• Three point tracking is achieved with padder capacitor
• Local oscillator frequency of receiver should be greater than received signal to get addicate frequency coverage without switching.
• If ac to dc ration of impedance is low in diode detector, then negative-peak clipping takes place.
• If a receiver has good blocking performance then receiver is unaffected by AGC derived by nearer transmission

Velocity factor of a transmission line depends on the dielectric material used. AM receiver having diode detector is used for SSB,SC. Standing wave ratio of a transmission line is infinity, if it is terminated with- open circuit/short circuit/pure reactance. Impedance inversion is obtained by a quarter wave line

Short circuit stub is better than open circuit stub, because oc stab is liable to radiate. For a transmission line load matching over a range of frequency double stub is used. To couple a coaxial line to a parallel wire line BALUN is used. Three types of energy loses in transmission line – radiation loss, I^2R loss, dielectric heating. High frequency waves are affected by solar cycle.

In ground wave propagation one who moving away from transmitter losses signal because of tilting. Microwave following curvature of earth is called ducting. Helical antenna is used for satellite tracking at VHF because of faradays effect. Ungrounded antenna near the ground act as array

Top loading is used to increase effective length of antenna. Wavelength of a wave in wave guide is more than free space. Transmission lines use principal mode of propagation so does not suffer with cut off frequency

If an EM wave reflects at waveguide wall then wavelength along the wave guide is greater than direction of propagation. When an EM wave reflects at a plane conducting wall, its velocity increases than that in the free space, it is called phase velocity. When free space wavelength of a signal is equal to cut off wavelength of a wave guide then group velocity is 0 , phase velocity is infinity, wavelength in wave guide becomes infinity. Chock flanges is used to compensate discontinuities in the joint of waveguide

Piston attenuator is a wave guide attenuator below cut off. Cylindrical cavity resonators are not used with klystron because it have harmonically related frequencies. Three or more holed directional couplers are used instead of two holed couplers to increase bandwidth. Manganese ferrite is used as circulator, isolator, phase shifter.

Circular wave guide have less attenuation than rectangular guide. Attenuator is used in traveling wave tube to avoid oscillation. Baud rate is equal to twice the bandwidth of ideal channel. Shannon Hartley law deals with noise. ASCII code provides parity check. Data transmission rate of modem is in bits per sec. The number of repeaters in coaxial cable link depends on bandwidth of channel. Microwave repeaters are 50kms apart, because of earth curvature. Telephone traffic is measured in ERLANG. To separate TDM/FDM receiver channels AND gate is used. PWM is used in telegraphy with a monostable. Companding is used to protect small signals in PCM from quantizing distortion. PCM requires large bandwidth.

Different Antennas and their key points

• Rhombic antenna – non resonant antenna
• Log periodic antenna – multiband HF receiving antenna
• Horn antenna – excited by wave guide
• PIN diode – microwave switch
• Telegraphy- FSK
• Troposcatter propogation – fading
• UHF range – space wave
• Tropospheric scatter – UHF

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