UNIT 1
MICROWAVE OSCILLATORS AND AMPLIFIERS
Microwave occupies a space in the electromagnetic wave spectrum in the frequency 300 MHz to 300GHz corresponding to a range of wavelength from 100 cm to 1 mm in free space having permittivity farad/m, permeability henry/mand.
Conductivity= Mho/m.
Short wavelengths of microwaves make it capable to freely propagate through the ionised layers of the atmosphere.
Advantages
A) Availability of increased bandwidth : The large bandwidth of microwaves i.e 1 GHz to 10 GHz gives the advantage that more information can be transmitted in low frequency ranges as the lower band of frequency is very crowded.
B) Directive properties are improved: Directivity increases and beamwidth decreases as frequency increases. To get sharp beams of radiation, the diameter of the antenna becomes very large at low frequency. But at microwave frequencies antenna size of several wavelengths give smaller beamwidths.
C) Fading effect: Fading effect is prominent at low frequency due to change in transmission medium, but at microwave frequencies, line of sight propagation, less fading effect is noticed.
D) Power : At microwave frequencies, transmitter and receiver power are very low.
Uses
Microwaves are used in long distance communications, radar, radio astronomy, etc.
Commercial and industrial applications use the heating property of microwaves in a) microwave oven at 2.4 GHz, 600 W b) drying machines for textile, food and paper industry c) biomedical applications.
Conventional vacuum tubes like triodes, tetrodes, and pentodes are used as a low output power at low microwave frequencies. But at higher microwave frequencies these tubes become less effective when used as amplifiers and oscillators. At UHF ( 300 - 3000MHz ) or microwave frequency, there are many factors which deteriorate the performance of conventional tubes. Therefore, special types of tubes like klystron and magnetron are used to overcome the high frequency limitations of conventional tubes.
Conventional vacuum tubes cannot be used for frequencies higher than 100 MHz for the following reasons :-
A) Inter - electrode capacitance effect
B) Lead inductance effect
C) Transit time effect
D) Gain bandwidth limitation effect
E) Effect due to RF loss
F) Effect due to Radiation loss
Transistors suffer from high frequency limitations. Capacitances between electrodes play an important part in determining high frequency response. Current gains of common base and common emitter configurations and acquire reactive components and make them complex and unusable.
Interelectrode capacitances in BJT is dependent on width of the depletion layer which is again dependent on bias considerations. Therefore, the effect is more pronounced than what it is with tubes. Also, finding the high frequency parameters becomes very difficult.
Electrode inductances cause the same effect as with tubes but as transistors are smaller in size, therefore electrode leads are short and somewhat reduces the effect of lead inductances.
The effect of transit time is somewhat identical, and the smaller distances travelled in transistors are nullified by the slower velocities of current carriers. At UHF and Microwave frequencies, the majority carriers suffer an emitter delay time and the injected carriers encounter the base transit time. Also, electrons and holes take some finite time to cross the collector.
Therefore, to design a transistor circuit at microwave frequencies, scattering or S - parameter two port network theory is used where the parameters are the forward and reverse transmission gains and the forward and reverse reflection coefficients which gives relatively easy measurement and plotting on the Smith chart.
UNIT 1
MICROWAVE OSCILLATORS AND AMPLIFIERS
Microwave occupies a space in the electromagnetic wave spectrum in the frequency 300 MHz to 300GHz corresponding to a range of wavelength from 100 cm to 1 mm in free space having permittivity farad/m, permeability henry/mand.
Conductivity= Mho/m.
Short wavelengths of microwaves make it capable to freely propagate through the ionised layers of the atmosphere.
Advantages
A) Availability of increased bandwidth : The large bandwidth of microwaves i.e 1 GHz to 10 GHz gives the advantage that more information can be transmitted in low frequency ranges as the lower band of frequency is very crowded.
B) Directive properties are improved: Directivity increases and beamwidth decreases as frequency increases. To get sharp beams of radiation, the diameter of the antenna becomes very large at low frequency. But at microwave frequencies antenna size of several wavelengths give smaller beamwidths.
C) Fading effect: Fading effect is prominent at low frequency due to change in transmission medium, but at microwave frequencies, line of sight propagation, less fading effect is noticed.
D) Power : At microwave frequencies, transmitter and receiver power are very low.
Uses
Microwaves are used in long distance communications, radar, radio astronomy, etc.
Commercial and industrial applications use the heating property of microwaves in a) microwave oven at 2.4 GHz, 600 W b) drying machines for textile, food and paper industry c) biomedical applications.
Conventional vacuum tubes like triodes, tetrodes, and pentodes are used as a low output power at low microwave frequencies. But at higher microwave frequencies these tubes become less effective when used as amplifiers and oscillators. At UHF ( 300 - 3000MHz ) or microwave frequency, there are many factors which deteriorate the performance of conventional tubes. Therefore, special types of tubes like klystron and magnetron are used to overcome the high frequency limitations of conventional tubes.
Conventional vacuum tubes cannot be used for frequencies higher than 100 MHz for the following reasons :-
A) Inter - electrode capacitance effect
B) Lead inductance effect
C) Transit time effect
D) Gain bandwidth limitation effect
E) Effect due to RF loss
F) Effect due to Radiation loss
Transistors suffer from high frequency limitations. Capacitances between electrodes play an important part in determining high frequency response. Current gains of common base and common emitter configurations and acquire reactive components and make them complex and unusable.
Interelectrode capacitances in BJT is dependent on width of the depletion layer which is again dependent on bias considerations. Therefore, the effect is more pronounced than what it is with tubes. Also, finding the high frequency parameters becomes very difficult.
Electrode inductances cause the same effect as with tubes but as transistors are smaller in size, therefore electrode leads are short and somewhat reduces the effect of lead inductances.
The effect of transit time is somewhat identical, and the smaller distances travelled in transistors are nullified by the slower velocities of current carriers. At UHF and Microwave frequencies, the majority carriers suffer an emitter delay time and the injected carriers encounter the base transit time. Also, electrons and holes take some finite time to cross the collector.
Therefore, to design a transistor circuit at microwave frequencies, scattering or S - parameter two port network theory is used where the parameters are the forward and reverse transmission gains and the forward and reverse reflection coefficients which gives relatively easy measurement and plotting on the Smith chart.
