Microwave Tubes

• Used for high power/high frequency combination

• Tubes generate and amplify high levels of microwave power more cheaply than solid state devices

• Conventional tubes can be modified for low capacitance but specialized microwave tubes are also used

Crossed-Field and Linear-Beam Tubes

• Magnetron is one of a number of crossed-field tubes– Magnetic and electric fields are at right angles

• Klystrons and Traveling-Wave tubes are examples of linear-beam tubes– These have a focused electron beam (as in a CRT)

Vacuum Tubes

•An electronic device in which electrons flow through vacuum is called vacuum tubes.

•A vacuum tube consists of a evacuated glass envelope which contains a cathode, an anode

and one or more electrodes called grids. •Diode, triode, tetrode, pentode

Vacuum Tubes

Features of Vacuum Tubes

• Vacuum tubes are voltage controlled device.• These can operate at very high voltages.• High power can be easily developed by

vacuum tubes.• According to the number of electrodes,

vacuum tubes can be classified as (i) Vacuum diode(ii) Vacuum triode

(iii) Vacuum Tetrode (iv) Vacuum Pentode• Conventional tubes such as triodes, tetrodes

and pentodes are useful only at low microwave frequencies.

• These tubes cannot operate at high frequencies due to their limitations at those frequencies.

High Frequency Limitations of Conventional Tubes

• The conventional tubes become less effective at microwave frequency range when these are used as an amplifier and oscillator. The limitations of conventional tubes at high frequencies is due to :

(a) Inter-electrode capacitance effect (b) Lead Inductance effect (c) Transit Time effect

Inter-electrode Capacitance Effect• The capacitance exists when two pieces of

metal are separated by a dielectric. Vacuum has a dielectric constant of 1.

• The elements of the triode are made up of metal and are separated by dielectric material .

• So there must exist capacitance between them. This capacitance is called interelectrode capacitance

Inter-electrode Capacitance Effect

Lead Inductance Effect

• The common lead inductance is the inductance associated with the common connection of vacuum triode.

• This effect is more when the frequency of the signal is high.

• As the frequency increases, the inductive reactance increases and due to high inductive reactance there is an input matching problem

• The lead inductance affects the performance of vacuum triode with most of input voltage lost across inductance and only small fraction of input reach to terminal for amplification.

• These inductances form unwanted tuned circuit with the capacitance and parasitic oscillations are produced. As frequency increases, the reactance increases.

Effect of Transit Time

• The time taken by an electron to travel from cathode to anode is called transit time.

• At low frequencies, the transit time is very small i.e. the electrons reach instantaneously the anode plate from cathode.

• At high frequencies, the transit time becomes large because the source driving the grid becomes loaded and the gain of the vacuum tube becomes less than unity.

Effect of Transit Time

• This loading is due to the dissipation of the power at the grid. The effect of loading is such that the noise in the circuit increases.

• To minimize this effect, the distance between the electrodes is to be reduced and high voltage must be applied.

• This will increase the interelectrode capacitance.

Klystron

• Used in high-power amplifiers• Electron beam moves down tube past several

cavities.• Input cavity is the buncher, output cavity is the

catcher.• Buncher modulates the velocity of the

electron beam

Klystron Tubes

• Klystron tube is a vacuum tube that can be operated either as an oscillator or as an amplifier at microwave frequencies. Two basic configurations of klystron tubes are :

1. Multicavity klystron which is used as a low power microwave amplifier.

2. Reflex klystron which is used as a low power microwave oscillator.

Klystron Tubes

Two cavity Klystron Amplifier

Application

As power output tubes1. in UHF TV transmitters2. in troposphere scatter transmitters3. satellite communication ground station4. radar transmitters

As power oscillator (5 – 50 GHz), if used as a klystron oscillator

Reflex Klystron oscillator

A reflex klystron consists of an electron gun, a cavity with a pair of grids and a repeller plate as shown in the above diagram.

In this klystron, a single pair of grids does the functions of both the buncher and the catcher grids.

The main difference between two cavity reflex klystron amplifier and reflex klystron is that the output cavity is omitted in reflex klystron and the repeller or reflector electrode, placed a very short distance from the single cavity, replaces the collector electrode.

Working of reflex klystron The cathode emits electrons which are accelerated

forward by an accelerating grid with a positive voltage on it and focused into a narrow beam.

The electrons pass through the cavity and undergo velocity modulation, which produces electron bunching and the beam is repelled back by a repeller plate kept at a negative potential with respect to the cathode.

On return, the electron beam once again enters the same grids which act as a buncher, their by same grids acts simultaneously as a buncher for the forward moving electron and as a catcher for the returning beam

The feedback necessary for electrical oscillations is developed by reflecting the electron beam, the velocity modulated electron beam does not actually reach the repeller plate, but is repelled back by the negative voltage.

Thus the repeller voltage is so adjusted that complete bunching of the electrons takes place at the catcher grids, the distance between the repeller and the cavity is chosen such that the repeller electron bunches will reach the cavity at proper time to be in synchronization.

Due to this, they deliver energy to the cavity, the result is the oscillation at the cavity producing RF frequency.

Application

The reflex klystrons are used in1. Radar receivers2. Local oscillator in microwave receivers3. Signal source in microwave generator of

variable frequency4. Portable microwave links5. Pump oscillator in parametric amplifier

Traveling Wave Tube (TWT)

• TWT is an amplifier that makes use of distributed interaction between electron beam and a travelling wave.

• It is mainly used for amplification of high frequencies. i.e. 3000 MHz or above.

• Its principle feature is based on a slow wave structure. the RF wave propagate at the speed of light, while electron beam propagate at much slow velocity. Therefore the mechanism that reduces RF wave phase velocity in a TWT is a slow wave structure

The basic structure of a TWT consists of a cathode and filament heater plus an anode that is biased positively to accelerate the electron beam forward and to focus it into a narrow beam.

The electrons are attracted by a positive plate called the collector, which has given a high dc voltage.

The length of the tube is usually many wavelengths at the operating frequency.

Surrounding the tube are either permanent magnets or electromagnets that keep the electrons tightly focused into a narrow beam.

Basic structure of a Traveling Wave Tube (TWT)

Features The unique feature of the TWT is a helix or coil that

surrounds the length of the tube and the electron beam passes through the centre or axis of the helix.

The microwave signal to be amplified is applied to the end of the helix near the cathode and the output is taken from the end of the helix near the collector.

The purpose of the helix is to provide path for RF signal.

The propagation of the RF signal along the helix is made approximately equal to the velocity of the electron beam from the cathode to the collector

FunctioningThe passage of the microwave signal down the helix

produces electric and magnetic fields that will interact with the electron beam.

The electromagnetic field produced by the helix causes the electrons to be speeded up and slowed down, this produces velocity modulation of the beam which produces density modulation.

Density modulation causes bunches of electrons to group together one wavelength apart and. these bunch of electrons travel down the length of the tube toward the collector.

Functioning

The electron bunches induce voltages into the helix which reinforce the voltage already present there. Due to that the strength of the electromagnetic field on the helix increases as the wave travels down the tube towards the collector.

At the end of the helix, the signal is considerably amplified. Coaxial cable or waveguide structures are used to extract the energy from the helix.

Performance characteristics

1. Frequency of operation : 0.5 GHz – 95 GHz2. Power outputs: 5 mW (10 – 40 GHz – low power TWT) 250 kW (CW) at 3 GHz (high power TWT) 10 MW (pulsed) at 3 GHz3. Efficiency : 5 – 20 % ( 30 % with depressed

collector)

Application of TWT

1. Low noise RF amplifier in broad band microwave receivers.

2. Repeater amplifier in wide band communication links and long distance telephony.

3. Due to long tube life (50,000 hours against ¼th for other types), TWT is power output tube in communication satellite.

4. Continuous wave high power TWT’s are used in tropo scatter links (due to larger power and larger bandwidths).

5. Used in Air borne and ship borne pulsed high power radars.

M-type tubes• High-power oscillator• Common in radar and microwave ovens• Cathode in center, anode around outside• Strong dc magnetic field around tube causes

electrons from cathode to spiral as they move toward anode

• Current of electrons generates microwaves in cavities around outside

Slow-Wave Structure

• Magnetron has cavities all around the outside• Wave circulates from one cavity to the next

around the outside• Each cavity represents one-half period• Wave moves around tube at a velocity much

less than that of light• Wave velocity approximately equals electron

velocity

Duty Cycle

• Important for pulsed tubes like radar transmitters

• Peak power can be much greater than average power