Microwave Active Devices: Types, Features & Operation

Generally microwave devices can be categorized as either passive components or active components. We have already talked about microwave passive components in this other article. This article focuses on microwave active devices. They may be classified into two types: low-power solid-state devices and high-power vacuum tube devices.

Communications involving the gigahertz bandwidths cannot employ conventional transistor or integrated circuit (IC) design because silicon devices suffer from stray capacitance between leads and the semiconductor elements themselves. Moreover, the movement of charge carriers from one region to the other takes a finite time referred to as the transit time. If the transit time is similar to the period of frequency being transmitted then phase shifts can occur which affect the signal adversely.

Solid-State Devices

Two electronic devices which overcome the major problem of transit time are the Gunn diode and transit time (IMPATT) diode. These two devices employ a negative resistance characteristic, that is, over a portion of the characteristic curve the resistance decreases as the voltage increases.

Gunn Diode

The Gunn diode is employed in oscillator and mixer applications and comprises of a thin slice of n-type gallium arsenide between two metal conductors. When a voltage is placed across the slice, oscillations occur, the frequency of which depends on the thickness of the slice. Additional, the period of the oscillations is equal to the transit time of electrons across the slice. Thus the geometry of the device is essential to its operation.

It also operates on several modes, and this allows the diode to be tuned when placed in a cavity. But, like all negative resistance devices, it needs a circulator to separate the input from the output signals.

IMPATT Diode

The IMPATT diode uses the delay time needed for an avalanche condition and the transit time to produce a negative resistance characteristic. It has a p-n junction and may be a four-layer device. The thickness of the intrinsic region is fabricated so that the transit time of an electron across this region is equal to half the period of the operating frequency.

This diode operates with reverse bias just below the breakdown region, and the fluctuation of the operating frequency, which is superimposed upon this bias, causes breakdown to occur once per cycle. When the junction breaks down, a burst of electrons enters the intrinsic region and appears at the other side half a cycle later. The resultant oscillations are dependent on the dimensions of the diode and the resonant frequency of the cavity which it is immersed.

Whilst IMPATT diodes have higher efficiency and greater power levels than Gunn diodes, they are noisier because of the avalanche effect.

Microwave Tubes

For extremely high-frequency applications (above 1 GHz), the inter-electrode capacitances and transit-time delays of standard electron tube construction become prohibitive. This is why high frequency electron tube designs have been developed to overcome these challenges.

When greater power is needed more complex devices must be employed. Furthermore, the process of miniaturization imposes problems at higher frequencies due to higher power dissipation. Because of these restrictions, alternative designs have been built such as multi-cavity magnetrons, klystrons, reflex klystrons and travelling wave tubes.

Multi-Cavity Magnetrons

This article introduces the fundamental principle of operation of a magnetron. You can read it to have a basic understanding of a magnetron.

The structure of a multi-cavity magnetron is illustrated below:

In the multi-cavity magnetron, the cavities are set in oscillation by the rotating electrons in the space between cathode and anode. When the electron path is favorable to adding electron energy to the field, the oscillations are sustained. Other electron paths take energy away from the field. The magnetron operating conditions must be adjusted so that more energy is added to the field than taken away. Maximum output occurs when the added energy reinforces the cavity oscillations.

The problem of transit time is overcome by adding end plates to the split anode and causing the electrons to spiral around the cathode as demonstrated below:

One of the major shortcomings of the magnetrons is that it is difficult to modulate them by any of the typical modulation techniques. Furthermore, the transit time sets a limit to the upper frequency which can be used. Nonetheless, very high-power applications such as modern radar would be impossible without the magnetron, which is capable of producing 10 MW more pulsed power.

Klystrons

The issue of transit time can also be overcome by employing a klystron. Though originally developed as a low-power device, modern tubes have high power handling capacity. Klystron tubes are superior to magnetrons in that they can function as an oscillator, amplifier or detector.

The klystron employs a process referred to as velocity modulation in which the velocity of the electron stream is varied as it moves through the tube. This process is illustrated by the figures below:

The figure above is an arrangement consisting of an electron beam of constant velocity directed towards two grid meshes. These grids extend out into the electron stream and a source RF is connected across the grids.

As the electrons pass through the grids they are affected by the field set up between them by the RF generator. An electron that leaves grid 1 just before that grid goes positive is drawn back to it when the grid becomes positive. At the same time the electron is repelled by grid 2, thus causing it to slow down. An electron that leaves grid 1 when it is just going negative is repelled by grid 1 and attracted by grid 2. This electron is speeded up. Electrons that pass through the grids when the grids are at zero potential are neither speeded up nor slowed down, and they join the slowed-down electrons. The speeded up electrons catch up with those that go through unaffected, and this manner bunches of electrons are formed.

Since the electrons spend only short time between the grids, there is no resultant current in the grids. Hence, the transit time of the electrons has negligible effect on the operation.

To be of greater use the electrons are converted into density modulated beam from which energy can be extracted using various techniques such as the drift-tube process.

Reflex Klystrons 

This type is a single-cavity klystron and operates as an oscillator only. It is a low-noise device and is used in low power applications below frequencies of 30 GHz.

In this microwave tube, electrons emitted from the heated cathode travel through the cavity grids toward the repeller plate, then are repelled and returned back the way they came (hence the name reflex) through the cavity grids. Self-sustaining oscillations would develop in this tube, the frequency of which could be changed by adjusting the repeller voltage. Thus, this tube functions as a voltage-controlled oscillator.

As a voltage-controlled oscillator, reflex klystron tubes were used as local oscillators for radar equipment and microwave receivers. Reflex klystrons have since been superseded by semiconductor devices in the application of local oscillators but amplification klystrons continue to find use in high-power, high-frequency radio transmitters and in scientific research applications.

Travelling-Wave Tubes

For very wide-band applications the klystron’s operation is a compromise between gain and bandwidth. If the gain is reduced enough to give the required bandwidth, the noise generated within the klystron may decrease the signal-to-noise ratio. Furthermore, in the klystron tube the coupling between the electron beam and the field is limited due to the narrowness of the resonator gaps. Thus, the transfer of signal is less efficient.

In order to overcome these deficiencies the travelling-wave tube was developed. The figure below illustrates such a system:

The structure comprises of an electron gun which produces a collimated beam of electrons. The electrons are accelerated along the tube towards the collector, which has a high positive potential. The slender section of the tube contains a helix of wire which is rigidly mounted on insulating supports in the long glass tube.

The electron beam travels through the helix to the collector. Outside the helix there are two short spirals wound on the glass at each end of the helix, and these serve as the input and output connections. Outside the tube is a focusing magnet which serves to collimate the electrons.

The helix has inductance and capacitance which are effective in slowing down any wave travelling in it, and thus it works as a delay line. This prevents one of the issues of the klystrons, where the interaction between the field and the electrons is very short. The interaction can take place through the entire length of this tube.

With a beam of electrons travelling down the helix, a wave is injected on the end of helix. This wave travels in the same direction as the electrons. By varying the collector voltage, we adjust the speed of the electrons so that the electrons tend to move slightly faster than the wave. When the wave has a positive maximum it tries to attract the electrons and hence speeds them up. If the wave has a negative maximum it tries to retards the electrons. Thus bunching occurs as in the klystrons. But as the electrons were travelling slightly faster than the wave, more energy is given up in the retarding than is extracted in the speeding up. This produces considerable gains up to 60 dB for frequencies of 100 GHz or more.

By accurate matching and by employing directional attenuators which will soak up the backward wave as it occurs it is possible to prevent backward waves in amplifiers. When functioning as an oscillator the backward reflection is allowed. The reflected wave then will travel forward in the right phase, be amplified and so forth, until the circuit reaches its peak amplitude of oscillation. This is referred to as backward-wave oscillator.

The collector voltage or voltage applied to the helix controls the speed of the electrons. Therefore, when the travelling-wave tube is used as an oscillator, the collector or helix voltage can be used to control the oscillator frequency. Additionally, the tube can be pulse-modulated, which is needed for certain applications.

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