Backward Wave Oscillators, BWOs

EIOs), backward wave oscillators (BWOs) or magnetrons are available. Their spectral characteristics may be favourable however, they typically require highly stabilized high-voltage power supplies. Still higher frequencies may be obtained using far-infrared gas lasers pumped for example by a CO- laser [49]. 

A family of vacuum-tube MMW sources is based on the propagation of an electron beam through a so-called slow-wave or periodic structure. Radiation propagates on the slow-wave structure at the speed of the electron beam, allowing the beam and radiation field to interact. Devices in this category are the traveling-wave tube (TWT), the backward-wave oscillator (BWO) and the extended interaction oscillator (EIO) klystron. TWTs are characterized by wide bandwidths and intermediate power output. These devices operate well at frequencies up to 100 GHz. BWOs, so called because the radiation within the vacuum tube travels in a direction opposite to that of the electron beam, have very wide bandwidths and low output powers. These sources operate at frequencies up to 1.3 THz and are extensively used in THZ spectroscopic applications [10] [11] [12]. The EIO is a high-power, narrow band tube that has an output power of 1 kW at 95 GHz and about 100 W at 230 GHz. It is available in both oscillator and amplifier, CW and pulsed versions. This source has been extensively used in MMW radar applications with some success [13]. 

The most commonly encountered MMW sources are now the Gunn and Impatt devices although the Backward Wave Oscillator (BWO) is still used for wideband spectroscopic studies. The Gunn and Impatt devices exhibit the property of negative resistance that makes them well suited as MMW oscillators. 

The local oscillator (LO) is locked by another frequency stabilizer to the master oscillator through another mixer (M2). Part of the 30 MHz signal that is fed into the second frequency stabilizer is split off to be used as a phase-coherent reference signal. Both the master and local oscillator are backward wave oscillators (BWO), and when locked, they have the frequency stability of the VFO. BWO s have good bandwidth, a relatively level power output, and can be swept easily. This last feature is convenient when working with cavities, as the cavity resonant shape and position are often adjusted. Cavity coupling information can also be obtained by sweeping across the cavity resonance. 

Usually a traveling wave tube means a forward wave tube. In the vacuum tube, the electron beam and forward waves interact with each other. There is a vacuum tube in which the electron beam interacts with backward waves. This type of tube is termed the backward wave tube. The backward wave tube is inherently highly regenerative (built-in positive feedback) therefore, it is usually an oscillator. Such an oscillator is termed a backward wave oscillator (BWO). The BWO has a traveling wave structure inside, but usually it is not called a traveling wave tube. Details of a BWO are presented in Chap. 6.4.3. The oscillation frequency of a BWO is dominated by the electron speed, which is determined by the anode voltage. Therefore, a BWO is a microwave frequency voltage controlled oscillator (VCO). 

Backward wave oscillator (BWO) In which the electron beam and microwaves travel in opposite direction to each other. 

Microwave volts e controlled oscillator (M VCO) A backward wave oscillator (BWO) is an example. 

Backward wave oscillators (BWO) A microwave oscillator tube that is based on a backward wave interaction. 

A typical microwave spectrometer is illustrated in Fig. 2. The essential elements of a microwave spectrometer are a microwave source, absorption cell, detection system, and a system for measuring the source frequency. Microwave sources—the klystron and, more recently, the backward-wave oscillator (BWO)—generate a very narrow band of frequencies so that the source is essentially monochromatic. Furthermore, the source frequency can be conveniently varied and is often phase stabilized to give good frequency stability. 

Einstein coefficient of stimulated absorption from state w to BWO backward wave oscillator... 

FIGURE 6.22 Schematic diagrams of backward wave oscillators (a) O-type linear BWO> (b) M-type radial BWO. 

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