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High-Power Vacuum Devices
Published in Jerry C. Whitaker, The RF Transmission Systems Handbook, 2017
The tetrode is a four-element tube with two grids. The control grid serves the same purpose as the grid in a triode, while a second (screen) grid with the same number of vertical elements (bars) as the control grid is mounted between the control grid and the anode. The grid bars of the screen grid are mounted directly behind the control-grid bars, as observed from the cathode surface, and serve as a shield or screen between the input circuit and the output circuit of the tetrode. The principal advantages of a tetrode over a triode include: Lower internal plate-to-grid feedbackLower drive power requirements; in most cases, the driving circuit need supply only 1% of the output powerMore efficient operation; tetrodes allow the design of compact, simple, flexible equipment with little spurious radiation and low intermodulation distortion
Vacuum Tube Principles
Published in Jerry C. Whitaker, Power Vacuum Tubes, 2017
The tetrode is a four-element tube with two grids [4]. The control grid serves the same purpose as the grid in a triode, while a second (screen) grid with the same number of vertical elements (bars) as the control grid is mounted between the control grid and the anode. The grid bars of the screen grid are mounted directly behind the control-grid bars, as observed from the cathode surface, and serve as a shield or screen between the input circuit and the output circuit of the tetrode. The principal advantages of a tetrode over a triode include the following: Lower internal plate-to-grid feedback.Lower drive power requirements. In most cases, the driving circuit need supply only 1% of the output power.More efficient operation. Tetrodes allow the design of compact, simple, flexible equipment with little spurious radiation and low intermodulation distortion.
Cathode Ray Tube Displays
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
The cathode generates the stream of electrons used to form the image-writing beam. The traditional cathode is a metal conductor such as nickel coated with a thin layer of oxide, typically a barium strontium compound. To reduce the voltage required to generate electron emission, the cathode is heated to 700 °C–1200 °C. Applications that require high brightness often use more advanced and expensive dispenser cathode designs to increase the beam current while maintaining reasonable cathode life. These designs incorporate complex cathode structures and materials such as barium ceramics, molybdenum, rhenium, and tungsten. Readers interested in advanced cathode designs and additional information on CRT materials technology should refer to References [6–10]. The flow of electrons from the cathode is controlled by varying the potential between the cathode and a series of control grids commonly known as G1 (the control grid) and G2 (the acceleration grid). A voltage potential of 100–1000 V between G2 and the cathode creates the potential necessary to pull a stream of electrons off the cathode, forming the beam. The beam amplitude can be controlled and even completely shut off by varying the potential on G1. Thus, the voltage at G1 controls brightness because the brightness is proportional to beam current. The design of the cathode with respect to impedance and loading influences the maximum rate at which the beam can be modulated. The cathode and its associated control grids can be designed to produce a crossover flow of electrons or a laminar flow. In crossover gun designs, the emitted electrons converge to a point in front of the cathode. By using electron optics, this beam spot is imaged onto the phosphor screen. Due to inherent advantages, the crossover design is widely used. A crossover beam is narrow, making it easier to deflect than a thicker beam, and the spot can be very small, improving resolution at the screen. In theory, a laminar flow design provides for the possibility of higher beam current from a similar-sized cathode. In practice, the improvement is not usually advantageous enough to offset the added difficulty of controlling a wider beam.
High-power shortwave DRM transmitter in solid-state technology
Published in Automatika, 2018
Goran Pavlakovic, Silvio Hrabar
RF exciter generates a carrier signal of precise and stable frequency (within the shortwave frequency range). This low-power signal is further amplified in the driver amplifier, which provides enough power to the control grid of the electron tube. This amplifier is usually a solid-state amplifier operating either in class AB or class B, designed in a push–pull configuration. The necessary impedance transformation from the control grid to the amplifier is utilized using a modified π matching network. A solid-state Pulse Step Modulator (PSM) is used to provide the high-level anode and screen grid voltages [16,17]. Modulated RF signal is further brought (via a coupling capacitor) to the output matching network (a triple π configuration). This network provides the necessary impedance transformation and, at the same time, suppresses unwanted harmonic components meeting the specific emissions standard. To provide the necessary digital modulation (OFDM), in-phase and quadrature components of the input signal are converted to polar amplitude and phase components and then applied to the electron tube. Amplitude signal (envelope) is used to modulate the power supply voltages for the anode and screen grid, while the phase signal is used to modulate the control grid of the electron tube. The electron tube is very efficient in the terms of high-power amplification in the shortwave frequency range. However, the electron tube suffers from short lifetime (typically up to 20,000 working hours). Today’s solid-state amplifying devices highly surpass this figure, but the possible output power of a single RF transistor is still lower than the output power of a single electron tube.