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Ceramic Microwave Components

  helix assembly image

Accuratus serves a broad spectrum of microwave component suppliers. Ceramics are widely used in the industry and Accuratus has been an active participant since its founding.

MW Power Tube Ceramics

Raw ceramic components and metallized hermetic ceramic components suitable for brazed assembly are widely used in power tubes or vacuum electron devices.

Collector ceramics require high dielectric strength and high thermal conductivity. Significant heat is generated in the collector as the kinetic energy of the electron stream is converted to heat. Getting the heat out of the collector efficiently is critical to avoid melting or severely damaging the collector assembly. Some of the ceramics here serve as part of the vacuum envelope requiring hermeticity also. The collector runs at several thousand volts below ground potential. Electrical puncture of the ceramic can lead to catastrophic device failure. Small discharges across the face of the ceramic can disrupt the function of the device, rendering it unusable in an application. Materials generally used in these applications are beryllium oxide and aluminum nitride owing to their excellent thermal conductivity, vacuum tightness and bulk and surface dielectric properties. Proper attention to detail to prevent compromise of these critical material characteristics is imperative for collector ceramics and is a hallmark of Accuratus’ service to the industry.

An electron gun is mounted at the end of the tube opposite the collector. Requirements here are generally less demanding however, the use of high quality hermetic metallized ceramics is important. There is little heat developed here and voltages are low. Aluminum oxide is generally the material of choice. The need to stack several of these components with critical inter-electrode spacing requires the precision machining capability of Accuratus and the careful metallization of the components by our industrial partners.

RF windows are produced for coaxial and waveguide inputs and outputs. The windows must be hermetic and are normally brazed to the metal tube envelope. Consistent dielectric properties and strongly adherent, vacuum tight metallizing are critical. Typical materials of construction are beryllium oxide and aluminum oxide although diamond windows for extreme power output are produced.

Loss elements for various tube types are produced. Ring shaped components to damp oscillations in klystron cavities are tailor made to customer specification. Severs and loss buttons to control cavity Q, absorb reflections from impedance discontinuities and drop interstage gain are made in volume quantities. Porous aluminum oxide is used for low power, less demanding applications. Higher power applications requiring tightly defined loss characteristics and dielectric properties are fabricated in customer supplied silicon carbide composite materials typically with oxides such as magnesia, beryllia and alumina added to develop specific properties. Where anisotropic material properties are a concern, Accuratus maintains orientation with respect to raw material pressing direction.

Helix traveling wave tubes are a special subclass of linear beam tubes offering high gain over wide bandwidths. These tubes use electrically long helical circuit elements mounted coaxially in a metal tube. The electrical circuit is comprised of the helical metal ribbon, ceramic supports and the metal outer tube all in a vacuum environment. Often, many tubes are connected in parallel to generate higher power in microwave systems thus requiring tight specifications on output power and phase. Consistent dielectric properties and tight geometric tolerances are necessary to assure the tube meets output specifications. Low dielectric loss for good efficiency and low dielectric constant to minimize loading of the circuit are important material properties. The ceramic materials must also be capable of transmitting significant heat from the helix. Typically, materials used include beryllium oxide and boron nitride with aluminum oxide used on some lower power low frequency legacy devices.

Ceramic Heatsinks and Heat Spreaders

Heatsinks are used throughout the industry, particularly in the high power segment, to transfer heat away from both active and passive devices. Important material properties include good to excellent thermal conductivity and good to excellent dielectric properties. Materials in common usage include aluminum oxide, beryllium oxide, boron nitride and aluminum nitride.

Common applications include:
— Resistor supports in high power combiners
— Dielectric supports for loads and terminations
— Transistor and diode bases
— Conduction cooled power tubes
— High power capacitor supports
— Coil forms in helical resonators
— High power dielectric resonator supports
— Substrates for hybrid microelectronics
— High frequency resistor cores

Ceramic Isolator and Combiner Components

Accuratus precision machines customer specified or customer supplied ferrite and dielectric components. Tightly held tolerances for components used in the GHz range are routinely and economically produced for end users as well as for other suppliers to the industry.

Ceramic Dielectric Tuning Elements

Precision components are produced for a number of variable capacitor manufacturers. Low and moderate dielectric constant materials having low loss are used; typically fused silica and aluminum oxide. Tight tolerances and uniform material electrical properties are mandatory for easy assembly and repeatable final product specifications.

Customer specified dielectric resonator pucks are also fabricated primarily for the high power terrestrial microwave communications market. Specific high dielectric constant components are produced to customer specification. Electrically insulating, low loss supports for the dielectrics are also produced. These may be required to minimize perturbation of the electromagnetic field in the vicinity of the dielectric. In high power applications they must transfer significant quantities of heat out of the dielectric to avoid tuning drift, loss of circuit Q and perhaps avoid catastrophic failure of the puck.

See also: Materials > Aluminum Nitride
See also: Materials > Boron Nitride
See also: Materials > Fused Silica
See also: Materials > Macor


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