Coaxial Electric Feedthroughs - Introduction
Coaxial is the classification given by MDC to all ISI™ feedthrough geometries containing two concentric conductors - one outer metal tube or shield conductor concentric with, and enclosing, a cylindrical center conductor. The inner and outer conductors are separated and insulated by an alumina ceramic dielectric. ISI™ Coaxial Feedthroughs are offered in conventional Grounded or Floating Shield configurations. They are also available in Single or Double Ended geometries and in most cases supplied with industry standard air-side, Coaxial Connectors.
Single Ended Coaxial Feedthroughs are used in applications where continuous shielding is not important, or where space is limited. Double Ended Coaxial Feedthroughs are used in applications where continuous air and vacuum RF or noise shielding is required.
The significant feature of all Coaxial Feedthroughs is their concentric geometry. Electrical signals passing through a center conductor are protected or shielded from external interference or noise by an outer tubular conductor. On the other hand, interference or noise generated by power in the center conductor is also suppressed by the outer conductor, thus shielding sensitive electronics external to the coaxial line. Floating Shield Coaxial Feedthroughs provide a second shield and dielectric layer, allowing the outer tubular shield to be used as a secondary conductor path. A potential drawback of Floating Shield Feedthroughs is that their ground shields do not completely cover the floating shield or center conductor and therefore do not have three concentric and continuously shielded conductor paths.
For cryogenic applications, particular ISI Coaxial Feedthroughs are ideal, due to their seal geometry and material selection. To quickly locate a product suitable for cryogenic use, look for part numbers printed in a cyan-blue color. Ceramic to metal seals should never be subjected to thermal gradients in excess of 25°C per minute - doing so can greatly reduce the life of the product and void the warranty.
ISI™ standard Coaxial Feedthrough types are as follows:
BNC - Bayonet Naval Connectors
These are generally used in 50Ω and 75Ω low power instrumentation transmission lines. Note that ISI BNC Feedthroughs are not impedance rated due to restrictions in geometry and the relatively high dielectric constant of alumina ceramics.
MHV - Miniature High Voltage
These are similar to BNC series feedthroughs. They do not intermate with BNCs and are used in medium power applications with higher voltage requirements.
SHV-5 - Safe High Voltage
Like MHV Feedthroughs these are rated up to 5000 volts. The difference between these feedthroughs and their MHV counterparts are the pin and contact configuration. SHV cable connectors have recessed female contacts with male mating pins located in the feedthrough. The exact opposite is true with MHV Feedthroughs. SHV cable connector center contacts do not protrude from connector ends as they do on MHVs. This makes them safer in a disconnected condition. SHV Feedthroughs are available in four voltage categories.
SHV-10 - Safe High Voltage
SHV-10 provides an intermediate 10kV solution for SHV coaxial feedthrough applications. The SHV-10 feedthrough complements the standard 5kV and 20kV products currently offered. SHV-10 feedthroughs are ideally suited for high voltage DC pulsed applications. The outer contact ground connection is maintained throughout the center contacts’ mating cycle. The cable connectors’ center contact is recessed to prevent shock hazards when the connectors are unmated.
SHV-20 - Safe High Voltage
SHV-20 feedthroughs and connectors feature special high voltage interfaces which provide reliable service in pulsed high voltage applications. These coaxial feedthroughs are designed for use where the normal operating voltage of standard coaxial connectors is inadequate. The SHV-20 feedthrough was designed to replace the existing SHV-15. Limitations in its design prevented the SHV-15 from utilizing the full potential of its cable connectors. The SHV-20 design incorporates an improved electrical interface which includes standard off-the-shelf connector contacts that greatly enhance electrical contact performance. The SHV-20 feedthroughs and connectors cannot be used with older SHV-15 components because contacts and sockets on SHV-20 products use larger pin diameters which will not mate with the older SHV-15 design.
SHV-B - Bakeable Safe High Voltage
The difference between these and other SHV Feedthroughs is the use of ceramic dielectric material in the air-side cable connectors. Ceramic insulation gives these connectors higher temperature capability than conventional Teflon® insulated products. SHV-B design is not an industry standard connection, but developed for the special requirements of high temperature applications in which connectors can remain engaged and operational at elevated temperatures.
ISI™ Type-N coaxial feedthroughs are 50 ohms style and suitable for use to 1 GHz. Units are designed and rated for high and ultra-high vacuum applications. Except for the nickel conductor units which are non-magnetic and constructed of materials suitable for cryogenic applications.
SMA – Subminiature Type-A
SMA-UHV feedthroughs are ideally suited for medium-to-high frequency signal transmission combined with the rigorous demands of ultrahigh vacuum environments. The design features of this group will vary, but the double-end SMA includes frequency service to 1 GHz, low VSWR low loss and 50 ohm matched impedance. For decades the SMA interface has been the connection of choice in accelerator facilities around the world. ISI's SMA products have provided reliable service to temperatures as low as -200 degrees C.
The high frequency SMA is built similarly to the UHV series SMA, but re-engineered to meet the increasing demands of RF communications system designers. The design parameters of the HF-SMA include high frequency service as high as 8GHz, low VSWR, low loss and 50 Ohms matched impedance. As a carefully impedance matched component, the HF-SMA offers optimum RF performance. The connector interface is designed in accordance with MIL-C-39012/58F and mates with both industry standard SMA and 3.5mm precision coaxial connectors. In order to achieve these HF performance parameters, the use of Teflon® dielectric spacer material is employed. Although the feedthrough meets the hermetic requirements for UHV environments, it can only be recommended for high vacuum environments where the use of Teflon® is permissible.
SMB - Subminiature Type-B
A close relative of the SMA, the SMB interface is also known as subminiature Type-B. Unlike the SMA, the SMB is designed for quick connecting and disconnecting, made possible with its spring retention connector. The SMB products are ideally suited for coaxial applications where space is a luxury. Their small footprint allows for high density arrangements.
ISI™ microdot style feedthroughs are the smallest threaded interface coaxial connectors available for UHV service. This connector interface has become the standard in the vacuum coating industry. It is widely used in the construction of crystal sensor deposition monitors. This product is complemented by prewired cable assemblies and in-vacuum microdot cables and accessories.
BS - Between Series Coaxial
As the name implies, these feedthroughs provide a means of changing coaxial connection styles ‘mid-stream’. In the vacuum coating industry, deposition monitors are used to measure the thickness of coatings deposited on a variety of components. The standard connection for these deposition monitors is BNC on air-side and microdot on vacuum-side. Throughout this Catalog, BS coaxial product headings (e.g. BNC-Microdot), represent respectively the air- and vacuum-side connection styles. Other standard options include BNC-A and MHV-A feedthroughs.
MDC offers three standard vacuum mount styles: Del-Seal™; Kwik-Flange™; and Weldable. Additional configurations available upon request.
Intended Operating Conditions
Electrical ratings are safe operating limits. These ratings are determined by various factors, including dielectric strength, geometry and system operating pressure. Please note that all ISI catalog products are electrically rated for operation with one side in dry atmospheric conditions and the other side in a vacuum environment with a maximum system pressure of 1 x 10-4 Torr. We advise that users make allowances for deviations from stated operating parameters and take adequate safety precautions when feedthroughs are operated at high voltages or high currents
Insulator Seal (ISI) brand catalog and custom engineered ceramic to metal seal products are manufactured to ISO 9001:2008 standard utilizing our qualified hermetic brazing and joining processes.