What is the longest Stub Length?

Posted on June 10, 2009
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MIL-STD-1553B specifies that the longest stub length is 20 feet for transformer coupled stubs, but can be exceeded. With no stubs attached, the main bus looks like an infinite length transmission line with no disturbing reflections. When a stub is added, the bus is loaded and a mismatch occurs with resulting reflections. The degree of mismatch and signal distortion due to reflections are a function of the impedance presented by the stub and terminal input impedance. To minimize signal distortion, it is desirable that the stub maintain a high impedance. This impedance is reflected back to the main bus.  At the same time, however, the impedance must be kept low so that adequate signal power will be delivered to the receiving end. Therefore, a tradeoff between these conflicting requirements is necessary to achieve the specified signal-to-noise ratio and system error rate performance (for more information, refer to MIL-HDBK-1553A).

What Type of Connectors are Employed?

Posted on May 28, 2009
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There are several types of connectors used on the bus and at the coupler stubs, the most common of which is the concentric twinax connector. The concentric twinax connector has three bayonet coupling slots/lugs known as TRB type (same envelope size as a coaxial BNC connector). The center contact is high (positive) connected to the twinax blue wire and the cylindrical contact is low (negative) connected to the twinax white wire. The body of the connector is bus shield. Additional mating interfaces include: (1) two-bayonet, (2) four-bayonet and (3) threaded.  Because bayonet-type connectors do not require safety wiring and can withstand severe shock and vibration, these are typically preferred over threaded types.  There is a subminiature version of the twinax concentric connector known as TRS type (same envelope size as TPS coaxial connectors). The mating interface options are the same as those for the TRB style listed above.  Other connector shell options employing concentric twinax contacts include D-subminiature and cylindrical multi-pin (for example, MIL-DTL-38999 or D38999.

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