This paper provides information about the physical aspects of the wired trigger bus used in the LXI Standard and provides some background on the decisions that were made concerning its implementation. The paper does not contain performance related information which can be found elsewhere.

The wired trigger bus (WTB) provides a means of connecting LXI Devices together in a way that allows them to exchange hardware trigger signals that have low timing uncertainty. It provides a standardized way of providing the trigger signals previously used on bench (and other) instruments based on a physical connection (such as a coaxial lead). These traditional methods were all based on ad hoc definitions requiring the user to understand the trigger signals generated by one device and what the receiving device needs to make a trigger event happen. The WTB also provides an equivalent function to the triggers used on modular backplanes, such as VXI and PXI. There are differences in performance since the distance is greater and cables are used, but there are also aspects which improve on these trigger connection systems since it includes an active Wired OR system which does not exist on PXI and is implemented on VXI via a much slower open collector drive system.

The physical interface for the WTB is based on the TIA/EIA-889 Multipoint Low Voltage Differential Signalling (M-LVDS) standard. This standard uses differential current driven signals to exchange data between semiconductor devices. It is intended primarily to interconnect devices on a PCB or devices over short distances with ribbon style cables. For instrumentation use the WTB working group of the LXI Consortium altered the way the transmitter and receiver devices work to allow it to provide a robust wired OR function and defined a cabling and connection standard to permit its use over long distances.

The WTB provides 8 physically independent trigger channels, matching the number of logical channels provided by the other trigger mechanisms in the LXI Standard. The channels are designated as LXI0 to LXI7. This is also the same number of triggers on the backplane of a VXI or PXI chassis.

Using LXI in Satellite System Testing

by Jochen Wolle, Rohde & Schwarz

Satellites are involved in a wide range of applications including scientific research, weather prediction, navigation, and military/aerospace. The greatest numbers of satellites that orbit the earth today are used for communications, enabling people across the globe to communicate with one another. In fact, more than half of the satellites in space are used for telecommunications. Beyond telecommunications, satellite communications are intrinsically tied to almost every aspect of our daily lives whether we realize it or not.

Perhaps the most tangible form of satellite communications is the transmission of our favorite television shows from around the world. Furthermore, airport command and control centers rely on satellite communications to ensure our loved ones return safely from their travels. And in times of natural disaster, such as the 2001 World Trade Center attack, the 2005 tsunami in the Indian Ocean, or Hurricane Katrina in 2006, satellites often are the only means of communications available.

A new two-tone method that makes use of LXI technology has been developed that allows users to measure relative group delay with high precision on converters and mixers. The measurement can be made without local oscillator (LO) access as is typically the case in satellite communications systems.