Expanding a Matrix Size

Users need to expand a matrix size for a variety of reasons, but the most common is that the matrix size they need is not available as a single device/module. This particularly true for PXI where the modular construction places limits on the size of matrix that can be implemented.

If you can, chose an integrated solution

While many devices/modules can be expanded users should remember this does not come as a very economic option unless the expansion capability has been designed to make expansion easy.

In the case of LXI the mechanical freedom allows much larger matrices to be more cost effectively implemented, and further expansion of an LXI Device to be accomplished by simply adding a cable assembly to link the two Devices. An example of this is the LXI matrix model 60-552, the simple addition of 78 pin D Type cable assembly allows the already large matrix (in this example up to 64x64).

In the case of PXI users should consider using a multi-slot BRIC module to implement larger matrices. 

Choosing an integrated matrix solution rather than using smaller devices connected together provides many user advantages:

• It save labor and wiring effort to connect terminal blocks together, saving costs in terms of labor and purchased connector accessories
• It is more reliable
• It requires no additional testing to check the switching system has been correctly configured
• The matrix can be programmed simply as a matrix rather than a collection of individual parts, making test program development faster, simpler t understand and less dependent on extensive development tools.
• Improved matrix performance because of shorter connection distances
• Simpler diagnosing of failure in he event of a switching system fault
• The possibility of fault diagnosis from a single test tool such as BIRST

Understanding the performance issues of expansion

In some cases though the user has no choice but to expand the switching system by the addition of other devices or modules. Users should be aware of the issues involved in expanding a matrix:

• Direct Connection. In this case the user directly expands the matrix by adding a direct cable to link either the Y or the X (or even both) axis of the matrix. It is the simplest expansion method to understand and one example is shown in the 60-552 data sheet.If just X expansion is required the Y bus connections of two Devices are linked together so the system has a single Y axis and two sets of X axis available (one set in each Device) . This method of expansion does impact the BW though, the Y axis now has twice as many relays connected to it plus a length of cable that connects the two devices. As a result the BW of the matrix will be reduced, but the functionality provided is of one matrix.Some type of BRIC matrix (those with wide analog bus) use this methodology.

Expanding a matrix in both the X and Y direction using a direct connection, in this case using an example of 60-552 which has dedicated connectors for expansion purposes

• Loop Thru Connection. In this case (assuming the X axis is to be expanded) the Y axis are again linked, but a relay is provided that routes the Y signal either to the matrix within the device or to a second matrix. The loading on the Y axis is much reduced since if the internal matrix is selected the connection is directly to the internal matrix, if the second matrix is selected then the connection routes via the interconnecting cable to second matrix. BW degradation is avoided, but the loop thru relay increases cost and tales space. Some functionality in the matrix is also lost since it is no longer as simple to connect the X or one matrix to the X of the second matrix. RF matrices use this system because they are primarily intended for use as Y to X connection matrices and all efforts have to be made to preserve the matrix BW.

Example of a Loop Thru connection on a 2x2 RF matrix (40-837)

• Isolation Relay. In this case the matrices include isolation relays that disconnect the matrix when the connection is not required. As the connecting cable between two traces is not disconnected there is some BW degradation, but much less than the Direct Connection example. Matrix functionality is fully maintained. This method of connection is used in BRIC's with 8 way and smaller analog backplanes and in LXI products such as 60-554.

Using isolation relays to expand a matrix, leaving the interconnection bus in place. This method is used in both LXI and PXI matrices for intermediate BW's.

Making it Easy to Expand

The easiest matrices to expand are those where the expansion is accomplished using a connector which is separate to the UUT connection (X or Y) since expansion can be easily managed without effecting the connections that the user is primarily interest. This is much easier to implement on LXI products that PXI products because of the front panel space issues created by the PXI standard. Some of the LXI products use these dedicated connections since it makes it possible to use standard of the shelf cabling accessories.

Summary

It is clear that with the exception of the Loop Thru Connection method expanding the size of matrix can have a significant impact on the BW of the matrix. It can also have an impact on other performance parameters such as noise immunity and path resistance. The degradation is dependent on how the user implements the expansion (cable lengths, use of shields etc). The complex the expansion method is to implement by the user the more likely the user will see unwanted performance degradation. That reinforces a clear message that choosing an integrated matrix of adequate size (whether in PXI or LXI form factor) is usually a better choice.