Programmable Switching With VXI A Tutorial

Switches and switching systems are often the most underrated and overlooked sections in the design of effective ATE. Without the correct switch architecture, it really doesn’t matter how good the instrumentation in your system is.

The digital voltmeters, oscilloscopes, counters, function generators, pulse generators and other programmable instruments for your system are relatively easy to select, especially since so many are on the market. The real challenge comes when you connect them to the device being measured or tested to obtain the maximum performance automatically–which depends on nonintrusive switching performance.

In the days of rack-and-stack test instruments, the switch was usually designed and manufactured by the in-house system design engineer. Now, VXI provides an excellent platform on which to deliver commercially available programmable switching.

By utilizing VXI switch modules, the industry has simplified the complexity of custom-designed switching-systems modules, and you can purchase switching solutions at a lower cost because of the reduction in nonrecurring engineering. The VXI platform allows for a building-block design in switching.

Unfortunately, all switches are not the same. Selecting the wrong type of switch or switch design could lead to poor performance of the ATE. In essence, system performance is just as dependent on the switch(es) as on the test equipment. The switch should be transparent to both the measurement and stimulus path.

The proper switching system requires a good understanding of the various types of switches and switch-system architecture. The most common types of switches are:

Single pole, single throw, commonly called form A.

Single pole, double throw, commonly called form C.

Double pole, single throw or two form As.

Double pole, double throw or two form Cs.

Shielding for these types of switches are none, electrostatic and coaxial.

The most common switching design types are:

General-Purpose: a collection of individual switches.

Tree: a collection of switches tied to a common point directly accessed by all switches.

Matrix: a collection of switches tied to more than one common point, usually called buses.

Star: a collection of switches tied to a common node. The node is not directly accessible except through a switch.

Each of these switch designs has particular usage in ATE. The general-purpose switch offers access to uncommitted switching. Often, this capability is used to make connections unrelated to connections to instrumentation.

The general-purpose switch provides the basic switch type for general usage. For example, you may need a temporary connection to ground or to tie two points on the device together during a test. The type of general-purpose switch selected depends on the signal being switched.

Always remember that the quality of the signal being connected depends on the quality of the switch being used. If the switch is making a simple ground connection, then the switch would be a simple unshielded form-A switch. If the connection is to a low-amplitude and low-frequency signal, where signal loss and noise must be kept to a minimum, then the switch should be electrostatically shielded.

If the connection is a very high frequency, or low level, or an environment where controlled impedance is desired, then consider coaxially shielded switches. As a rule of thumb, the switch should be considered as a continuation of the cables being used to connect the device.

The switch tree is typically used as a scanner. This style of switching allows multiple points to be connected to a common path. A one-of tree allows only one point to be connected to the common path at a time. An any-of tree allows multiple points to be connected to the common path.

A one-of tree will often be used in the measurement portion of switching (typically a scanner). An any-of tree will often be used in the stimulus portion of switching where multiple points must be connected to the source at the same time.

Switch trees are usually applied as dedicated connections to a source or measurement instrument. The switch tree provides multiple access points to an instrument. A collection of switch trees connected to a collection of instruments would be the basis of simple ATE.

Dedicating the switch tree to an instrument eliminates the use of the switch tree for other purposes. To overcome this loss of resources, add another tree to the common path.

For example, a 12 x 1 tree will allow 12 possible points to be connected to the common path which may be connected to a digital voltmeter (Figure 1). These 12 points could not be used for any other purpose.

On the other hand, a 12 x 1 x 3 allows the same 12 points to be connected not only to the digital voltmeter, but also to a counter and an oscilloscope, for instance (Figure 2). However, only one signal can be connected to these instruments at a given time.

The switch matrix overcomes this limitation because there are multiple common paths. Multiple common paths mean that many paths can use the same points.

In the example, the 12 x 1 x 3 would become a 12 x 3 which would allow the 12 points to be connected to any of the three common buses (Figure 3). Now, it will be possible to connect different points to the instruments without shorting points together. This switch matrix design uses more switching, and reduces both the total number of available points and bandwidth (because of the stub lines that are created).

A star switch is a special type of switch tree. There is no common path (bus), but there is a common node. This type of switch is especially useful in high-frequency applications where stub lines are undesirable (Figure 4).

In the example of the 12 x 1 x 3, the two switch trees could be considered a special form of a 15-pole star switch of which 12 poles are the switch points and three poles are the instruments. Star switching is particularly useful for selectively connecting switch trees or matrices while maintaining maximum frequency performance.

By combining the different styles of switching, a high-performance general-purpose ATE switch system can be achieved. A good all-around ATE switch would provide switch points for both low- and high-frequency paths.

The number of required points would be provided by switch trees of both coaxial and noncoaxial paths. These trees would be connected to a matrix to provide maximum versatility in the use of these trees.

This matrix would become the intermediate-level switch or I-Level. The I-Level switch connects to the instruments. The size of the I-Level switch depends on the number of simultaneous signals required within the system design.

Special consideration must always be given in the selection of the I-Level. It does not make good sense to connect a 350-MHz coaxial switch tree to a 5-MHz I-Level switch and then expect a 300-MHz oscilloscope to see any signal that is useful.

The ideal approach is to use the highest bandwidth possible in all cases, but this criteria can become very impractical and costly. As a good rule of thumb, always try to make the I-Level a higher-performance switch than the trees to which it is attached.

The I-Level does not necessarily need to be a single matrix. It could be multiple matrices which connect to the switch trees isolated by star switches to maintain bandwidth performance.

A general measurement application which has a voltmeter, counter and a digitizer would look like Figure 5. Now the low-frequency tree will support measurements for low-frequency signals (no longer just a voltmeter switch). The low-frequency tree(s) provides support for two-wire or four-wire measurements and guarding. The coaxial switch trees provide support switching for high-frequency measurements made by the counter or oscilloscope.

The I-Level switching adds the next level of versatility. The I-Level has both high-frequency and low-frequency matrices as well as the star switching. Always remember that the I-Level must be a higher-performance switch than the switch trees that attach to it.

In the example, if the low-frequency switch trees have a bandwidth of 30 MHz, the low-frequency I-Level matrix bandwidth should be greater, such as a 50-MHz minimum. The high-frequency trees (>350 MHz) are connected to the star switches (>800 MHz) to allow minimum degradation when connecting to multiple instruments.

The high-frequency matrices are more difficult to design than a high-frequency tree and generally have lower bandwidth performance than the high-frequency tree for the same number of switches. In this case, the high-frequency matrix (>200 MHz) channels signals from the high-frequency paths to the lower-frequency paths and vice versa.

The goal of a good overall general-purpose switch design is achieved because:

The low-frequency section is kept clean. A design which starts with 30-MHz performance will not affect voltmeter measurements which generally go up to 1 MHz. Additionally, the shielding provided to achieve the 30-MHz performance also supports low-level, low-frequency signal measurement, a function often overlooked.

Good switching techniques should not only consider the signal being measured or transmitted but also the elimination of other signals, such as noise coming from the system environment, ground loops and the switching power supply within the VXI chassis.

The high-frequency switch paths are kept at an optimum level because the 350-MHz trees are connected to the 800-MHz star switches to reach the oscilloscope and counter. In other words, the path gets better as the signal proceeds toward the instruments.

The high-frequency I-Level matrix allows for connections between the low-frequency paths and the high-frequency paths. To observe a low-frequency signal on the 300-MHz oscilloscope, the signal travels from the 30-MHz low-frequency tree to the 50-MHz low-frequency matrix to the 200-MHz high-frequency matrix to the 800-MHz star switch to reach the oscilloscope. The performance of the path improves as the signal travels toward the high-frequency instruments, minimizing the effect of the switching on the actual signal being measured.

Taking the reverse direction–that is, connecting a coaxial signal to the voltmeter for precision voltage measurements in the 1-MHz domain–is also ideal. The coaxial paths start at the 350-MHz tree, to the 800-MHz star switch, to the 200-MHz high-frequency I-Level matrix, to the 50-MHz low-frequency matrix and finally to the voltmeter. In this direction the low-frequency coaxial signal is unaffected by the switching.

When designing your switching solution, ensure that the number of switches that the signal passes through from the input ports to an instrument is the same, regardless of the path. This type of design is called a balanced design, and is applicable whether you are using switch trees, switch matrices or star switches (Figure 6a).

The 9 x 1 switch tree in Figure 6a depicts how each path to the common output will require two relays, ensuring that the characteristics and performance of each path will be identical. This technique will equalize the resistance within the path and minimize the variation in DC losses that occur when selecting different signal paths to the instrument.

In Figure 5, the path resistance (or number of relays) from any of the inputs to the common should be the same. For this example, let’s assume that it takes one relay to pass through each section of a tree or matrix and two relays to pass through a star switch.

The number of relays from any low-frequency input to the voltmeter would be two relays. The path resistance from any of the low-frequency inputs to the counter or the oscilloscope would be seven relays. The number of relays from any high-frequency input to the voltmeter would be six relays. The number of relays from any high-frequency input to the oscilloscope or counter would be three relays. Typically, when you have created such an environment, the bandwidths of each section are known, and the signal losses are predictable and can be compensated for through software.

A switch system is unbalanced if it is created by consecutively adding more relays in a daisy chain (Figure 6b). Such switch trees add a different number of switches for different configurations and, as a result, the path resistance(s) is going to be different for each switch input in each section.

For example, the 9 x 1 switch shown in Figure 6b has been constructed by adding two 3 x 1 trees to the initial 3 x 1 tree. Because of the complexities and variations of the number of switches in each resulting path, the performance of the same switch type will vary depending upon the path. The resulting software calibration tables become both difficult and cumbersome to develop and maintain.

An easy way to identify unbalanced trees and matrices is when the specifications read that you can achieve “up to” a specified bandwidth. That specified up-to bandwidth is often the optimal switch route which you must identify.

With all other paths, the bandwidth will be less than the optimal route; in many cases, significantly less. With an unbalanced switch system, you must select which path of the unbalanced tree is going to provide the best performance and then the next best, etc.

Then to achieve measurement consistency, attempt, through software control, to compensate for the differences. The difficulty of this task is compounded when only data on the inputs and the common is provided, not to mention the challenges that occur when you add an instrument or expand the tree.

The 9 x 1 switch tree in Figure 6a is balanced. The bandwidth performance is the same regardless which path the signal travels. You do not have to compensate for any unknown or unplanned resistance, and your software solution is consistent for all paths.

By combining both stimulus and measurement resources through an I-Level type switch, station self-test and measurement of stimulus, prior to connections to the test unit, can be achieved. All of these capabilities are possible with VXI as a platform and using currently available VXI switch modules. The performance of the ATE is no better than the path the signal takes to get to the instruments in the system. Remember, that path includes the switching and the cabling within the system.

About the Author

Jeff Lum is President and CEO of Ascor. He has more than 20 years of experience in designing and manufacturing electronic switching, and holds two patents. Mr. Lum graduated from San Jose State University with a B.S. degree in electrical engineering. Ascor, Inc., 47790 Westinghouse Dr., Fremont, CA 94539-7485, (510) 490-8819.

Copyright 1995 Nelson Publishing Inc.

October 1995

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