Device Selection in a Hybrid System

In a hybrid test system, a strong foundation built on measurement devices is a must. This article was designed to help you find the best measurement devices for your automated test system.

Introduction to Hybrid Test Systems

When designing automated test equipment (ATE) systems, it is important to ask: “Will this system provide the testing we need for this project, and will it last for the duration of my test needs?” This is a fairly important question to answer, since a “no” to this question means the design process is not complete and requires more time to be put into the ATE system. For most system designers a response of “yes” to this question means their job is complete and they can move onto the next phase of the test system. Then the main concern is the lifecycle of the test components vs. the lifecycle of the test system. If the lifecycle of the individual components surpasses that of the test system then extra components are usually purchased and stored until needed. This severely limits the system’s flexibility, and might not be cost effective.

Now, broaden the question to “Will this system provide the testing we need for this project as well as give me the flexibility to adjust to future needs? Will this system have the longevity to last through the entire project and have the ability to adapt for use for new test systems?” Many test systems today would not fill these requirements. This is where a hybrid test system has extreme advantages over a typical ATE system. Hybrid test systems utilize multiple ATE platforms including VXI, PXI, and GPIB. This allows them to make use of existing testing components, as well as provide the ability to use newer components when they become available.

This flexibility is increased by using a layered system architecture. As shown in Figure 1, there are five layers in the layered hybrid system architecture. This provides a few levels of abstraction between the software applications and the hardware. This enables replacing hardware with newer hardware or adding new testing functionality without having to rearchitect the entire system. This gives developers a huge advantage over existing test system architectures.

Learn more about Hybrid Test Systems >> Extend Test System Longevity
Throughout the rest of this article we will discuss the Device I/O layer in this architecture. This layer deals with the actual hardware devices you will be using for testing purposes. Important factors of this layer include making sure your device will meet testing needs, as well as be scalable to different system needs. These considerations include measurement functionality, timing and synchronization, mix and volume, and the lifecycle of the products under testing. Through all of this you want to make sure that you are able to maintain the investment you make in this test system, so system longevity is also a major factor. By choosing your measurement devices wisely you will have an excellent start to building your hybrid test system.

Device I/O Layer Requirements

An important aspect of designing the best possible test system is system longevity. You want to maximize the investment you make in your new test system and facilitate easy upgrades in the future as your measurement needs change. You want to simplify the maintenance process of this test system by ensuring that replacing pieces on the Device I/O layer requires minimal changes in the higher levels. Planning for future measurement needs sreamlines the process of maitenance and upgrades.

Another important aspect of designing the best test system is making sure that it will be flexible enough to fit changing testing needs, as well as scalable enough to prevent your testing application from outgrowing your testing system. The nature of a hybrid system will allow you to make minimal system changes, even when your test application grows in diversity and/or scale. Since you will have many different technologies available under the umbrella of your one test system, you should have the capabilities of expanding the test system to fill new needs as they arise.


When designing a test system, the goal is to design the best test system to fit your measurement needs. This means that the Unit Under Test (UUT) should determine the requirements for your hardware. Those requirements will determine the actual instruments needed for this application. Once you know which instruments you need, you can then determine which ATE buses you want to use based on several issues including price, performance, timing requirements, and considerations for future upgrades. Considering multiple buses allows you to use existing hardware, as well as provides the flexibility to add more advanced hardware to your system in the future. As your measurement requirements change throughout your testing process, you will need to go through the cycle shown in Figure 2 again and decide if your current test system fulfills the UUT requirements.

As mentioned above, you want your test system to be able to handle changes in measurement needs in the future. Thus, when designing your test system, you should prepare it to not only meet the basic measurement requirements of the UUT, but to also have the ability to adapt to growing system measurement needs. This means going beyond just features like resolution, frequency, sampling rate, and channel count and looking for complete measurement functionality. With virtual instrumentation, engineers can create a user-defined system that can adjust to changing measurement needs. By using modular hardware, development software, and PC technologies, this software-defined approach gives greater flexibility than vendor-defined box instruments by giving the user the data and processing power to acquire any measurement needed. This approach is beneficial in terms of both time and money when designing your new system with the capability to expand for future needs.

Important Device Metrics

Let’s touch on a few of the key points that will allow your new system to have the reliability, scalability, flexibility, and longevity that a hybrid system is capable of.

· Basic Measurement Functionality. Making sure that the system you design meets all of the basic functionality (for instance resolution, frequency, accuracy, and sample rate) required by your UUT ensures that your system will serve the current needs.

· Easily Increased Channel Count. Having a test system that allows you to easily increase the channel count for the measurements you are taking ensures scalability. If your test requires twice as many channels than originally planned for, using modular hardware architectures like PXI and VXI allow you to easily add instrument modules to increase the system channel count. Then, with the proper software tools, increasing the number of channels to read can be accomplished in a matter of minutes in software.

· Timing and Synchronization. There are many things to keep in mind in regards to timing. Will your test need triggering? Will multiple devices have to share a clock? Will you have synchronous or asynchronous events? Can you use hardware timing and synchronization to improve the throughput of the system? The answers to these questions will help you choose the correct ATE buses for your system. Figure 3 gives a comparison of many ATE buses that are available.

	GPIB	VXI	USB	TCP/IP Ethernet	Standard PCs	CompactPCI/ PXI
Transfer Width (bits)	8	8, 16, 32, 64	Serial	Serial	8, 16 (ISA) 8, 16, 32, 64 (PCI) 8, 16, 32, 64 (PCI-X 1.0 and 2.0)	8, 16, 32, 64
Throughput (Mbytes/s)	Up to 8	Up to 160	Up to 1.5 (USB 1.0) Up to 60 (USB 2.0)	Up to 1.25 (10BaseT) Up to 12.5 (100BaseT) Up to 125 (1000BaseT)	1-2 (ISA) 132-512(PCI) 264-1024 (PCI-X 1.0) 1064 - 4264 (PCI-X 2.0)	Up to 528
Timing and Synchronization	None	Defined 10 MHz Reference Clock 8TTL Trigger Lines 2 ECL Trigger Lines Local Bus(Sub- microsecond)	None	IEEE-1588 Synchronization Protocal (microseconds)	Proprietary	None for CompactPCI Defined for PXI 10MHz Reference Clock 8 TTL Trigger Lines STAR Trigger Local Bus (nanosecond)
Control Loop Rates	Seconds	Microseconds	Seconds	Seconds	No	No (CompactPCI) Microseconds (PXI)
Standard Software Frameworks	VISA Available	VXIplug&play Defined	VISA available	VISA available	None	None (CompactPCI) Defined (PXI)
Modular	No	Yes	No	No	No	Yes
EMI Shielding	Optional	Defined	Optional	Optional	Board Specific	Module Specific

· Broad Range of Measurement Devices. Having multiple types of ATE buses available allows you to have a broad range of instrumentation to use in your test system. For instance, by using a hybrid system you use a VXI spectrum analyzer in the 26 GHz range for an RF application and use PXI modules for a high channel count application to tightly synchronize thousands of chaneels. The ability to combine hardware adds a huge amount of both flexibility and scalability.

· Ease of Use. Having the most powerful, accurate, and cheap device in the world does not help too much if you are unable use it. While designing your test system look for measurement devices that are easy to use, have good support documentation available, and are known to be reliable. Another consideration is how easy it is to replace a measurement device when the time comes to do so.

· Interconnectivity. A very important aspect of having a hybrid system is making sure that all of the components can interface with each other. You need to make sure that the devices you purchase are designed to communicate with the rest of the system that you have designed. If the instrument you're researching uses GPIB and you do not have any way to connect GPIB to your system, you may want to consider A) getting a different instrument that connects via means that are already available in your system or B) getting a GPIB controller for your current system.

Summary

Traditional testing methods can no longer provide for the increased needs of today's test systems. Buying a new desktop instrument for every type of measurement that needs to be done is simply out of the question. Moving to a software-based Hybrid System can greatly decrease the money and time spent on developing a test system. If this hybrid test system is developed with thoughts about future testing needs in mind, the cost of upgrades and time to implement changes in the future will also decrease greatly.

An important part of designing this hybrid test system is thorough research on the Device I/O layer of the five-layer architecture for hybrid systems. There are many things to take into consideration when choosing measurement devices for your hybrid test system. Hybrid test systems provide a very scalable system by using modular technologies like PXI and VXI. They also allow for great flexibility by allowing many different types of instruments on different buses to all be interconnected. This will allow for sharing triggering and timing lines between different instrument buses. This also allows new devices added to a certain bus to share data with devices on other types of ATE buses. If proper care is put into selecting these devices you will ensure that your system will meet current and future testing needs.

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