NI PXI Chassis Design Advantages

NI has a variety of powerful PXI Express and PXI chassis for measurement and automation applications (see Table 1). NI chassis feature high-performance backplanes and a rugged, reliable mechanical package. You can find the right NI chassis for your application whether you need a portable, benchtop, rack-mount, or embedded system, or a system with integrated signal conditioning. NI PXI Express chassis are compatible with PXI Express and CompactPCI Express modules. In addition, all NI chassis work with both PXI and CompactPCI modules. In this paper, learn about the NI design features that increase cooling performance, improve the acoustical performance, and enhance the reliability of the power supply.

NI chassis are designed and validated to meet or exceed the cooling requirements for the most power-demanding PXI modules. The PXI specification requires a minimum of 25 W of power be available to each peripheral slot and that each slot be able to dissipate that same amount of heat. The PXI Express power specification increased this requirement by about 20 percent, stipulating that the chassis must provide a minimum of 30 W of power and dissipate the associated heat.

PXI Express Chassis designed by NI exceed PXI Express requirements by providing at least 38.25 W of power and cooling in every peripheral slot; some chassis push slot cooling capacity even further and can provide 58 W or 82 W of cooling to a single slot. This extra power and cooling makes advanced capabilities of high-performance modules, such as digitizers, high-speed digital I/O, and RF modules, possible in applications that may require continuous acquisition or high-speed testing. Chassis vary in total power, so it is always best practice to perform a system-level power budget when configuring a new system.

^{Figure 1: NI PXIe-1062Q features the patented rear cooling fan design.}

Many NI PXI chassis feature a patented rear cooling fan design, as depicted in Figure 1, where air from the rear of the chassis (2) is forced through a turning vane and evenly distributed across all module slots (1). This provides increased cooling and fewer stagnant spots in airflow compared to chassis designs where the fans are located directly underneath the modules. Fans at the rear of the chassis also help reduce the amount of electrical noise from the fans’ motors that the modules are subjected to.

NI also offers slot blockers—a plastic modular filler PXI card that can be populated in unused slots of the chassis. This enhances the airflow in the populated slots by reducing airflow bypass in the empty slots resulting in a reduction of the temperature rise of electronic components on installed modules by up to 20 percent.

Table 1 shows the cooling performance difference that these design advantages help NI PXI chassis to achieve in comparison to another major vendor. This is evaluated by measuring the average component temperature of the components on a PXI module inserted into the chassis. The NI PXIe-1075 chassis can deliver the same cooling performance for the Auto fan speed setting and better cooling performance for High.

^{Table 1: Comparison of the cooling performance difference between NI PXI chassis and another major vendor.}

While some NI chassis can provide even higher cooling, all NI PXI Express chassis are capable of delivering at least 38.25 W of cooling to each slot. It is important to note that this specification is valid for a chassis filled with modules each requiring 38.25 W of power dissipation. The same is not always true of chassis offered by other major vendors. Although these vendors claim the ability to power and cool greater than 38.25 W, it is not for every slot, and often requires the chassis adhere to limiting or difficult-to-achieve caveats.

Finally, as evidenced by Table 2 above, even at 38.25 W of cooling per slot, NI PXI Express chassis are sometimes capable of better cooling performance than other vendors who specify greater than 38.25 W of cooling. 

Even with their advanced cooling performance capabilities, NI PXI chassis are designed to minimize the overall system acoustic emissions. This is important as PXI systems are used in both rack-mounted automated test and benchtop validation environments, where acoustic emission requirements differ. Combining fan speed control, the type of fan used, and fan mounting method make cooling optimization possible while minimizing the acoustic noise emitted.

Many NI PXI chassis permit you to select between two fan speeds, High or Auto, via a fan speed selection switch. When set to Auto, the chassis fan speed is controlled proportionally to the ambient air temperature as read at the chassis fan inlet. Below a reading of 30 °C, the chassis cooling system operates in an acoustic performance zone, where acoustic emissions are minimized. As the measured ambient temperature rises above 30 °C, the chassis cooling fan speed is increased accordingly. When fan speed is set to High, the chassis provides maximum air flow independent of the ambient temperature. This mode is most appropriate for applications where acoustical noise is not of concern, and where you intend to enhance the long-term life of the PXI modules in the system by cooling them to a greater extent.

^{Table 2: Compare the sound pressure level of PXI Express chassis.}

^{* Sound pressure level is defined at operator position in accordance with ISO 7779}

^{** 38 W cooling profile mode only; higher cooling profiles have a higher sound pressure level}

NI implements pulse-width modulated (PWM) fans in many of its PXI chassis to reduce acoustic emissions further than that of traditional voltage-controlled fans. PWM signal control of the fan permits the NI chassis designer to use a wider range of the fan’s RPM settings, thus making it possible to fine tune the chassis’ acoustic emissions and cooling performance.

To meet (and exceed) the cooling requirements of the PXI specification, the fans selected for implementation within an NI PXI chassis must be powerful. Many NI chassis use fan mounts that are manufactured of vibration dampening materials to isolate the chassis frame from mechanical vibration in the fans, further reducing the acoustic noise. In many NI PXI chassis, these mounts, and thus the fans, are placed in the rear, helping to reduce the amount of electrical noise (EMI) transmitted to the PXI modules.

NI PXI chassis are designed to minimize the overall system acoustic emissions, and can do so while delivering advanced cooling performance. Per Table 2, some chassis fans have been further optimized for benchtop performance, such as the PXIe-1084 in its 38 W cooling mode. Note that 10 dB here translates to a perceived noise difference of 2X.

NI maintains design ownership of the instrument-grade power supplies in many of its eight to 18-slot PXI and PXI Express chassis. As a result, NI can guarantee long-term availability of these power supplies and fewer design changes to its chassis due to power supply manufacturer changes. In contrast, other PXI vendors who rely solely on a standard PC power supply have little to no control over the quality of their power supplies.

The instrument-grade power supplies implemented within NI PXI chassis are optimized to meet the unique power requirements of PXI as opposed to ATX power supplies, which are designed for general use in personal computers. They are custom-designed for NI chassis to meet and exceed the PXI specification minimum power requirements. With these power supplies, NI PXI Express chassis can deliver at least 38.25 W to all modules in a filled chassis. Some chassis push this even further, such as the PXIe-1095, which can deliver 82 W of power to all modules in a filled chassis.

^{Table 3: NI PXI Express chassis are designed to exceed PXI specification for minimum current.}

In its product documentation, NI specifies the total power available to the modules from the chassis power supply. In contrast, many other vendors list the output of the power supply. By subtracting the power consumed by the chassis components such as the fans and backplane from the total power from the power supply, the remaining power available for the controller and modules is specified. NI PXI chassis manuals clearly indicate the current on each voltage rail and the maximum power dissipation per slot.

The PXI platform specifications, as defined by the PXI Systems Alliance, require that a PXI Express chassis deliver 650 W of power on the 3.3 V and +12 V backplane rails to power the system controller and module slots. It is important when comparing PXI chassis from different vendors to do so using the “power supplied to the backplane” (or similar) specification, and not just the total power or power per slot. NI makes sure to realistically and consistently define power specifications whether it is total power, power supplied to backplane, or power per slot. Other vendors often market misleading power specifications that cannot be achieved under typical installation/operation environments.

There is an increasing demand in emerging applications for PXI-based systems for high ambient temperature (up to 55 °C) operation. PXI chassis from NI can meet this demand with minimal power derating. Power derating refers to the loss in power supplied to the chassis slots when operated at higher temperatures or other extreme environmental specifications. Many other vendors’ PXI chassis meet the required PXI specification for available power at lower ambient operating temperatures (20–35 °C) but may become unstable or inoperable at higher temperatures (>40 °C).

^{Figure 2: NI PXI delivers specified power to the backplane over full temperature range.}

As NI PXI chassis implement instrument-grade power supplies, they are capable of providing the minimum power requirement over the entire specified operating temperature range (0–50/55 °C) with no power derating. To reiterate, with an NI PXI chassis you can operate a chassis filled with modules at the highest temperature specified on the data sheet (please refer to product manuals for operating temperature ranges for specific NI PXI chassis models).

The electrical noise generated by moving mechanical features within the chassis, specifically cooling fans, can degrade the measurement accuracy of PXI and PXI Express peripheral modules. To prevent this, many NI chassis not only place cooling fans in the rear of the chassis, but also implement a dedicated 12 V power supply to power the chassis cooling fans, system controller slot, and in some cases the power supply fans, to avoid coupling noise from these components into the rails powering the measurement modules.

Most NI chassis also feature remote sensing of the output voltage on the backplane power rails, to compensate for voltage drops. This design feature is important for PXI and PXI Express chassis particularly for applications with high-power modules, as it provides better regulation at the backplane when there are large load swings.

For systems where high availability is a concern, NI has designed an easy-to-replace power supply and fan shuttle for most of its 8-slot and higher chassis, in the event of a power supply failure. You could replace the shuttle from the rear by sliding out/in the failed/replacement power supply shuttle after removing the thumb screws that secure it in place. This design facilitates a mean time to repair (MTTR) of the power supply to less than five minutes. If the chassis is implemented in a rack-mount installation, as long as the back of the chassis is accessible, you can replace the power supply and fan shuttle without removing modules or reconnecting any I/O.

A key advantage of a PXI system is the integrated timing and synchronization capabilities. A PXI chassis incorporates a dedicated 10 MHz system reference clock, PXI trigger bus, star trigger bus, and slot-to-slot local bus, while a PXI Express chassis adds a 100 MHz differential system clock, differential signaling, and differential star triggers to address the need for advanced timing and synchronization.

^{Figure 3: PXI Express timing and synchronization features are key advantages.}

The phase noise and stability of the backplane system reference clocks are important characteristics of the PXI chassis, as they indicate how reliably you can expect to synchronize modules within the system. With the NI PXI choice of components and backplane design, phase noise performance of the PXI Express 100 MHz differential system clock on an 18-slot chassis is close to 1,000x (30 dB) better than other vendors’ chassis in the same class.

You can phase-lock-loop (PLL) the 10 MHz and 100 MHz system reference clocks to a higher stability clock source than that which is provided on the chassis backplane. This helps higher sample rate PXI modules to better align their samples across multiple instruments. The PLL circuitry of the NI PXI chassis is designed to suppress more noise when locking to an external reference, thus permitting cleaner transmission of the higher stability clock source. With other vendors’ chassis, depending on the system clock source phase noise required by the application, you may need to phase-lock the external reference clock to each module individually, rather than at a system-level to the chassis backplane, resulting in an increase in system complexity and cost.

^{Figure 4: Compare the PXI Express reference clock performance.}

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