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Modular Instruments – Do they have any limitations?

-November 12, 2012

Perhaps the most common question asked of me is to identify the measurement limits of modular standards like PXI, VXI, or AXIe.  Is there some fundamental frequency limit, parametric accuracy limit, or other measurement limitation that keeps traditional instruments as the only solution for a particular application?

I’m going to put aside two pragmatic factors that do impact the choice between modular and traditional instrumentation, availability and use model, and cover them in later articles.  For today, I want to focus exclusively on measurement capability.

Are there measurement limitations to modular instruments? My short answer is “very few, and becoming fewer”.  

Let’s see why.  Any open modular standard is a set of rules that describes a module’s mechanical dimensions, electrical and power interfaces, and communication bus.  Do these restrict the type of instruments that can be designed?  Let’s look at each aspect.

Size.  VXI and AXIe modules deploy circuit boards that are roughly the size of those found in traditional instruments, so you wouldn’t expect a size constraint for these.  However, PXI is today’s most popular standard and has significantly smaller volume than the two larger modular formats.  The small envelope size is challenging, but it is not a barrier to most instrumentation. As a proof point, recent introductions of PXI RF instruments deploy two shielded printed circuit board assemblies within a single 0.8 inch module width.   This breakthrough density is enabled by Moore’s Law, which allows designers to essentially double functional density every 18 months.  Subsequently, size is becoming less of a constraint, as something which is marginal today can be feasible a year from now.  Also, while PXI is small, the ability to use multiple slot widths if needed gives designers an option if they can’t fit within a single slot width.

Cooling.  However, there is a cooling limit that places practical boundaries on certain instrument types.  3U PXI (the most common size) can routinely cool 30 watts per slot.  Though there are modules that reach somewhat higher power densities, there is a fundamental physical limit on power and cooling. Because of this, expect to see high-powered power supplies to remain marketed as traditional instruments. While dense low-power voltage sources and SMUs (Source Measure Units) can make sense in PXI, the cooling density is a strict physical limitation for the higher power portion of the product line.

Digital Density.  Are there other limitations due to the size and cooling?  There is probably a similar density constraint for high-speed digital electronics.  While Moore’s Law has enabled the size of electronics to decrease, the power density per volume of space is actually increasing.  Digital pin electronics, like those found in semiconductor ATE or logic analyzers, demand high power just as power supplies do, along with the airflow needed to cool them.  This is one reason for the existence of the 200 watt/slot AXIe standard.   Additionally, a single board coplanar design allows digital signal integrity that is difficult to achieve across multiple printed circuit assemblies.  This is due to digital signals now being in the microwave region.  Practically speaking, this is not a limit on the top digital speed or processing you may find on a single pin of PXI, just the practical channel density.  And it certainly isn’t a limit for modular instruments altogether, since AXIe delivers the large board area and power density if PXI isn’t enough.  The bottom line is that between PXI and AXIe, modular systems can meet the needs of high-speed digital instrumentation.

Power.  How about the modular system power supplies themselves? After all, the highest voltage supplies in PXI are just 12 volts, isn’t there another limit here?  Again, Moore’s Law to the rescue, this time with high density switching power supplies.  These power supplies not only generate all the voltages needed for instrumentation, they also allow isolated floating instruments.  AXIe has taken this to an extreme by delivering a single 48 volt power rail to all slots, requiring a switching power supply on all modules.  High voltage means low current flow for the same wattage.  Furthermore, by accepting the challenges of switching power supplies in the first place, conductive emissions in PXI or AXIe are dealt with directly, instead of assuming analog-clean power rails are delivered to each slot.  

PCI Express.  Finally, what about the high-speed communication bus on the backplane?  Won’t it cause interference with sensitive measurements? Not with careful design.  Modern PXI and AXIe backplanes are based on PCI Express, a very high-speed serial bus.  Serial buses have two key advantages over their older parallel ancestors.  First, there are fewer lines to radiate.  Second, being differential pairs, they actually emit less radiation than earlier designs.  Good backplane design coupled with good module design has enabled increasingly sensitive parametric measurements to be delivered in modular form factors.

Reviewing my above comments you will see I never mentioned a frequency limit to modular instruments.  That’s because there aren’t any.  I never mentioned a parametric sensitivity limit either.  Again, because there aren’t any.  While modular form factors can present challenges to the highest performance measuring instruments, I’ve witnessed clever engineers design breakthrough modular products that were previously thought impossible.  Within the power and cooling constraints mentioned above, expect modular instruments to be able to deliver any of the measurement precision and performance you find with today’s traditional instruments.  

A modular world without limits?  Not quite.  In future blog posts I will comment on commercial availability and use models.  These are pragmatic factors that also determine where modular and traditional instruments are deployed.  Stay tuned.

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