Keeping It Clean

Powering ATE is a dirty job, and don�t let anyone tell you to ignore it.

You�ve just won a program to replace an older weapons system with the latest upgrades. But when it comes to support, you find marketing plans to update only the ATE�s programmable supplies, computer, and instrumentation. The older system had linear power supplies and data acquisition rates that really are in the cellar.

New processors, programmable switching supplies, and high-speed Ethernet controls will save you a bundle on throughput. But what will they do to noise susceptibility?

Starting with a foundation of properly conditioned power will eliminate the phantom failures that haunt most new projects. Providing the appropriate conditioning, along with proper grounding, will effectively mitigate a future breakdown of system components and shelter you from those beyond-your-control environmental hazards that occur long after the system has been installed.

Present-day electronic systems and components are far more sensitive to noise than their predecessors. To begin with, switching logic levels are down around 3 V on most of the new semiconductors, and it doesn�t take much to mistake a stray harmonic for a timing pulse.

The vulnerability problem becomes more complex when you consider higher data rates and DAC resolutions that have moved from 12 b to 20 b. High-speed microprocessors manage PID loops with update rates that are multiples of those used in the 1990s, and the high-speed switching circuits used in programmable DC and AC supplies create nonlinear loading on your power bus at frequencies that were never considered during the original ATE design.

These elements come together in ways that may bypass or even resonate with your old power conditioners, making the test system even more susceptible to electrical noise and transients. Noise and transients can trigger semiconductor switching at the wrong time, interrupt digital clock timing or sequencing, or cause errors in your acquired data. Best case, these things are merely disruptive. Worst case, they are destructive.

Operating environments also are changing. ATE systems must frequently be transportable, so weight is a major issue. Shelter-mounted systems like clean-room systems are starved for space.

In many combat or Third World operations, your only power sources come from Vietnam era gen-sets or local power of questionable quality. The
208-V three-phase power often looks more like 200 V, and the 60-cycle generator you were promised turns out to be 50 Hz from the British battalion across the field. What�s worse, about the time you run your lines across the ground to your test site, some high powered transmitter turns on, radiating every inch of the power lines each time it sweeps.

Facilities for ATE support everything from PCBs to jet-engine test cells. In today�s world, more and more of them are being remotely located, and stability of the power source often is an afterthought.

What are the symptoms? Phantom failures. You chase them from instrument to controller and back again, replacing boards in your VXI chassis, switching power supplies, and rewiring the backplane only to find the problem resurfaces on another run.

System errors may become prevalent on your monitor. Warning signs, like discolored capacitors or melted insulation on a neutral leg, point strongly toward common-mode noise and high-impedance returns. Over time, you notice unexpected performance degradation, a sure sign of transient problems, and likely the loss of what transient voltage surge suppression (TVSS) protection you may have had, a fault that generally is transparent to the system users.

The Problems
Where do power problems originate? Problems addressed by conditioners consist primarily of transients in the form of ring pulses or unidirectional transients, harmonics, filter resonance, high-impedance grounding, and brownouts. There are four sources of the noise:

� Noise generated from the power lines as they distribute power to your facility.
� Noise generated by your neighbors on the power grid, be they down the street or in the next room.
� Load-side noises generated by test equipment within your system or even the units under test and then fed back across the ATE power bus or ground to the controller and displays that orchestrate your test program sets.
� The connections intended to provide clean, low-impedance power; the grounds; and neutral returns can create problems that relate to wiring practices as well as proximity of the power conditioner to what it is protecting.

Here are just a few types of power-related problems that can occur in your application along with some commonly applied solutions:

� High-Frequency Electrical Noise�typically addressed with low-pass filters and clean grounding
� Spikes and Transients�typically addressed with low-pass filters, TVSS, and clean grounding
� Voltage Fluctuations�typically addressed with voltage regulation and uninterruptible power supplies (UPS)
� Incorrect Line Voltage�typically addressed with step-up or step-down transformers or voltage regulation
� AC Line Sags and Swells�typically addressed with voltage regulation and UPS
� Generally Unstable Power�typically addressed with filtering, suppression, transformers, isolation, clean grounding, voltage regulation, and UPS

Managing these power problems is critical. You must know enough about the issues presented by every new piece of hardware in your system to provide your specific power subsystem with the right mix of solutions.

Source Saboteurs
Line-side noise generators like lightning strikes, peak loading due to regional air-conditioning demands, and power-factor capacitor bank switching are examples of sources of electrical noise on the line that can infiltrate your sensitive electronics. Figure 1a shows severe voltage sags. Even if your regional electric utility guarantees a level of power quality, it often has no control over these transient issues.