At first, the results looked very bad, with plenty of failure points.
However, an identical setup, but without the cable, indicated the
true noise source was at 180 MHz, which is the 10th harmonic of the
ALE in the digital system. The cable spread and amplified the 180-MHz
noise across the spectrum, making the results look very bad. In both
instances, the scanner was powered by a 9-V battery. Large resonances
around 30 to 50 MHz are usually caused by cables. The noise at those
frequencies often disappears when the cable is removed.
To determine if an emissions problem is actually a cable problem, try
placing snap-on ferrites on the cables. The ferrites will shift the resonance
point of the cables, producing a noticeable effect. The position of the
ferrite on the cable is critical, and will have to be optimized to remove
the problem. Some companies (PC monitor makers, for example) sell their
products with ferrites molded into the cable just for this reason. Placing
common-mode filters or LC filters in series with pc-board connectors can
reduce EMI caused by cabling.
Surprising as it may sound, the effects of software could pose serious
issues when testing for emissions and susceptibility. We have seen noticeable
differences in EMI performance with different software architectures.
When you think about it, this makes sense. When EMI testing first begins
on a prototype, the system typically runs a stripped-down version of the
system software. Then, when the product is submitted for final qualification,
the full operating system is ready. Radiation is primarily generated by
the switching of CMOS gates within microprocessors, and ASICs and their
associated interconnections. The released software may toggle certain
pins differently (a watchdog timer, for instance) than the prototype software.
An operating system tends to make periodic calls to certain functions,
as opposed to a single threaded system. These factors sometimes affect
the frequency content on the address and data buses.
Engineers designing products that must meet EMC regulations need be
aware of the big picture before they start designing. Designers must understand
which certifications the product is required to meet, and the exact configurations
that will be tested. Then EMI development and test time must be factored
into the schedule. It's worth spending an extra week designing a pc-board
to save weeks in the EMI lab.
For companies contemplating buying a GTEM cell of their own, the math
is simple. Symbol has enough projects to keep its GTEM cell running every
day. And, the time-saving advantage of having a way to measure EMI in
your own lab is priceless. For the specific types of products that we
manufacture, the FCC has recognized that a GTEM cell produces results
that correlate to those of an OATS. Thus, the company has the advantage
of certifying its own products.
At first, the results looked very bad, with plenty of failure points.
However, an identical setup, but without the cable, indicated the
true noise source was at 180 MHz, which is the 10th harmonic of the
ALE in the digital system. The cable spread and amplified the 180-MHz
noise across the spectrum, making the results look very bad. In both
instances, the scanner was powered by a 9-V battery. Large resonances
around 30 to 50 MHz are usually caused by cables. The noise at those
frequencies often disappears when the cable is removed.
To determine if an emissions problem is actually a cable problem, try
placing snap-on ferrites on the cables. The ferrites will shift the resonance
point of the cables, producing a noticeable effect. The position of the
ferrite on the cable is critical, and will have to be optimized to remove
the problem. Some companies (PC monitor makers, for example) sell their
products with ferrites molded into the cable just for this reason. Placing
common-mode filters or LC filters in series with pc-board connectors can
reduce EMI caused by cabling.
Surprising as it may sound, the effects of software could pose serious
issues when testing for emissions and susceptibility. We have seen noticeable
differences in EMI performance with different software architectures.
When you think about it, this makes sense. When EMI testing first begins
on a prototype, the system typically runs a stripped-down version of the
system software. Then, when the product is submitted for final qualification,
the full operating system is ready. Radiation is primarily generated by
the switching of CMOS gates within microprocessors, and ASICs and their
associated interconnections. The released software may toggle certain
pins differently (a watchdog timer, for instance) than the prototype software.
An operating system tends to make periodic calls to certain functions,
as opposed to a single threaded system. These factors sometimes affect
the frequency content on the address and data buses.
Engineers designing products that must meet EMC regulations need be
aware of the big picture before they start designing. Designers must understand
which certifications the product is required to meet, and the exact configurations
that will be tested. Then EMI development and test time must be factored
into the schedule. It's worth spending an extra week designing a pc-board
to save weeks in the EMI lab.
For companies contemplating buying a GTEM cell of their own, the math
is simple. Symbol has enough projects to keep its GTEM cell running every
day. And, the time-saving advantage of having a way to measure EMI in
your own lab is priceless. For the specific types of products that we
manufacture, the FCC has recognized that a GTEM cell produces results
that correlate to those of an OATS. Thus, the company has the advantage
of certifying its own products.