Nuts and Volts
TruthinSpecifications?
Given the lax inspection policies of the FDA, I don’t havemuch faith in package labels. With the current
economy, there’s simply too much pressure to skimp on
ingredients or falsely elevate nutritional claims. I can’t prove it
— I just don’t have the analytical tools on hand to determine
the quantity of digestible protein in a fish stick or energy bar.
The same pressures on the food industry no doubt have an
effect on the electronics industry. But this is more than a
feeling — I’ve proven it to myself. A recent book project of
mine involved the teardowns of two dozen electronic
devices, from a stereo amplifier and ultrasonic humidifier to a
power conditioner. In several cases, the teardowns revealed
manufacturers were either intentionally or unintentionally
misleading consumers with product specifications that were
confusing, incomplete, or simply false. For example, one
manufacturer listed the frequency response of their stereo
amp at 20 Hz–20 kHz. You might assume this represents 3
dB points, but you’d be guessing. Without additional data,
there’s no way to fully evaluate the amplifier or compare its
frequency response specifications with those of other amps.
Another example is the total harmonic distortion plus
noise (THD+N) figure of 0.1%. Again, the issue is
incompletely specifying the measurement. One of the key
parameters in measuring THD+N is the bandwidth. You could
assume that it’s the bandwidth of the amplifier at 3 dB points,
but that’s not stated anywhere. As a result, the manufacturer
has quite a bit of wiggle room when it comes to delivering on
the specs.One of the worst deviations from published specs
was for an expensive power conditioner. The documentation
stated that the device employed special high-energy MOVs
with built-in thermal fuses. As a result of the teardown,
however, I found ordinary MOVs — one of which was placed
directly across the hot and neutral lines, without the safety
factor afforded by a thermal fuse. Not only are the
specifications erroneous, but the device represents a fire risk.
This power conditioner didn’t have a UL (Underwriters
Laboratories) listing — a red flag for any consumer product.
The role of the UL is to evaluate the safety of a device, not its
functions or adherence to published specifications. However,
when it’s missing, you should be suspect of the specifications
in general. So, let’s say electronic device specifications are
erroneous 10% of the time. What are we to do? Obviously,
you can’t tear down every device you buy, simply to verify
the specifications. Besides, many of my teardowns refuse to
be put back together again. My approach is to simply
question every specification I see, especially if the device is
from a no-name manufacturer. Consider that some
manufacturers of audio amplifiers simply copy the
specifications for the integrated amplifier chips used in their
amps and apply those specifications to the entire audio
system. However, chips never provide the full specs when
used in a real circuit. Power may not be sufficient or properly
regulated, thermal conditions may not be optimum, and the
load impedance may fluctuate with
frequency (as is normal for a typical
speaker). The same goes for RF wireless
systems. Whether you’re evaluating a
piece of test gear or a consumer device,
if the specifications match the
theoretical performance of the
underlying chips, you shouldn’t trust the
specifications. Test equipment is a
special case, thanks to the NIST
(National Institute of Standards and
Technology) traceable calibration. NIST
offers calibration certification for
equipment ranging from temperature
probes to voltmeter accuracy. I rarely
pay the premium for the NIST
calibration, but view the option of NIST
calibration as a sign of quality. A poorly
built instrument is rarely capable of
performing to NIST standards. If you
have stories of questionable
specifications
Enregistrer un commentaire