14 Dec
2004
14 Dec
'04
9:08 a.m.
>>>> "Eric" == Eric Smith
<eric(a)brouhaha.com> writes:
Eric> Tim wrote:
> For not-so-heavy transformers, higher frequencies
are desirable,
> thus the 400 Hz AC power on military aircraft.
Eric> And civilian aircraft as well. When I flew from San Francisco
Eric> to London in 2002, I had to make arrangements to use my CPAP
Eric> machine. They were concerned because the machine is rated for
Eric> 50 to 60 Hz. But it uses a switching power supply which is
Eric> actually perfectly happy to run on 400 Hz, so I was able to use
Eric> it. It's irritating that the manufacturer defines the rating
Eric> based on what power they expect the user to have, rather than
Eric> practical operational limits.
Makes perfect sense. If you put it on the data sheet, you have to
test it. Presumably it says 50 or 60 Hz because that was tested,
while 400 Hz was not.
Eric> Speaking of annoying specifications, I was just reading the
Eric> manual for an HP logic analyzer circa the mid-1990s. They make
Eric> a distinction between "specifications" and
"characteristics",
Eric> defining the former to be something they actually test the
Eric> instrument against, and the latter to be non-tested and
Eric> non-guaranteed. They list the maximum clock fequency and
Eric> minimum clock pulse width as specifications. But they list the
Eric> number of channels and the memory depth as "characteristics",
Eric> which means that if the analyzer only had 67 channels instead
Eric> of the "characteristic" 68, they could still claim that it met
Eric> the specifications. In practice I'm sure they wouldn't try to
Eric> screw customers on that point, and couldn't get away with it
Eric> anyhow because the channel count and memory depth are
Eric> prominently featured in the sales literature. But after they
Eric> go to all the trouble of explaining a distinction between
Eric> specifications and characteristics, it seems really bizarre
Eric> that they would put the channel count in the latter category
Eric> since it is clear that they do actually have to test them all.
For that case, it looks like a typo. But electronic circuits often
have properties that are established by design but not 100% tested.
Look at an IC data sheet for example. Properties like input
capacitance are often "characteristics" while rise time and max clock
frequency are "specifications" that are tested for each chip.
paul