Hi, Ed. I wasn't suggesting the EUT would be an exposure problem, as
the usual emissions are far too low; just that a mannequin made for
whole body SAR would be almost ideal. You got around that by measuring
the difference in emissions. *Bingo!*
The point I wanted to make was that 461 tests need to closely
approximate the actual use environment, which a passive charge
discharge/recharge cycle doesn't do for actively communicating smart
batteries.
Cheers,
Cortland Richmond
KA5S
On 8/24/2012 1313, Ed Price wrote:
Re: "Smart" Batteries
*Cortland:*
**
*I didn't get into other details, but yes, we were concerned about
human RF exposure at the 225 MHz data link frequency. However, the
modulation scheme was very low duty cycle. First, we had a message
rate of once per 10 seconds. Then, the individual participant was
assigned a time-slot that allowed for roughly a 9 millisecond total
message. And the message itself was composed of frames of data, which
consisted of digital words made of digital bits (offs and ons). You
could sure see it with a Peak detector, but the Average was
undetectable. We did measurements with a QP and Average detector, plus
measurements with a bolometer type power density meter, in addition to
calculating the power from Peak measurements and typical duty cycle
values. Every way we looked at it, the human exposure was very low.*
**
*We tested the soldier-worn system on a mannequin to 461 conditions;
there were never any connecting wires, although we did have to supply
real-time GPS to the EUT. It was also helpful that the soldier-worn
harness also had optical ports that we could use. I would have
preferred a mannequin that was more representative of a human torso,
but out PVC pipe and foam rubber mannequin produced emission results
very similar to a man-worn setup. We also did extensive antenna
pattern testing with real humans crawling around in the dirt. The
battery is never charged while on the soldier, so the man-worn
equipment really has only one mode of operation. The batteries are
usually installed before the training session, but a long session
might require a field re-supply, so a quantity of batteries could be
transported, typically on an HMMWV. Batteries are never charged in the
field, or while in the harness, mainly because it's easier to move
charged batteries than the chargers themselves.*
**
*As an aside, we sold systems to the British, and they had us include
enhancements such as gadgets that simulated land mines & IED's, so not
everything was man-worn. And of course, there were other devices in
this product family that were intended for vehicles and weapons, but
that testing was similar to traditional 461 testing.*
**
*The concern about "very long" cables as part of the EUT may be going
away. System designers are finally embracing optical cables instead of
using a fire-hose sized signal and control cable bundles. OTOH, I was
seeing a rise in designs that tried to use COTS Wi-Fi (or similar) to
network very local boxes instead of using signal & control cables.
There are a lot of EMC problems with this, so we will still have lots
of job security. **J***
**
*Ed Price*
*El Cajon, CA*
*USA*
-
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