On Thu, 15 Jul 2010 00:37:22 -0700, Richard Clark
wrote:

On Wed, 14 Jul 2010 23:02:21 -0700, Jeff Liebermann
wrote:

However, the -30dB
drop of the iPhone 4 requires some extra circuit design screwups.
Unfortunately, detecting the regeneration is going to require internal
probes, test fixturing, and plenty of expensive test equipment.

I goofed slightly. The Anandtech article shows -24dB maximum signal
loss.

Regenerative designs inhabit the naked edge of
amplification/oscillation.

Not designs. I don't think it was intentional. Someone else
mentioned that Apple was so much into security on the iPhone 4, that
prototypes were tested in a similar case made to look like an iPhone
3G. That implies that there was little field testing in the new case.

You've already said as much. It has to
be, thus, a design feature and not a hack.

I beg to differ. Nobody wants oscillatory effects in the front end of
their receiver. They're unstable, vary with temperature, are
difficult to control in manufacturing, and will probably make the
device fail Part 15 incidental radiation test. Designing a front end
that's unconditionally stable with any antenna load does not yield the
optimum sensitivity. If you want the best performance, amps that are
unstable at specific loads will give better NF and gain.

Otherwise every third one
is going to break into oscillation and the FCC (or test lab) would
have certainly picked up on that.

Maybe. However, one would think that someone would have done a field
test with the final iPhone 4 and noticed that touching the antenna gap
produced a major signal drop. Also, FCC test are not preformed with
human hands wrapped around the phone due to the limited supply of
dismembered hands suitable for testing.

Further, if it is designed in, you
can observe that in the design. Special testing methods are too
elaborate to explain this. Regen is not a design feature left to the
vagaries of production variation in parasitic coupling.

That might partly explain why some users claim that there's no call
drop effect when they touch the antenna gap. Of course, users in
strong signal areas are not going to see a dropout, while weak signal
areas are going to be much worse.

Incidentally, when Consumer Reports ran a test with the rubber ring
thing, they slipped and didn't have the technician stand in front of
the test equipment this time.
http://blogs.consumerreports.org/electronics/2010/07/apple-iphone4-iphone-4-bumper-case-fixes-antenna-issue-problem-signal-loss-tested-verified-consumer-reports-labs-quick-fix.html
http://802.11junk.com/jeffl/crud/iPhone4-cmu200.jpg (normal)
http://802.11junk.com/jeffl/crud/iPhone4-cmu200-hand.jpg (with hand)
It's a Rhodes and Schwarz CMU200.
http://www2.rohde-schwarz.com/product/CMU200.html
I'm not familiar with the operation. The "Reported Power" appears to
be the signal generator output power (because it doesn't change when
the antenna is touched). However, I can't devine what the "Avg" or
"Peak Burst Power" means.

Also, note that the difference in (something) between the first photo
(+16.4dBm) and the hand on antenna measurement (-11.7dB) indicates a
-28dB drop in signal level. Ouch.

If I'm wrong and Apple does use parasitics to encourage regenerative
feedback, then they may introduce a lanyard for hanging the phone
around the neck and call it the lavaliere option. No doubt they will
sell plenty of mirrors to see it, and someone will come out with an
image reversal app. Can a stylus be far behind? Whoops! It will
have to be chopstick for squeezing and expanding images.

We'll find out after the press conference on Friday. My suggestion
was to ship a rubber glove with the phone. I also suggested gluing a
plastic thumb tack to warn against holding it improperly. Also, a low
signal warning tone. Others have suggested tape, rubber prophylactic
bump guards, duct tape, warning labels, rebates, bribes, and external
antennas:
http://gizmodo.com/5581253/there-fixed
Unfortunately, that will require running the FCC type certification
tests again.



--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558