On 08/14/2010 07:10 PM, J. Forster wrote:

This is the incident I described, but notice that there where three
sources, two of which was different antennas with the same amplifier...
both instances had the problem. I have seen another report from this
incident where they pointed out the differences in component for the
same design. A very benign subtle difference...

Doesn't seem to find it right now.

Cheers,
Magnus

On 8/14/2010 10:08 AM, J. Forster wrote:

This is, easily said, a wonderful goal, and absolute fantasy.  It's
optimistic at best to expect someone to anticipate all contingencies.
It's certainly good practice to specific critical parameters, but it's
rarely makes economic sense to specify every possible detail.

As to relying upon unspecified parameters, most datasheets are woefully
incomplete.  If you are going to use any significant number parts, it's
unlikely that you'll be able to get everything specified, much less get
compliance commitments for each parameter.  Few vendors are willing to
do the testing required to guarantee a substantial number of parameters,
and the simple reason is no one is willing to pay for it.

I've spent quite a bit of time dealing with maintenance of military
systems that would be long obsolete in any other business.  After
obsolescence, the number one problem was parts that meet all published
specs, but had changed performance so much (for better or worse) that
they no longer functioned in the application.  A common problem is Ft or
gain, but leakages are often orders of magnitude different. As often as
not, they were much worse.

--
mailto:oz@ozindfw.net
Oz
POB 93167
Southlake, TX 76092 (Near DFW Airport)

On 8/14/2010 12:10 PM, J. Forster wrote:

I wish it were a one off.  I and friends at cell ops chase these things
all of the time in the cellular and public safety bands. This one just
happened to be in a location that covered a wide area in a densely
populated area.

--
mailto:oz@ozindfw.net
Oz
POB 93167
Southlake, TX 76092 (Near DFW Airport)

I can't recall hearing of other wide-area jamming of GPS, but they may not
have reached the media.

Certainly, that incident alone demonstrates the vulnerability of GPS and
argues against the shutdown of LORAN.

-Jo0hn

===========

OK, important uncontrolled parameters.

For example, I'd consider things like hFE; VCEsat; VCBO, fT and others
important, but not the package capacitance in a low frequency transistor.
There are clearly unimportant parameters and essentially irrelevant ones.
That's where experience and good judgement comes in.

If your circuit is not stable with a high fT part, that needs to be tested
or the design fixed.

If your design is that critical, you may have to do incoming
inspectrion/selection or send the parts to a company that does.

Certainly, old Ge power transistors have ICBO issues.

-John

===========

Hi

It's a very rare thing to see jelly bean parts screened for RF parameters. Much more common to catch and fix an issue at the board level. Pretty rare to see discrete RF anymore anyway.

Bob

On Aug 14, 2010, at 3:56 PM, "J. Forster" jfor@quik.com wrote:

I've seen audio range power amps that will oscillate on a part of a cycle
because an output device with a higher fT was installed. Older vintage
parts with the same type JEDEC number never did that.

-John

============

On 8/14/2010 2:56 PM, J. Forster wrote:

Right, but the definition of "high ft" varies over time.  Few reasonable
designers as recently as 10 years ago would have anticipated the ft of
today's CMOS processes.  It's also likely they wouldn't have expected
their designs to have lasted 10 years, and the vast majority haven't.

Oddly enough, it seems like most of the really long life designs are the
lower volume ones.  These are usally the same ones that won't justify
extensive up front analysis and cost unless they are DoD or aerospace
applications.

But the portion of the discussion that is the root of this branch was
precisely about designers being surprised by dramatic and unanticipated
changes in component performance. Few rational companies are going to
test for parameters in that category.

I think we are almost making the same point here.  Certainly we agree.
My point is that there is an economic tradeoff.  There are a number of
parameters that are critical to any circuit's operation that it's
reasonable to decide are not likely to vary outside critical
parameters.  It makes no economic sense to test these.

--
mailto:oz@ozindfw.net
Oz
POB 93167
Southlake, TX 76092 (Near DFW Airport)