[time-nuts] Simulation

[Bob Camp]

Hi

Simulation might or might not have helped.

1) Was Vbe breakdown even included in the Spice model
2) If so did it ring bells (rare) or did it just clip without error ( common )
3) Would the same designer who didn't understand it in the first place have seen it clipping at -5 and concluded "looks to be in spec at -5"
4) Would any of it be reviewed in light of the new transistor or was the guy on another project by then

My favorite in the absolute max Vbe category is the typical class C RF amp. Look at the spec, check a few thousand working boards. Scratch head and move on. Lots of reverse bias on the base and they run forever.

 Bob



On Aug 14, 2010, at 7:47 PM, "Alan Melia" <alan.melia at btinternet.com> wrote:

> Surely all this is a case of an engineer being able to
> read......specifications :-))
> Manufactures specify to sell parts, most of the important parameter are
> minimum values, and the range is as wide as it can be. If you want a
> parameter "bracketed" then you must specify that when buying and pay the
> premium or take the expense of testing incomming batches for sensitive
> parameters. I used to give lectures to new engineers on reading
> specifications .....and what "Absolute maximum"  actually means. Clever
> engineers also love using operations that rely on unspecified parameters.
> For example using a stock switching diode as a spike quencher in a relay
> driver. The usually work (but not always!) but are generally not specified
> for that service.
> 
> Spice parameters are probably a few "typical" samples of even "the one I
> happened to choose to measure"!!
> 
> I had a case of some +/-12v transistor logic which made it was into a big
> telecoms project. the prototypes worked fine. By the time of bulk
> manufacture the supplier of the the main transistor had had several cost
> improvement stages. The result was the kit was unreliable. The problem was
> that first the base junction were swung to -12v by the "0" logic state and
> second from a transistor with a fairly simple round dot geometry emitter,
> the device had been replaced by an interdigital high ft chip (it still met
> the basic "greater than" specs.)
> 
> The interdigital device suffered severe loss of gain after a period of
> avalanching at several millamps (I think it also suffered electromigration)
> much more than the circular geometry transistors. I think this was due to
> the increased emitter periphery length. It was solved by placing a simple
> diode in the emitter leg, or a clamp on the base. The engineer had not read,
> or not understood the meaning of  the Vebmax = 5v parameter, and his
> prototype had worked. The production side were not very happy but eventually
> were forced to junk thousands of 8inch square pcbs and do a re-layout. This
> was in the days before computer simulation !! It might have helped!
> 
> Alan G3NYK
> 
> 
> ----- Original Message ----- 
> From: "jimlux" <jimlux at earthlink.net>
> To: "Discussion of precise time and frequency measurement"
> <time-nuts at febo.com>
> Sent: Saturday, August 14, 2010 10:42 PM
> Subject: Re: [time-nuts] Simulation
> 
> 
>> Hal Murray wrote:
>>>> Line got moved to other side of big ocean. Process got " tweaked" beta
> is
>>>> now 4x what it was.
>>> 
>>> I'm pretty sure that military grade parts have paperwork and processes
> to
>>> cover that case.  I expect it costs a lot.
>>> 
>>> I think the same sort of service is available to high volume customers
> at a
>>> less than military price.
>> 
>> just so..
>> 
>> In the space business, we call it "traceability to sand"... you haven't
>> lived til someone has a failed 2n2222, somwhere on some piece of
>> critical hardware, and they issue a GIDEP alert, and then the mission
>> assurance folks call you up and ask, "you don't by any chance have
>> 2N2222's in your flight hardware do you?".. then there's the whole
>> manufacturer and date code hunt.. Looking through the build
>> documentation to find out. (or worse yet, if you had decided for some
>> reason to use the prototype, which you didn't keep such good records on,
>> but which you have photos of, and trying to read the date codes off the
>> assembled item with a magnifying glass)
>> 
>> 
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