[time-nuts] Simulation

[Bob Camp]

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 at quik.com> wrote:

>> On 8/14/2010 10:08 AM, J. Forster wrote:
>>> FWIW, IMO any engineer who uses undocumented or uncontrolled parameters
>>> or
>>> instructions in a production design is a fool.
>>> 
>>> If you are that silly, you must fully specify the selection criteria.
>>> 
>>> -John
>> 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.
> 
> 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.
> 
>> 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.
> 
> If your design is that critical, you may have to do incoming
> inspectrion/selection or send the parts to a company that does.
> 
>> 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.
> 
> Certainly, old Ge power transistors have ICBO issues.
> 
> -John
> 
> ===========
>> 
>> --
>> mailto:oz at ozindfw.net
>> Oz
>> POB 93167
>> Southlake, TX 76092 (Near DFW Airport)
>> 
>> 
>> 
>> 
>> 
>> 
>> 
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> 
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