[time-nuts] 4046 variations (was EPE GPS....)

Robert Atkinson robert8rpi at yahoo.co.uk
Sun Sep 27 20:44:06 UTC 2009

I've started an interesting discussion! I don't know the details of King's selection process, it may have been that that batch would work in either application. Light aircraft avionics designs can be quite interesting. Older designs (I've not looked at any recent ones) would oftrn use chips that had temperature specifications well below the specification of the unit. The "professional" sets would use the industrial or military spec parts. I've designed aircraft equipment where only I or someone with access to the design analysis whould understand why a particular part was selected. The part might look "wrong" without this information. One case was a 3.15A fuse in series with a 27R resistor at the 28V supply input. The fuse can never blow (no the aircraft didn't have 115V 400Hz supplies).
The reason was a pater exercise to obtain intrinsic safety approval without formal testing. The rules said a specific type of fuse must be used at the input, the minimum rating of that approved type of fuse was 3.15A!  

Robert G8RPI.

--- On Sun, 27/9/09, Lux, Jim (337C) <james.p.lux at jpl.nasa.gov> wrote:

> From: Lux, Jim (337C) <james.p.lux at jpl.nasa.gov>
> Subject: Re: [time-nuts] 4046 variations (was EPE GPS....)
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Date: Sunday, 27 September, 2009, 6:32 PM
> On 9/27/09 9:54 AM, "Javier Herrero" <jherrero at hvsistemas.es>
> wrote:
> > Yes, of course, some applications needs the parts that
> are in the better
> > side of the distribution curve for a particular
> parameter and for a
> > particularly stringent application, as you mention :),
> but I think that
> > a 4046 seems a part that should not have to be so
> critical that even in
> > the same lot number some work, some don't, and it is
> hard to believe
> > that no alternatives existed on that time that could
> have avoided the
> > high costly selection process.
> > 
> > Regards,
> > 
> > Javier
> > 
> > Lux, Jim (337C) escribió:
> >>
> >> 
> >> Not always marginal or poor design. 
> Sometimes, you can't get the
> >> performance needed with (any) part that simply
> meets the datasheet specs, so
> >> hand selection is needed.  Picking matched
> pairs is a time honored method
> >> for instance.. In some cases, the mfr does the
> picking for you (RF power
> >> transistors). In another case, there's an
> extremely low noise FET used in
> >> some charge amplifiers that not only has to come
> from one mfr
> >> (notwithstanding the 2N JEDEC number) but you want
> to buy a bunch and pick
> >> the quietest one.  High value resistors and
> leakage currents also come to
> >> mind.
> That was an aircraft radio, and while I don't know a whole
> bunch about what
> certification requirements they have, but for spaceflight,
> we often use
> older (design-wise) parts which have a "flight heritage",
> even though the
> performance is not what you could get with a more modern
> part (even if the
> new part is available as Class S). (anyone know of a flight
> qualified opamp
> to replace the venerable OP27.. Not that the OP27 isn't
> good, but a lower
> power, lower noise, wider band device is always nice to
> find)
> I could see the radio having been designed years and years
> ago, when the
> 4046 was all you could get, and even with the cost of hand
> selecting, it was
> still better than the alternative (dozens of carefully
> selected crystals and
> multideck rotary switches).  Move forward 10-15 years,
> and rather than go
> through the considerable effort and cost to "certify" a new
> design, they
> just stick with what they have.  Then, move forward
> another 10 years, and
> someone needs that radio repaired (I had an early 70s Piper
> Cherokee in the
> 80s with the original radios, and I'll bet the next owner
> kept them for a
> while, too.
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