[R-390] Re: 200 KHz crystals again

[Tim Shoppa]

Bob wrote:

>  I'm also not quite sure why a  
> "free" cell phone TCXO is worse than one you pay for. If 1 and 10 MHz  
> miniature TCXO's (not XO's) are out there free I haven't seen it ....

Not for free, but they are readily available, along with a lot of
other ones that will divide easily down to 100kc.

Depending on "free" ones is not so good because these sorts
of projects depend on a specific surplus source that usually
doesn't last for more than a year or two.

But a TCXO is way over-engineering IMHO. Tweaking
100kc (or 1Mc or whatever) cal crystals works just fine, even
easier to adjust today than 50 years ago.

> There have been a number of papers published on
> crystal reliability.  Do a Google on "Frequency Control
> Symposium". A good library may have  
> copies as well. I have not seen any that substantiate
> a *reliability* issue on crystals run at 85 versus crystals
> run at 45C. I also have not seen any indication of it in
> any of the work we have done.

My big problem (working on equipment from 5 to 50 years
old) is ovens that get stuck in the "on" position despite
the fact that the thermostat was supposed to turn them off.
Everything bakes. The modern high-tech high-temp insulating
materials (e.g. kapton) turn into broken little flakes. Older
stuff just turns into burnt-looking crud.

You might think that since I'm working with older equipment that
they're all mechanical thermostats, but you'd be wrong. I've
seen a lot of really nice HP OCXO's turn into burnt lasagna when
the solid-state temperature control circuitry got stuck "on".

Many modern crystals are remarkably well-sealed comapred to
the state-of-the-art from 50 years ago. Usually the crystal
and its hermetic seals survive the baking quite nicely, it's just
everything else that is burnt up.

Tim.