Re: Personal Beacon shutdown

gm0uhc <gm0uhc@...>

Martin . . .

I second what Chris says regards GPS locking. For some time I viewed
it as a 'black art' but it is nowhere near as intimidating as it
sounds. Also, I would have to agree to some extent with Chris's hint
that the quality of crystals these days isn't what they are cut
(pardon the pun) out to be. I have a crystal cut for 40 deg.
operation that is somewhat 'unruly, but my 'cupido' PLL board soon
keeps it in check!! I'd go for GPS locking, you won't regret it!


--- In, Chris Bartram <yahoo@...> wrote:


Personally, I'd bite the bullet and lock the beacon to GPS.

There are a number of reasons for this.

Firstly, GPS receivers are relatively cheap, even those intended
timing/frequency stabilisation applications are starting to
approach the
price of a decent custom crystal. It's now quite usual to find a
10MHz output
on 'timing' GPS receivers. There are a number of reliable designs
from eg.
CT1DMK and G4JNT which can be used to lock an existing crystal
oscillator to
a stable source.

Secondly, having recently had some very relevant professional
experience -
which I can't talk about in detail - I've come to the conclusion
that the
performance of custom crystals from most suppliers is getting worse
and worse
as the demand for them decreases.

Thirdly, operating even a good 20degree C crystal at an elevated
is not necessarily a Good Idea. It can work, but, crystals 'cut'
operation at room temperature sit on an inflexion of their
temperature/frequency characteristic, and so change frequency
little with temperature over a few degrees. Trying to operate the
crystal at
an elevated temperature will place it on a slope, and it becomes
much more
difficult to stabilise the frequency. Crystals cut for operation at
temperatures are available, but even from 'professional' suppliers,
they are
_much_ more susceptible to rapid aging effects than a room

If you really don't want to go to the trouble of locking the
oscillator, one
technique which used to be used very successfully by a number of
people, but
which seems to have been forgotten, was to bury crystal oscillators
a metre
or so below the surface. At that depth the annual change in
temperature is a
degree or so, and discounting aging effects, oscillators will drift
little indeed...

Vy 73


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