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!

73,
Ian, GM0UHC

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

Martin,

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
for
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
temperature
is not necessarily a Good Idea. It can work, but, crystals 'cut'
for
operation at room temperature sit on an inflexion of their
temperature/frequency characteristic, and so change frequency
relatively
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
elevated
temperatures are available, but even from 'professional' suppliers,
they are
_much_ more susceptible to rapid aging effects than a room
temperature
crystal.

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
very
little indeed...

Vy 73

Chris
GW4DGU

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