The crystal oscillator engineer speaks
Message #2 Posted by NH on 18 May 2003, 4:28 p.m.,
in response to message #1 by Thomas Reins
Hello,
I have done design work on little crystal oscillator
circuits previously. Some of them were for
32.768 kHz, others were for 14.31818 MHz .
Anything I came up with was strictly ho-hum, using
a dedicated CMOS cell that was designed on a
silicon chip layout software. Little more than
a 2-transistor logical inverter really.
You bias it to the halfway point, like, if it
is a 3V supply, then you bias it to 1.5V .
It acts like a little amplifier, and with the
right load capacitors, you get a reliable
oscillator. If you mind your P's and N's,
maybe low-current consumption also.
BACK TO YOUR CALCULATOR WATCH, I have 3 suggestions,
and the 3rd suggestion is the one to focus on:
1. you might conceivably try to change out
the crystal. Probably it is in some tiny oddball
metal-can, but I have seen some very small
packages for 32.768 kHz. NO, I dont even know
if your HP-01 runs on 32.768 kHz it might be
something else. Also, if you did replace it the
result may be worse than if you left it alone.
You could solder the original back in, unless you
had destroyed it with the extra handling, which
is a 50-50 chance.
2. If you can see little load capacitors
(typically there are two) each of which goes
from either terminal of the crystal, to ground,
then you may have something. Increasing the
capacitance typically slows it down, less capacitance
increases the speed. Since yours is running too
slow, you'd need to have less capacitance.
BAD NEWS, these capacitors are often integrated
onto the custom silicon die (thats how I did
the ones I designed).
3. You could leave it alone. If you lose
one second per day, that's nuthin'. Twice a year
you have to reset it for daylight savings time.
And what most of us do with a watch that runs a
little slow, is you set it about 3 minutes fast.
After a couple months, it will be right on, and
after that, it will be a little slow.
Perfectionism is a dangerous game when dealing with
old nostalgic items. You have something thats
pretty good, and it works. Start messing with it
and you break it, then you have nothing (because
its been discontinued by those clueless marketeers
so there is no factory building you more).
Good luck,
- Norm
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