When LCD wristwatches became common in the seventies we, in the
frequency and timing group of a space tracking facility, investigated
the possibility of adjusting our new watches against our standard.
We found that a a small copper plate, about 1 X 2 cm, resting against
the display and connected to a scope probe was able to pick up enough 32
KHz energy to be displayed in the scope. Then connected the vertical
output to an HP 5245L counter referenced to our standard and set the
gate time to 10 seconds and got the frequency. We learned that the
watch had to be worn in order to operate at the right temperature, the
body acting as an oven, so you has to wear it backside in order to
access the trimmer (yes, at that time those watches had and adjusting
trimmer, maybe heritage from the mechanical ones, laser trimming arrived
later).
Our group became very popular and busy adjusting every watch our
colleagues bought to a few seconds per month.
Ignacio, EB4APL
On 03/03/2013 16:29, Magnus Danielson wrote:
On 03/03/2013 03:46 PM, Jim Lux wrote:
On 3/3/13 1:00 AM, Poul-Henning Kamp wrote:
Content-Type: text/plain; charset=ISO-8859-1
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In message <657D7F7CC03849419A2A90752E6A60A6@pc52>, "Tom Van Baak"
writes:
When playing with watches a while ago I tried to pick up any 32
kHz signal but failed. Those with 1 Hz stepper motors were easy,
but LED or LCD displays were too electro/magnetic/acoustic quiet
for me to ever detect anything.
Most LCD and LED clocks have a shielding metal-coating on the front
glass, exactly to eliminate all EMI/EMC issues.
Yes, but perhaps there's enough leakage to make this work. After all,
the EMI requirement (assuming it's running at 32 kHz) isn't particularly
stringent and because the fob is small, the radiated field at any
distance is going to very small. OTOH, I can put a probe or coil right
on or around the fob.
I'll let you all know what I detect when I try it tomorrow.
An electrostatic shield will not contain the H-field from the shifting
currents.
Cheers,
Magnus
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