At 04:59 PM 8/12/02 +0100, you wrote:

>A good frequency counter will probably trigger from a pickup coil such as
>mentioned, but then it will need a proper ovened timebase which is regularly
>referenced to a standard to get anything meaningful of the accuracy you are
>looking for. Remember that you will really need a standard good for 0.1Hz in
>12MHz to make your efforts worth while.

Anything that gets close enough to the crystal to read the signal will
probably affect the frequency by more than 0.08ppm. If the capacitor
affects the frequency by, say, 20ppm/pF, then changes in stray capacitance
of more than 4 FEMTOFARADS (0.004pF) will affect the frequency more than 1Hz.

NS patented a neat way of calibrating LED watches way back, they used the
mux frequency of the display as a non-contact way of transmitting the
frequency externally. You could do that with the "power on" LED in a product,
just run it at 50% duty cycle at 4MHz and use a PIN photodiode to pick up
the signal. Visible LEDs typically switch in around 100ns, so that should
not be a problem.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
9/11 United we Stand

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