[time-nuts] is there a "best bet" advanced hobbyist buildable GPSDOdesign?
SAIDJACK at aol.com
SAIDJACK at aol.com
Tue Dec 11 21:52:13 EST 2007
In a message dated 12/11/2007 15:39:50 Pacific Standard Time,
slburris at gmail.com writes:
>OK, so for the DAC piece, why not just use an NXP LPC ARM chip for the
>microcontroller, and use a 32bit PCM output followed by a low pass filter as
>the VXCO EFC? The DAC just needs high resolution, not accuracy, right?
>Or would the switching noise from the processor modulate the control
>voltage?
Hi Scott,
one problem with using all 32 bits in the NXP LPC PWM output is that the
counter can only run at typically 60MHz, the maximum CPU clock.
With a full 32 bits count time you would only get 2^32 / 60MHz pulses per
second, or in other words (at 50% setting) a square wave with 71.6Hz.
That's pretty hard to filter out to get a very clean DC voltage so that
there won't be a spur at 72Hz offset on the OCXO output. This would require
large-capacitor filters.
bye,
Said
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