[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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