[time-nuts] V standards

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Dec 2 07:18:45 UTC 2008


Hi Bruce,
 
yup, I thought that circuit and it's claim to 32 bit resolution was over  the 
top too, I didn't actually read the details.
 
Coarsedac changes are an issue as you mentioned, but one can get around  them 
by scaling the two dacs in a way to only use a couple of bits from the  
coarsedac, and operate most of the time using just the fine dac.

This is similar to a mechanical coarse adjustment (such as on the  10811A) 
setting the approximate voltage range, with the EFC done by  the fine dac.
 
If you use a 16 - 18 bit fine dac, and 6 - 8 bits of a coarse dac then  this 
works quite well, and with a low aging SC-cut crystal you may never see a  
coarsedac change, while still having over 21+ bits of overall resolution.
 
It's fairly straight forward to improve the resolution of a good Dac by 2,  3 
or even 4 bits using PWM, see Tom's description of how this is done in a  
Z3801A on leapsecond.com.
 
So in short, one could use two identical 16 bit dacs, scale the resistors  so 
that only the upper 6 - 7 bits are used as a coarsedac, and use the fine dac  
as a 20 bit dac with PWM resolution-enhancement (16 to 20 bits). This would  
yield more than 26 bits theoretically from 2 inexpensive 16 bit dacs, and the  
noise could be removed by low pass filtering using an analog filter with  say 
less than 0.3Hz cutoff frequency similar to the Z3801A circuitry. In this  
case the coarsedac can probably even be exchanged for a simple high-stability  
pot.
 
It does require a bit of real time firmware to do the PWM enhancement of  
course.
 
bye,
Said
 
 
In a message dated 12/1/2008 22:47:29 Pacific Standard Time,  
bruce.griffiths at xtra.co.nz writes:

The  major advantage of such a DAC is the inherent monotonicity which
cannot be  achieved and maintained (around coarse DAC transitions)
without frequent  calibration when the outputs of 2 16 bit DACs are  combined.

Bruce




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