[time-nuts] V standards

[WarrenS]

>Bruce Wrote: 
> "Sometimes its cheaper and more convenient to build  a high 
> resolution DAC than use a pot and a conventional DAC. ...

Very true, And don't forget also more fun, more challenging, and more better!
which all very import  requirements in this field, maybe more so than performance.

Assuming  100 to 1000 seconds are the typical max tracking times 
practical in the standard GPSDO's due to temperature changes, 
initial aging, etc  as stated by Ulrich in the posting  
http://www.febo.com/pipermail/time-nuts/2008-December/034823.html 
and IF IE-10 is the best you get from the GPS signal in these same 
time averages as stated on Page #3 of  
http://www.thinksrs.com/downloads/PDFs/Catalog/PRS10c.pdf

Doesn't this imply that a Dac with more than about 12 bits becomes 
an overkill at 100 seconds tracking when using a 1E-7 turning range OSC, 
and around 18 bits for the same Osc when tracking at 1000 Sec? 

WarrenS
**********************
WarrenS wrote:
> Concerning the Need for very high resolution Dacs in GPSDO.
>
>   
>> Bruce said: "However there are low noise OCXOs with EFC adjustment 
>> ranges of 1E-6 or more, (that would benefit from >20 bit Dacs)
>>     
>
> >From what I have seen so far,  the higher the accuracy of the OSC 
> used in a GPSDO the LESS there is the need to use a high resolution Dac.
> The FS725 rubidium with it's 2e-9 external EFC range, would not seem 
> to need more than the most simple 12 to 16 bit EXTERNAL tracking Dac. 
> True that its internal Dac needs to have very high resolution if it is used 
> to lock an internal VCXO that has a range of say 1e-6.
>
> Question:  Is the Aging rate of these low noise OCXO units poor enough  
> that you could not use a couple of fixed precision resistor and/or a pot 
> for course adjustment and reduce the EFC range of the DAC by say 1/100 ?. 
> Are there low noise OCXO (that are being used in GPSDO) whose long term 
> drift over say 6 months would need more than 1E-8 or so of  'automatic' turning?
>
> And concerning the 10811A. Should one consider reducing its EFC 
> tuning range by say 2 to 20 to ease the requirements of the Dac? 
> Even if the EFC tuning range was reduced by just a factor of two, 
> It could then be done with just a standard 0 to 5 volt Dac  
> instead of the -5 to +5 that I saw suggested by its spec.
> I would think that a well aged unit would be much better than its worse case 
> spec of 1E-7 per year, And even if not, I'm guessing  that many of the time-nuts, 
> would be very welling to trade off  it needing a have few extra manual adjustments 
> in order to get better performance.
>
> WarrenS
>
>   
Warren

Using a pot plus a relatively low resolution DAC is a classical
technique which can be quite useful if the pot is readily accessible and
one doesn't have too many OCXOs etc to adjust.
Sometimes its cheaper and more convenient to build a high resolution DAC
than use a pot and a conventional DAC.
This is particularly true if the OCXO is inaccessible or at least
relatively inaccessible.

Bruce