[time-nuts] GPS: ADEV or MDEV?

[Brooke Clarke]

Hi Tom:

Appendix H of Linear App Note 86 "A Standards Lab Grade 20-Bit DAC with 
0.1ppm/C Drift" on pdf page 43 describes the 1,000 X amplifier they used and 
the need for an out of production analog scope with to see sub 1 uV noise as 
well as the need for a totally shielded measurement system.

In table H4 (pdf pag 45) they list the SR560 as a suitable amplifier that 
includes the needed filtering.
http://www.thinksrs.com/products/SR560.htm

I don't have this stuff but was just reading the app note after the reference 
here to the LTZ1000 DC reference.  It uses their 24 bit A/D converter and a PIC 
with a couple of D/A converters to get the 20 bits.  It could be modified to 
get more bits with less precision.

One of the things I've looked at is how do you generate the EFC control voltage 
for a GPSDO.  Stock D/A converters when scaled for the full voltage range of 
the oscillator will be way too coarse.  If you use a manual pot to allow them 
to run where the LSB is a little smaller than the desired frequency step size 
then you need to set the pot maybe every 6 months or so to follow the aging of 
the crystal.

The FTS4060 combines two 16 bit D/A converters and only needs manual tweaks 
about once a year.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.pacificsites.com/~brooke/PRC68COM.shtml
http://www.precisionclock.com



Tom Van Baak wrote:
>>>Can someone do a similar numerical analysis of these two
>>>factors: tempco and noise? It's not that I don't believe you;
>>>it's just that I'd rather see numbers and plots than words.
>>>  
>>
>>Tom attached plot of zener noise is also from Eickes 1964 paper.
>>Bandpass is a little unclear as its determined by the response of a 
>>galvanometer
>>
>>Bruce
> 
> 
> Good. Thanks for the cool plots. OK, my eyeball sees:
> 
> 5 uV peak-peak, maybe 2 uV rms.
> 10 uV peak-peak, maybe 4 uV rms.
> < 1 uV peak-peak, about 0.2 uV rms
> 4 uV peak-peak, guess 1 uV rms
> 3.5 uV peak-peak, < 1 uV rms
> 10 uV peak-peak, 1 uV rms
> 3 uVpp per spec
> 
> This suggests that one could get Vref noise down to about
> 1 uV (or better if you measure rms). Does anyone on the list
> have actual experience with this issue?
> 
> Anyway, assume a full 1 uV of noise and assume a 10811A
> with an EFC sensitivity of 1e-6/volt. Then Vref noise translates
> to 1e-12 in frequency -- which is worth considering since it's
> in the same ballpark as good OCXO noise. Maybe not the
> dominant factor, but a potential contributing factor.
> 
> For example, below is a 10-minute frequency plot of a typical
> hp 10811A which has a low ADEV of about 2e-12 (tau 1 to 10
> seconds). The scale is 1e-11 / division.
> 
> http://www.leapsecond.com/museum/hp10811/log14669v.gif
> 
> So you can imagine that Vref noise on the order of 1e-12 might
> just start being noticeable in the plot, meaning that if you want
> short-term performance down in the low 12's and if you have
> an OCXO that even gets that low, then you want to keep your
> Vref (and DAC, filters, etc.) clean to microvolt levels.
> 
> It's just a back of the envelop calculation so I'm not really sure.
> Someone correct me if you see something wrong.
> 
> Related - does anyone have equipment in your home/lab that
> can directly measure, at uV or sub-uV levels, noise on the
> EFC line? I wonder how clean a Z3801A EFC is, for example.
> 
> Also an idea - is there an easy way to artificially increase Vref
> noise, by say 10x, so one could see if that change made any
> measurable change in OCXO output ADEV or phase noise?
> 
> /tvb
> 
> 
> 
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