[time-nuts] Re: PICDIV stability

[Tom Van Baak]

All -- The 2012 test results for the T2-mini, which contains a PIC 
divider chip, is here:

http://leapsecond.com/pic/jitter/

It's about 1 ps, or sqrt(2) less because it was comparing two T2-mini 
against each other with a common reference. Also note that this 
measurement is the sum total of the Wenzel sine-to-square circuit 
onboard the T2mini, the PIC divider chip itself, and the 74AC04 buffer chip.

I also included some plots of a baseline test to show that the Wenzel 
ULN (Ultra Low Noise) reference and the Miles' TimePod analyzer are not 
the limiting factor in the test.

Hal -- The pD17 PIC divider used in the T2-mini has a single output. See 
T2-mini schematic in the above URL. The PIC code is here:

http://leapsecond.com/pic/src/pd17.asm

Bruce -- I agree with your comments. Thanks for posting that.

Attila -- I have not measured the voltco. Note the T2-mini has an 
onboard regulator. I also have not measured tempco. Although the jitter 
is about 1 ps the wander over that 10 minute run is about ±6 ps (2.4 ps 
rms). Look at the phase plot in the test results. This is also why the 
ADEV plot has that characteristic plateau from tau 2 to 20 s.

IIRC, the test was done causally on a floor in open air so walking, 
breathing, drinking coffee, and checking email are known to wiggle 
things at the picosecond level. Someone could look into this more if 
they wish. I would be interested to know how much of the wandering is 
due to the voltage regulator vs. Wenzel transistor circuit vs. the PIC 
vs. the 74AC chip.

/tvb


On 1/7/2022 12:40 PM, Hal Murray wrote:
>> The two biggest outside influences on the PICDIV are supply voltage and temperature.
> Another interesting influence is the number of outputs that are switching and
> the load on them.  In particular, if you have several outputs running at
> different frequencies, the clock-out delay should be slightly longer when 2
> outputs switch when compared to when only one is switching.
>
> Has anybody measured that on a PIC? (or similar chip)
>
> I think one of tvb's picDEVs has several outputs.
>

On 1/7/2022 5:00 PM, Bruce Griffiths wrote:
> That entire thread is full of misinformation and should be ignored unless one understands the difference between random and data dependent jitter.
>
> For a well designed divider with a single output frequency only the random jitter spec is significant.
>
> One doesn't need a bunch of expensive LeCroy gear to measure RJ of such dividers as its PN manifestations are readily apparent and measurable.
>
> Using one of the supposedly super low jitter flipflops isn't a panacea. In practice unless an appropriately designed ZCD is used the wideband input noise of the very fast FF will dominate and produce much more jitter than expected due to the relatively slow slew rate of the outputs of most 10MHz sources.
>
> Bruce