[time-nuts] Re: PICDIV stability

[Bob kb8tq]

Hi

There are a lot of different 78x05 devices out there and various outfits
pay more or less attention to the tempco on various die shrinks / redesigns.
You can see a *wide* range of temperature performance ( > 10:1) between
different examples from vendor A vs vendor B. 

With any linear regulator, self heating can degrade the “input side” rejection.
Since there are lots of ways to load a regulator, this just piles in on top of the
tempco stuff.

If the 74AC04 *is* the dominant source of error (and I’m not at all sure it is), 
there are chips with significantly lower propagation delay. Has your 74AC04
been through a shrink that cuts it’s delay in half? Who knows …. Who’s 
“king of the hill” this month? Buy a bunch of chips and test them. 

It all gets really tangled …...

Bob

> On Jan 8, 2022, at 2:41 AM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> Tom
> 
> The voltage coefficient of delay for a 74AC04 is around -300ps/V so with a tempco of -1.1mV/k for the output of a 7805 this results in an induced delay tempco of around +0.33ps/K for the 74AC04 due to the voltage regulator tempco. The typical propagation delay of the 74AC04 is around 4ns with an associated tempco of around 12ps/K. Thus the actual propagation tempco dominates over induced tempco. I would expect a similar result for the PIC clock to output propagation delay.
> 
> Bruce
>> On 08/01/2022 16:05 Tom Van Baak <tvb at leapsecond.com> wrote:
>> 
>> 
>> 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
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