[time-nuts] Questions about TAC frontend, and some measurements

FabioEb at quipo.it FabioEb at quipo.it
Sat Dec 22 13:34:12 UTC 2012

I answer here to Bob Bill and Magnus.

>I think I would grab some sort of USB thermometer and start logging 
> the room temperature.
>CMOS input op-amps are a pretty good way to buffer the integrating 
> capacitor.
>They are cheap and have very low bias currents.

The suspect is temperature, the first
thing I'm suspecting is the FE5680A temp coefficient.
I didnt grasp the "real numbers", so I tried estimating
the local drift, i.e. the drift value every 2k samples.
Here the results:
The drift stays around -3.2x10^-10 then
abruptly goes to -2.4x10^-10, so if the culprit
is the 5680, it's frequency should change about 1x10^-10,
if I didnt screw up all the calculations.
Does this make sense?

As for the buffer opamp, I will try with MCP6001,
cheap and it's input impedance is so high I will be
limited by the pcb...
By the way, my LM358 seem to be injecting 1.5nA
into the ramp capacitor until it levels to around 1-1.5V.

> Like Bob said, start logging the temperature.
> Since you have about 86400 s period on this behaviour, I expect that
> heating up in the morning (sun or just habits of humans roughly
> aligned with sun patterns) be the reason, so this would be 
> temperature
> dependent. Plotting supply voltage may be another reason.

Magnus, I will log some temperatures and voltages.

>> scope probe set to 10x, DC coupled.
> Do you really get 1-2 cycle long difference measures that way?
> You risk a high non-linearity at the small difference side otherwise,
> as it takes time to wake the transistors.
> As I commented, you might want 1-2 cycles to pass, so adding a second
> DFF might be needed for that task.

So if I'm understanding you are suggesting to measure on the
second 10MHz edge, instead of the first, I would have 100 to 200nS
instead of 0 to 100nS. I didnt think about this, I like the idea!

> Like that you try your interpolator wings!

Sorry, I didnt undestand this part.

> I do recommend you to check out the Wenzel clock input stage, which
> is being deployed in the TADD-2 divider. Squares up sine clocks
> nicely.
> Cheers,
> Magnus

>Hi Fabio,
>I am not crazy about your 10 MHz input circuit.  You might want to 
> consider
>investigating John Miles input arrangement at the following web site:
>I used it to drive an input to a divider chip without the output 
> resistor or

Magnus and Bill, the input stage I'm using was inspired by
the wenzel second schematic on this page:
But you both are right, I'm starting to see that it's
not that stable.
I will try the discrete solution on the wenzel page.
Is the transformer mandatory or I can avoid it?
In case I have some IF-cans but I've never used and
dont know much about them.

Thank you all,

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