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

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 22 17:25:39 UTC 2012

Dear Fabio,

On 12/22/2012 02:34 PM, FabioEb at quipo.it wrote:
> I answer here to Bob Bill and Magnus.
>> Hi
>> 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.
>> Bob
> The suspect is temperature, the first
> thing I'm suspecting is the FE5680A temp coefficient.

When it comes to phase, your interpolator may also be sensitive.

> I didnt grasp the "real numbers", so I tried estimating
> the local drift, i.e. the drift value every 2k samples.
> Here the results:
> http://www.flickr.com/photos/14336723@N08/8296002061/
> 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?

Sounds a bit on the high side.

> 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!

Indeed. Some even let one more edge go and measure between 200 and 300 ns.

>> Like that you try your interpolator wings!
> Sorry, I didnt undestand this part.

Trivial, I like that you experiment and build your own interpolator 
design, build experience.

>> 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:
>> http://www.ke5fx.com/ac.htm
>> I used it to drive an input to a divider chip without the output
>> resistor or
>> capacitor.
>> Bill....WB6BNQ
> Magnus and Bill, the input stage I'm using was inspired by
> the wenzel second schematic on this page:
> http://www.wenzel.com/documents/waveform.html
> 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.

Good. It amplifies up the clock so that you will have low jitter.

> Is the transformer mandatory or I can avoid it?

You can avoid it, just make sure that you get the transistors properly 
biased, so DC blocking cap and some resistors.

> In case I have some IF-cans but I've never used and
> dont know much about them.

It's relative benign transistors being used.

Good luck and look forward to your progress reports.

You got me inspired to try something myself. :)


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