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

Bob Camp lists at rtty.us
Sat Dec 22 13:52:07 UTC 2012


It is often harder to measure a pulse that goes from 0 to 100 ns than it is to measure one that goes from 100 to 200 ns. 

The resolution on the 0 to 100 measure will be 2X, but the non-linearities at zero are quite difficult to deal with. The 100 to 200 measure can get to the same resolution with some analog tricks. If you are running into an ADC, the resolution may already be "good enough". There may be no benefit from making it 2X better. 

For the measurement you are trying to do, 0.1 ns is probably good enough. A 10 bit ADC would do that at 100 ns span. A 12 bit ADC would do it at a 200 ns span. 


On Dec 22, 2012, at 8:34 AM, 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.
> 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?
> 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:
>> 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.
> 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,
> Fabio.
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