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

Fabio Eboli FabioEb at quipo.it
Wed Dec 26 10:26:32 UTC 2012

Hello, hope you all had a happy Christmas.

Back to the topic.
Bob Camp asked:
>One very simple question - how good would it do if you just did it all 
> with logic gates? Tri-state buffers and things like that….
>Now that you are up to a 100 to 200 ns long pulse, a lot of the fiddly 
> stuff about "can't get a 2 ns pulse through it" goes away.
>I'm not suggesting you tear up what you have. It's just something else 
> to try and compare

Bob, are you hinting to something like the last mail from Bruce?
I.e. to use a tristate buffer to charge the capacitor?
If not can you explicit what are you thinking? :)

Thanks also to Alan Melia and Tom Miller for the details about
bjt saturation .

Bruce, about the tempco of the current generators,
There is the led in series with a BE junction.
The blue leds should have a tempco in mV per °C similar
to th BE junction, dont know the red ones.
Would it be better to use something like a 4.7 or 5.1V
zener? If I remember correctly these zener voltage
shuld cancel most of the BE tempco.
And what about a TL431 instead of the led+bjt?

The Avago diodes are pretty costly :)
Is that circuit working like the internals of ECL logic

> The simplest (lowest part count and least number of power supplies)
> consists of a tristate buffer driving an RC circuit.
> The PPS signal is connected directly to the buffer input whilst the
> output of the PPS synchroniser (at least 2 stages to minimise the
> probability of metastabilty at the synchroniser output) drives the
> buffer tristate control input.

A 2 stage syncronizer is composed of 3 FF?
I.e. clock in parallel to 3 FF, PPS to the
first D, Q from the first to D of the second,
same from the second to the thid, and Q from
the third to out. Let's assume that the inputs
from PPS and 10MHz are fast enough, what can still
generate metastability? Setup time violation?

> The RC network starts charging when the PPS signal goes high and
> stops when the synchroniser output goes high.
> The capacitor charging is nonlinear but this is easily corrected in 
> software.
> The capacitor is connected between the input of a capacitive charge
> redistribution ADC and ground.
> Software correction for the effect of charging the charge ADC input
> capacitance is also required.

I see you are stressing the fact of using a capacitive charge
redistribution adc. I dont know much about the internals
of the ADC devices, can you suggest a partnumber for an example?

> Suitable fast single gate tristate drives are readily available.
> With low tempco resistors and capacitors the TAC gain tempco can be
> 200pmm/C or less.
> The only disadvantages are the increased software complexity and the
> need for an extra bit of ADC resolution to maintain TAC resolution.

The 3-state buffer + R-C seem an elegant solution for a microcontroller
based thing, I'v given an eye to logic buffers, and seem that all
suggest that the Hi-Z state leackage current is not very well
specified, but something around 1uA, that means that cap's voltage 
the pulse can rapidly (and unpredictabily?)change due to leackage.
I imagine also that the leackage of the buffer will vary with 

The ADC of the micro is pretty fast, I shuld check the datasheet
but I remember around 1uS per conversion, what would happen connecting
directly the micro ADC to the charged cap? And sync the ADC to sample
immediately (few uS) after the pulse. Could the loading from the
s/h capacitance be corrected in fw?

> Bruce

By the way, I updated my miserable schematic, I tried a simple
mod to avoid the saturation of the switches. Only because I had
it already built: http://pastebin.com/9VHkhmSv

Now I'm chasing the origin of the drift variation, logging
the temperatures and voltages. More on this as soon as I
have some data.

Thank you all,

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