[time-nuts] Trigger circuit for TIC

[Tobias Pluess]

Dear colleagues
I am currently thinking about a concept for my own TIC. While I gathered
some knowledge during my GPSDO project about using the TDC7200 as
interpolator, I have not yet a good idea how I should design the input
stage of a TIC. It should be somewhat universal, i.e. it should be usable
with analog sine waves as well as with digital signals (TTL).

So the trigger input circuit shall accept a signal of less than approx. 5
volts amplitude, and provide a digital output signal (3.3V logic level)
which can then be used for further processing. It would be good if I could
directly feed it with the output signals from a DMTD, but could also
connect the 1PPS output signals of GPSDOs to it.
I know that trigger jitter can be a problem, especially when the signals
have small amplitudes and are sine instead of square. So what should I
consider to minimise this effect?
I have seen some schematics where people used ADCMP comparators, but I have
forgotten which one exactly and I cannot remember where I saw it. On the
other hand, on other pages I read that comparators are not recommended
because they have a higher jitter, and it was recommended to use high-gain
amplifiers instead.
So what is the "proper" way to design such a trigger circuit which is
usable for sine as well as square waves?
(if the jitter was not critical, I would simply use some comparators.)

Background: I want to construct something like a timestamping counter which
allows me to caracterise my GPSDO I built. For this, I would like to do
different things: first, I want to compare the 1PPS signals of different
GPSDOs with respect to mine (5V TTL signals), but on the other hand, I also
want to use the DMTD method which provides signals with maybe some 10Hz
frequency and several 100mV amplitude. It would be good if there was a
trigger circuit which could be used for both.



BR
Tobias
HB9FSX