[time-nuts] Trigger circuit for TIC

[Bruce Griffiths]

There is no universal circuit that minimises jitter for all signals without some tuning to suit the input frequency and threshold crossing slew rate. The LTC6957 works well for higher frequency sine waves. The low frequency sine wave outputs of a DMTD require a set of cascaded lowpass filtered limiters akin to those detailed on Oliver Collins classic paper on the design of zero crossing detectors before the LTC6957.  

Bruce
> On 03 June 2020 at 21:49 Tobias Pluess <tpluess at ieee.org> wrote:
> 
> 
> 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
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