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

FabioEb at quipo.it FabioEb at quipo.it
Fri Dec 21 12:43:53 UTC 2012

Hello, while waiting fot the final doom, or a new
job (tough times here) here is another update of
the work I'm doing, sorry for the looong mail,
hope I'm not boring the readers.

I have a question about a some measurements I
made, and I'd like an opinion about a frontend
schematic I designed.

First the question about the problem:
in this graph (bottom right) there is the time
interval between the PPS from the PA6H GPS
module and the 10MHz form the FE5680A,
uncrambled and corrected for linear drift.
The graph periodically makes big steps, and
this happens in the morning hours, in the
few captures I made so far the fact happens
around 6am-8am in the morning:
another previous capture with span enough to
include 2 mornings:

I will make more tests to check if the problem is in
the gps receiver or in the FE5680 or in the way I'm taking
measurements. This will take a while.
Maybe there is a simple explanation that I cannot see
since I'm a total newbie i this field.
Where should I search first for the problem?

How I'm taking the measurements:
the measurements are taken with the racal 1992
connected to the point OutD in the centre of the
frontend I'm building:
The OutD will spit out a short negative pulse,
the width of this pulse is the same (or very near)
than the phase time interval between the rising
edges of the pps and the 10MHz.
The counter logs the pulse width using a 100MHz
scope probe set to 10x, DC coupled.

The plots are made using this script I wrote for the task :
The (rude) script  tries to unscramble the data, remove
outliers and correct for linear drift.
If it's useful I will upload the raw log data.

The frontend circuit:
As I wrote before I'm trying to feed a microcontroller
with 10MHz from a Rb oscillator and a PPS pulse from
a GPS module and see if I can obtain a good starting
point for building my own GPSDO.
Now I'm testing a front end that will present to the
micro both the PPS and 10MHz nicely squared, and
an analogue representation of the time interval between
the rising pulses of the sources.

What do you think of the circuit I designed?
(thanks to many resources coming from
this list, I passed much time on ko4bb site
and many others I dont even remember, thank you
all!) here the asc file for LTSpice:
I'm using components I had around or scavenged
in scrap electronics I had.
The TAC seem to work, but now I need a better
opamp (the LM358 has too much current
flowing in-out of the inputs), here a pair
of captures taken directly from C1 capacitor, 90nS pulse:
and a 50nS one:
The red trace is the input GPS PPS.

Fabio Eboli.

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