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

Bob Camp lists at rtty.us
Fri Dec 21 12:51:03 UTC 2012


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.


On Dec 21, 2012, at 7:43 AM, FabioEb at quipo.it wrote:

> 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:
> http://www.flickr.com/photos/14336723@N08/8294131424/
> another previous capture with span enough to
> include 2 mornings:
> http://www.flickr.com/photos/14336723@N08/8294131660/
> 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:
> http://www.flickr.com/photos/14336723@N08/8293076065/
> 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 :
> http://pastebin.com/XmKQp9gR
> 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:
> http://pastebin.com/94H78jxs
> 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:
> http://www.flickr.com/photos/14336723@N08/8293075961/
> and a 50nS one:
> http://www.flickr.com/photos/14336723@N08/8294131200/
> The red trace is the input GPS PPS.
> Fabio Eboli.
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