Hi 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. Bob On Dec 21, 2012, at 7:43 AM, [email protected] 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. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
