I sure do not have an answer for you to your question. Though it would seem that if you have a good ref like the Tbolt be done with it. Those are some impressive pictures, detail, and work. Regards Paul WB8-TSL
On Wed, Mar 2, 2011 at 10:19 PM, Robert Watzlavick <[email protected]>wrote: > Hi, > I'm new to the list and I'd appreciate some advice regarding my > application. One of my hobbies is building and testing liquid fuel > rocket engines (http://www.watzlavick.com/robert/rocket). As part of my > instrumentation system, I have some Datum 9550 Video Time Overlay units > which stamp time from IRIG-B onto NTSC video from various cameras > positioned around the static test stand. I also have a Datum bc635PCI > card which generates the IRIG-B signal and outputs a TTL start pulse to > synchronize the data acquisition system so I can associate a particular > video frame with the measured data. The data system uses National > Instruments hardware (PXI-6030E, SCXI-1520) and can accept a start > trigger and a reference clock. I discovered that the accuracy of the > built-in 10 MHz reference clock in the PXI chassis is sufficiently > different from the flywheeling bc635 card that even over a few minutes, > they can drift by up to tens of milliseconds. That sort of negates the > goal of lining up the video with the data so I routed the 10 MHz out > from the bc635 into the PXI chassis CLKIN to lock them together. That > seems to work fine as even after several hours, the data and video are > still synchronized. When I did that experiment, I didn't have any > "good" 10 MHz reference sources to compare against other than the PXI > chassis which is rated at +/- 25 ppm and 5 ps RMS of jitter. The bc635 > has a VCXO which probably isn't that good either. I looked at the 10 > MHz out from the bc635 and it looked pretty clean and stable so I > assumed it was good enough. > > A few months ago, I decided I needed a better 10 MHz reference source > because I was trying to calibrate some pressure transducers against a > Paroscientific DigiQuartz which has a frequency output as a function of > pressure. On a related note, I had always wanted a GPS-based NTP server > of my own (just because!) so I picked up a Datum TymServe TS2100 w/GPS > from eBay. As an added bonus, it has a GPS-synchronized IRIG-B output I > could use for the rocket testing and a disciplined 10 MHz I could use > with my HP5316A and the Paroscientific. All good, or so I thought, > until I compared the 10 MHz out from the TS2100 to the OCXO in the > 5316A. When locked to GPS, the TS2100 jumps all over the place relative > to the 5316A. I know it's the disciplining algorithm because when I > turn it off, it's actually pretty stable compared to the 5316A. I see > the same thing when I compare the ref clock out from the bc635 to the > 5316A on a scope - it seems to be correcting once a second but the phase > is all over the place, sometimes moving 2-3 cycles before it reverses. > I'm not sure how to quantify it but I'm wondering whether the jitter > from the TS2100 or bc635 will be enough to screw up the reference clock > on the PXI chassis for the data system. The data sheet on the PXI-1042 > chassis says it uses a PLL to phase lock to an incoming clock so maybe > it dampens out the jitter and it's not a problem. > > But, I'm the type that likes to look at the details so I bought a > Thunderbolt from eBay and I was able to confirm it is definitely the > TS2100 and bc635 that has the issue. No amount of playing with the gain > and filter settings in these units cuts down on the phase shifts so I'm > guessing it's a byproduct of the cheap VCXO they use. I was thinking of > upgrading both of those units to an OCXO to improve the stability, an > MTI-240 for the TS2100 and an MTI-210 for the bc635. The boards appear > to be pin compatible with these units so they should just work. I have > the schematic for the bc635 and it looks straightforward to change it > out but on both units I'll have to reprogram the OCXO gain settings each > power cycle because I can't figure out how to make them stick in the > EEPROM. I've talked to the MTI rep and he is going to see if he can > waive the $1500 min order requirement for me. No idea how much the > oscillators will cost, probably more than I want to pay. Has anybody > successfully upgraded the oscillators in these units? > > For now, running the entire data system locked to GPS isn't necessarily > a requirement (cool but not really needed) so I could use the 10 MHz > from the Thunderbolt to run into the bc635 and on to the PXI chassis. > I'd just have to manually set the time in the bc635 and let it > flywheel. Another option is to jam-sync the bc635 to GPS and then let > it flywheel during the test so it's 10 MHz output doesn't jump all over > the place. > > Sorry for the long post but I'd appreciate any comments or advice you > could offer an apprentice time-nut. > > Thanks > -Bob > K5RLW > > _______________________________________________ > 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.
