[time-nuts] Oscillator questions

paul swed paulswedb at gmail.com
Thu Mar 3 03:55:26 UTC 2011


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 <rocket at watzlavick.com>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
>
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