Bruce Griffiths wrote: > Ulrich > > The Thunderbolt manual makes it clear that the PPS output is divided down > from the 10MHz OCXO output via the CPU and its support circuitry... > If the CPU has an internal PLL and no external resynchronising flipflop, then > it is possible that the rms PPS jitter may be as high as 300ps or so... A > gate array with insufficient ground and Vcc pins could also have a similar > output jitter. The jitter produced by whatever circuitry is used to shape the > OCXO output into a logic level square wave can also contribute significant > jitter if the signal amplitude is relatively low at the input to the shaper. > > The solution for your purposes is to use a resynchronising flipflop to remove > this jitter before making your phase error measurements within your external > OCXO discipling circuitry. > > Bruce > > > Addendum:
Are you using linear or switching power supplies to power the Thunderbolt? If you are using switching supplies, can you power it with linear supplies and measure the ADEV /jitter characteristics (PPS vs 10MHz)? Bruce > > ________________________________ > From: Ulrich Bangert <[email protected]> > To: Time nuts <[email protected]> > Sent: Saturday, 24 January, 2009 4:21:10 AM > Subject: [time-nuts] What is the real source of the TBOLT's PPS output? > > Gents, > > considered an OCXO with an OAVAR of or better say 1.0E-11 @ 1 s (as used > in the TBOLT) and a divider chain to generate an 1PPS from the 10 Mhz > oscillator signal. What OAVAR @ 1 s would we expect if we compare the 10 > MHz to the PPS derived from it? > > I know, the answer is not given easily, but: If the divider chain is > engineered correctly (i.e. a synchronous and not a ripple divider) then > we are basically comparing the 10 Mhz with a delayed version of itself. > The delay will be the typical clock-to-output propagation delay of a > d-flipflop of the dividers chain's semiconductor family. > > Since we are basically measuring the propagtion delay of a semiconductor > I would expect to measure an OAVAR that is almost independend from the > OAVAR of the 10 MHz clock and which is dominated by the jitter that is > typical for this semiconductor family. In other words: I would rather > expect to measure a LOWER OAVAR than to measure an OAVAR that is higher > than that of the 10 MHz clock. > > Would I measure a SIGNIFICANT HIGHER OAVAR than that of the 10 MHz > clock, this were an indication that the divider chain (beneath the delay > introduced by it) would contribute a significant amount of instability > to the PPS. I encourage you to perform such an comparing measurement on > your TBOLT. Use the PPS output to start a TIC and the 10 MHz output to > stop the TIC and record the results. > > I did so and received the results to be seen in TBOLT_External.PDF. @ 1 > s the OAVAR is 3.3E-10 which is an almost unbelieveable bad value when > one takes the 1.0E-11 @ 1 s of the 10 MHz clock into account. If THIS > PPS were derived from the 10 MHz then Trimble's engineers had done a > very bad job on the TBOLT. Which is not what I believe! But then: If it > is NOT derived from the OCXO, where does it come from then? > > Is it perhaps the PPS coming from the receiver ??????? Well, considered > that a high-grade implementation of an GPSDO with a M12+ may be able to > produce a GPS PPS with an stability of 2.0E-9 @ 1 s (inclusive > SAW-correction), then 3.3E-10 @ 1s were an incredible GOOD value for the > TBOLT's receiver. Can that be????? This encouraged me to have a look > again to the measurements that have been discussed in another thead: > TBOLT-internal measurments as reported with the TBOLT monitor program. > TBOLT_Internal.PDF shows the results of such an internal measurement > while the TBOLT was in the disciplined state. @ 1 s we notice an OAVAR > of 2.1E-10 which is even a tick better than the external measured > 3.3E-10. > > This similarity makes me believe that what comes from the PPS output of > the TBOLT is basically the GPS PPS and NOT ONE DERIVED FROM THE 10 MHZ. > Unfortunately the manual makes no absolute clear statement about this > question but there is one clue of which I believe it seconds my opinion. > > The manual says about the PPS: > > BNC Connector TTL levels into 50 ohm 10 microseconds-wide > pulse with the leading edge synchronized to GPS or UTC within > 20 nanoseconds (one sigma) in static, time-only mode. > > That is: The synchronisation of the PPS to GPS/UTS is dependend ONLY > from the receiver's time-only working mode, nothing else, especially NOT > from the state of the PLL controlling loop. If the PPS were derived from > the 10 MHz, then everything described about the regulation loop, i.e. > the whole PPS pulse shifting action to start with an low offset of +/- > 50 ns with and the disciplining starting after that would lead IMHO at > least SOMETIMES to effects that the internally generated PPS may be far > without the window specified. If Trimble can specify this, it MUST be > the true GPS PPS! > > On the other hand, if this were true we could turn off all our GPS > receivers and instead supply every GPS-PPS that we need from the TBOLT, > which should be better an factor of almost 10! > > Your opinions on that topic are welcome as always! > > Best regards > > Ulrich Bangert > www.ulrich-bangert.de > Ortholzer Weg 1 > 27243 Gross Ippener > _______________________________________________ > 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.
