[time-nuts] What is the real source of the TBOLT's PPS output?

[Bruce Griffiths]

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 <df6jb at ulrich-bangert.de>
> To: Time nuts <time-nuts at febo.com>
> 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 
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