[time-nuts] Thunderbolt question regarding PPS and 10MHz phase relationship

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Sep 19 23:53:18 UTC 2009


Bob

One way of reducing the effective noise/jitter on the PPS signal when
using it with the 5370 to measure the instability of a source with
respect to the Thunderbolt is to use the PPS to ARM the 5370 whilst the
10MHz from the thunderbolt starts the 5370 and the source (or a
frequency generated by dividing down the source under test) being
compared stops the 5370. This reduces the measurement noise by an order
of magnitude.

Bruce

Robert Darlington wrote:
> Ah ha!  That explains the changing numbers I see on the 5370B (jitter), and
> explains why I see no change on the scope other than the jitter.  Reading
> the manual, I don't see any clear explanation like what you just gave me.  I
> see my mistake in manual interpretation but not until after reading your
> reply.  Thank you.
>
> -Bob
>
>
>
> On Sat, Sep 19, 2009 at 5:18 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
>   
>> wrote:
>>     
>
>   
>> Bob
>>
>> Robert Darlington wrote:
>>     
>>> Hello all,
>>>
>>> I'm trying to adjust the time between the leading edge of the PPS square
>>> wave and the 10MHz zero crossing outputs on a Trimble Thunderbolt.  The
>>> manual seems to indicate the cable delay compensation setting will do
>>>       
>> this.
>>     
>> No, it doesnt.
>> This only adjusts the timing of the output PPS signal with respect to
>> the GPS time as seen by the receiver.
>> It is only intended to compensate for delay in the cable (and
>> potentially the active antenna group delay) between the antenna and the
>> receiver.
>> The PPS output is synchronous with the 10MHz signal and the delay from
>> the preceding zero crossing of the 10Mhz signal to the leading PPS edge
>> cannot be altered.
>> The PPS signal has considerable jitter (~300ps rms ) with respect to the
>> 10MHz signal zero crossing.
>>     
>>> As I understand things, this is also called the "PPS Offset" in the
>>>       
>> manual.
>>     
>>> I've changed this from 58ns to -58ns with no visible change over the
>>>       
>> course
>>     
>>> of a 12 hour period on a scope.  My test setup is using an old Tek TDS
>>>       
>> 210
>>     
>>> scope triggering on the PPS on channel 1 through a 50 ohm terminator,
>>> trigger set to 400mV and horizontal position adjusted so that 400mV
>>>       
>> crosses
>>     
>>> the center vertical graticule marking.   Channel 2 is attached to the
>>>       
>> 10MHz
>>     
>>> signal through another pass-through 50 ohm terminator.  What I see on the
>>> screen looks like 15ns delay between the triggered PPS signal and the
>>>       
>> zero
>>     
>>> crossing 10MHz signal.  My 5370B shows 18ns delay between them when the
>>> start input level is set for 400mV.
>>>
>>> The manual indicates that it could take a while to see a change, but how
>>> long is a while?  My tbolt has default settings for everything except the
>>> PPS Offset value (again, now sitting at -58ns).  I expected to at least
>>>       
>> see
>>     
>>> some change over the course of 12 hours or so, but I get nothing.  Am I
>>> completely misunderstanding something?  I accidentally cycled the power
>>> after making the settings change after the tbolt was up for a couple of
>>> months.  I learned to not use a normal serial cable with an APC ups, it
>>>       
>> cut
>>     
>>> the power on the UPS immediately upon plugging it into the back of my PC!
>>> Settings were verified after power was brought back up.  Total downtime
>>>       
>> was
>>     
>>> perhaps 10 seconds, but that was enough for it to lose almanac data.
>>>
>>>
>>> Thanks,
>>> Bob, N3XKB
>>>
>>>       
>> Bruce
>>
>>
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>>     
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