[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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