[time-nuts] Absolute phase

[Jerry Hancock]

When I first powered them on together, they seemed to drift all over the place with regards to relative phase.  For the past hour now, they are keeping around 7 degrees and sometimes dropping to around 4.  I think that is pretty amazing given all the stuff between the source and final display.  I have the cursors on them now and they are sitting at 2ns difference.  I am going to figure out how to plot this possibly using Timelab and my HP 5371a.


> On Nov 17, 2017, at 2:29 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
> 
> Hi
> 
> As the “filter” (or control loop) in some GPSDO’s scales, it impacts the degree 
> of “agreement” between the GPSDO’s. Not all GPSDO’s scale filters so not all 
> exhibit the behavior. The HP / Symmetricom  Z38xx units are one family that 
> do this kind of scaling. 
> 
> As the scale moves longer (time) or narrower (frequency) the ADEV improves.
> This also depends a bit on the quality of the OCXO you have and the environment
> the GPSDO is in. Somewhat counter intuitively, the level of agreement gets worse
> as the ADEV improves. As the time agreement gets worse and ADEV gets better, 
> the frequency accuracy (in the frequency nut sense) gets better. 
> 
> Again - not all GPSDO’s do this sort of thing. 
> 
> Bob
> 
>> On Nov 17, 2017, at 5:15 PM, Arthur Dent <golgarfrincham at gmail.com> wrote:
>> 
>> I have 2 antennas mounted on opposite ends of a roof and both of them feed
>> commercial GPS DA/splitters and I can have as many as 10 receivers running
>> at one time for testing. I have also used one of the high frequency type F
>> TV passive splitters with one D.C. feed through and added 200-300 ohm
>> resistors from the other outputs to ground. All this has seemed to work
>> just fine but one of the older receivers apparently radiated its L.O. out
>> the antenna coax and would interfere with a couple of other receivers I
>> connected to the same DA.
>> 
>> 
>> 
>> Connecting one 10 Mhz references to the external trigger on my scope and
>> feeding 2 other GPS receivers to the input channels (all from the same
>> antenna DA), I can watch the slow drift at 2 ns/div with respect to the
>> trigger and sometimes one receiver drift one way as the other receiver
>> drifts in the opposite direction and sometimes they drift the same way.  The
>> drift is generally less than 2 ns but it is there and I assume it depends
>> on what the internal ‘housekeeping’ of the receiver is doing and what birds
>> they are using. So bottom line, they aren’t ‘locked’ to each other but are
>> generally close.
>> 
>> 
>> -Arthur
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