[time-nuts] Absolute phase

Bob kb8tq kb8tq at n1k.org
Fri Nov 17 17:29:15 EST 2017


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. 


> 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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