[time-nuts] UBLOX GPS board testing

[Ulrich Bangert]

Jochen,

> With a Z3815A (Furuno receiver, E1938A oscillator) I noticed 
> the PPS to jump up and down by 2 ns pk-pk in average.

Were the PPS derived from the OCXO's 10 MHz output then 2 ns changes were
the result of 2% frequency changes which is FAR beyond the EFC capabilities
of any OCXO that I know. If it really comes from the E1938A then I suspect a
bad oscillator.

On the other hand 2 ns pk-pk is close to the specs of a good timing receiver
with applied sawtooth correction. So your PPS may not come from the OCXO but
direct from the GPS receiver in conjunction with some software controlled
delay line for the sawtooth correction. 

You can easily check whether this is the case: Leave the Z3815 off for some
time while you leave a second GPS receiver up and running. Immediately after
returning power to the Z3815A start to measure the time intervals between
their PPSs. If the time interval settles to a more or less "fixed" value
(disregarding jitter) in say some seconds, this would be a indicator that my
second guess is right. If in contrast to that the PPS comes from the OCXO
then it is subject to the Z3815A's regulation loop and will you will see a
shift over minutes to hours until it settles to a fixed value regarding the
second receiver.

Best regards
Ulrich Bangert


> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Jochen Frieling
> Gesendet: Mittwoch, 22. Dezember 2010 01:39
> An: time-nuts at febo.com
> Betreff: Re: [time-nuts] UBLOX GPS board testing
> 
> 
> 
> On Tue, 21 Dec 2010 15:26:54 -0600, John Green wrote:
> > I tried substituting the 1 PPS output from the Z3801 and 
> comparing it 
> > to its own 10 MHz output and find the same jumpy behavior as I get 
> > with the UBLOX boards. Well, not exactly the same but 
> pretty much. Now 
> > I am confused. I expected the 1 PPS to be in lock step with the 10 
> > MHz.
> 
> With a Z3815A (Furuno receiver, E1938A oscillator) I noticed 
> the PPS to jump up and down by 2 ns pk-pk in average. On a 
> plot of phase difference to a more stable 10 MHz source, the 
> upper and lower values formed two lines, leading to the 
> assumption of it being quantization steps. (The time interval 
> measurement resolution was significantly better, using a HP 
> 5370B.) I have since dropped the idea of using this PPS for 
> stability measurements by feeding it to the TIC and will use 
> a PPS externally divided down from 10 MHz instead.
> 
> 
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