[time-nuts] pps vs. 10 MHz timing

SAIDJACK at aol.com SAIDJACK at aol.com
Fri Dec 12 22:12:29 UTC 2008


Hello Matt, Magnus,
 
unfortunately there will be a phase shift between the 1PPS and the  10MHz 
rising edge due to the nature of how we generate the 1PPS pulse from the  clean 
OCXO 10MHz signal.
 
But this trade-off allows us to offer the 1PPS phase-shift option via  the 
SERV:1PPS command.
 
We can simply shift the 1PPS by 16.66ns steps with the software command,  
which would not be possible if we had to align the signal to the 10MHz, 100ns  
period.
 
The system works by multiplying the 10MHz signal up to 60MHz using a PLL,  
then using a 60E6-to-1 divider with arbitrary phase alignment (selected by the  
user) to generate the clean 1PPS output. This allows a resolution of 16.66ns 
on  where to place the clean 1PPS using the serv:1pps command.
 
As Magnus has suggested, if you need a strict phase alignment, then using  
FF's to latch the 1PPS rising edge with the 10MHz signal can phase-align the two 
 signals. Due to metastability being possible, metastable-hardened FF's (from 
NXP  etc) should be used. Alternatively an external 10E6-to-1 divider could 
also be  used for this, clocked by the 10MHz signal. There would still be at 
least a  clock-to-output phase shift when using FPLD's etc to implement this  
divider.
 
BTW: by using the serv:1pps command for the coarse setting, and the  GPS 
antenna-delay command for fine-setting the user can place the 1PPS  pulse anywhere 
with 1ns resolution on the Fury GPSDO. The coarse 16.66ns steps  will be 
instantaneous, the 1ns antenna-delay steps will take some time for the  PLL to 
shift.
 
Hope that helps,
bye,
Said
 
 
 
 
In a message dated 12/12/2008 12:33:25 Pacific Standard Time,  
magnus at rubidium.dyndns.org writes:

Matt  Ettus skrev:
> On my Oscilloquartz GPSDO, the 10 MHz output goes low at  very close to
> the same time as the 1 PPS output goes high.
>  
> On my Fury, the 10 MHz sine wave is just dropping off of its max  high
> voltage as the 1 PPS goes high.  The 10 MHz CMOS output goes  high just
> shortly before the 1 PPS.
> 
> What do other  GPSDOs do?

You can expect anything. Some buffers the PPS from the GPS  engine and 
then make some more or less elaborate 10 MHz training  algorithm. Don't 
expect rising edges to coinside for all of  them.

> Clearly, any device trying to latch the 1 PPS signal using  the 10 MHz
> clock will need to choose which clock edge to use,  depending on which
> type of GPSDO is used.  How is this normally  handled?



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