[time-nuts] 1PPS users?

Bob Camp kb8tq at n1k.org
Mon Dec 19 07:06:50 EST 2016


Hi

The vast bulk of all GPSDO’s sold over the last three decades were sold because the first
CDMA specification required that the basestation operate without a GPS signal for
24 hours. During that time the alignment of the Gold Codes is required to be within
10 us of “GPS time”. Once the codes go out of alignment, the cell phone takes an
interference hit, so the the basestation must be shut down. 

The time and code alignment requirement has “evolved” over time. Various 3G, 4G, 5G, 6G ..
systems have tighter timing specs (or apparently none at all). Technology has also evolved.
A self contained GPSDO located at each and every transmitter is no longer the only way to do this. 
We have 1588 and a number of other tools that didn’t exist in the 1980’s. 

Bob

> On Dec 18, 2016, at 11:20 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> Hi Scott,
> I do understand the reasons that these users want this quality of output.  It's who these users are, what fields, industries, etc, that I didn't quite understand.
> Bob -----------------------------------------------------------------
> AE6RV.com
> 
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/GFS-GPSDOs/info
> 
>      From: Scott Stobbe <scott.j.stobbe at gmail.com>
> To: Bob Stewart <bob at evoria.net>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
> Sent: Sunday, December 18, 2016 8:22 PM
> Subject: Re: [time-nuts] 1PPS users?
> 
> Part of the reason 1PPS needs to be so clean is because you are continuously integrating phase noise of the LO (hopefully an OCXO). While 10 uS is pretty trivial off a gps receiver. Without gps, 10 us over 24 hrs with a plane jane AT-cut crystal subject environmental dynamics becomes ludicrous. More than likely the short term stability of GPSDO is cleaner than it needs to be for many applications, but that is so holdover over an hour or day remains reasonable.
> On Sun, Dec 18, 2016 at 6:16 PM, Bob Stewart <bob at evoria.net> wrote:
> 
> One thing I've never really understood is who actually uses the high-quality 1PPS output from a GPSDO.  I have spent a lot of time, effort, and money on developing my GPSDO without a whole of thought to the user base.  It was just a quest for the best result I could obtain with a particular technology.  The frequency standard users was a no brainer.  Everyone who wants a frequency standard eventually understands they need to get a GPSDO, or an Rb, or a Cs.  And that's all I thought I had: a good frequency standard.  And then Tom prodded me a bit and showed me the shortcomings of what I was doing, and I did something about it.  So, if an NTP user can get his time fix directly from a noisy receiver, who actually needs a time-accurate, low jitter 1PPS pulse?
> Bob - AE6RV
>  ----------------------------- ------------------------------ ------
> AE6RV.com
> 
> GFS GPSDO list:
> groups.yahoo.com/neo/groups/ GFS-GPSDOs/info
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