Re: [time-nuts] 1PPS users?

[Bob Camp]

Hi


> On Dec 19, 2016, at 11:55 AM, David J Taylor  
> wrote:
> 
> From: jimlux
> 
> And pretty much any of these will ingest a 1pps + message, rather than,
> say, a 10 MHz reference.
> 
> Why not just use the raw GPS 1pps (which is typically good to a few tens
> of ns)?  GPS coverage/outage and random variation in the GPS 1pps
> 
> A lot of them use a disciplined oscillator to generate the 1pps, so the
> oscillator is essentially a long term averager/low pass filter on the
> raw GPS output -
> =
> 
> Having the GPSDO/PPS is handy when someone parks outside your location and 
> carries a GPS jammer.

Unless the jammer causes the GPS receiver to go a bit nuts and the GPSDO is 
dumb enough to 
follow it. Don’t ask how I know about this particular situation :)

Bob

> 
> Cheers,
> David
> -- 
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-tay...@blueyonder.co.uk
> Twitter: @gm8arv 
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Re: [time-nuts] 1PPS users?

[Bob Camp]

Hi

> On Dec 19, 2016, at 10:11 AM, Tom Van Baak  wrote:
> 
>> I believe the power industry is using GPS provided time to stamp power
>> events for analysis as well.
> 
> Chris,
> 
> Yes, in fact one of the first applications of hp's SmartClock was just that:
> 
> "Accurate Transmission Line Fault Location Using Synchronized Sampling"
> http://leapsecond.com/hpan/an1276-1.pdf

There is an FCS paper that pre-dates that by about a decade. The guys at
Quebec Hydro set up GPS to monitor the phase state in their main distribution
network and wrote the paper. I don’t have an FCS DVD handy to dig it up. I
also have no idea if there is a copy outside a pay wall. They went into a bit
more detail on what they saw. It gives you a bit better feel for how it works 
in the real world. 

Bob


> 
> 
> Bob,
> 
> For more background on 1PPS and time synchronization see:
> 
> "GPS and Precision Timing Applications"
> http://www.hpmemoryproject.org/an/pdf/an_1272.pdf
> 
> "The Global Positioning System and HP SmartClock"
> http://www.hpl.hp.com/hpjournal/96dec/dec96a9.pdf
> 
> Other useful hp app notes for time nuts:
> http://leapsecond.com/hpan/list.htm
> 
> /tvb
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Re: [time-nuts] 1PPS users?

[David J Taylor]

From: jimlux

And pretty much any of these will ingest a 1pps + message, rather than,
say, a 10 MHz reference.

Why not just use the raw GPS 1pps (which is typically good to a few tens
of ns)?  GPS coverage/outage and random variation in the GPS 1pps

A lot of them use a disciplined oscillator to generate the 1pps, so the
oscillator is essentially a long term averager/low pass filter on the
raw GPS output -
=

Having the GPSDO/PPS is handy when someone parks outside your location and 
carries a GPS jammer.


Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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Re: [time-nuts] 1PPS users?

[Tom Van Baak]

> I believe the power industry is using GPS provided time to stamp power
> events for analysis as well.

Chris,

Yes, in fact one of the first applications of hp's SmartClock was just that:

"Accurate Transmission Line Fault Location Using Synchronized Sampling"
http://leapsecond.com/hpan/an1276-1.pdf


Bob,

For more background on 1PPS and time synchronization see:

"GPS and Precision Timing Applications"
http://www.hpmemoryproject.org/an/pdf/an_1272.pdf

"The Global Positioning System and HP SmartClock"
http://www.hpl.hp.com/hpjournal/96dec/dec96a9.pdf

Other useful hp app notes for time nuts:
http://leapsecond.com/hpan/list.htm

/tvb
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Re: [time-nuts] 1PPS users?

[jimlux]

On 12/19/16 6:28 AM, Chris Caudle wrote:

On Sun, December 18, 2016 10:20 pm, Bob Stewart wrote:

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.


Magnus wrote a paper recently on effects of the GPS time slip (recent
error in data upload) on the broadcast industry which uses GPS time to
synchronize widely separated transmitters so that they can overlap their
broadcast coverage areas without interference.

I believe the power industry is using GPS provided time to stamp power
events for analysis as well.




And pretty much any of these will ingest a 1pps + message, rather than, 
say, a 10 MHz reference.


Why not just use the raw GPS 1pps (which is typically good to a few tens 
of ns)?  GPS coverage/outage and random variation in the GPS 1pps


A lot of them use a disciplined oscillator to generate the 1pps, so the 
oscillator is essentially a long term averager/low pass filter on the 
raw GPS output -

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Re: [time-nuts] 1PPS users?

[Chris Caudle]

On Sun, December 18, 2016 10:20 pm, Bob Stewart wrote:
> 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.

Magnus wrote a paper recently on effects of the GPS time slip (recent
error in data upload) on the broadcast industry which uses GPS time to
synchronize widely separated transmitters so that they can overlap their
broadcast coverage areas without interference.

I believe the power industry is using GPS provided time to stamp power
events for analysis as well.

-- 
Chris Caudle


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Re: [time-nuts] 1PPS users?

[Tim Shoppa]

You know, there's a reason this list is called "time-nuts" and not
"frequency-nuts" :-).

But sometimes I wonder if "phase-nuts" might be a better term.

It is so incredibly useful to put your best 1 PPS into a scope and use that
to watch for systemic effects on your second-best clock. That's why we then
end up with a third, fourth, fifth, etc. clock :-)

So many newcomer GPSDO makers seem to just do frequency lock but rarely do
they or their users know the difference.

At same time there are so many non-ntpd NTP implementations that just jump
instead of slew the clock.

Tim N3QE

On Sun, Dec 18, 2016 at 6:16 PM, Bob Stewart  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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Re: [time-nuts] 1PPS users?

[Bob Camp]

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 <b...@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.sto...@gmail.com>
> To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency 
> measurement <time-nuts@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 <b...@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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> 
> 
> 
> 
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Re: [time-nuts] 1PPS users?

[Esa Heikkinen]

Hi Bob,


One thing I've never really understood is who actually uses the high-quality 
1PPS output from a GPSDO.


I use it for NTP server, like many others also do around the world.

--
73s!
Esa
OH4KJU
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Re: [time-nuts] 1PPS users?

[Michael Wouters]

One application is advanced network diagnostics eg cards like this:
https://www.endace.com/endace-dag-high-speed-packet-capture-cards.html

So for a 40 GbE card, time-stamping 1 kilobyte packets demands
sub-microsecond accuracy, if you want to compare at different points
in your network.

Cheers
Michael

On Mon, Dec 19, 2016 at 10:16 AM, Bob Stewart  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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Re: [time-nuts] 1PPS users?

[Bob Stewart]

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.sto...@gmail.com>
 To: Bob Stewart <b...@evoria.net>; Discussion of precise time and frequency 
measurement <time-nuts@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 <b...@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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Re: [time-nuts] 1PPS users?

[Hal Murray]

preilley_...@comcast.net said:
> I would like to get better than the +-10 nS that the better receivers
> provide. 

A GPSDO generally avoids the sawtooth offset.


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] 1PPS users?

[Scott Stobbe]

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  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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Re: [time-nuts] 1PPS users?

[Peter Reilley]

There was a discussion here a while ago about synchronizing radio 
telescopes that
were separated by some miles.   The 1PPS from GPS was suggested as a 
possibility.


I am working on a project to do location by triangulation that uses the 
1PPS signal.
I would like to get better than the +-10 nS that the better receivers 
provide.


Pete.


On 12/18/2016 6:16 PM, Bob Stewart 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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Re: [time-nuts] 1PPS users?

[John Ackermann N8UR]

For amateur use, PPS comparison requires less equipment and can be more 
accurate than trying to measure RF rates like 10 MHz.


When comparing two PPS signals, phase slips are very infrequent so you 
can observe drift rate over minutes/hours/days with an oscilloscope or 
simple time interval counter to get much better resolution than most 
frequency counters can provide.  (Of course, a GPS works as well for 
this as a GPSDO, albeit with more short term jitter.)


John

On 12/18/2016 08:33 PM, Bob Stewart wrote:

Hi Jim,
Thanks Jim,

So, what I'm seeing so far, assuming I'm interpreting it correctly, is big 
budget commercial and government applications, generally clustering around 
time-controlled multiplexing, as well as the niche that is the space industry.  
Then there's the hobbyist, such as Eric who wants to control a Fedchenko clock, 
or similar type of application, such as whatever sort of spread spectrum that 
ham radio may morph into, or perhaps the low S/N EME guys.  Any others?
Bob

   From: jimlux <jim...@earthlink.net>
  To: time-nuts@febo.com
  Sent: Sunday, December 18, 2016 6:56 PM
  Subject: Re: [time-nuts] 1PPS users?

On 12/18/16 3:16 PM, Bob Stewart 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?


Anyone who needs to trigger events at a precise time or log the
occurrence of an event uses the 1 pps - the serial port (or other
interface) gives you the "at the tone the time will be" message, and the
edge of the 1pps is the "tone".


I've got several systems flying in space (or soon to fly in space) that
use the 1pps from GPS to calibrate their internal clocks and/or to
provide an absolute time reference (along with the aforesaid time message).

For example, one needs to have your carrier frequency within a certain
tolerance for communications with the ground stations: you can either
fly a precision oscillator with an oven (big, heavy, high power) or you
can measure a not-so-precision oscillator (small, cheap, low power)
against a 1pps, and adjust your frequency that way.

I grant you that this is really more like *building* a GPSDO than
*using* a GPSDO.

I've used the 1pps from a GPSDO as a common trigger to synchronize
timing and timestamping for separate systems.



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Re: [time-nuts] 1PPS users?

[Bob Stewart]

Hi Jim,
Thanks Jim,

So, what I'm seeing so far, assuming I'm interpreting it correctly, is big 
budget commercial and government applications, generally clustering around 
time-controlled multiplexing, as well as the niche that is the space industry.  
Then there's the hobbyist, such as Eric who wants to control a Fedchenko clock, 
or similar type of application, such as whatever sort of spread spectrum that 
ham radio may morph into, or perhaps the low S/N EME guys.  Any others?
Bob

  From: jimlux <jim...@earthlink.net>
 To: time-nuts@febo.com 
 Sent: Sunday, December 18, 2016 6:56 PM
 Subject: Re: [time-nuts] 1PPS users?
   
On 12/18/16 3:16 PM, Bob Stewart 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?

Anyone who needs to trigger events at a precise time or log the 
occurrence of an event uses the 1 pps - the serial port (or other 
interface) gives you the "at the tone the time will be" message, and the 
edge of the 1pps is the "tone".


I've got several systems flying in space (or soon to fly in space) that 
use the 1pps from GPS to calibrate their internal clocks and/or to 
provide an absolute time reference (along with the aforesaid time message).

For example, one needs to have your carrier frequency within a certain 
tolerance for communications with the ground stations: you can either 
fly a precision oscillator with an oven (big, heavy, high power) or you 
can measure a not-so-precision oscillator (small, cheap, low power) 
against a 1pps, and adjust your frequency that way.

I grant you that this is really more like *building* a GPSDO than 
*using* a GPSDO.

I've used the 1pps from a GPSDO as a common trigger to synchronize 
timing and timestamping for separate systems.



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Re: [time-nuts] 1PPS users?

[Chris Albertson]

Who uses 1PPS?  In industry they are used to phase lock various
oscillators.   I would bet most of those oscillators are used in the
telecommunications industry both for bit rate clocks and for carrier
frequency synthesis.

We also see a lot of 1 PPS used for NTP servers that in turn are used to
keep computer internal time of day clocks running at the correct rate.



On Sun, Dec 18, 2016 at 3:16 PM, Bob Stewart  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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-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] 1PPS users?

[Eric Scace]

Speaking as an owner of a well-behaved mechanical clock:
Clock rate performance over time can be done based on time interval 
measurements anchored to a solid frequency standard. No 1pps needed per se.
Bring a clock to time is simplified with a timescale-accurate 1pps source.

> On 2016 Dec 18, at 18:16 , Bob Stewart  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

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Re: [time-nuts] 1PPS users?

[Bob Camp]

Hi

Roughly 99% of all GPSDO’s are only used for the PPS output. They go into cell 
sites and the reason
they exist is to sync up the Gold Codes on CDMA. That’s also why you see a 
*lot* of PPS only Rb’s
on the surplus market. The ones that don’t get used for cell towers, mostly go 
into other com systems 
that for some reason need to have accurate time (also involves codes …).

Bob

> On Dec 18, 2016, at 6:16 PM, Bob Stewart  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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Re: [time-nuts] 1PPS users?

[jimlux]

On 12/18/16 3:16 PM, Bob Stewart 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?


Anyone who needs to trigger events at a precise time or log the 
occurrence of an event uses the 1 pps - the serial port (or other 
interface) gives you the "at the tone the time will be" message, and the 
edge of the 1pps is the "tone".



I've got several systems flying in space (or soon to fly in space) that 
use the 1pps from GPS to calibrate their internal clocks and/or to 
provide an absolute time reference (along with the aforesaid time message).


For example, one needs to have your carrier frequency within a certain 
tolerance for communications with the ground stations: you can either 
fly a precision oscillator with an oven (big, heavy, high power) or you 
can measure a not-so-precision oscillator (small, cheap, low power) 
against a 1pps, and adjust your frequency that way.


I grant you that this is really more like *building* a GPSDO than 
*using* a GPSDO.


I've used the 1pps from a GPSDO as a common trigger to synchronize 
timing and timestamping for separate systems.




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Re: [time-nuts] 1PPS users?

[Hal Murray]

b...@evoria.net said:
> 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?

Most kernels have an option to capture a time stamp from a PPS signal at 
interrupt time.  That is much more accurate than the timing you get from user 
mode on a serial data stream.

There is also a mode where the whole NTP PLL processing is done in the 
kernel.  I don't see why that should make as much of a difference as it does, 
but I haven't tracked down the details.  (It's not in the typical Linux 
kernel.  You have to build your own.)


Most low cost GPS receivers have crappy timing on the serial port.  Really 
crappy.  It wanders with a time scale of hours so you can't filter out the 
jitter by averaging for a minute or two.  PPS on that sort of unit makes it 
much more interesting.


-- 
These are my opinions.  I hate spam.



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