Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Neil Green]

On 29 Feb 2016, at 10:46 am, Hal Murray  wrote:

> What distro are you starting with?
> 
> I'm using Debian.  Their kernel includes PPS support, both Wheezy and Jessie.

I’m using Raspbian Jessie. PPS over GPIO is now recognised in the stock kernel 
but kernel PPS isn’t. I compile a kernel that disable dynamic ticks and enables 
KPPS.

>> I had a DS3231 lying around so, ...
> 
> Are you setup to make long term measurements?  It will be interesting to see 
> how it drifts.

Measurements will be purely anecdotal, and I most likely won’t start observing 
until I’m happy that I’ve optimised the circuit. This is probably frustrating 
to an experienced time nut, but I lack any sophisticated measuring equipment 
beyond NTP’s loopstats/peerstats/clockstats files.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Hal Murray]

nc...@hotmail.co.uk said:
> and building a custom kernel for the Raspberry Pi to include KPPS

What distro are you starting with?

I'm using Debian.  Their kernel includes PPS support, both Wheezy and Jessie.


> I had a DS3231 lying around so, ...

Are you setup to make long term measurements?  It will be interesting to see 
how it drifts.

I was surprised how easy it was to see the big steps in frequency as it 
corrected for temperature changes.
  http://users.megapathdsl.net/~hmurray/time-nuts/Temp-tcxo32khz-a.png



-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Neil Green]

On 25 Feb 2016, at 9:07 pm, Jim Harman  wrote:

> On Thu, Feb 25, 2016 at 1:08 PM, Neil Green  wrote:
> 
>> I’ve decided to build this:
>> 
>> http://www.hackersbench.com/Projects/1Hz/
> 
> Good start!
> 
> Once you have built it and observed how sensitive it is to temperature
> variations, you might want to take a look at this:
> 
> https://www.adafruit.com/products/3013

So, after some fighting and frustration and sleeping on it, I successfully 
assembled the circuit I linked to above and powered it from a regulated 5V 
breadboard power supply:

http://86.17.250.240/IMG_1750s.jpg

I had a DS3231 lying around so, as per Jim Harman’s suggestion I used its 32K 
output instead of a cheapo 32.768Hz crystal. The results were initially as 
disappointing as I’d expected but, after ensuring that the oscillator *and* the 
32k output of the DS3231 were enabled, and building a custom kernel for the 
Raspberry Pi to include KPPS processing, things improved dramatically. A 
typical output of 'ppstest /dev/pps0':

source 0 - assert 1456678708.00032, sequence: 3205 - clear  0.0, 
sequence: 0
source 0 - assert 1456678709.01034, sequence: 3206 - clear  0.0, 
sequence: 0
source 0 - assert 1456678710.00034, sequence: 3207 - clear  0.0, 
sequence: 0
source 0 - assert 1456678711.01035, sequence: 3208 - clear  0.0, 
sequence: 0
source 0 - assert 1456678712.04033, sequence: 3209 - clear  0.0, 
sequence: 0
source 0 - assert 1456678713.01035, sequence: 3210 - clear  0.0, 
sequence: 0
source 0 - assert 1456678714.00033, sequence: 3211 - clear  0.0, 
sequence: 0
source 0 - assert 1456678715.01035, sequence: 3212 - clear  0.0, 
sequence: 0
source 0 - assert 1456678716.00034, sequence: 3213 - clear  0.0, 
sequence: 0
source 0 - assert 1456678717.01034, sequence: 3214 - clear  0.0, 
sequence: 0
source 0 - assert 1456678718.01034, sequence: 3215 - clear  0.0, 
sequence: 0
source 0 - assert 1456678719.01034, sequence: 3216 - clear  0.0, 
sequence: 0
source 0 - assert 1456678720.00035, sequence: 3217 - clear  0.0, 
sequence: 0
source 0 - assert 1456678721.00034, sequence: 3218 - clear  0.0, 
sequence: 0
source 0 - assert 1456678722.01034, sequence: 3219 - clear  0.0, 
sequence: 0
source 0 - assert 1456678723.02034, sequence: 3220 - clear  0.0, 
sequence: 0
source 0 - assert 1456678724.00033, sequence: 3221 - clear  0.0, 
sequence: 0

As you can see, the numbers are jumping about a bit but they are comparable to 
the GPS PPS output I have running of another Pi. The next stage is to tidy 
everything up, simplify where possible, shrink it and try to squeeze it into an 
enclosure. After that I'll maybe try to get my head around a 10mHz to 1hz 
version.

Thanks to everyone who offered advice. It was all helpful.

Neil.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Jim Harman]

On Thu, Feb 25, 2016 at 1:08 PM, Neil Green  wrote:

> I’ve decided to build this:
>
> http://www.hackersbench.com/Projects/1Hz/
>
> which has an easy to follow diagram. Parts have cost less than £5. I’m not
> expecting great things from it by any means, but it’s something I can get
> my head around and should improve my understanding (a little).
>

Good start!

Once you have built it and observed how sensitive it is to temperature
variations, you might want to take a look at this:

https://www.adafruit.com/products/3013

This breakout is based on the DS3231 chip, which is a temperature
compensated 32 KHz real time clock with a built in crystal and optional
battery backup.  It also has a 1 pps output.

It is reasonably stable and accurate out of the box, but if you want to
adjust it you can send it commands from a microprocessor that tweak its
frequency up and down in increments of about 0.1 ppm (about 3 seconds per
year). It does this by switching in and out tiny on-chip capacitors.


-- 

--Jim Harman
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Neil Green]

On 24 Feb 2016, at 6:50 pm, Hal Murray  wrote:

> How good is your antenna?  How often does your current GPS setup run out of 
> satellites?

It’s a puck antenna placed in a window. I rent, so having an external antenna 
installed unfortunately isn’t an option. I’ve never known it to run out of 
satellites, though.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Neil Green]

On 24 Feb 2016, at 4:30 pm, Chris Albertson  wrote:

> A simple GPSDO is, well, simple.  All you do is compare the phase of
> the PPS to the phase of the 10MHz crystal and then adjust the
> frequency of the crystal to keep the phase difference constant.   Can
> be done all in software by a $2 micro controller chip.  By Time Nut
> standards the performance will not be top notch but compared to your
> NTP server it could literally be 10,000 times better   A while back I
> tried to build the cheapest and simplest GPSDO possible and except for
> the price of the crystal and GPS receiver I spent about $5.
> Performan ce was such that when I put the cheap GPSDO  10MHz output
> and my Thunderbolt both on a dual trace scope I could see some
> movement in phase over hours long time but over days in stays locked
> 
> First divide the 10MHz crystal to say about 1Mhz using a TTL divider.
> Some (most) micro processor chips have a way that an external signal
> can trap the current value of a counter.  Let both the PPS and the
> 1Mhz do this and look at the difference in counts.  Adjust the crystal
> so the difference remains constant.   Don't worry about details, that
> is for latter make it as simple as possible.  Improvement is for
> version #2.

This appeals to me, but I think it’s currently beyond my level of knowledge. 
However, I’ve decided to build this:

http://www.hackersbench.com/Projects/1Hz/

which has an easy to follow diagram. Parts have cost less than £5. I’m not 
expecting great things from it by any means, but it’s something I can get my 
head around and should improve my understanding (a little). 
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Neil Green]

On 24 Feb 2016, at 3:08 pm, David J Taylor  
wrote:

> A BeagleBone Black might give half the jitter compared to the RPi, but not 
> the order of magnitude you may be seeking.
> 
> http://www.satsignal.eu/ntp/BBB-vs-RPi.html
> 
> Perhaps purely for serving the pool, the RPi is more than adequate.

I have a Beaglebone Black lying around. I’ve used it as a time server before 
but I didn’t find it significantly better than the Raspberry Pi. That was a 
while ago, though, so the software may have improved since then. I might have 
another crack at it over the weekend.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Chris Albertson]

My DAC is the ones built into the processor chip.  They are only 8-bits
wide but you can combine two of them by adding First reducing one of them
by a factor of 256 then adding them.  There is a small discontinuity but
software addresses that  Si I get a decent 15-bit DAC for free

As for the counter.  I was wrong.  It runs at full clock speed and it too
is built into the CPU chip.   Using an AVR/Arduino processor

This is actually applicable to a high-end NTP server.   The CPU clock on
the NTP server can be GPS disciplined.  If you do that then NTP has very
little to do as the internal clock will no longer change with the room
temperature.   After all those cheep canned crystals are the reason most
computers need NTP.  So you replace the cheap crystal on you Raspberry Pi
with a GPSDO.   What's the point of a sub uSec NTP server?  None, that is
the nuts part of TN.

On Wed, Feb 24, 2016 at 11:53 AM, Nick Sayer via time-nuts <
time-nuts@febo.com> wrote:

>
> > On Feb 24, 2016, at 8:30 AM, Chris Albertson 
> wrote:
> >
> > On Wed, Feb 24, 2016 at 6:22 AM, Neil Green  wrote:
> >
> >>
> >> What would be my next step up be, hardware-wise, in terms of improving
> precision, stability, etc? A GPSDO? Budget is limited as far as these
> things go - about £150 UK/$210 US.
> >
> > Given that you already have the GPS and PPS, you can build your own
> > GPSDO for very little cost.  The most expensive part is the crystal
> > and as long as you are not going for top of the line performance a
> > cheaper crystal will do. (You asked about the NEXT step not the LAST
> > step)
> >
> > A simple GPSDO is, well, simple.  All you do is compare the phase of
> > the PPS to the phase of the 10MHz crystal and then adjust the
> > frequency of the crystal to keep the phase difference constant.   Can
> > be done all in software by a $2 micro controller chip.  By Time Nut
> > standards the performance will not be top notch but compared to your
> > NTP server it could literally be 10,000 times better   A while back I
> > tried to build the cheapest and simplest GPSDO possible and except for
> > the price of the crystal and GPS receiver I spent about $5.
>
> The BOM for my GPSDO is similar. It’s not quite $5 until you also exclude
> the DAC ($7), but the GPS module and oscillator are well over 90% of the
> price.
>
> > Performan ce was such that when I put the cheap GPSDO  10MHz output
> > and my Thunderbolt both on a dual trace scope I could see some
> > movement in phase over hours long time but over days in stays locked
> >
> > First divide the 10MHz crystal to say about 1Mhz using a TTL divider.
> > Some (most) micro processor chips have a way that an external signal
> > can trap the current value of a counter.  Let both the PPS and the
> > 1Mhz do this and look at the difference in counts.
>
> For mine, the PPS “captures” a 16 bit timer that is clocked directly from
> the oscillator with no prescale. An overflow interrupt extends that timer
> to 32 bits, which is enough for 10 MHz to run for a few minutes before
> overflowing - plenty of time to capture phase deltas of a 1 PPS signal.
>
> From what I have learned about the whole thing so far, the real limiting
> factor is the phase granularity you can get with just a 1 PPS source. If
> the GPS module output, say, a 1 kHz signal, a GPSDO would be a lot easier
> to keep stable at low tau. Mine has a “hump” at sort of 10^2-10^3 tau that
> I can’t really do a lot about given that the only thing I have to
> discipline against is a PPS rising edge. I’m sampling 10 second periods,
> which gives me a base detection threshold of 10 ppb. I track a window of 10
> of those for the visual feedback. There, I can measure 1 ppb, but it takes
> 1000 seconds to do it. My newer version clocks at 20 MHz in an attempt to
> double the granularity, but from what I’ve been able to measure, it doesn’t
> do a whole lot about the “stumbling around” at low-ish (10^1-10^3) tau. The
> performance lower than that is dominated by the oscillator - in my case
> that’s either around 10^-10 or 10^-11 depending on whether you spend the
> extra $100 or not. :)
>
> My understanding is that better GPSDOs are able to provide for more
> granular phase detection. My system is granular to +/- 1 whole cycle per
> second of sampling. Something clocking a lot faster could come up with a
> system for fractional granularity. I could do that by clocking the logic
> faster and then dividing the output - exactly what I do for the 20 MHz
> variant - except that the 8 bit AVRs I’m using at the moment top out at 20
> MHz (at least, they’re not supported higher).
>
> It also doesn’t help me personally that my 53220A sort of peters out
> around 10^-11 anyway. I can’t really distinguish a Thunderbolt from a
> Connor Winfield OH300 based GPSDO, which clearly isn’t reality.
>
> Getting back to the R.Pi stratum 0 NTP daemon, I’ve got a PI Zero in the
> NTP pool as well. As others have pointed 

Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Martin Burnicki]

Pete Stephenson wrote:
> On Wed, Feb 24, 2016 at 3:22 PM, Neil Green  wrote:
>> I currently operate a stratum 1 NTP server in the NTP pool using a U-Blox 
>> Max-7Q GPS module with PPS attached to, variously, a Raspberry Pi via GPIO 
>> or a Celeron mini PC via serial DB-9. The machine does nothing but serve 
>> time to the pool. Operating systems of choice are Debian or FreeBSD.
>>
>>
>> What would be my next step up be, hardware-wise, in terms of improving 
>> precision, stability, etc? A GPSDO? Budget is limited as far as these things 
>> go - about £150 UK/$210 US.
> 
> For what purpose?
> 
> In my experience, NTP is able to sync other systems to within 10-100
> microseconds of an NTP server on a LAN. The vagaries of the internet
> mean that it's unlikely for NTP clients to sync any closer than a few
> tens of milliseconds.

It can be better, but that's not guaranteed. I'm located in Germany and
have a setup measuring the offset of some NTP servers in the U.S.
against a built-in GPS card, and the offset is below 2 ms most of the time.

Martin

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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Jim Harman]

On Wed, Feb 24, 2016 at 2:53 PM, Nick Sayer via time-nuts <
time-nuts@febo.com> wrote:

> I can measure 1 ppb, but it takes 1000 seconds to do it ... My
> understanding is that better GPSDOs are able to provide for more granular
> phase detection.


It doesn't take a lot of hardware to get about 1000x better than that. The
Arduino-based design Lars Walenius posted a while ago can take a reading
every second with a resolution of about 1 nsec. It uses an HC390 divider to
reduce a 10 MHz OCXO to 1 MHz. The 1 MHz and pps pulse from the GPS feed an
HC4046 phase comparator, producing a 1 pps pulse 0-1 usec wide, with the
width proportional to the phase difference. This feeds one diode and an RC
network to make a ramp with its peak height (0 - 2.5V) proportional to the
phase difference. This feeds the processor's 10-bit A/D converter, which is
set for 2.5V full scale.The 1 pps signal triggers the A/D, capturing the
height of the pulse. One more resistor discharges the capacitor between
pulses. Because the discharge time constant is much longer than the
interrupt latency and A/D conversion time, the A/D captures the peak value.

I have been experimenting with this design for some time, with a lot of
learning along the way. Of course there are always potential improvements,
but the current design works surprisingly well. If someone can provide
access to a good set of test gear (I am in CT), I could better quantify the
performance.

My latest schematic is attached.


-- 

--Jim Harman


Arduino GPSDO Schematic v4.pdf
Description: Adobe PDF document
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Bob Camp]

Hi

WWVB DSO’s were a pretty common thing back in the 70’s and 80’s. You could hold 
fractions of a ppm with them. With manual intervention / scheduling you could 
get into 
the “couple ppb” range on a good week. 

Comparative numbers would be 1x10^-11 on a GPSDO. All the same qualifiers about 
“is that really this or that” apply equally (or more so) to it’s WWVB cousin. 

Bob

> On Feb 24, 2016, at 5:22 PM, Hal Murray  wrote:
> 
> 
>> From what others on the list have said before, WWVB offers performance
>> that's at least a couple orders of magnitude worse than GPS, even if you
>> correct for all of the expected diurnal variations in LF propagation. Given
>> that a fairly pedestrian GPS module offers a nominal PPS accuracy of ~10ns
>> for $25... 
> 
> Yes.  But it would be fun to measure the diurnal variations.
> 
> I thought WWVB was actually good at that.  It's ground wave rather than 
> bouncing off the ionosphere.  But I'd like to measure both WWVB and WWV.
> 
> Has anybody built a WWV(B)DSO?  You would need a stable oscillator so you 
> could average over days or weeks rather than hours.  I'm thinking of a 
> surplus rubidium.  Maybe a manual screwdriver adjustment would be good enough 
> and a PIC/AVR to make a PPS and fake GPRMC.
> 
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Bob Camp]

Hi

The tick is a burst of audio at a fairly low frequency. You are going to need 
pretty
good conditions to get 0.1 ms. The fade process over much of the day will 
spread 
that out a *lot*.

Bob

> On Feb 24, 2016, at 5:11 PM, Tim Shoppa  wrote:
> 
> Hal -
>  In my experience over more than a decade, the ntpd WWV audio refclock has
> jitter circa 0.1ms.
> 
>  This is not nanosecond-time-nut PPS territory. But it is more than good
> enough for WAN ntpd.
> 
>  I use a Ten-Tec RX-320 as a cheap frequency-agile receiver for WWV. In
> between 5MHz/10MHz/15MHz usually one or two bands are open from me to
> Colorado.
> 
>  On the subject of WWVB - if you are near Colorado then you can have 24x7
> reachability, day or night. For me on the east coast, WWVB 60kHz is
> reliable in darkness but not in daylight.
> 
> Tim.
> 
> On Wed, Feb 24, 2016 at 1:56 PM, Hal Murray  wrote:
> 
>> 
>> tsho...@gmail.com said:
>>> My opinion if you want to serve reliable time through a longer GPS
>> outage:
>>> add a WWV or WWVB based radio clock. e.g. a shortwave radio and https://
>>> www.eecis.udel.edu/~mills/ntp/html/drivers/driver36.html
>> 
>> Do you have any graphs comparing WWV or WWVB to GPS when your GPS is
>> working
>> correctly?
>> 
>> When I run out of other things on my list, I'd like to collect that data.
>> What receivers do people recommend?
>> 
>> I have one of the low cost WWVB receiver modules but I never got any useful
>> data out of it.  Maybe it's time to try again.  It might work better on the
>> end of a long cable to get it away from all the EMI in my office.
>> 
>> 
>> --
>> These are my opinions.  I hate spam.
>> 
>> 
>> 
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Bob Camp]

Hi

If you are looking at pool service, the first question would be how you connect 
to the backbone. 

If you are running something asymmetric (DSL / cable modem) you already have 
enough of an 
offset that it alone it far bigger than any other error in your system. Even 
with fancier connections,
asymmetry can be an issue. 

Next thing to look at is how heavily loaded your connection becomes. A lot of 
links have limited 
bandwidth in both directions, but traffic loads that are much heavier in one 
direction. It does not
take much single direction buffering to take that up to a significant level.

Bob

> On Feb 24, 2016, at 9:22 AM, Neil Green  wrote:
> 
> I currently operate a stratum 1 NTP server in the NTP pool using a U-Blox 
> Max-7Q GPS module with PPS attached to, variously, a Raspberry Pi via GPIO or 
> a Celeron mini PC via serial DB-9. The machine does nothing but serve time to 
> the pool. Operating systems of choice are Debian or FreeBSD.
> 
> 
> What would be my next step up be, hardware-wise, in terms of improving 
> precision, stability, etc? A GPSDO? Budget is limited as far as these things 
> go - about £150 UK/$210 US.
> 
> 
> I appreciate this is basic stuff compared to the usual discussions but this 
> doesn't seem the right question to ask on the NTP lists. Any help 
> appreciated. Thanks.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Tim Shoppa]

Hal -
  In my experience over more than a decade, the ntpd WWV audio refclock has
jitter circa 0.1ms.

  This is not nanosecond-time-nut PPS territory. But it is more than good
enough for WAN ntpd.

  I use a Ten-Tec RX-320 as a cheap frequency-agile receiver for WWV. In
between 5MHz/10MHz/15MHz usually one or two bands are open from me to
Colorado.

  On the subject of WWVB - if you are near Colorado then you can have 24x7
reachability, day or night. For me on the east coast, WWVB 60kHz is
reliable in darkness but not in daylight.

Tim.

On Wed, Feb 24, 2016 at 1:56 PM, Hal Murray  wrote:

>
> tsho...@gmail.com said:
> > My opinion if you want to serve reliable time through a longer GPS
> outage:
> > add a WWV or WWVB based radio clock. e.g. a shortwave radio and https://
> > www.eecis.udel.edu/~mills/ntp/html/drivers/driver36.html
>
> Do you have any graphs comparing WWV or WWVB to GPS when your GPS is
> working
> correctly?
>
> When I run out of other things on my list, I'd like to collect that data.
> What receivers do people recommend?
>
> I have one of the low cost WWVB receiver modules but I never got any useful
> data out of it.  Maybe it's time to try again.  It might work better on the
> end of a long cable to get it away from all the EMI in my office.
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Hal Murray]

> From what others on the list have said before, WWVB offers performance
> that's at least a couple orders of magnitude worse than GPS, even if you
> correct for all of the expected diurnal variations in LF propagation. Given
> that a fairly pedestrian GPS module offers a nominal PPS accuracy of ~10ns
> for $25... 

Yes.  But it would be fun to measure the diurnal variations.

I thought WWVB was actually good at that.  It's ground wave rather than 
bouncing off the ionosphere.  But I'd like to measure both WWVB and WWV.

Has anybody built a WWV(B)DSO?  You would need a stable oscillator so you 
could average over days or weeks rather than hours.  I'm thinking of a 
surplus rubidium.  Maybe a manual screwdriver adjustment would be good enough 
and a PIC/AVR to make a PPS and fake GPRMC.



-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Nick Sayer via time-nuts]

> On Feb 24, 2016, at 8:30 AM, Chris Albertson  
> wrote:
> 
> On Wed, Feb 24, 2016 at 6:22 AM, Neil Green  wrote:
> 
>> 
>> What would be my next step up be, hardware-wise, in terms of improving 
>> precision, stability, etc? A GPSDO? Budget is limited as far as these things 
>> go - about £150 UK/$210 US.
> 
> Given that you already have the GPS and PPS, you can build your own
> GPSDO for very little cost.  The most expensive part is the crystal
> and as long as you are not going for top of the line performance a
> cheaper crystal will do. (You asked about the NEXT step not the LAST
> step)
> 
> A simple GPSDO is, well, simple.  All you do is compare the phase of
> the PPS to the phase of the 10MHz crystal and then adjust the
> frequency of the crystal to keep the phase difference constant.   Can
> be done all in software by a $2 micro controller chip.  By Time Nut
> standards the performance will not be top notch but compared to your
> NTP server it could literally be 10,000 times better   A while back I
> tried to build the cheapest and simplest GPSDO possible and except for
> the price of the crystal and GPS receiver I spent about $5.

The BOM for my GPSDO is similar. It’s not quite $5 until you also exclude the 
DAC ($7), but the GPS module and oscillator are well over 90% of the price.

> Performan ce was such that when I put the cheap GPSDO  10MHz output
> and my Thunderbolt both on a dual trace scope I could see some
> movement in phase over hours long time but over days in stays locked
> 
> First divide the 10MHz crystal to say about 1Mhz using a TTL divider.
> Some (most) micro processor chips have a way that an external signal
> can trap the current value of a counter.  Let both the PPS and the
> 1Mhz do this and look at the difference in counts.

For mine, the PPS “captures” a 16 bit timer that is clocked directly from the 
oscillator with no prescale. An overflow interrupt extends that timer to 32 
bits, which is enough for 10 MHz to run for a few minutes before overflowing - 
plenty of time to capture phase deltas of a 1 PPS signal.

>From what I have learned about the whole thing so far, the real limiting 
>factor is the phase granularity you can get with just a 1 PPS source. If the 
>GPS module output, say, a 1 kHz signal, a GPSDO would be a lot easier to keep 
>stable at low tau. Mine has a “hump” at sort of 10^2-10^3 tau that I can’t 
>really do a lot about given that the only thing I have to discipline against 
>is a PPS rising edge. I’m sampling 10 second periods, which gives me a base 
>detection threshold of 10 ppb. I track a window of 10 of those for the visual 
>feedback. There, I can measure 1 ppb, but it takes 1000 seconds to do it. My 
>newer version clocks at 20 MHz in an attempt to double the granularity, but 
>from what I’ve been able to measure, it doesn’t do a whole lot about the 
>“stumbling around” at low-ish (10^1-10^3) tau. The performance lower than that 
>is dominated by the oscillator - in my case that’s either around 10^-10 or 
>10^-11 depending on whether you spend the extra $100 or not. :)

My understanding is that better GPSDOs are able to provide for more granular 
phase detection. My system is granular to +/- 1 whole cycle per second of 
sampling. Something clocking a lot faster could come up with a system for 
fractional granularity. I could do that by clocking the logic faster and then 
dividing the output - exactly what I do for the 20 MHz variant - except that 
the 8 bit AVRs I’m using at the moment top out at 20 MHz (at least, they’re not 
supported higher).

It also doesn’t help me personally that my 53220A sort of peters out around 
10^-11 anyway. I can’t really distinguish a Thunderbolt from a Connor Winfield 
OH300 based GPSDO, which clearly isn’t reality.

Getting back to the R.Pi stratum 0 NTP daemon, I’ve got a PI Zero in the NTP 
pool as well. As others have pointed out, there isn’t a whole lot of value to 
be found in attempting to do better over the internet. Attached is the current 
pool monitoring graph for mine. The picture actually is a lot worse than the 
norm - which is represented by the right half of the graph, for the most part. 
Still, you can see that even in the most regular periods, it’s -3 or -4 ms +/- 
1 ms, and that’s with a monitoring station only 300 miles or so away.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Esa Heikkinen]

What would be my next step up be, hardware-wise, in terms of improving 
precision, stability, etc? A GPSDO? Budget is limited as far as these things go 
- about £150 UK/$210 US.


I have Symmetricom TS2100 with OCXO and GPS upgrades as primary server, 
accessible only at local network. It has about +-50 ns nominal PPS 
accuracy. Then there's another server based on Raspberry Pi2, which is 
accessible from Internet. The PPS from symmetricom is fed to Rpi2 and 
the actual time and date information is passed from TS2100 to Rpi2 with NTP.


Rpi2 can serve about 1 NTP queries per second, according my stress 
tests with ntpload.c test program by Kasper Pedersen. For stress test, 
several PC's are needed, each running few threads of ntpload.c, all 
bombarding the server under test simultaneously. Rpi2 has quad core 
processor, but unfortunately it looks that ntpd can utilize only one 
core. With about 1 ntp queries per second there's only one core with 
100% load and others are basicly idling. Good thing is that you can run 
another tasks simlataneously, without noticeable slowdown.


I also joined the NTP pool (only Finnish one) but then I noticed that 
it's was really bad idea to run any NTP server behind router/firewall. 
When there's rush peaks in NTP pool, there will be several hundreds (I 
had more than 600) NTP queries per second and each from different 
address of course. This will create a lot of sessions in router NAT 
table, in my case there was more than 13000 active sessions according to 
conntrack -C. This may overwhelm the router and in any case it will eat 
lot of RAM on the router. The lesson learnt was that NTP server should 
have it's own direct WAN access without any router. That might also 
decrease the delays. This will be my next setup. The Rpi2 NTP server 
itself seems to be very stable and there was no issues when I did the 
stress test with continuous ~1 queries/sec for about one day.


Regards,

--
73s!
Esa
OH4KJU

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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Hal Murray]

tsho...@gmail.com said:
> My opinion if you want to serve reliable time through a longer GPS outage:
> add a WWV or WWVB based radio clock. e.g. a shortwave radio and https://
> www.eecis.udel.edu/~mills/ntp/html/drivers/driver36.html 

Do you have any graphs comparing WWV or WWVB to GPS when your GPS is working 
correctly?

When I run out of other things on my list, I'd like to collect that data.  
What receivers do people recommend?

I have one of the low cost WWVB receiver modules but I never got any useful 
data out of it.  Maybe it's time to try again.  It might work better on the 
end of a long cable to get it away from all the EMI in my office.


-- 
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Hal Murray]

nc...@hotmail.co.uk said:
> What would be my next step up be, hardware-wise, in terms of improving
> precision, stability, etc? A GPSDO? Budget is limited as far as these things
> go - about £150 UK/$210 US. 

How good is your antenna?  How often does your current GPS setup run out of 
satellites?


-- 
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Chris Albertson]

On Wed, Feb 24, 2016 at 6:22 AM, Neil Green  wrote:

>
> What would be my next step up be, hardware-wise, in terms of improving 
> precision, stability, etc? A GPSDO? Budget is limited as far as these things 
> go - about £150 UK/$210 US.

Given that you already have the GPS and PPS, you can build your own
GPSDO for very little cost.  The most expensive part is the crystal
and as long as you are not going for top of the line performance a
cheaper crystal will do. (You asked about the NEXT step not the LAST
step)

A simple GPSDO is, well, simple.  All you do is compare the phase of
the PPS to the phase of the 10MHz crystal and then adjust the
frequency of the crystal to keep the phase difference constant.   Can
be done all in software by a $2 micro controller chip.  By Time Nut
standards the performance will not be top notch but compared to your
NTP server it could literally be 10,000 times better   A while back I
tried to build the cheapest and simplest GPSDO possible and except for
the price of the crystal and GPS receiver I spent about $5.
Performan ce was such that when I put the cheap GPSDO  10MHz output
and my Thunderbolt both on a dual trace scope I could see some
movement in phase over hours long time but over days in stays locked

First divide the 10MHz crystal to say about 1Mhz using a TTL divider.
Some (most) micro processor chips have a way that an external signal
can trap the current value of a counter.  Let both the PPS and the
1Mhz do this and look at the difference in counts.  Adjust the crystal
so the difference remains constant.   Don't worry about details, that
is for latter make it as simple as possible.  Improvement is for
version #2.

The next step up would be something like the Thunderbolt or a much
better DIY GPSDO.






-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Tim Shoppa]

GPSDO's are very nice if you want a bench source for calibration purposes.

On the subject of NTP servers... GPSDO's would be relevant when doing
holdover through a GPS outage measured in days.

My opinion if you want to serve reliable time through a longer GPS outage:
add a WWV or WWVB based radio clock. e.g. a shortwave radio and
https://www.eecis.udel.edu/~mills/ntp/html/drivers/driver36.html

Tim N3QE

On Wed, Feb 24, 2016 at 9:22 AM, Neil Green  wrote:

> I currently operate a stratum 1 NTP server in the NTP pool using a U-Blox
> Max-7Q GPS module with PPS attached to, variously, a Raspberry Pi via GPIO
> or a Celeron mini PC via serial DB-9. The machine does nothing but serve
> time to the pool. Operating systems of choice are Debian or FreeBSD.
>
>
> What would be my next step up be, hardware-wise, in terms of improving
> precision, stability, etc? A GPSDO? Budget is limited as far as these
> things go - about £150 UK/$210 US.
>
>
> I appreciate this is basic stuff compared to the usual discussions but
> this doesn't seem the right question to ask on the NTP lists. Any help
> appreciated. Thanks.
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[Pete Stephenson]

On Wed, Feb 24, 2016 at 3:22 PM, Neil Green  wrote:
> I currently operate a stratum 1 NTP server in the NTP pool using a U-Blox 
> Max-7Q GPS module with PPS attached to, variously, a Raspberry Pi via GPIO or 
> a Celeron mini PC via serial DB-9. The machine does nothing but serve time to 
> the pool. Operating systems of choice are Debian or FreeBSD.
>
>
> What would be my next step up be, hardware-wise, in terms of improving 
> precision, stability, etc? A GPSDO? Budget is limited as far as these things 
> go - about £150 UK/$210 US.

For what purpose?

In my experience, NTP is able to sync other systems to within 10-100
microseconds of an NTP server on a LAN. The vagaries of the internet
mean that it's unlikely for NTP clients to sync any closer than a few
tens of milliseconds.

The Pi is a decent system, but its ethernet port is connected via USB,
so accuracy isn't as good as it would be on a different, non-USB
system. It's quite satisfactory for serving time to the internet using
NTP.

A GPSDO is handy if you need better precision (but the Pi and the
internet can't deliver that better precision, so this is not a major
advantage for NTP) or holdover in case of issues with GPS. Many also
emit a stable reference frequency: this is useful for providing a
common reference for test equipment like frequency counters, or for
radio/microwave systems that need very high stability. If you don't
have such needs, a GPSDO will be of limited use.

> I appreciate this is basic stuff compared to the usual discussions but this 
> doesn't seem the right question to ask on the NTP lists. Any help 
> appreciated. Thanks.

Welcome, and thanks for providing a server in the NTP pool.

Cheers!
-Pete

-- 
Pete Stephenson
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Re: [time-nuts] Next step up from basic GPS/PPS timekeeping

[David J Taylor]

I currently operate a stratum 1 NTP server in the NTP pool using a U-Blox 
Max-7Q GPS module with PPS attached to, variously, a Raspberry Pi via GPIO 
or a Celeron mini PC via serial DB-9. The machine does nothing but serve 
time to the pool. Operating systems of choice are Debian or FreeBSD.


What would be my next step up be, hardware-wise, in terms of improving 
precision, stability, etc? A GPSDO? Budget is limited as far as these things 
go - about £150 UK/$210 US.


I appreciate this is basic stuff compared to the usual discussions but this 
doesn't seem the right question to ask on the NTP lists. Any help 
appreciated. Thanks.

___

Neil,

A BeagleBone Black might give half the jitter compared to the RPi, but not 
the order of magnitude you may be seeking.


 http://www.satsignal.eu/ntp/BBB-vs-RPi.html

Perhaps purely for serving the pool, the RPi is more than adequate.

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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[time-nuts] Next step up from basic GPS/PPS timekeeping

[Neil Green]

I currently operate a stratum 1 NTP server in the NTP pool using a U-Blox 
Max-7Q GPS module with PPS attached to, variously, a Raspberry Pi via GPIO or a 
Celeron mini PC via serial DB-9. The machine does nothing but serve time to the 
pool. Operating systems of choice are Debian or FreeBSD.


What would be my next step up be, hardware-wise, in terms of improving 
precision, stability, etc? A GPSDO? Budget is limited as far as these things go 
- about £150 UK/$210 US.


I appreciate this is basic stuff compared to the usual discussions but this 
doesn't seem the right question to ask on the NTP lists. Any help appreciated. 
Thanks.
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