Re: [time-nuts] ✘NEO-M8N vs. NEO-M8T

[Gregory Maxwell]

On Mon, May 21, 2018 at 5:54 PM, Gary E. Miller  wrote:
> Also, how does that get me to the gola of a good PPS to feed into the
> Linux PPS kernel module?  I doubt Linux would accept a patch to put
> gpsd, and more, into the kernel to read GPS and adjust the PPS.

Considering that sawtooth error is something found in virtually every
GPS receiver I've previously been somewhat surprised that the linux
PPS stuff did not have support for it.

My best guess is that the magnitude of sawtooth error is just not
large enough to matter for typical applications of linux PPS.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Furuno GT-8031 breakout board

[Gregory Maxwell]

Hi Bob,

If you do a group buy of GT-8736 I'm game for 10.

On Thu, Feb 15, 2018 at 6:55 PM, Bob Darlington  wrote:
> I bought some of the Furuno GT-8736 boards for $35 a pop, qty 1 (or 2 as
> was the case).  And was quoted at $26.91 a pop if I buy 100.   If there's
> interest, I'm happy to coordinate a group buy at cost.  Just paid off that
> credit card yesterday so why not?   If there are more desirable boards from
> Furuno, let me know and I'll see what I can do.
>
> -Bob
>
> On Thu, Feb 15, 2018 at 3:59 AM, ew via time-nuts 
> wrote:
>
>> Talking about Furuno has any one looked at the GT-87. I have known about
>> it for years but found no way to buy some now DigiKey has them for $ 100,
>> Buerklin in Germany for half that price. Saw tooth is +- 1.7 nsec we are
>> not going to bother with correction. We are in the process to lay out a
>> board any recommendations are appreciated
>> Bert Kehren
>>
>> In a message dated 2/15/2018 12:39:48 AM Eastern Standard Time,
>> hol...@hotmail.com writes:
>>
>>
>>  I just did a small adapter board that converts the 2x5 pin 2mm header on
>> the Furuno GT-8031 to a 1x9 pin 0.1" connector with the pinouts of the
>> Adafruit Ultimate GPS. There a couple of minor pin differences... the
>> Adafruit FIX pin is not used and the Adafriuit 3.3VREG output pin is used
>> as the Furuno antenna power connection. The Furuno can be soldered to the
>> board or a female connector could be installed.
>>
>> I'm getting ready to order a few boards. If interested, contact me off
>> list.___
>> time-nuts mailing list -- time-nuts@febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>> mailman/listinfo/time-nuts
>> and follow the instructions there.
>>   1 Attached Images
>>
>> ___
>> time-nuts mailing list -- time-nuts@febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/
>> mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Why discipline Rubidium oscillator?

[Gregory Maxwell]

On Mon, Nov 20, 2017 at 8:28 PM, Jerry Hancock  wrote:
> Bob, I was referring to the rubidium standard of 6834682610.904 Hz.  For some 
> reason I thought it was closer to 9Ghz.
>
> I assume then rubidium standards oscillate (if that is the correct term) 
> somewhere around that number but not exact or is it in the detection where 
> things fall down?

I think you are confused by the difference between primary and
secondary standards.

A typical rb gas cell is a secondary standard.  Its exact frequency is
distorted by a number of factors like gas pressure, interaction with
the cell walls, and ambient magnetic fields which cannot be canceled
by the design of the standard.  This is why it is useful to discipline
a telecom rb against GPS, disciplining can be accomplished through
control of a biasing magnetic field.

Something like a cesium beam standard is able to internally cancel
most of these biases "under standard conditions".  A drift free
frequency source can also be constructed using rubidium, such as
rubidium fountains just as a secondary standard could be constructed
using cs-- like cs gas cell standards (such as the sa.45 CSAC).

[Hopefully I haven't mangled things].
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

[time-nuts] Novatel Dual frequency GNSS receivers on ebay

[Gregory Maxwell]

There is an ebay listing for "Novatel GPS-702-GG with SPAN-CPT Single
Enclosure GNSS/INS Receiver + Cable" with a fairly large number
available.

This is a Novatel OEM628 dual frequency receiver (supports GPS,
Glonass, SBAS, apparently including L1C and L2C), plus a three fiber
ring gyros (with bias performance that blows away any mems gyro I've
ever used) and an 3-axis mems acceletrometer in an aluminum case, plus
a decent dual frequency antenna.  This is a generation-ish old kit.
The industrial casing conspires to make it look somewhat less modern
than it actually is.

The receivers have external clock input (though not plumbed to the
outside of the case) which appears to work though I didn't try much
with it yet. Mine came with 2013-ish firmware but easily upgraded to
current (2016) firmware. There is a windows based firmware update tool
which talks to it over serial and is very straight forward (The
firmware update OEM6631.zip can be found via google).

You can communicate with them over serial in ascii, there is extensive
firmware documentation that goes over every command
https://www.novatel.com/assets/Documents/Manuals/om-2129.pdf  some
of which are specific to other modules. There is also a separate
manual for the inertial navigation specific features (NovAtel SPAN-CPT
Users manual.pdf)

The external clock should allow you to hang it off a more stable
oscillator which will improve the stability of the GNSS results, and
_I presume_ improve the quality of the PPS output-- the firmware
manual and operating manual are thin on details, and mostly just go
into telling you how to adjust the kalman filter constants for
different clock types.

These also appear to support the novatel 'align' mode where you serial
connect two receivers separated by a short baseline and get really
accurate absolute headings; I'm planning on trying that that but
haven't set it up yet.

Looks like uber (last position was ubers offices in denver) had a
fleet of these things. The couple I got run great, including the IMU,
the antennas obviously spent a long time outside, but work fine. The
cable they come with is weird, but I had no problem chopping one end
off and figuring out the pinout (see bottom).

The novatel OEM6 is well supported by rtklib and I was able to get
post-processed positions very easily.

Seller takes best offers a fair amount below the $649 asking price.
Looks like they may have another 30 or so of them.

May be useful for doing time transfer especially with the clock input.
Just using it to get nice dual band observations to precisely survey
an antenna location for a traditional GPSDO may improve GPSDO
performance by a fair amount.

Here is the signals and wire colors on the cables mine came with.
YMMV, I'd suggest not blindly trusting that colors match on other
units.These cables don't plumb out many of the signals from the
module (in particular, they don't carrying COM2, which is why I
haven't tried multi-receiver headings yet, since I'd need to figure
out how to talk to it over USB if com1 is in use for that), I'm unsure
if they're wired through the to external connector.

01 white  power return (-)
02 brown  9-18 VDC power input (+)
03 yellowCOM1 RS232 TX
05 pink   COM1 RS232 RX
09 green  COM1 GND
10 black  USB D+
11 purple USB D-
12 yellow brnstp  USB GND
15 redODO SIGA
16 blue   ODO SIGA-inv
29 grey pinkstp   PPS (high resistance? 80 ohm)
30 whitw grnstp   Event1
31 red blustp signal ground
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

[time-nuts] The Witching Hour; short fiction that will likely amuse time-nuts

[Gregory Maxwell]

Found on the web:

http://slatestarcodex.com/2013/11/03/the-witching-hour/
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Optical Cesium or maybe Cesium "light"!

[Gregory Maxwell]

On Sat, Mar 18, 2017 at 2:48 AM,   wrote:
> Looks like Oscilloquartz is getting ready to sell this commercially!

http://www.chronos.co.uk/files/pdfs/itsf/2015/day2/1410_High_performance_optically-pumped_cesium_beam_clock-PBerthoud-Oscilloquartz.pdf

Two year old deck with a fair amount of overlap, though a few more
engineering diagrams.

Heres to hoping they fit it with an unreliable LCD backlight so
they'll be available as cosmetically defective cheap surplus in the
future. :)
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] hm H Maser

[Gregory Maxwell]

On Wed, Jan 11, 2017 at 12:25 AM, Bob kb8tq  wrote:
> I have a pile of stuff. You have a pile of stuff. Others each have their pile 
> of stuff. Doing
> a design that works only with my pile is possible. Doing a design that works 
> with my pile
[...]
> You have to do it with a fairly standardized
> design. That means buying (at the very least) kits of parts. Like it or not, 
> the parts kit for a
> Rb will be cheaper than the parts kit for any of the other devices…..

I read the occasional posts by PHK on his efforts to upgrade the
electronics in his 5065a and Corby's SUPER physics package upgrade
with great interest.  I have wondered if the end result may be that
incremental upgrades to someone elses classic design, adding on modern
synthesizers and digital control, etc. Might eventually result in a
'Ship of Theseus' oscillator, which in its final form is buildable
from relatively easily sourced parts (plus perhaps a rubidium cell
that could be group bought at non-absurd prices).

Presumably taking an already established design and improving it
incrementally has lower risk and costs than a new design. In
particular, it can start off with 5065a as "my pile" inputs, but by
the end it doesn't have them anymore... and not just lest risky but
also a more natural way to divide the effort up into less
professionally-sized chunks.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Line Voltage - USA

[Gregory Maxwell]

On Mon, Jan 2, 2017 at 4:49 AM, Bill Byrom  wrote:
> Most US homes and small businesses are powered by what is commonly
> called a "split-phase" 240 V feed. The final distribution system
> transformer has a 240 V center-tapped secondary. The center tap is
> grounded, and three wires are fed to the building (actually it might be
> up to around 6 houses):
> (1) Leg L1 or phase A (red wire) -- This wire will measure 120 V to the
> neutral or 240 V to Leg L2.
> (2) Neutral (white wire) -- This wire is grounded at the distribution
> system and at the service entrance to the building.
> (3) Leg L2 phase B (black wire) -- This wire will measure 120 V to the
> neutral or 240 V to Leg L1.

When someone here previously mentioned observing high voltage, one
possible cause for this in this common "split-phase" configuration  is
that if the neutral wire is overloaded, damaged, poorly connected, or
otherwise has high resistance,  the voltage on the two legs will swing
wildly and in opposite directions depending on load.

So, e.g. if you put a 1kw load on L1 while L2 is nearly unloaded then
perhaps L1s voltage drops to 108v while L2 rises to 132v.

The reason for this is that, e.g. imagine that the neutral were
removed completely you would effectively be connecting your appliances
in a parallel-series circuit (all on L1 in parallel, all on L2 in
parallel, the both in series) across the 240v feed.

I've had issues with neutrals several times in the past, and in one
instance, temporarily dealt with it by moving as much of the load to
240v as I could,  manually balancing the remaining loads, and then
using a digital multi-meter to dynamically control some additional
load to keep the voltage sane on each side.

I think the fact that you can end up with a much higher voltages at
the outlet if the neutral has problems is one of the more unfortunate
properties of the split-phase approach.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Google public NTP service

[Gregory Maxwell]

On Wed, Nov 30, 2016 at 2:21 PM, Michael Rothwell  wrote:
> ... was just announced.
> https://cloudplatform.googleblog.com/2016/11/making-every-leap-second-count-with-our-new-public-NTP-servers.html?m=1

Obvious outcome is obvious. Leap smear prevented faults between google
systems but then created the problem that other things don't agree
with google's timestamps-- and the leapseconds still cause problems
for many other parties.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

[time-nuts] Sapphire whispering gallery mode masers?

[Gregory Maxwell]

The recent mention of WGM sapphire oscillators and recent threads
about time-nut constructable secondary frequency standards reminded me
of a paper I ran into a while back about a WGM maser.

The mechanical of the device were similar to an ordinary WGM
oscillator: cryocooled sapphire crystal in a vacuum. But
electronically it was pretty different, a dopant in the crystal could
be RF pumped at some frequency far away from one of the normal high-Q
modes of the oscillator and formed a three-stage laser whos emission
bandwidth included the normal (~10GHz) high-Q WGM, which it would then
happily lase at.  They reported that the signal power was
significantly higher than a AHM.

The advantage of the construction is that the maser level is
controlled by saturation, and so it was not very sensitive to the
intensity stability of the pump.

It seemed to me that it might be a possible candidate for a difficult
but achievable 'home' experiment for an oscillator with exceptional
short to medium term stability in a way that something like a hydrogen
maser isn't.  (Then again, I've never worked with anything cooler than
LN2).

Unfortunately I can't find the paper, but I doubt I dreamed it.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Tbolt issues

[Gregory Maxwell]

On Fri, Sep 2, 2016 at 12:51 AM, Charles Steinmetz
 wrote:
> Now shorten the observation time to 20nS.  We see 1/5 of a complete cycle
> (72 degrees, 0.4 pi radians) of the wave.  No matter which particular 72
> degrees we see, we simply don't have enough information to reliably deduce

I do not see why you argue that.

For the purpose of discussion, lets assume you have a noiseless signal
which is stationary in frequency and amplitude over 20nS starting at
the zero crossing. Given these strong priors (single tone, constant
frequency which is not higher than one half cycle in our 20nS window,
constant amplitude, noiseless) there is exactly one frequency
consistent with any of those two observations.  If the starting phase
is unknown, I believe you need one additional observation to end up
over-determined and have an unambiguous solution again.

This kind of strong prior assumption is why sinusoidal estimators and
PLLs are able to extract tones with precision far beyond what you
would expect from taking a DFT from equivalent amount of data.

In reality, there is phase noise, non-linearities, harmonics, tidal
variations, and whatnot that make these assumptions untrue... but how
far they corrupt these assumptions depends on how useless the results
are.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

[time-nuts] Ashtech Z-12 firmware?

[Gregory Maxwell]

I recently picked up a couple non-functional Ashtech Z-12 receivers
with the external frequency input (these appear to PLL an internal
TCXO; I understand there are other models where they replace the TCXO
completely).

I was hoping to use them both better data sources for common view time
transfer with better performance than the GPS L1/CA  that I've been
using; as well as using them for ground truth data for a SDR
semi-codeless CRPA receiver I'm slowly building.

These units were hanging at startup at "Downloading channel". I
determined that they would work a single time if the memory were
cleared (by holding an arrow, right for external up for internal,
during start). Because the internal 3.6v primary cell batteries were
reading about 40mv I took a guess that the low battery was causing
memory corruption across restarts.

New batteries and a bit of soldering later, and they are working
reliably across restarts, locking sats on L1/L2, talking on RS232,
etc. (hurrah).  Hopefully this message in the archives is useful if
someone else encounters similar symptoms.

Before I go further I was wondering if anyone knew of a source for
firmware for these old units?  One of mine is marked "Z-12 700751-6
(b)", unfortunately it seems ashtech has been sold many times, and the
old web/ftp sites have been scattered to the wind.

The nearest I was able to find was this page
http://kb.unavco.org/kb/article/astech-micro-z---firmware,-firmware-release-notes,-firmware-upload-programs-and-how-to-use-them-368.html
 with micro-Z firmware.

[If there is a relevant GNSS nuts list, I'd be glad to hear about it--
my interest is in CVTT but perhaps somewhere else there is a den of
people obsessed with old survey hardware that would prefer these
questions.]
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] NUT4NT: Four-channel All-frequency GNSS RF-to-Bits

[Gregory Maxwell]

On Sat, Aug 13, 2016 at 11:53 PM, Bob Camp  wrote:
> Hi
>
> It’s not real clear what the second chip on the board does. If it is just a 
> bit to Ethernet converter
> then you are dealing with 2 bit data out of each of the four channels. You 
> aren’t just doing tracking
> in that case ….4 channels at 2 bits -> 8 bits per clock. Clock at 38 to 100 
> MHz. That could turn out
> to be a very crowded Ethernet connection.

I assume it's like the GNSS firehose (
http://pmonta.com/blog/2012/06/04/gnss-firehose/ ) or the other
existing GNSS SDR receivers: The device samples some bandpass around
the relevant GNSS frequency(/ies), sends a digitized signal for you to
despread and track.

2bits * 4ch * 20MHz * 2 (nyquist) = 320Mbit/sec, no big deal for
gigabit ethernet or USB3 (or even USB2 for that matter, at least
ignoring overheads).

>From there you can have arbitrarily complex processing on the host--
but from that setup you can't easily produce a PPS signal-- the host
will operate asynchronously with the signal, with lots of delay and
jitter.

I've long though a better design for a GPSDO is instead of having the
GPS produce a PPS, you have the GPS contain a TIC and feed it a PPS,
and capture those timestamps along with the gps observations.  The GPS
solution would then also include the observed offset.  Beyond
eliminating the extra complexity of sawtooth correction, this could
produce better results under some signal conditions because averaging
error signal produced from single fixes will not always produce the
same result as running a larger model over more observations (because
the errors can be multimodal). (besides-- for precise timing against
an local atomic standard... you probably don't want to perturb the
local clock, but instead track corrections to some reference time.)

In either case (PPS out or PPS in) I believe some actual hardware
support is needed to tie the PPS pulses to the GPS sampling clock, in
any of these SDR GPS approaches... though I think not much.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] NUT4NT: Four-channel All-frequency GNSS RF-to-Bits

[Gregory Maxwell]

On Thu, Aug 4, 2016 at 11:08 PM, Daniel Mendes  wrote:
>
> A new interesting toy soon to be crowdsourced:
>
> https://www.crowdsupply.com/amungo-navigation/nut4nt

A shame, it looks like it can be externally clocked, but I don't see a
way to get in and measure a PPS signal.

Considering the increasingly frequent jamming I see doing a multiband
CRPA timing receiver sounds like a fun project; but I'm not quite
seeing how to do really overkill timing with this device. :)

I guess on the NT1065 you'd want use the clk with a counter to capture
the position of a PPS or external reference with respect the the
sample clock?... and hopefully the signal processing chain from clock
to the ADC has a constant delay?
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Leap second to be introduced at midnight UTC December 31 this year

[Gregory Maxwell]

On Thu, Jul 21, 2016 at 5:27 PM, Tom Van Baak  wrote:
> Time to mention this again...
>
> If we adopted the LSEM (Leap Second Every Month) model then none of this 
> would be a problem. The idea is not to decide *if* there will be leap second, 
> but to force every month to have a leap second. The IERS decision is then 
> what the *sign* of the leap second should be this month.

I think dithered leapsecond would be a massive improvement over what
we have now, I'd even pay for travel to send someone to advocate it at
whatever relevant standards meeting was needed.

But I still think it is inferior to no leapseconds at all (and
adjusting timezones after the several thousand years required to make
that needed).  The reason I hold this is three fold:

(1) The complexity to deal with leapseconds remains in LSEM. It won't
be as much of a constant source of failure but correct handling is a
real cost that goes into the engineering millions of
products/projects. Some of the current practical methods of handling
leap seconds (like shutting off/rebooting critical systems or
discarding data) would also be less reasonable with LSEM. They might
be less needed with LSEM but I cannot say that they would be
completely unneeded*.

(2) I'm not aware of any application that cares greatly about the
precise alignment with the _mean_ solar day that doesn't need to go on
and apply location specific corrections.  Those applications could
easily apply the UTC to mean solar adjustment along with their
location specific adjustment.

(3) Obtaining the leap second sign requires a trusted data source.
When UTC has leap seconds a happily ticking atomic clock cannot keep
second level alignment with UTC without some trusted source of data.
Existing mechanisms for distributing leap second information have
communicated false information in several well known events in the
past, and these were accidents not malicious cases. LSEM would improve
this somewhat, since there would always be an update you just need to
learn the sign, so communications failures could be detected and
alarmed. But the need for this trusted input still creates a security
vulnerability window that could be avoided. Systems where their
security/integrity depend on time sync, could be pushed 24 seconds out
of sync in one year by an attacker that can control their leap
observations-- creating error greater than their free running
oscillators might have absent any sync at all.  This is especially
acute in that in a decade or two we might have 1PPB solid state
optical clock chips which are inexpensive and allow for "set once and
run forever without sync" operation for a great many applications --
getting potentially 0.5 ppm of error added on top from the lack of
leap seconds would basically limit the civil time performance of
unsynchronized clocks what you can get from a TCXO bought off mouser
for a couple bucks today.

So: It's my experience that the current handling of leap seconds is a
slow motion disaster. LSEM would be a big improvement-- reducing the
costs to the "intended costs" by eliminating many of the extra costs
created by inadequate testing. But it would not eliminate the base
cost of continuing to have our civil time perturbed by leap seconds to
begin with-- costs that are only increasing as atomic oscillators come
down in price and applications with tight synchronization requirements
become more common.


(*) once a month is not adequate testing to ensure freeness of fringe
race conditions. Meaning that at a minimum large scale life or large
value handling systems that might be impacted would still get the
reboot treatment in LSEM, but now with disruption every month.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] LEA-M8T

[Gregory Maxwell]

On Fri, Apr 8, 2016 at 12:06 AM, Tom Van Baak  wrote:
> Meanwhile it would not surprise me if each GNSS system gives a slightly 
> different position and a slightly different time than GPS does.

One could easily imagine a system that when signals from both
constellations were strong it fit a model matching a secondary system
to a primary.  Then when the primary was degraded/unavailable used the
secondary data with the model-- which would hopefully fix any bias
between the systems-- assuming that it's fairly stable. But that would
be a fairly specialized mode of operation.

If the solutions were pooled with weighing and knowledge of their
process errors adding more shouldn't make it worse... but might not
add anything: the ideal weight might be zero.

I wonder how much of these negative experience with glonass are due to
non-calibrated antenna: Glonass does frequency division multiplexing,
and so the antenna's angle/frequency dependent phase can harm
performance... many GPS antenna greatly attenuate the glonass signal
too.

For time-nuts operation where the GPS is conditioning one (or more)
nice OCXO or atomic clocks in a world where tens of gigaflops of CPU
are among the least expensive toys we can buy, I imagine that fairly
different signal processing approaches would be ideal: e.g. instead of
just computing second by second solutions and putting the error into a
PLL, one could collect hours of observation data and produce after the
fact correction data that uses prior assumptions about the stability
of the local oscillator.  Such a processing mode could be more robust
against disturbance from multipath and SV failure, as that data could
be excluded as hopelessly inconsistent compared to modes that didn't
assume a stable local clock and didn't use potentially hours of
observation that watched the SV move across the whole sky.

It's somewhat annoying that this kind of experimentation first would
require something like getting a software GPS implementation working
well; when you just want to try changing around secondary processing
after all the coorelators and such.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

[time-nuts] Springer textbooks >10 years old now available for download as PDF at no cost

[Gregory Maxwell]

The Quantum Beat
http://link.springer.com/book/10.1007/978-1-4757-2923-8

Frequency Standards and Metrology
http://link.springer.com/book/10.1007/978-3-642-74501-0

Frequency Measurement and Control
http://link.springer.com/book/10.1007/3-540-44991-4

(and, many many other books on control theory, and other time-nuts
relevant works.)
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] GPS Disciplined TCXO

[Gregory Maxwell]

On Fri, Oct 23, 2015 at 11:08 PM, Nick Sayer via time-nuts
<time-nuts@febo.com> wrote:
>
>> On Oct 23, 2015, at 2:09 PM, Gregory Maxwell <gmaxw...@gmail.com> wrote:
>>
>> On Tue, Oct 20, 2015 at 8:53 PM, Bryan _ <bpl...@outlook.com> wrote:
>>> Saw this on the Hackaday site if anyone is interested.
>>> https://hackaday.io/project/6872-gps-disciplined-tcxo
>>
>> Will this design that uses the output of the DAC directly not run into
>> problems with non-monotonicity and/or dead-zones in the DAC output?  I
>> would expect a PLL to behave very poorly if there is any
>> non-monotonicity in the least significant bit of the DAC.
>
> The datasheet claims the DAC is inherently monotonic. It’s a $7 part, so I 
> don’t have much reason to look sideways at that claim.

Indeed!  However, the spec sheet shows (e.g. figure 10) a differential
non-linearity of 0.2 .. -0.2  LSB,  meaning that when the PLL makes a
single step the result may be 20% greater or lower than expected,
which probably isn't good for stability though not the PLL
breaking-ness of a non-monotone response.

> That strikes me as familiar - a little like how Arduino fakes analog output 
> by running PWM into an LPF.

It's a common technique, (it and ones like it) also used internally in
high bit depth DACs.

> If you look at the AD5061 datasheet, there is unfortunately a relatively 
> significant (to my eyes, at least) update glitch. I suppose it’s quick enough 
> that the RC filter would get rid of most of it, but it is an extra noise 
> source if you do it frequently, like you’re suggesting.

Ouch, that is a fairly substantial spike compared to 1lsb... it's
short at least, but if you are only updating once a second I'd wonder
if that would not have a measurable impact on stability.

A potential advantage of running at a constant high rate is that
rather than taking the impact of that glitch once per second, the
glitch happens constantly and so its effect can just be averaged out
by the PLL.  (e.g. it becomes equivalent to just scaling the output
voltage by its average effects).
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] GPS Disciplined TCXO

[Gregory Maxwell]

On Tue, Oct 20, 2015 at 8:53 PM, Bryan _  wrote:
> Saw this on the Hackaday site if anyone is interested.
> https://hackaday.io/project/6872-gps-disciplined-tcxo

Will this design that uses the output of the DAC directly not run into
problems with non-monotonicity and/or dead-zones in the DAC output?  I
would expect a PLL to behave very poorly if there is any
non-monotonicity in the least significant bit of the DAC.

My intuition would be to use TPDF dither against a higher resolution
internal value (perhaps 32 bits) with noise shaping so that the dither
has no power at DC (and not much near it), followed by a RC low-pass
filter. With this approach the fact that available DAC parts run at
speeds far faster than needed for this control helps overcome the fact
that highly monotone parts are less common (last I looked).  This
would also help give much finer control without compromising tuning
range.

[Disclaimer: I've never built a GPSDO]
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] KS-24361 REF-0 standalone

[Gregory Maxwell]

On Fri, Aug 7, 2015 at 6:06 PM, paul swed paulsw...@gmail.com wrote:
 Looking forward to the notes.
 Yes it could be fairly simple if what ref 0 wants is a string that
 essentially says the system is fixed with 3 d accuracy. Perhaps after that
 the ref 0 makes no checks other then the string keeps coming with the
 correct quality. Not to push a particular proc but any of the low end ones
 will do that stunt very easily.
 That would be pretty sweet.

Does it also take and apply sawtooth correction data?  It would be
pretty great to e.g. be able to use the sawtooth data that comes out
of the modern ublox recievers (even the non-timing models).
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] End Of The World

[Gregory Maxwell]

On Wed, Jul 1, 2015 at 1:03 AM, Bob Camp kb...@n1k.org wrote:
 Hi

 So are we all still here? Any portion of the group blasted into non-existance 
 by the leap second please speak up :)

 ===

 Any observations of anomalous behavior yet?


I was eagerly connecting to various things to watch for problems when
everything stopped responding precisely at the end of the leapsecond.

Turned out my cell phone-- an old android 2.3 device on the sprint
network-- dropped off the network (looked like it was up but couldn't
make or receive calls and no data was getting through).  I waited 15
minutes to see if it would recover on its own, but it did not during
that time.

OTOH if it went out for 15 minutes otherwise I might not have thought
to much of it-- so no proof that it was due to the leapsecond; very
coincidental timing if not. :)
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Using CPLD/FPGA or similar for frequency

[Gregory Maxwell]

On Tue, Jun 9, 2015 at 11:40 AM, Alan Ambrose alan.ambr...@anagram.net wrote:
 How about a 1pS resolution TIC? :)

 Or a 12 digit frequency counter? :) :)

 It's not a proper time-nut project unless there's a nutty element...

Well, how complex? Front end with a fast ADC and make a DSP DMTD device?

In terms of simpler things that (AFAIK) one can't go out and buy:  a
TIC with 4 or 8 inputs would be an interesting piece of time nut
gear.even if it was 'just' 1ns resolution

Surplus lab TICs are easily had but become quite a pile of equipment
when you want to concurrently measure a half dozen oscillators.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Current state of optical clocks and the definition of the second

[Gregory Maxwell]

On Tue, Jan 13, 2015 at 5:46 PM, Dr. David Kirkby (Kirkby Microwave
Ltd) drkir...@kirkbymicrowave.co.uk wrote:
 I had a brief read. Equation 1 made me wonder what could be achieved
 with a cheap HeNe laser. It should be fairly easy to mix a couple of

See Sams Laser Faq section on stabalized HeNe Lasers:
http://www.repairfaq.org/sam/laserchn.htm#chnsshnl1

This uses zeeman splitting to get two different polarization modes
lasing at slightly different wavelengths. (this is described in more
detail elsewhere in the FAQ about some commercial lasers that use this
effect.)

There are do it yourself at home grade (/ easily available surplus
parts) things you can do to get the short term linewidth down to about
4MHz or so, sadly 4MHz out of 470THz is only 1e-8 or so, so not super
competitive with some off the shelf OCXO.

You're also then stuck with an optical standard, and the down
conversion to microwave is decidedly more complex. It would probably
be a fun time-nuts project, even given the not amazing performance, if
not for the difficulty in down-converting to something that could feed
a counter.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Current state of optical clocks and the definition of the second

[Gregory Maxwell]

On Mon, Jan 12, 2015 at 12:34 PM, Attila Kinali att...@kinali.ch wrote:
 I just stumbled over this [1] nice article by Fritz Riehle that might be
 of interest to others as well.

I've seen less discussion of non-atomic stable optical oscillators.
Most (all?) of these optical atomic standards are passive atomic
clocks and need a free running oscillator.

Seems that the state of the art in stabilized lasers has improved a
lot lately, e.g. there are commercial available 1550nm devices which
have a =3Hz line-width: http://stablelasers.com/products.html (well
on a short term basis, the medium term performance is not so
impressive)

Considering the rarity and extreme cost of H-masers, or just really
exceptional quarts oscillators; might it be the case that optical LOs
start looking interesting for applications which just need stability
(or being steered by other sources; e.g. GPSDL)?
(They can be down-converted to microwave frequencies using an optical
comb; a mode-locked laser whos pulses are phase locked to an incoming
beam.)

Certainly just the local oscillator is _closer_ to something a
time-nut might experiment with than a complete optical atomic standard
(if still not quite in reach).
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] June 30 2015 leap second

[Gregory Maxwell]

On Fri, Jan 9, 2015 at 3:21 PM, Martin Burnicki
martin.burni...@burnicki.net wrote:
 Systems which are simply time clients can receive the leap second warning
 via the usual protocols like NTP or PTP/IEEE1588.

Indeed, they can. Even when there hasn't been a leap-second.
Practically all internet (and otherwise?) time distribution is
unauthenticated, the leap second itself is unauthenticated.

It's fragile enough that there have been accidental false leap-second events.

... one of many reasons I'd prefer leap seconds went away though I've
personally had great fun observing them in the past. (and, I suspect,
they may have been one of the first reasons I became interested in
precise time keeping).
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Fwd: CGSIC: FW: New NANU 2014090

[Gregory Maxwell]

On Wed, Dec 17, 2014 at 12:01 AM, Jim Lux jim...@earthlink.net wrote:
 On 12/16/14, 3:36 PM, Magnus Danielson wrote:
 what about one of the software receivers? I would think that making  L2 and
 L5 filters isn't that tough, so all you need is the back end.

I'm pretty sure GNSS-SDRLIB supports L2C,  you can see it working with
a ~$400 bladerf

http://www.rtl-sdr.com/real-time-gps-positioning-bladerf/

Though getting dual band, in phase, requires something like a $1100
USRP B210... and that that point you're getting in to the price-points
where you can find surplus survey receivers and such.

From a timenuts perspective, it may be more interesting to go the SDR
route, since the survey receiver is not likely to have a lot of
affordances for timing applications and there is a lot of interesting
things you could do in software.

There have been some people talking about GNSS targeted SDRs, e.g.
things with 4x  in phase down mixers and backing ADCs that could
simultaneously capture every current/known GNSS signal and send them
down to the computer, at a reasonably low price... but I think none
have moved past the prototyping phase.

The nearest I'm aware that seems actually available is
http://www.onetalent-gnss.com/ideas/software-defined-radio/sdrnav40
but I don't really know anything about it. (e.g. if it's real, what
the software support story is, etc.)
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] 58503a and Yixunhk

[Gregory Maxwell]

On Sun, Dec 14, 2014 at 1:38 AM, Adrian rfn...@arcor.de wrote:
 Same here. I have a Z3805A from this vendor that works flawlessly, and I
 know of other people that purchased from him without any problems.

 To call it a cam when a HP unit comes with a remanufactured box is quite
 a harsh statement, IMHO.

 I'm glad they are putting those ugly units into a neat and practical box
 that is made with attention to every detail.

Almost $800 for a box that arrived full of rust (and faulty to boot)
is a bit crazy though.

I bought a z3805a (in an ugly old rackmount case) from this seller for
$180 a while back and it works great. I expected it to be beaten up
old surplus stuff, and it was but still quite functional. I wouldn't
be shocked if thats whats getting remade here.

I did so after researching the seller somewhat (he's been mentioned on
time-nuts a few times before) and knowing what I was getting into...
e.g. that it would be some surplus with some amount crap-shoot
potential, but that he was also experienced in selling these things.

Opened it up and checked for rust, because water damage had been
specifically cited in some other post.

Indeed, when I got it, I first tossed it on a somewhat inadequate
power supply and had it not come up... he responded quickly and
competently to my message and offered a replacement if it was bad.

All that said, I'd think I'd be pretty torqued if I'd bought the $800
box and got a box of rust. There is something unethical about making
somethings external condition not match the internal condition and
selling it for a high price... unless it's clearly marked as a
re-manufacturing of surplus hardware.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] gravity, space and time

[Gregory Maxwell]

On Fri, Dec 12, 2014 at 8:42 PM, folkert folk...@vanheusden.com wrote:
 Hi,

 If I understood it well, we should occasionally encounter gravitational
 waves going through, well, the whole galaxy. As time and space are
 intertwined, those ripples may be measured somehow I guess.
 Isn't this that we as time nuts community can help the scientific
 world with? E.g. create some kind of grassroots effort where our very
 accurate clocks can detect this? I can imagine all kinds of reasons
 that existing infra for this may not always be able to detect this on
 its own.
 What do you think?

The waves fall of with distance just as other undirected radiations
do.  As a result they should be incredibly weak when we observe them
(if not, we have worse concerns!).

There are observatories working on detecting these with incredibly
sensitive equipment.  Search for LIGO  for an example.

Even assuming we all had H-masers at home (I wish... anyone know what
a VCH-1008 costs? is it too much to dream that it has a small price to
match its small size?), I'm not aware of any way we could usefully
measure gravitational waves, even ignoring their weakness, just due to
a lack of precise time transfer with enough time resolution.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Beaglebone NTP server

[Gregory Maxwell]

On Wed, Dec 10, 2014 at 9:58 PM, Brian Lloyd br...@lloyd.aero wrote:
 Well, I am hoping to get to the point where the path to using the BBB as an
 NTP server using the PRU for more precise timing, and using the LTE-lite to
 provide the 1pps and time data. Of course, the LTE-lite can also provide
 1pps and 10MHz to my workbench. (It's all going to go in a 1U box in the
 rack next to my workbench.)

 Yes, I know that all the information is out there and much if it can be
 gleaned from this list by following several threads back through several
 months. But that brings to mind playing Adventure. Has anyone compiled a
 how-to for turning a BBB into an NTP server using the PRU for timing? Seems
 like a straight-forward and useful turn-key kind of thing.

Getting a BBB to take 10MHz refclk input (in the fashion of
http://www.febo.com/pages/soekris/)
and being able to timestamp _multiple_ PPS signals via the PRUs would
make for a pretty awesome time-nuts toy.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Re: [time-nuts] Did a member of time-nuts buy this?

[Gregory Maxwell]

On Sun, Dec 7, 2014 at 7:03 PM, Bob Camp kb...@n1k.org wrote:
 Of the $20K to $30K that a new tube costs, I doubt the material and basic 
 assembly adds up to over $5K. The rest of the cost is the final assembly / 
 test / yield / re-test / tooling / labor. That’s doing them in as high a 
 volume as anybody does them. You will need either a couple of H-Masers or a 
 set of Cs’s running and in good condition simply to make sure you got them 
 put together right ….

I'm sure that assembly/test/yield/re-test/tooling/labor are all
killers, but why the comment on h-masers?

CS beam standards are largely self-calibrating, thats why they can be
primary references.  For checking against systemic error you might
want a CS ensemble, but everyone has access to one of those for long
term measurement via GPS.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

[time-nuts] Did a member of time-nuts buy this?

[Gregory Maxwell]

Bob Camp wrote:
 Unless you are making a GPS receiver from scratch (which you might be), there 
 is a certain “trust factor” that comes into using a GPS for timing. Since you 
 can’t play with the firmware, you trust that the guy who wrote it did a good 
 job.

As compared to internet facing software embedded systems seem to be
unusually fragile, consider this paper on GPS receivers with
adversarial signals:
http://www.andrew.cmu.edu/user/tnighswa/GPS_CCS.pdf

And the trust with using GPS goes beyond the quality of the
construction of the receiver:  You're trusting the the GPS
constellation is working and correct (see the recent GLONASS failure)
and you're trusting that there aren't random jammers going by, you're
trusting that there isn't someone in physical proximity manipulating
the signal intentionally (see the paper above), or even just random
truckers going by with jammers (there have been past threads on
time-nuts) about this.  IIRC the stated US policy with respect to GPS
signal integrity is that it may be intentionally degraded (and can be
degraded in a geographically targeted manner) for e.g.
political/military objectives, so you trust that you won't be the
target or collateral damage of any such degradation or that it won't
be severe enough to effect you.

GPS driven timing works amazing well under most conditions most of the
time and at a very low cost. The trade-off is that you're taking more
fringe risk and greater trust. I sometimes worry that we're building
too much public infrastructure which is depends on a single system (or
on space based timing in general, since Kessler syndrome, while
unlikely, is a risk that exists) now that loran is gone in the US. Of
course, the attractiveness of GPS makes this self-fulfilling: Solid,
long living, CS primary frequency sources would probably be much less
expensive of GPS didn't cover so much of the commercial demand for
them.  There are newer receivers (e.g. ublox m8) that are concurrent
mult-gnss which might help, or maybe not: who knows what the receiver
will do if one system starts emitting crap?  I am not especially
confident that the software in these systems is well baked under
exceptional conditions.

If you're working on things with no availability requirements, no
real-time requirements (e.g. able to go download after-the-fact GPS
reliability and precise ephemeris from NGS), and aren't doing anything
where your timing is likely to be intentionally attacked, say for
test-lab purposes... then these issues may be less of a consideration.

In the context of time-nuts though many people are interested in the
art and science of precise time/frequency for pretty much its own
sake... and the driving need for the lowest phase noise or best adev
at some window might just be because it's possible.  In that light,
the extremes of autonomy, reliability, avoidance of systemic risk, and
surviving attacks are also interesting parameters that I find to be
interesting to explore, and they're ones which perhaps have inadequate
commercial attention on them these days since it seems people are
often (a little too) willing to trust and then point fingers when
things fail.

[Or at least this is an area I personally find interesting ... I wrote
this back in 2011 not so long after I started reading time-nuts:
https://people.xiph.org/~greg/decentralized-time.txt  before I knew
common-view time-transfer was already a thing, and when I knew a
little less nothing than the nothing I know now about time/frequency
standards.]

In terms of the 5061A  at least some of the old surplus units floating
around out there are non-working for silly reasons,  e.g. left
sitting for a long time, and they'll actually lock up fine if left
with the ion pump running for a few days, or the OCXO put back on
frequency, or the gain adjusted though I wouldn't spend $1k just
to find out. I picked up a 5061B for basically shipping costs a while
back and it was up and running reliably after some minor repairs...
though the beam current is low and it likely doesn't have much life
left in the tube. It's hard to deny how interesting and finely built
these devices are, objects of techno-lust in their own right, even in
surplus-and-maybe-not-reliable and impossibly-expensive-to-refurbish
condition.

As an actual lab tool-- rather than a science project, sadly, I do
have to agree that you're better off with a GPSDO than a surplus CS
unless you happen to get really lucky in the surplus gear lottery. Of
course, none of this is mutually exclusive. It's possible and
reasonable to have both.
___
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.