really slow PLL

On Friday, 22 July 2022 at 11:24:11 UTC+1, Don Y wrote:
> On 7/22/2022 2:36 AM, Martin Brown wrote: 
> > On 21/07/2022 18:41, bitrex wrote: 
> >> On 7/21/2022 1:21 PM, Martin Brown wrote: 
> > 
> >>> IEEE488 was good in its day but a bit long in the tooth now. Still on some 
> >>> test equipment in service today and was provided as standard on NEC 9801 
> >>> PC's in Japan although hardly ever used by their customers. 
> >>> 
> >>> The cables and connectors could only be described as a bit clunky! 
> >>> They really didn't get on with metal swarf being around but were OK in clean 
> >>> dry electronics/physics labs - much less so in chemistry ones... 
> >> 
> >> I feel like there wasn't really a good relatively inexpensive standard for 
> >> interfacing PC peripherals until USB. Serial and parallel were slow 
> >> (occasionally some devices supported ECP, I remember having to enable it in 
> >> the BIOS sometimes), and external SCSI wasn't really well-suited to anything 
> >> but external disk drives. 
> > 
> > There were bidirectional parallel ports that could run fairly quick. 
> > A parallel port interface for a CD drive was just about doable on a bog 
> > standard PC parallel port.
> But old ports (without queues) were a bitch for the processor to service.
> > SCSI was quite good for external bulk data devices like scanners too. It's 
> > disadvantage was cost.
> And cable length. Aside from differential (even costlier), you were 
> stuck to "arm's reach". At least serial and USB can be pushed to 
> longer lengths (despite limitations of their respective standards).
> > Token ring WAN over cheap twisted pairs took over for a while in my neck of the 
> > woods before truly fast Ethernet really got going. 
> > (it was an academic predecessor of IBM's token ring offering)
> IBM's suffered from the same sort of costs as SCSI with their big, 
> klunky connectors.
> > Early fast ethernet distribution was on expensive thick heavy coax cable marked 
> > up with where to tap into them. The cost difference for wiring up was enormous. 
> > Physically manhandling the cable was a PITA.
> Ah, yes. "Orange hose" and vampire taps. I suspect one could make a pretty 
> little structure (think: Lincoln Logs) out of lengths of that! A likely factor 
> in its lack of ubiquity. 
> 
> I rely on network connectivity for an increasing number of appliances, now. 
> Scanners (direct network connections or USB to SBC host), disks (iSCSI), 
> printers (direct network connections or dedicated print server appliance), 
> other computers, etc. 
> 
> But, all of the T/TX technologies make cabling a PITA. Every cable has to 
> travel all the way to a switch, even if some other networked device is 
> nearby (e.g., 10Base2 would tolerate device insertion with just a short 
> length of coax -- though the entire network was disrupted in the process!) 
> I have thick bundles of CAT5e affixed to the undersides of my workbenches 
> as the cables from each device (on or under the bench) join the bundle 
> as it travels to the east or west switch. Add a new device? Ugh! 
> 
> IMO, most interfaces fall down (in practical terms) when it comes to the choice 
> of connector. Either too cheap (flimsy) or too costly. The RJ45/8P8C wins in 
> terms of cost... but is a nuisance when the locking tab inevitably breaks off 
> (even if "guarded"): "Great! Now I either repair the cable or replace it!" 
> 
> And, inevitably, connectors require a bit of "fiddling" to ensure oriented 
> correctly and mated well. How many times do you discover a bent pin on a 
> HD SCSI cable only because the interface refuses to run properly? (and 
> trying to repair said pin is essentially impossible). 
> 
> And, of course, the wide choice of "standards" (SCSI cables being among the 
> worst for lack of uniformity: Old Sun, Apple, SCSI, SCSI2, SCSI3, VHDCI, 
> etc.) 
> 
> The ideal connector wouldn't have a top or bottom (etc.) Something like 
> a phone plug where all you have to do is line up plug to hole and jam 
> it home. Considering most connectors reside on the backs of equipment, 
> expecting the user to be able to VIEW the mate while attempting to connect 
> is wishful thinking!
Yes, I discovered the perils of blind plugging when I tried to plug something
in deep inside a rack cabinet.  My finger discovered an unguarded fan and it
initially felt like an electric shock.  I pulled my arm back so fast I hit my face
and got a nosebleed!
John

Martin Brown wrote:
> On 22/07/2022 03:44, Clifford Heath wrote:
>> On 22/7/22 03:10, Phil Hobbs wrote:
>>> Gerhard Hoffmann wrote:
>>>> Am 21.07.22 um 16:15 schrieb Phil Hobbs:
>>>>
>>>>> I wonder if there's an advantage to using the closure phase for an 
>>>>> array that large.  With 17 oscillators you've got 136 independent 
>>>>> phase differences, so maybe there's a way to get 22 dB instead of 
>>>>> 12 dB improvement.
>>>>
>>>> -v ?
>>>>
>>>> what do you mean with closure phase? Where do the 22 dB come
>>>> from?
>>>>
>>>> The idea was simply to have all 16 regulated to the be
>>>> synchronous and then feed them into a 16-to--1 Wilkinson
>>>> combiner. The phase noise should average out among the
>>>> 16 units. Just as proof of concept. The MTI-260 are quite ok,
>>>> but with bleeding edge oscillators that could be interesting.
>>>> In the region where you just cannot improve an oscillator.
>>>>
>>>>> Cheers
>>>> Gerhard
>>>
>>> Sure.  Thing is, that wastes a lot of information that you could 
>>> maybe use to get 10*log(136) = 21.3 dB improvement instead of 
>>> 10*log(17) = 12.3 dB.  [136 = N(N-1)/2 when N = 17.]
>>>
>>> Closure is a really cute idea, which I first came across in the 
>>> context of very long baseline interferometry (VLBI) radio telescopes. 
>>> See the discussion from BEOS 3e here:
>>>
>>> <https://electrooptical.net/www/sed/closure.png>.
>>
>> Interesting, thanks.
>>
>> Some frequency synthesiser chips employ proprietary majick to reduce 
>> the phase noise associated with integer divide/multiply ratios. 
>> Polyphase oscillator and slipping by partial cycles I think. Perhaps 
>> they're doing something like closure against the different clock phases?
> 
> Quite probably - it has been known for a long time in radio astronomy 
> first derived by Jennison in 1958 at Jodrell Bank for 3 antennae. This 
> is the original ground breaking paper for anyone interested
> 
> https://articles.adsabs.harvard.edu//full/1958MNRAS.118..276J/0000276.000.html 
> 
> 
> (easier to understand versions exist today). WIki isn't bad:
> 
> https://en.wikipedia.org/wiki/Closure_phase
> 
> It allows you to get a good phase observable uncontaminated by the phase 
> error at each node for every distinct subset of 3 nodes. There is a 
> corresponding closure amplitude for distinct subsets of 4 nodes.
> 
> Obviously the bigger N is the more useful observables you can get which 
> is why the big dish telescopes sometimes stay on target and in the loop 
> for perhaps longer than they really ought to in deteriorating weather.
> 
> This book reviews most of the classical tricks used in VLBI and 
> interferometry from the period when they had just become routine:
> 
> Indirect Imaging: Measurement and Processing for Indirect Imaging
> Editor-J. A. Roberts
>  &nbsp;0 ratings by Goodreads
> ISBN 10: 0521262828 / ISBN 13: 9780521262828
> Published by Cambridge University Press, 1984
> 

The real power comes from the number of independent observables from N 
instruments going like N**2, so that you win SNR like N**2 instead of N.

Quite a startling improvement for moderate-to-large N!

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

On 7/22/2022 5:12 AM, John Walliker wrote:
> Yes, I discovered the perils of blind plugging when I tried to plug something
> in deep inside a rack cabinet.  My finger discovered an unguarded fan and it
> initially felt like an electric shock.  I pulled my arm back so fast I hit my face
> and got a nosebleed!

Too funny!  :>

I had a similar experience groping for the right place to plug
a device and happened upon an exposed bus bar.  Only 12V (at 100A)
but sufficient to vaporize the leads on the device I was plugging!

And, cause me to retract my arm in utmost haste -- whacking
my elbow in the process.

Moral of story:  shut down power before making changes (even
if that sequence takes a fair bit of time to complete!)

Don Y wrote:
> Don wrote:
>> Don Y wrote:
>>> bitrex wrote:
>>
>> <snip>
>>
>>>> Yeah I don't quite get it, either. My rack of synthesizers can each playone
>>>> voice of the Maple Leaf Rag via MIDI and they all stay synced together really
>>>
>>> How is "really well" defined?  In the domain of human auditory perception?
>>
>> In this case, isn't "really well" defined as an absence of sour note(s)?
>
> That assumes the synthesis uses the same clock as timing.  I think the
> discussion here has been wrt durations/intervals.
>
> How sensitive are *your* ears to noticing small differences in pitch,
> absence a comparative reference?  Can you discern a difference of a few
> cents ("perfect pitch")?

Can't everyone's ears (except perhaps the autistic tone-deaf and such)
hear a sour note relative to the preceding note? Do you need to name a
note (perfect pitch) in order to hear its sourness? 
    It's all but impossible for me personally to ignore the sourness of 
cringeworthy, awkward note(s). Sour notes make me want to get out of 
earshot.
    
Danke,

-- 
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

On Fri, 22 Jul 2022 10:24:04 +0100, Martin Brown
<'''newspam'''@nonad.co.uk> wrote:

>On 21/07/2022 12:42, jlarkin@highlandsniptechnology.com wrote:
>> On Thu, 21 Jul 2022 12:06:26 +0100, Martin Brown
>> <'''newspam'''@nonad.co.uk> wrote:
>> 
>>> On 21/07/2022 01:22, John Larkin wrote:
>
>>>> If a follower is told to start locking, it could timestamp the first
>>>> incoming 1 PPS with a giant counter clocked by its local 40 MHz VCO.
>>>> If a later 1 PPS edge appears to arrive too soon, we could speed up
>>>> our VCXO by, say, 1 PPM, and vice versa. So longterm it walks into
>>>> alignment with the 1 PPS and eventually dithers a microsecond per
>>>> second. Noise on the coax gets fixed over time too.
>>>
>>> Have a free running counter on each of the followers and use the value
>>> of that after 1s, 10s, 100s to determine the correct tweak to apply
>>> locally. Tweaks of 1ppm at a time is rather crude you should be able to
>>> determine the right amount to tweak it by better than that.
>>> (especially over longer timebases)
>> 
>> I wouldn't expect my VCXO to be more than 10 PPM off at the start of
>> the lock request. So I can walk it to within 1 PPM, namely 1
>> microsecond error, in at most 10 seconds using 1 PPM jogs. If the osc
>> were 50 PPM off, it would take 50 seconds to catch up to the external
>> pulse.
>
>You would have lower systematic error after lockin if you made the 
>adjustments by reading the full width counter as a two's compliment 
>number when the synch pulse arrives (and using a 2^N counter).

Yes, it could evaluate the magnitude of the time error and close a
classic almost-analog control loop. The plant is already an integrator
so it could be a simple proportional loop.

>
>>>> That's better than just measuring the 1 Hz period once a second,
>>>> tweaking the clock, and then throwing away that measurement. I want a
>>>> time lock, not a frequency lock.
>>>
>>> Then you probably want to measure the cumulative error over many
>>> seconds. Each unit can work out how long it can free run without
>>> exceeding tolerance once it has the rough and ready count from the first
>>> second. After that you have a good idea of how many seconds you can free
>>> run for without having any ambiguities from residual drift.
>> 
>> Yes, I don't want to measure period once a second. I want to compare
>> time alignment forever after receiving the first 1 pps pulse.
>> 
>> It's actually simple: first received pulse, start a mod 40 million
>> counter. Every time it rolls over, do an early/late compare to the 1
>> PPS input, and jog the 40 MHz VCXO 1 PPM in the right direction.
>> 
>> The compare is a dflop, d is the msb of the counter, clock is the 1
>> PPS input. Occasional metastability is fine; it indicates success.
>> 
>> It doesn't even need to be a 40 million counter. Something a fraction
>> of that will do. 10,000 maybe.
>
>Unless you are very wedded to base 10 it might well work better to use a 
>hardware 16 or 24 bit counter and allow it to free run. The master clock 
>time pulses could be at some suitable power of 2^N x 0.1us.


My xo is 40 MHz and 1 PPS is convenient, GPS compatible. 


>
>Under software control you could even do quick corrections in the first 
>second to get the basic frequency of all clocks approximately right.

The loop will be software, once an FPGA provides the measurements. All
the VCXOs will park, open-loop before we try to lock, at some
calibrated nominally correct frequency, not many PPM off absolute.

>
>The master could produce a set of pulses that were 1/16s, 1/8s, 1/4s, 
>1/2, 1s long at the outset. That would speed up the lock in time.
>
>> Maybe the counter can just free-run, never get initialized. Gotta do
>> the math on that after I wake up.
>
>If you want to get the clocks on the various boxes as close to in synch 
>as possible then it makes sense to correct any errors quickly.
>
>Power of 2 steps decreasing to some floor level being most favourable.

It's only a power supply!

If it takes 10 seconds to achieve dither-lock, starting 10 PPM out,
the worst time error is still way under a millisecond.

On 7/22/2022 7:17 AM, Don wrote:
> Don Y wrote:
>> Don wrote:
>>> Don Y wrote:
>>>> bitrex wrote:
>>>
>>> <snip>
>>>
>>>>> Yeah I don't quite get it, either. My rack of synthesizers can each playone
>>>>> voice of the Maple Leaf Rag via MIDI and they all stay synced together really
>>>>
>>>> How is "really well" defined?  In the domain of human auditory perception?
>>>
>>> In this case, isn't "really well" defined as an absence of sour note(s)?
>>
>> That assumes the synthesis uses the same clock as timing.  I think the
>> discussion here has been wrt durations/intervals.
>>
>> How sensitive are *your* ears to noticing small differences in pitch,
>> absence a comparative reference?  Can you discern a difference of a few
>> cents ("perfect pitch")?
> 
> Can't everyone's ears (except perhaps the autistic tone-deaf and such)
> hear a sour note relative to the preceding note? Do you need to name a
> note (perfect pitch) in order to hear its sourness?

Perfect pitch is more than just "naming a note".

>      It's all but impossible for me personally to ignore the sourness of
> cringeworthy, awkward note(s). Sour notes make me want to get out of
> earshot.

How "sour" does the note have to be before it is perceptible, as such.
A cent?  Two?  Fifty?  A semitone??  (about a 25 cents is typical for
the average, non-musician, listener to be able to detect -- without
context; i.e., if the "previous note" was similarly sour, your estimation
of the correctness of the following note can perceive both as correct...
like singing in an entirely different *key*!)

<https://neurosciencenews.com/pitch-detection-music-21087/>

This is a reference note (middle C) followed by the same note "soured"
by 12 cents:

<https://en.wikipedia.org/wiki/File:Sines_12_cent_difference.wav>

Chances are, you can't tell the difference hearing them
in sequence.  If you heard just *one*, you'd not be able to tell if it
was correct, or not.  The third sound sample plays both simultaneously
so you can hear them beating against each other -- the difference then
becomes very noticeable!

Here's *one* cent difference:

<https://en.wikipedia.org/wiki/File:Sines_1_cent_difference.wav>

And 24 cents (about the point of "normal" perception):

<https://en.wikipedia.org/wiki/File:Sines_24_cent_difference.wav>

If your device's *timing* was off by 0.05%, would that be consequential?

Don Y wrote:
> Don wrote:
>> Don Y wrote:
>>> Don wrote:
>>>> Don Y wrote:
>>>>> bitrex wrote:
>>>>
>>>> <snip>
>>>>
>>>>>> Yeah I don't quite get it, either. My rack of synthesizers can each play one
>>>>>> voice of the Maple Leaf Rag via MIDI and they all stay synced together really
>>>>>
>>>>> How is "really well" defined?  In the domain of human auditory perception?
>>>>
>>>> In this case, isn't "really well" defined as an absence of sour note(s)?
>>>
>>> That assumes the synthesis uses the same clock as timing.  I think the
>>> discussion here has been wrt durations/intervals.
>>>
>>> How sensitive are *your* ears to noticing small differences in pitch,
>>> absence a comparative reference?  Can you discern a difference of a few
>>> cents ("perfect pitch")?
>>
>> Can't everyone's ears (except perhaps the autistic tone-deaf and such)
>> hear a sour note relative to the preceding note? Do you need to name a
>> note (perfect pitch) in order to hear its sourness?
>
> Perfect pitch is more than just "naming a note".
>
>>      It's all but impossible for me personally to ignore the sourness of
>> cringeworthy, awkward note(s). Sour notes make me want to get out of
>> earshot.
>
> How "sour" does the note have to be before it is perceptible, as such.
> A cent?  Two?  Fifty?  A semitone??  (about a 25 cents is typical for
> the average, non-musician, listener to be able to detect -- without
> context; i.e., if the "previous note" was similarly sour, your estimation
> of the correctness of the following note can perceive both as correct...
> like singing in an entirely different *key*!)
>
> <https://neurosciencenews.com/pitch-detection-music-21087/>
>
> This is a reference note (middle C) followed by the same note "soured"
> by 12 cents:
>
> <https://en.wikipedia.org/wiki/File:Sines_12_cent_difference.wav>
> Here's *one* cent difference:
>
> <https://en.wikipedia.org/wiki/File:Sines_1_cent_difference.wav>
>
> And 24 cents (about the point of "normal" perception):
>
> <https://en.wikipedia.org/wiki/File:Sines_24_cent_difference.wav>
>
> If your device's *timing* was off by 0.05%, would that be consequential?
    
Very interesting information. Thank you. 
    It's easy for me to hear the one cent difference, how about you? My
audio perception comes in handy when it's time to tune a keyboard. Some-
times musicians purposefully vary tones by a few cents in order to 
produce vibrato.

In regards to your 0.05% device timing question, the answer is: it 
depends. A 0.05% device timing variance in my Power Bank Keepalive:

https://crcomp.net/mp3mod/index.php

for instance, is inconsequential.
    
Danke,

-- 
Don, KB7RPU, https://www.qsl.net/kb7rpu
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.

On 7/22/2022 8:07 AM, Don wrote:
> Don Y wrote:
>> Don wrote:
>>> Don Y wrote:
>>>> Don wrote:
>>>>> Don Y wrote:
>>>>>> bitrex wrote:
>>>>>
>>>>> <snip>
>>>>>
>>>>>>> Yeah I don't quite get it, either. My rack of synthesizers can each play one
>>>>>>> voice of the Maple Leaf Rag via MIDI and they all stay synced together really
>>>>>>
>>>>>> How is "really well" defined?  In the domain of human auditory perception?
>>>>>
>>>>> In this case, isn't "really well" defined as an absence of sour note(s)?
>>>>
>>>> That assumes the synthesis uses the same clock as timing.  I think the
>>>> discussion here has been wrt durations/intervals.
>>>>
>>>> How sensitive are *your* ears to noticing small differences in pitch,
>>>> absence a comparative reference?  Can you discern a difference of a few
>>>> cents ("perfect pitch")?
>>>
>>> Can't everyone's ears (except perhaps the autistic tone-deaf and such)
>>> hear a sour note relative to the preceding note? Do you need to name a
>>> note (perfect pitch) in order to hear its sourness?
>>
>> Perfect pitch is more than just "naming a note".
>>
>>>       It's all but impossible for me personally to ignore the sourness of
>>> cringeworthy, awkward note(s). Sour notes make me want to get out of
>>> earshot.
>>
>> How "sour" does the note have to be before it is perceptible, as such.
>> A cent?  Two?  Fifty?  A semitone??  (about a 25 cents is typical for
>> the average, non-musician, listener to be able to detect -- without
>> context; i.e., if the "previous note" was similarly sour, your estimation
>> of the correctness of the following note can perceive both as correct...
>> like singing in an entirely different *key*!)
>>
>> <https://neurosciencenews.com/pitch-detection-music-21087/>
>>
>> This is a reference note (middle C) followed by the same note "soured"
>> by 12 cents:
>>
>> <https://en.wikipedia.org/wiki/File:Sines_12_cent_difference.wav>
>> Here's *one* cent difference:
>>
>> <https://en.wikipedia.org/wiki/File:Sines_1_cent_difference.wav>
>>
>> And 24 cents (about the point of "normal" perception):
>>
>> <https://en.wikipedia.org/wiki/File:Sines_24_cent_difference.wav>
>>
>> If your device's *timing* was off by 0.05%, would that be consequential?
>      
> Very interesting information. Thank you.
>      It's easy for me to hear the one cent difference, how about you? My
> audio perception comes in handy when it's time to tune a keyboard. Some-
> times musicians purposefully vary tones by a few cents in order to
> produce vibrato.

Could you hear a middle C "soured" by one cent when it follows a
(correct) C-sharp immediately preceding it?  Or, vice versa?

*I* can't.  My threshold is closer to 10 cents and rely on electronic
devices when tuning instruments.

And, if every note was "off key" by 10 cents, I'd not recognize the
tony.

> In regards to your 0.05% device timing question, the answer is: it
> depends. A 0.05% device timing variance in my Power Bank Keepalive:
> 
> https://crcomp.net/mp3mod/index.php
> 
> for instance, is inconsequential.

In the context of this thread, it likely has an impact.  A cent is
about 500PPM (though in the frequency domain)

On Fri, 22 Jul 2022 06:20:58 -0000 (UTC), Cydrome Leader
<presence@MUNGEpanix.com> wrote:

>jlarkin@highlandsniptechnology.com wrote:
>> On Fri, 22 Jul 2022 00:08:56 -0000 (UTC), Cydrome Leader
>> <presence@MUNGEpanix.com> wrote:
>> 
>>>jlarkin@highlandsniptechnology.com wrote:
>>>> On Thu, 21 Jul 2022 07:43:18 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
>>>> wrote:
>>>> 
>>>>>Am 21.07.22 um 01:20 schrieb John Larkin:
>>>>>> 
>>>>>> 
>>>>>> Suppose I have several rackmount boxes and each has a BNC connector on
>>>>>> the back. Each of them has an open-drain mosfet, a weak pullup, and a
>>>>>> lowpass filtered schmitt gate back into our FPGA.
>>>>>> 
>>>>>> I can daisy-chain several boxes with BNC cables and tees.
>>>>>> 
>>>>>> Each box has a 40 MHz VCXO and I want to phase-lock them, or at least
>>>>>> time-align them to always be the same within a few microseconds,
>>>>>> longterm.
>>>>>
>>>>>I have a backburner project of locking 16 MTI-260 oscillators
>>>>>slooowy to another one, and when they are in sync, combine
>>>>>them  with an array of Wilkinsons. That should have a nice
>>>>>effect on phase noise by averaging over 16.
>>>>>The CPLD has enough resources to implement that as a delay
>>>>>locked loop with 1 pps, but low hanging fruit first.
>>>>>
>>>>>> 
>>>>>> I could call one the leader (not "master") and make the others
>>>>>> followers (not "slaves") and have the leader make an active low pulse
>>>>>> maybe once a second. A follower would use her (not "his") clock to
>>>>>> measure the incoming period and tweak its local VCXO in the right
>>>>>> direction. That should work.
>>>>>> 
>>>>>> Don't GPS receivers lock their 10 MHz oscillators to a 1 PPS pulse
>>>>>> from the satellites?
>>>>>
>>>>>No. There is no 1PPS pulse from the sat nor the need for exactly 10 MHz. 
>>>>>The sats transmit a pseudo noise sequence that is
>>>>>aligned to the second of their local clock source.
>>>>>The GPS receiver knows the polynomial and runs a local copy of
>>>>>the polynomial. It knows by cross correlation if the local
>>>>>pseudo noise is the same as that of the sat and therefore knows
>>>>>the start of the second. Usually that won't be the case.
>>>>>Then the receiver delays its own polynomial by omitting a
>>>>>clock to the shift register that generates it and tries again.
>>>>>Sooner or later it will fit.
>>>> 
>>>> Where does the 10 MHz come from?
>>>
>>>https://en.wikipedia.org/wiki/GPS_disciplined_oscillator
>> 
>> "GPSDOs typically phase-align the internal flywheel oscillator to the
>> GPS signal by using dividers to generate a 1PPS signal from the
>> reference oscillator, then phase comparing this 1PPS signal to the
>> GPS-generated 1PPS signal and using the phase differences to control
>> the local oscillator frequency in small adjustments via the tracking
>> loop."
>> 
>> That's what I meant: the 10M xo is locked to the 1 PPS GPS output.
>> 
>> The GPS 1 PPS is perfect (by definition) long-term but terrible
>> short-term, so the XO or rubidium has to be very good itself, and the
>> loop has to be very slow. Big flywheel.
>
>GPS timing isn't completely perfect in reality. Antennas blow off roofs, 
>contractors cut cables etc. Even losing sync for a minute is sort of a big 
>deal. As you mentioned, jitter is the real problem. There are tradeoffs to 
>making a flywheel thats too heavy so to speak.
>
>For really fussy stuff, one might have multiple GPS receivers and a quorum 
>of local 10Mhz oscillators. In fact, 10Mhz is a dinosaur relic for modern 
>stuff too. We've got racks of 10Mhz oscillators and equipment to monitor 
>any phase shift between local oscillators and GPS sources. It's all going 
>to the dumpster when somebody finally notices it's been powered down and 
>forgotten about.
>
>Fairly accurate nS resolution timing is possible in computers these days, 
>with the right tricks.

I triggered a scope from a very good ovenized XO and looked at the
rising edge of a rubidium. The edge looked solid, as if it was
internal triggered. Checking every 20 minutes or so, it ws slowly
creeping across the screen, at 5 ns/cm.

>
>> I'll be doing something similar, locking my 40 MHz clock to some 1 PPS
>> input, the difference being that I don't mind a few us of jitter, so I
>> can lock quick and crude. 
>
>Do you have to worry about fun issues like an the timestamp of a signal 
>being received before it was even transmitted between pieces of equipment?

It a multi-channel power supply! 

>
>I like the toggle switches on the USNO hydrogen masers:
>
>https://www.cnmoc.usff.navy.mil/Organization/United-States-Naval-Observatory/Precise-Time-Department/The-USNO-Master-Clock/The-USNO-Master-Clock/
>
>They were originally made by some weird company called Sigma Tau. Somehow, 
>Microchip owns them now. New models have a new paint job, but still look 
>like they might be a transit case for a Dalek.

On Fri, 22 Jul 2022 10:37:53 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

>Am 22.07.22 um 04:47 schrieb jlarkin@highlandsniptechnology.com:
>
>>>> Where does the 10 MHz come from?
>>>
>>> https://en.wikipedia.org/wiki/GPS_disciplined_oscillator
>> 
>> "GPSDOs typically phase-align the internal flywheel oscillator to the
>> GPS signal by using dividers to generate a 1PPS signal from the
>> reference oscillator, then phase comparing this 1PPS signal to the
>> GPS-generated 1PPS signal and using the phase differences to control
>> the local oscillator frequency in small adjustments via the tracking
>> loop."
>> 
>> That's what I meant: the 10M xo is locked to the 1 PPS GPS output.
>
>No. The 1pps is asserted when the CPU thinks it's closest to
>the "right" clockcycle. It could be off by half a cycle.
>There is no need for 10 MHz, one could have chosen a nice
>multiple of the desired baud rate.
>

Our GPS receivers output 1 PPS and 10 MHz. Argue with Wikipedia.