On Thu, 21 Jul 2022 18:04:22 -0700, Don Y <blockedofcourse@foo.invalid> wrote:>On 7/21/2022 7:04 AM, bitrex wrote: >>> You really need to define longterm before the problem becomes well posed. Do >>> you mean hours, days, weeks or months of runtime? >> >> 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?Sound travels about a foot per millisecond, so we are used to multiple time delays. A marching band sounds coordinated to us. I am thinking about vision a lot lately too. Each eye acquires a differently scaled image that changes constantly. There are millisecond-level changes in focus and parallax. I can put my glasses on, or not, and magnification surely changes. Somehow a brain acquires two images and distorts them in real time so that all the bits align. One has much better resolution using two eyes than either alone can provide. Nice trick. Sound must work like that to, extensive post-processing of acquired inputs. Probably time shifting parts of the spectrum, or something more complex, multiple cross-correlations.
really slow PLL
Started by ●July 20, 2022
Reply by ●July 22, 20222022-07-22
Reply by ●July 22, 20222022-07-22
Don Y wrote:> 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)Your question about a one cent following note difference perception is interesting. And, it needs to be followed up by me, so to speak. :) For some unknown reason, the only thing to truly satisfy me is to tune instruments by ear. 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.
Reply by ●July 22, 20222022-07-22
On a sunny day (Fri, 22 Jul 2022 09:01:52 -0700) it happened jlarkin@highlandsniptechnology.com wrote in <vnhldhh5vjech8rgksjrmss5djbgfndtqj@4ax.com>:>On Thu, 21 Jul 2022 18:04:22 -0700, Don Y ><blockedofcourse@foo.invalid> wrote: > >>On 7/21/2022 7:04 AM, bitrex wrote: >>>> You really need to define longterm before the problem becomes well posed. Do >>>> you mean hours, days, weeks or months of runtime? >>> >>> 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? > >Sound travels about a foot per millisecond, so we are used to multiple >time delays. A marching band sounds coordinated to us. > >I am thinking about vision a lot lately too. Each eye acquires a >differently scaled image that changes constantly. There are >millisecond-level changes in focus and parallax. I can put my glasses >on, or not, and magnification surely changes. Somehow a brain acquires >two images and distorts them in real time so that all the bits align. > >One has much better resolution using two eyes than either alone can >provide. > >Nice trick. Sound must work like that to, extensive post-processing of >acquired inputs. Probably time shifting parts of the spectrum, or >something more complex, multiple cross-correlations.Yes vision is a very interesting thing, been working on that for most of my life (TV etc) As I am not from this planet as you likely guessed I can see things from sound only But today it got even weeder... I had the TV on, switched it off, sat on the bench relaxing with eyes closed all of the sudden I visualized a field of purple I think it was flowers... Very bright and strong.. switched on the TV and there is was : same field. Still puzzled, only thing on was the sat receiver (connected via HDMI to the TV) but TV was off. I have tried some processing sound to vision but I think we need a neural net copy from the brain's visual part. RF to vision? Work on the 4G tower here ended at 6 PM today, at about 5 my 4G signal was back at 100% was depending on signal from a far away tower for the last 3 or 4 days,,, dropped when it rained.. but.. being impatient .. had checked on the ad dropped in my mailbox for fiber connection.. checked out their site, not bad and just as expensive but much faster than 4 G and no data limit.. but may take month before they are at my house installing stuff.,. fiber as backup not a bad idea... not very portable though. As to depend on GPS or GLONASS or what have you these days for professional test equipment seems a no-no to me, often there is no coverage, And as to connections, BNC sucks. Ethernet cable connectors suck too but you can do a whole lot with those connections and protocols and you can get good time signals from the net. I have a Casio radio watch accurate to the second.. sure you can jam that 77.5 kHz too but so far it has always worked here, what do you US guys use? WWVB? Automatic summer-winter time switch too
Reply by ●July 22, 20222022-07-22
jlarkin@highlandsniptechnology.com wrote:> 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 believe they somehow pair the rubidium clocks with another quartz crystal, even in those new tiny physics modules. I've not had a chance to tear apart a rubidium clock yet, or the ovenized stuff. They come in little metal boxes, that remind me of TV tuners.>>> 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!You were also asking about timestamping in other posts. Not entirely sure what you're upto in the end, but it might be interesting.
Reply by ●July 22, 20222022-07-22
jlarkin@highlandsniptechnology.com wrote:> 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.What he's trying to say is it may not be perfect 10 million cycles between every 1PPS pulse. For most stuff, this is good enough. For the metrology folks, using strong words like exactly or in this case "locked" will always cause an argument.
Reply by ●July 22, 20222022-07-22
On Friday, 22 July 2022 at 17:02:04 UTC+1, jla...@highlandsniptechnology.com wrote:> On Thu, 21 Jul 2022 18:04:22 -0700, Don Y > <blocked...@foo.invalid> wrote: > > >On 7/21/2022 7:04 AM, bitrex wrote: > >>> You really need to define longterm before the problem becomes well posed. Do > >>> you mean hours, days, weeks or months of runtime? > >> > >> 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? > Sound travels about a foot per millisecond, so we are used to multiple > time delays. A marching band sounds coordinated to us. > > I am thinking about vision a lot lately too. Each eye acquires a > differently scaled image that changes constantly. There are > millisecond-level changes in focus and parallax. I can put my glasses > on, or not, and magnification surely changes. Somehow a brain acquires > two images and distorts them in real time so that all the bits align. > > One has much better resolution using two eyes than either alone can > provide. > > Nice trick. Sound must work like that to, extensive post-processing of > acquired inputs. Probably time shifting parts of the spectrum, or > something more complex, multiple cross-correlations.There is cross-correlation between left and right ears in the brainstem which is involved in determining the direction of sound sources. Detectable time differences are remarkably small. (I would have to look it up to give a number.) John
Reply by ●July 22, 20222022-07-22
fredag den 22. juli 2022 kl. 16.46.36 UTC+2 skrev Don Y:> 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?https://youtu.be/AFaRIW-wZlw?t=54
Reply by ●July 22, 20222022-07-22
On Thu, 21 Jul 2022 18:21:51 +0100, Martin Brown <'''newspam'''@nonad.co.uk> wrote:>On 21/07/2022 16:31, John Walliker wrote: >> On Thursday, 21 July 2022 at 15:42:40 UTC+1, bitrex wrote: >>> On 7/21/2022 10:21 AM, Lasse Langwadt Christensen wrote: >>>> torsdag den 21. juli 2022 kl. 16.04.42 UTC+2 skrev bitrex: >>>>> On 7/21/2022 7:06 AM, Martin Brown wrote: >>>>>> On 21/07/2022 01:22, John Larkin wrote: >>>>>>> On Wed, 20 Jul 2022 19:32:20 -0400, Phil Hobbs >>>>>>> <pcdhSpamM...@electrooptical.net> wrote: >>>>>>> >>>>>>>> John Larkin wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> 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 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? >>>>>>>>> >>>>>>>>> My system should work from a 1 PPS GPS pulse too, all boxes as >>>>>>>>> followers. >>>>>>>>> >>>>>>>>> The PLL algorithm might be interesting. >>>>>>>>> >>>>>>>> >>>>>>>> It's certainly possible. However, within whatever tiny loop bandwidth >>>>>>>> you wound up with, the lockers would still have >>>>>>>> >>>>>>>> 20 log(40e6) = 152 dB >>>>>>>> >>>>>>>> higher phase noise than the lockee. >>>>>>> >>>>>>> GPS has that problem too. >>>>>>> >>>>>>>> >>>>>>>> It would be interesting to do the math to see whether it's possible to >>>>>>>> generate a concensus lock for the group: if you get everybody close >>>>>>>> enough, just sum their sine wave outputs and lock each one of them to >>>>>>>> that, with some bit of AC coupling or something so that they don't all >>>>>>>> wander together off to the edge of the tuning range. >>>>>>>> >>>>>>>> Maybe have one doing the locking with a phase shifter and the others >>>>>>>> with VCOs, or something like that. >>>>>>>> >>>>>>>> Definitely a partly-baked idea, but surely one could do better than >>>>>>>> 152 dB! >>>>>>>> >>>>>>>> Cheers >>>>>>>> >>>>>>>> Phil Hobbs >>>>>>> >>>>>>> Each box is basically a multichannel power supply, but channels can be >>>>>>> programmed to do stuff in timed sequences. I want different box >>>>>>> outputs to time align within, say, one millisecond longterm once >>>>>>> programs are kicked off together. So, many microseconds of equivalent >>>>>>> RMS phase noise is OK as long as we stay time aligned longterm. >>>>>> >>>>>> You really need to define longterm before the problem becomes well >>>>>> posed. Do you mean hours, days, weeks or months of runtime? > >>>>> 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 well, at least over a timespan of several >>>>> minutes. >>>> >>>> but they are anot free runnign are they? they are all reacting to midi >>>> >>> There's a system clock in each one surely but they don't try to sync >>> their system clocks, they receive an instruction "do X for Y ms" and >>> their processor figures out how long Y ms is, and does it for that long. >>> >>> It is literally good enough for rock & roll, but whether it's good >>> enough for power supply sequencing IDK, there is gonna be some latency. >>> >>> HP used to have GPIB on their power supplies, I've never used it but I >>> expect it wasn't really useful for tight synchronization. >> >> The Group Execute Trigger command does allow quite tight synchronisation >> between different GPIB devices. > >GPIB flat out on a good day could manage 1Mbyte/s but in real world >situations with interconnect cabling you would be lucky to get 500kb/s. >It's best feature was that it ran at the maximum speed the receiving >device could handle (assuming that the controller was fast enough). > >Synchronisation to a GET command would be probably be better than 1us >but would depend on the decoding time in each individual box. Some GPIB >devices were rather pedestrian at accepting commands. > >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.Ever read the actual 488 spec? There is a state diagram that could wreck your sleep for a week. 488 has a hardware "accepted" line, but for some reason SCPI in other contexts is send-and-pray. 488 is rare on new instruments, which are ethernet and USB. A Rigol scope makes a great USB power supply for fans and charging phones.> >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...
Reply by ●July 22, 20222022-07-22
On Fri, 22 Jul 2022 16:41:40 -0000 (UTC), Cydrome Leader <presence@MUNGEpanix.com> wrote:>jlarkin@highlandsniptechnology.com wrote: >> 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 believe they somehow pair the rubidium clocks with another quartz >crystal, even in those new tiny physics modules. I've not had a chance to >tear apart a rubidium clock yet, or the ovenized stuff. They come in >little metal boxes, that remind me of TV tuners.The rubidium physics/optics is noisy and nasty short-term. It usually disciplines a very good VCO or VCOCXO. I have a schematic somewhere around here; I'll look for it.> >>>> 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! > >You were also asking about timestamping in other posts. Not entirely sure >what you're upto in the end, but it might be interesting.Trying to get multiple power supplies and loads to act together so people can run, say, a modestly complex power sequence and load test on some gear and loop that for days or weeks and stay coordinated. Each box has 8 plugins, which naturally share the same timebase, but I want to extend time coordination to multiple boxes. So, time lock their XOs long-term, and start all the individual sequencers simultaneously. My customers complain about how nasty it is to organize a heap of purchased ac and dc supplies and dummy loads and relay drivers, to millisecond resolution, with a mess of weird interfaces. Here's the basic idea. It's still evolving. https://www.dropbox.com/s/5byvlk8uh9n23e3/P940_PD14np.pdf?dl=0
Reply by ●July 22, 20222022-07-22
On Fri, 22 Jul 2022 16:54:07 -0000 (UTC), Cydrome Leader <presence@MUNGEpanix.com> wrote:>jlarkin@highlandsniptechnology.com wrote: >> 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. > >What he's trying to say is it may not be perfect 10 million cycles between >every 1PPS pulse. For most stuff, this is good enough. For the metrology >folks, using strong words like exactly or in this case "locked" will >always cause an argument.No, the 1 PPS is sometimes generated by uP code, and sometimes jumps back and forth around ticks of some other clock. It's like a rubidium, ugly and nasty but long-term average excellent. I get the impression that a GPS box that outputs 10 MHz, gets the 10M by locking to the 1 PPS. The nasty 1 PPS is the bottom line of the GPS frequency standard. Real stability on less-than-weeks time frames comes from the flywheel effect of a very good 10 MHz oscillator and a very slow control loop. Which is my problem, absolutely time locking a bunch of 40 MHz oscillators from a 1 PPS pulse generated by, essentially, a relay driver.
