On 7/21/2022 4:33 AM, John Walliker wrote:> On Thursday, 21 July 2022 at 07:49:43 UTC+1, whit3rd wrote: >> On Wednesday, July 20, 2022 at 4:21:08 PM UTC-7, 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. >> If you can tolerate 'a few microseconds' on a 40 MHz signal, that's not a phase-lock >> problem, it's a frequency-lock problem. Why not just run an up/down counter >> to generate a correction voltage for each non-leading VCO? > > If you have an ethernet interface to each unit then Precision Time Protocol > should do exactly what you want. > https://en.wikipedia.org/wiki/Precision_Time_Protocol > JohnYeah, that sounds like the ticket to me also. Trying to use each box's system clock for purposes of keeping "user-space" tasks in sync across boxes makes me uncomfortable, sounds like a srs hack. If you need to tightly synchronize events between physically separate hardware why not use a standard designed for the task rather than some roll-your-own shit
really slow PLL
Started by ●July 20, 2022
Reply by ●July 21, 20222022-07-21
Reply by ●July 21, 20222022-07-21
Gerhard Hoffmann wrote:> Am 21.07.22 um 13:19 schrieb jlarkin@highlandsniptechnology.com: > >> >> Where does the 10 MHz come from? > > Choise of implementer. One local clock generator is needed. > This clock determines short term stabiity and phase noise. > > My Lucent KS24361 uses 5 MHz MTI-260 double ovens; for > redundancy/holdover it has a 2nd unit with another crystal > oven without a receiver. > > The redundancy units were really hard to sell without the > receiver; that's why I have 20 of these MTI-260, got a good > price. :-) > > They were new old stock built by HP/Agilent for Lucent as > replacement parts. They have never been on a telecom tower > in China like most of those one gets on ebay. > > I have expanded the Lucent to 10 MHZ and with a distribution > amplifier: > > < http://www.hoffmann-hochfrequenz.de/downloads/DoubDist.pdf > > > cheers, GerhardI 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. 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
Reply by ●July 21, 20222022-07-21
On 7/21/2022 10:12 AM, bitrex wrote:> On 7/21/2022 4:33 AM, John Walliker wrote: >> On Thursday, 21 July 2022 at 07:49:43 UTC+1, whit3rd wrote: >>> On Wednesday, July 20, 2022 at 4:21:08 PM UTC-7, 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. >>> If you can tolerate 'a few microseconds' on a 40 MHz signal, that's >>> not a phase-lock >>> problem, it's a frequency-lock problem. Why not just run an up/down >>> counter >>> to generate a correction voltage for each non-leading VCO? >> >> If you have an ethernet interface to each unit then Precision Time >> Protocol >> should do exactly what you want. >> https://en.wikipedia.org/wiki/Precision_Time_Protocol >> John > > Yeah, that sounds like the ticket to me also. Trying to use each box's > system clock for purposes of keeping "user-space" tasks in sync across > boxes makes me uncomfortable, sounds like a srs hack.At minimum it likely won't scale very well. Why implicitly discourage one's customers from buying only a limited number of units
Reply by ●July 21, 20222022-07-21
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
Reply by ●July 21, 20222022-07-21
bitrex wrote:> 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 >>> <pcdhSpamMeSenseless@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...superficially at least it sounds like he wants a sequencer. > > Using the nuts & bolts system clock for synchronization of "user tasks" > also makes me uncomfortable, if the device behavior only need to align > to the millisecond why not trigger them using some simple network > protocol and let their hardware figure out how long a millisecond is > independently. Do the timings of these boxes need to be tighter than the > Maple Leaf Rag? > >Given that it's so simple to do it right, why not do that? 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
Reply by ●July 21, 20222022-07-21
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.
Reply by ●July 21, 20222022-07-21
On Thu, 21 Jul 2022 09:27:39 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>jlarkin@highlandsniptechnology.com wrote: >> On Wed, 20 Jul 2022 20:28:35 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> John Larkin wrote: >>>> On Wed, 20 Jul 2022 19:32:20 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@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. >>>> >>>> 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. >>>> >>>> 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. >>>> >>> >>> Absolutely. The scary 152 dB number doesn't mean that doing something >>> like that is automatically a bad idea. >>> >>> Being an old RF and ultrastable laser guy, though, it does make my ears >>> perk up. ;) >>> >>> Cheers >>> >>> Phil Hobbs >> >> I like thermostats, single-bit-feedback control loops. >> >> We have a couple of boxes that do fan control based on interior >> temperature. Once a second, if it's above the setpoint, ratchet fan >> speed up some fixed amount, 1% maybe. If it's cooler than the >> setpoint, step fan speed down. There's no acoustic drama and it's >> perfectly stable. >> >> It dithers around the setpoint but nobody notices. >> >> This is immune to classic control theory so the concept annoys some >> people but it works great. > >A real old time control guy like Tim Wescott would probably be a fan >too--the great virtue of a bang-bang controller is that (as you say) >it's highly resistant to variations in the _plant_. > >Your furnace doesn't go nuts when you have a Christmas party, even >though all those people generate a lot of heat, and there's lots of >opening and closing of doors and ovens. > >Cheers > >Phil HobbsMy power supply box has 8 plugin modules of various types. Some don't need much air but some really do. The two big fans are howlers at 48 volts. Each module can present one bit to the motherboard software: I want more air, or I don't. If any board wants more, ratchet the fans up a bit. If none want more, jog the fans down.
Reply by ●July 21, 20222022-07-21
On Thu, 21 Jul 2022 09:36:31 -0400, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>Martin Brown wrote: >> On 21/07/2022 01:22, John Larkin wrote: >>> On Wed, 20 Jul 2022 19:32:20 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@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? >> >>> 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) >> >>> 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. >> >> This is an ancient trick from physics which avoids the smartest students >> from having to laboriously count every pendulum swing when determining g >> to maximum possible precision in a given time. It used to be (and >> probably still is a favourite exam practical). Components needed are >> very cheap and the whole thing is a good test of experimental technique. >> > >It's not as efficient as 'dry labbing'. ;)We cut our EE labs and went sailing or something, and faked all the reports one night at the end of the semister. Of course we got As.
Reply by ●July 21, 20222022-07-21
On Thu, 21 Jul 2022 10:15:20 -0400, bitrex <user@example.net> wrote:>On 7/21/2022 10:12 AM, bitrex wrote: >> On 7/21/2022 4:33 AM, John Walliker wrote: >>> On Thursday, 21 July 2022 at 07:49:43 UTC+1, whit3rd wrote: >>>> On Wednesday, July 20, 2022 at 4:21:08 PM UTC-7, 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. >>>> If you can tolerate 'a few microseconds' on a 40 MHz signal, that's >>>> not a phase-lock >>>> problem, it's a frequency-lock problem. Why not just run an up/down >>>> counter >>>> to generate a correction voltage for each non-leading VCO? >>> >>> If you have an ethernet interface to each unit then Precision Time >>> Protocol >>> should do exactly what you want. >>> https://en.wikipedia.org/wiki/Precision_Time_Protocol >>> John >> >> Yeah, that sounds like the ticket to me also. Trying to use each box's >> system clock for purposes of keeping "user-space" tasks in sync across >> boxes makes me uncomfortable, sounds like a srs hack. > >At minimum it likely won't scale very well. Why implicitly discourage >one's customers from buying only a limited number of unitsTime synchronization of programmable power supplies and loads is precisely one selling feature that my customers want but nobody else seems to make. It works fine in one box but I want to extend the function to multiple boxes in a rack. The controller in each box is a MicroZed and doesn't support the PTP thing, and my customers may not be able top provide it anyhow. The 1 PPS thing works with just a BNC cable. Besides, what I do is design things.
Reply by ●July 21, 20222022-07-21
jlarkin@highlandsniptechnology.com wrote:> On Thu, 21 Jul 2022 09:27:39 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> jlarkin@highlandsniptechnology.com wrote: >>> On Wed, 20 Jul 2022 20:28:35 -0400, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> John Larkin wrote: >>>>> On Wed, 20 Jul 2022 19:32:20 -0400, Phil Hobbs >>>>> <pcdhSpamMeSenseless@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. >>>>> >>>>> 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. >>>>> >>>>> 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. >>>>> >>>> >>>> Absolutely. The scary 152 dB number doesn't mean that doing something >>>> like that is automatically a bad idea. >>>> >>>> Being an old RF and ultrastable laser guy, though, it does make my ears >>>> perk up. ;) >>>> >>>> Cheers >>>> >>>> Phil Hobbs >>> >>> I like thermostats, single-bit-feedback control loops. >>> >>> We have a couple of boxes that do fan control based on interior >>> temperature. Once a second, if it's above the setpoint, ratchet fan >>> speed up some fixed amount, 1% maybe. If it's cooler than the >>> setpoint, step fan speed down. There's no acoustic drama and it's >>> perfectly stable. >>> >>> It dithers around the setpoint but nobody notices. >>> >>> This is immune to classic control theory so the concept annoys some >>> people but it works great. >> >> A real old time control guy like Tim Wescott would probably be a fan >> too--the great virtue of a bang-bang controller is that (as you say) >> it's highly resistant to variations in the _plant_. >> >> Your furnace doesn't go nuts when you have a Christmas party, even >> though all those people generate a lot of heat, and there's lots of >> opening and closing of doors and ovens. >> >> Cheers >> >> Phil Hobbs > > My power supply box has 8 plugin modules of various types. Some don't > need much air but some really do. The two big fans are howlers at 48 > volts. > > Each module can present one bit to the motherboard software: I want > more air, or I don't. If any board wants more, ratchet the fans up a > bit. If none want more, jog the fans down. >Yup. We do the Class H supplies for our TEC driver boards like that--if the linear amp rails, immediately jack up the supply by 0.5 V or so, then gradually ramp it down again. We could use two ADC channels, of course, but one comparator is simpler and works very well. 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
