Hi Ulrich: Thanks very much for your email of 16 December it's a big help for me to understand how to use Allan plots. I would like to learn more about their application to Time Interval Counters. For example I have the SR620 and although the one shot resolution is 1 ps the one shot precision is specified as 20 ps. What test can be done to determine an Allan plot for a TIC?
Have Fun, Brooke Clarke w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Ulrich Bangert wrote: >Brooks, Brooke and Bruce, > >1) I do not want to talk bad Brooks Shera's design. In fact i admire it >a lot for its simpicity. It was the first to be published in amateur >literature and that makes it easily the best available in amateur use >for a long time. And I learned lots from it. Indeed i needed weeks to >understand how some subtle ingredients go ahead hand in hand to make the >whole thing work, the short measurement times that i talked about being >one of them. The original QEX publication was surely a breakthrough in >amateur technology. > >2) One of the things that the original publication lacks is a in-depth >rule on how to set the loop time constant correct for a given LO. When i >was new into this topic it was kind of my belief that choosing this >parameter correct was the 'black art' in constructing a good frequency >standard and I wanted to learn more about it. Today i know, that only >ONE SIMPLE RULE applies to this question despite the fact that some math >for drawing tau-sigma-diagrams is indispensable. > >3) This rule is: An OXCO has a banana like tau-sigma-diagram with the >lowest ADEV anywhere between 10-100 s. A GPS receiver's >tau-sigma-diagram is a straight line with a slope of -1 starting >anywhere from ADEV 2E-8 @ 1 s to 1E-7 @ 1 s depending on the receiver. >Note that these receiver figures apply to raw, not sawtooth corrected >values. Now have a look to where the lines meet each other. Left from >that point the OCXO's ADEV is smaller then the GPS receiver's. Right >from that point the GPS receiver's ADEV is smaller the the OCXO's. There >is no guessing or speculating at all: The loop time constant MUST be set >to where the meeting point is. If it is set to anything else this will >make the ADEV of the standard's output more worse than is necessary. >Note that the OCXO's tau-sigma is already on its ASCENDING slope where >the lines meet. > >4) From that simple rule a complete briefing for the construction of a >good frequency standard may be deduced: > >a) Because left of the meeting point the standard's output stability is >only a function of the OCXO's stability and NOTHING ELSE choose the best >available LO in terms of ADEV up to the expected meeting point of the >lines. For this purpose a GOOD xtal oscillator may by all means be >better than a Rb! Perhaps the people that are going to discipline a Rb >with GPS may be disappointed: While the Rb is much easier to discipline >due to its smaller sensibility to environmental changes a good xtal >oscillator (the key word is: good. And good means: better than a >HP10811) may outperform a Rb based standard in terms of ADEV for short >observation times. @1 to some 10 s the HP10811 is better in ADEV than >most Rbs. However up from there its ADEV goes up steeper than that of an >thermically better managed USO like a FTS1000/1200. An even better >choice but beyond the financial scope of most of us were a BVA based >oscillator. > >b) Because the meesting point is always on the the OCXO's ascending >slope choose the best available receiver in terms of how high it's -1 >slope tau-sigma is located. The less high the absolute position of his >tau-sigma is, the more left (=earlier) the meeting point will be. The >more left the meeting point is the less the overall ADEV of the >standard's output will be deteriored by the OCXO for observation times >near the meeting point due to its ascending ADEV slope. > >c) The TIC measurement resolution must be high enough to not deterior >the phase measurements by the sheer measurement 'granularity'. > >Some graphics might be helpfull in understanding this. Have a look to >page 22 of > >http://www.ulrich-bangert.de/AMSAT-Journal.pdf > >which i wrote for the German AMSAT journal. Don't worry over the German, >just look to the pictures. In this graphic both the tau-sigma of a >HP10811 and a M12+ are drawn into the same diagram and according to 4) >it becomes immediatly clear why we want the OCXO as stable as possible >before the meeting point and the receivers tau-sigma as low as possible >to make the meeting point as early as possible. > >Exactly this is the point where i fear that you, Brookes, are the victim >of a basic misconception, at least your comment makes me think so: > > > >>>I believe the sawtooth correction is of little or no value for a >>>GPSDO, >>>which typically requires a low pass filter between the GPS >>> >>> >>1pps and the >> >> >>>disciplined oscillator. This filter is quite effective in >>> >>> >>removing the >> >> >>>sawtooth quantization introduced by the GPS rcvr clock, >>> >>> >>just as it removes >> >> >>>the similiar quantization caused by my phase detector. >>> >>> > >This indicates that you are believing that it can all be done with low >pass filtering. And this is wrong for two reasons: > >a) As Bruce and TVB have pointed out there are 'anomalies' in a GPS >receiver's raw pps (well documented on TVB's web pages) where the idea >that lowpass filtering the raw phase data will do the job is simply >unsustainable. > >b) Low pass filtering is a trade with nature: You can get better >precision due to low pass filtering but you have to pay for it in terms >of time that you have to wait for the samples to avarage over. Look >again at page 22 of > >http://www.ulrich-bangert.de/AMSAT-Journal.pdf > >and ask yourself what the noisefloor of you circuit would look like in >this diagram. I tell you: Even if you had the best current GPS receiver >available your phase measurements would be dominated by a noisefloor >induced by the 4E-8s single shot resolution of your TIC giving a >straight line starting at 4E-8 @ 1 s and having a slope of -1 i.e. a >line that runs parallel to the M12+ graph but a factor 2 higher in >absolute terms. Low pass filtering = averaging means nothing else than >running up and down the line. Go to any point of time on the horizontal >axis and draw a vertical line there. Where this line meets your >noisefloor draw a horizontal line and on the vertical axis read the >precision that you gain if you average over that time. It is as easy as >that. And to find out when you reach a certain precision go to that >precision on the vertical axix and draw a horizontal line. Where this >line meets your noisefloor draw a vertical line and read the necessary >averiging time on the horizontal axis. And note that this horizontal >line drawn in the last example has crossed the M12+'s line by a factor >of 2 earlier! That is: the sheer measurement resolution of 4E-8 s has >DOUBLED the averaging time necessary to come to a certain given >precision. > >At a first glance this may be not so impressive: Instead of 10 s we have >to wait 20 s with your circuit to get the precision that the receiver >alone has already after 10 s. Why do I make that heck out of it? Don't >we have these 10 additional seconds? Please read on: The M12+'s >sigma-tau shown un the diagram is the one for the raw phase data. If the >sawtooth correction is taken into account the line starts at an ADEV of >2E-9 @ 1 s. Unfortunately its slope is less than -1 so the factor 10 >increase in precision does not hold for all oservation times. At >observation times of app. 1 day the two lines meet, giving an >improvement in using the sawtooth corrected values only for observation >times < 1 day. In > >http://www.ulrich-bangert.de/html/photo_gallery_44.html > >you can see the sigma-taus for the raw and the corrected data from a >M12+ drawn into the same diagram. With a good OCXO the meeting point >between receiver tau-sigma and OCXO tau-sigma is in to order of 1000 s. >1000 s are small against a day, that means that almost the full possible >improvement in ADEV by using the sawtooth corrected values apply to the >case of a loop time constant of 1000. This factor of 10 in conjunction >with the factor of 2 that we had before results in the factor of 20 that >i claim that the noisefloor ot your circuit is inferior to that of a >modern GPS receiver. And of course my claim stays intact! > >Some of you may now scratch your head and say: "Well...yes 20 is a >handfull! With the Shera circuit we will have to wait 20 times the time >that is necessary due to GPS 1pps jitter alone, but isn't it more >important that we reach this precision/stability (in a sense these two >words are synonyms in this discussion) AT ALL with the Shera circuit?" > >This is EXACTLY where the misconception starts. If someone is claiming: >"I can average over 30s to get an improved measurement precision." I am >going to ask him: "Hey, why don't you average over 300 s, giving you an >additional factor 10 improvement." The answer might be: "Yes, perhaps I >could do. It depends.." My next question were: "Depends? Depends on >what? If every factor 10 in measurement averaging results in a factor 10 >in measurement precision, why not even average over 30000 or 300000 s >??" I know the next answer very well in advance: "Oh no, i can't do >THAT. While the argument of improving the measurement precision is >right, i can't make use of this precision because my OCXO has drifted >too much away if I wait THIS long!" AAHH! You have to take your OCXO >into account? And yes, that is correct, but it is correct in a different >sense than you may think! > >It is correct in the sense that i tried to explain before: The >tau-sigmas of the OCXO and the receiver meet each other and where they >meet depends ONLY on > >a) receiver quality in terms of ADEV > >b) OCXO quality in terms of ADEV > >c) TIC's noise floor > >In reality you are not free to choose "I want to average over 30 s" or >"I want to average over 100 s". Instead the simple rule DICTATES that >you HAVE to set your averaging time to the meeting point's x-axis value >and to nothing else. There is simply no use in saying: "But with such >and such averaging times i would reach a precision of such and such". >You cannot choose! The physical properties of your receiver, your ocxo >and your TIC dictate it! > >Since we now know what 'averaging' is all about let us now consider >again at which ADEV the two tau-sigmas meet. Clearly we want to make the >ADEV at this point as small as possible as it represents a local maximum >in the overall tau-sigma of the standard's output. Since we are on the >ascending slope of the OCXO our interest must be that the lines meet AS >EARLY as possible. Since we cannot do anything on the -1 slope of the >receiver's tau-sigma we achieve this only by shifting the absolute >position of the tau-sigma as low as possible. This in turn is achieved >by using the best available receiver AND using the sawtooth correction. >A TIC resolution of 4E-8 shifts the meeting point a factor of 20 more to >the right than would be necessary with a good receiver. Since I admire >it a lot what you do, Brookes, i would be glad if you could gain the >insight that averaging over raw phase data is something VERY DIFFERENT >from using sowtooth corrected values. > > > >>Hi Ulrich: >> >>I think the answer is what other low cost options >>are available? I would like to have a more modern >>TIC capability to add to the clock I'm working on. >>But although there's been a lot of discussion about >>different ways of making TIC measurements, it's not >>clear to me how to do it on a budget. >> >>For example the TIC232 circuit by Richard H McCorkle >>is easy to implement, but how good is it's noise floor. >>See: >> >>http://www.piclist.com/techref/member/RHM-SSS-SC4/TIC232.htm >> >>Then there's the low cost frequency counting TIC that appeared >>in QEX that we know trades performance for low cost so it's >>not a candidate. >> >>Any ideas on what circuits have a noise floor that's compatible >>with the M12+T or it's newer equivalents and at the same time are >>in the low cost category? >> >> > >Brooke, looking at the web page and the circuit diagram I second >everything that Bruce has already said to it. This one uses a 16 MHz TIC >time base and therefore its performance is even worse compared to >Brooks's circuit. This one has its tau-sigma starting point at 62E-9 @ 1 >s, abt. 30 times worse than the M12+. > >If it can be done 'on a budget' as you say depends a bit on what you >would call 'a budget' but it can surely not being done better if you >have the Shera design prices in your head! In my own DIY GPDSO I do it >using a delay chain out of the fastest interconnection elements >available in a ALTERA Flex10K10 gate array, giving 110 ps resolution. >That chip is surely not more than 50 US$ in single quantities. >Unfortunately the delay of a single element of this delay line depends >on chip temperature and supply voltage so that the lines need to be >'calibrated' on a cyclic base. While this is done in the controllers >firmware it makes the whole circuit a bit tricky. I currently try to >migrate the design into a XILINX Spartan III fpga XC3S400 worth 25 US$ >in single quantities. Let us see what 2007 has to bring for us. > > > >>One can only achieve the subnanosecond resolution required to avoid >>degrading the performance of an M12+ by using a clock >>frequency of 1GHz or more. Thus this method is probably too >>expensive and difficult to implement. >> >> > >Bruce, the clue is NOT to go out for a high clock frequency. Instead >search for sub-clock interpolation schemes. Lots of them are available! > >Best regards >Ulrich Bangert, DF6JB > > > > >>-----Ursprüngliche Nachricht----- >>Von: [EMAIL PROTECTED] >>[mailto:[EMAIL PROTECTED] Im Auftrag von Dr Bruce Griffiths >>Gesendet: Samstag, 16. Dezember 2006 02:00 >>An: Brooks Shera; Discussion of precise time and frequency measurement >>Betreff: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS >>locking circuit >> >> >>Brooks Shera wrote: >> >> >>>----- Original Message ----- >>>From: "Ulrich Bangert" <[EMAIL PROTECTED]> >>>To: "'Discussion of precise time and frequency measurement'" >>><[email protected]> >>>Sent: Friday, December 15, 2006 05:47 >>>Subject: Re: [time-nuts] LPRO-101 with Brooks Shera's GPS >>> >>> >>locking circuit >> >> >>>....... >>> >>> >>> >>>>I second Bruces's opinion about what is an overshot or >>>> >>>> >>not. When ps >> >> >>>>reolution is ready available then why not use it? I attach >>>> >>>> >>a online >> >> >>>>output from my DIY GPSDO from a few minutes ago that shows >>>> >>>> >>the M12+'s >> >> >>>>signal properties when measured with abt. 110 ps >>>> >>>> >>resolution against a >> >> >>>>FTS1200. The yellow line reperesents a prefiltered version of the >>>>sawtooth corrected values (blue). The filter time constant >>>> >>>> >>is 1/3 of >> >> >>>>the loop time constant as in a PRS-10. The yellow values >>>> >>>> >>are the ones >> >> >>>>to feed the regulation loop. >>>> >>>> >>>> >>> >>> >>> >>>>What I wanted to explain is the Shera concept noise floor >>>> >>>> >>is a factor >> >> >>>>20 above what a modern receiver can deliver (again inc. >>>> >>>> >>the sawtoth >> >> >>>>correction). And yes, you are right: There were different numbers >>>>when this concept was thought out! And exactly because different >>>>number were there when this concept was thougt out I am >>>> >>>> >>going to ask >> >> >>>>why people still built it today. >>>> >>>> >>>> >>> >>> >>> >>>>Best regards >>>>Ulrich Bangert, DF6JB >>>> >>>> >>>> >>>I believe the sawtooth correction is of little or no value for a >>>GPSDO, >>>which typically requires a low pass filter between the GPS >>> >>> >>1pps and the >> >> >>>disciplined oscillator. This filter is quite effective in >>> >>> >>removing the >> >> >>>sawtooth quantization introduced by the GPS rcvr clock, >>> >>> >>just as it removes >> >> >>>the similiar quantization caused by my phase detector. >>> >>>For example, reading from your graph I averaged the raw >>> >>> >>data (as best >> >> >>>I >>>could by reading the blue line). The running average of >>> >>> >>the raw data over >> >> >>>40 sec (starting at 12:31:30) was -4.5 nsec, after 60 sec >>> >>> >>it was -4.2 nsec. >> >> >>>These values appear to be indistinguishable from the values >>> >>> >>you get by >> >> >>>averaging the "sawtooth corrected" data (the yellow line). >>> >>>It appears from your plot that the sawtooth correction has >>> >>> >>contributed very >> >> >>>little or nothing that averaging does not already provide. Have I >>>misunderstand something? >>> >>>I believe that your "noise floor is a factor 20 above what a modern >>>receiver >>>can deliver" statement is incorrect. With an HP 5720B >>> >>> >>(about 100 psec >> >> >>>resolution), I have measured the phase difference between >>> >>> >>the GPS 1pps and >> >> >>>the phase of a 5 MHz oscillator controlled by my >>> >>> >>controller. This data has >> >> >>>been compared with simultaneous phase serial output from >>> >>> >>the controller as >> >> >>>determined its maligned 24 MHz asynchronous internal phase >>> >>> >>measurement >> >> >>>circuitry. >>> >>>ADEV Stable 32 plots of both data sets are essentially identical. >>>From this >>>I conclude that nothing would be gained, for the purpose of >>> >>> >>discipling an >> >> >>>oscillator, by using a more elaborate and expensive phase >>> >>> >>detector (the >> >> >>>phase detector in my controller costs $6.61, including >>> >>> >>$5.35 for the dual 24 >> >> >>>MHz osc that is shared as the PIC clock). It was my goal >>> >>> >>when I designed >> >> >>>the controller was to make the design transparent to the >>> >>> >>builder and to use >> >> >>>as few parts as necessary consistant with performance >>> >>> >>limited only by >> >> >>>available GPS receivers and VCXOs. When I wrote the QST >>> >>> >>article I was >> >> >>>totally ignorant of the fact that I could buy the HP58503 >>> >>> >>with similiar >> >> >>>performance for $5400. >>> >>>Your earlier comment about the capture range of the phase >>> >>> >>detector is >> >> >>>well >>>taken. For the past several years the PIC software I >>> >>> >>provide has included >> >> >>>an option designed for use with inexpensive TCVCXOs. It >>> >>> >>requires only an >> >> >>>external 128 divider chip and produces EFC voltages >>> >>> >>suitable for inexpensive >> >> >>>oscillators. It works very well and provides sufficient >>> >>> >>performance for >> >> >>>many applications. >>> >>>Regards, Brooks >>> >>> >>> >>> >>> >>> >>> >>> >>---------------------------------------------------------------------- >> >> >>>---------- >>> >>> >>> >>> >>> >>>>_______________________________________________ >>>>time-nuts mailing list >>>>[email protected] >>>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> >>>> >>>> >>>_______________________________________________ >>>time-nuts mailing list >>>[email protected] >>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> >>> >>> >>> >>Brooks >> >>Your comparison of your circuit with measurements taken with >>the "5270" >>(is this a typo? did you mean 5370? which is known to have >>differential >>non linearities well in excess of 100 picoseconds, at >>least according >>to the designers - some later modifications to the circuitry reduced >>this effect somewhat) demonstrates very little unless the >>measurements >>were corrected for the sawtooth error. >> >>The only true test is to compare a sawtooth corrected >>GPSDOCXO alongside >>a sawtooth corrected GPSDOXO. Both should of course use equivalent >>performance oscillators and GPS timing receivers. >> >>The short plot that Ulrich furnished doesn't include any >>hanging bridges >>which occur when the GPS oscillator drifts through a harmonic >>of 1Hz. Most M12+ GPS timing receivers produce sawtooth >>correction errors in >>which such "hanging bridges" are not infrequent. >> >>No one is disputing that with an low performance oscillator its not >>worth going to too much trouble in improving the phase >>detector performance. However some of us have oscillators >>with much better performance than >>such cheap oscillators. We also have a need to achieve an oscillator >>instability of less than a few parts in 1E12 which your >>circuit cannot >>reliably provide in the presence of hanging bridges and aberrant PPS >>pulses which do occur from time to time. >> >>The existence of a commercial GPSDOCXO that achieves an Allan >>variance >>of 2E-13 or better from tau = 1 sec to 1 year, indicates that it is >>possible to do much better than your circuit is capable of. >>All we are >>doing is exploring cheaper ways of approaching this >>performance within a >>factor of 10 or so. >> >>Bruce >> >>_______________________________________________ >>time-nuts mailing list >>[email protected] >>https://www.febo.com/cgi-> bin/mailman/listinfo/time-nuts >> >> >> > > >_______________________________________________ >time-nuts mailing list >[email protected] >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > > _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
