Re: [time-nuts] femtosecond jitter anyone?
Mike Monett wrote: Chris The biggest problem with the OCXO is probably that it has a square wave output. With careful design it is possible to achieve a jitter of a few tens of femtosec for a logic level output from a limiter, but the OCXO designers are unlikely to have used such a limiter. [...] Bruce Bruce This would be an excellent subject for a tutorial on precision system design. Do you have any links to support your claim of a tens of femtosec for a logic level from a limiter? I am not aware of any logic family that can support that jitter performance. When you post items that stretch the state of the art, it would be nice if you would show us all how to do the same. Mike ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. Some ECL devices have jitter specs in the 100 to 200fsec range. see: http://www.onsemi.com A jitter of a few tens of femtosec is achieved by some clock drivers: http://www.analog.com/en/clock-and-timing/clock-generation-and-distribution/adclk905/products/product.html http://www.analog.com/en/clock-and-timing/clock-generation-and-distribution/adclk925/products/product.html Some ADC's have internal sampling jitter of a few tens of femtosec: http://www.analog.com/en/analog-to-digital-converters/ad-converters/ad9461/products/product.html However a very clean low noise source is required. Achieving a jitter of less than a 1picosec at 10MHz with a well designed limiter/filter cascade and a good source isn't too difficult, however the intrinsic random jitter of most common logic families is indeed a limiting factor. (HCOS inverters have typical random jitter of ~ 4ps, ACMOS inverters have an intrinsic random jitter of ~ 1ps faster CMOS families have lower random jitter). However such jitter can only be achieved if the logic gate input signal slew rate is fast enough or the gates input noise will increase the jitter. The achievable jitter increases as the input signal slew rate decreases (ie a s the frequency decreases for a fixed amplitude sinewave input). 100ns jitter at 1Hz due to the limiter noise is routine (around 10ns should be possible). JPL achieved 100ns decades ago. 10ns jitter at 10Hz due to limiter noise is relatively easy whilst a potential jitter of around 1ns rms is achievable. However a clean low noise input signal is required. Bruce ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Fluke, and E1938
Thanks for the tip Magnus! One comment on the 1938 discussion: I recently fired-up my E1938 disciplined by our Fury GPSDO. It took 3+ days for the retrace (aging) to slow down significantly. Now it's working really well. Said In a message dated 4/11/2009 13:53:41 Pacific Daylight Time, mag...@rubidium.dyndns.org writes: You might have better success with finding them rebranded as Fluke. This is a result of the old Fluke-Philips cooperation. Look for Fluke PM 6690. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] time-nuts Rockwell Jupiter 12-channel GPS receiver OEM module
Mike Monett wrote: If nobody has already mentioned it, Fluke.1 has the Rockwell Tu00-D205 high performance 12-channel GPS receiver OEM modules for $9.99 ea with free shipping worldwide, item number: 290306684157 These appear to have the 10KHz output described at http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm This website sells them too: http://www.weirdstuff.com/cgi-bin/item/20164 I wonder if it is suitable for use in a GPSDO? It looks quite different to the Jupiter T used in James Miller's design and the other Jupiters that Fluke.1 has sold in the past. -- Linux 2.6.26 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] time-nuts Rockwell Jupiter 12-channel GPS receiver OEM module
Hi Gents, It is actually a Rockwell Microtracker gps module and not a JUPITER board!! 5channel +5V board... also the 20pin header is different then the JUPITER. I still have a few docs . Please contact offline if you need more. Rgds Ernie. -Original Message- From: Eamon Skelton nos...@oceanfree.net To: Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Mon, 13 Apr 2009 11:43 am Subject: Re: [time-nuts] time-nuts Rockwell Jupiter 12-channel GPS receiver OEM module Mike Monett wrote: If nobody has already mentioned it, Fluke.1 has the Rockwell Tu00-D205 high performance 12-channel GPS receiver OEM modules for $9.99 ea with free shipping worldwide, item number: 290306684157 These appear to have the 10KHz output described at http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm This website sells them too: http://www.weirdstuff.com/cgi-bin/item/20164 I wonder if it is suitable for use in a GPSDO? It looks quite different to the Jupiter T used in James Miller's design and the other Jupiters that Fluke.1 has sold in the past. -- Linux 2.6.26 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Characterising frequency standards
Steve Rooke wrote: Hi Mark, 2009/4/13 Mark Sims hol...@hotmail.com: Hello Steve, Try this... take Tom's sample data set, run the numbers. Then, using a good random number generator, make another data set by randomly throwing out half (or more) of the samples (to simulate a non ZDT counter). Run the numbers again. See how they change. This should give you a good idea of how using a standard counter would affect your adev numbers. But randomly throwing out data points would introduce ZDT. It would introduce dead-time, it would not introduce zero dead-time (ZDT). Dropping every second sample of a phase/time-error series can maintain the zero dead-time property, but you loose the resolution for higher taus. The whole point I was making was that the data set is well defined the missing data occurs every other sample therefore tau0 = 2 x (sample period of each sample). You can reduce the dataset size that way if you had phase/time-error samples and attain twice the tau, yes. The downside is that you also reduce the degrees of freedom in the dataset and thus the statistical precission. With a large enought dataset this may not be much of an issue. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Characterising frequency standards
Steve, Steve Rooke wrote: Bruce, 2009/4/12 Bruce Griffiths bruce.griffi...@xtra.co.nz: Steve Steve Rooke wrote: If I take two sequential phase readings from an input source and place this into one data set and aniother two readings from the same source but spaced by one cycle and put this in a second data set. From the first data set I can calculate ADEV for tau = 1s and can calculate ADEV for tau = 2 sec from the second data set. If I now pre-process the data in the second set to remove all the effects of drift (given that I have already determined this), I now have two 1 sec samples which show a statistical difference and can be fed to ADEV with a tau0 = 1 sec producing a result for tau = 1 sec. The results from this second calculation should show equal accuracy as that using the first data set (given the limited size of the data set). You need to give far more detail as its unclear exactly what you are doing with what samples. Label all the phase samples and then show which samples belong to which data set. Also need to show clearly what you mean by skipping a cycle. Say I have a 1Hz input source and my counter measures the period of the first cycle and assigns this to A1. At the end of the first cycle the counter is able to be rest and re-triggered to capture the second cycle and assign this to A2. So far 2 sec have passed and I have two readings in data set A. Strange counter. Traditionally counters rests after the stop event have occured, since they cannot know anything else.The Gate time gives a hint on the first point in time it can trigger, the gate just arms the stop event. There is no real end point. It can however rest and retrigger the start event ASAP when gate times are sufficiently large. It's just a smart rearrangement of what to do when to achieve zero dead-time for period/frequency measurements. You could also use a counter which is pseudo zero dead time in that it can time-stamp three values, two differences without deadtime but has deadtime after that. Essentially two counters where the stop event of the first is the start event of the next. I now repeat the experiment and assign the measurement of the first period to B1. The counter I am using this time is unable to stop at the end of the first measurement and retrigger immediately so I'm unable to measure the second cycle but is left in the armed position. When the third cycle starts, the counter triggers and completes the measurement of the third cycle which is now assigned to B2. This is what most normal counters do. For the purposes of my original text, the first data set refers to A1 A2. Similarly the second data set refers to B1 B2. Reference to pre-processing of the second data set refers to mathematically removing the effects of drift from B1 B2 to produce a third data set which is used as the data input for an ADEV calculation where tau0 = 1 sec with output of tau = 1 sec. You would need to use bias adjustments, but the B1 B2 period/frequency samples is badly tainted data and should not be used.having a deadtime at the size of tau0 is serious bussness. Removing the phase drift over the dead time does not aid you since if you remove the phase ramp of the evolving clock, that of f*t or v*t (depending on which normalisation you prefer), you have the background phase noise. What we want to do is to characterize this phase noise. Taking two samples of it back-to-back and taking two samples with a (equalent sized length) gap becomes two different filters. Maybe some ascii art may aid: __ __ | |__ |__| y1 y2 y3 A1 A2 A2-A1 = y2-y1 vs. __ ____| |__ |__| y1 y2 y3 B1B2 B2-B1 = y3-y1 Consider now the case when frequency samples has twice the tau of the above examples _ __ | |__ |_| y1y2 y2-y1 These examples where all based on sequences of frequency measurements, just as you indicate in your caes. As you see on the differences, the nominal frequency cancels and the nominal phase error has also cancled out, so there is nothing to compensate there. Drift rate would however not be canceled, but for most of our sources, the noise is higher than the drift rate for shorter taus. Time-differences allows us to skip every other cycle thought. I now collect a large data set but with a single cycle skipped between each sample. I feed this into ADEV using tau0 = 2 sec to produce tau results = 2 sec. I then pre-process the data to remove any drift and feed this to ADEV with a tau0 = 1 sec to produce just the tau = 1 sec result. I now have a complete set of results for tau = 1 sec. Agreed, there is the issue of modulation at 1/2 input f but ignoring this for the moment, this should give a valid result. Again you need to give more detail. In this case the data set is constructed from the measurement of the cycle periods of a 1Hz input source where even cycles are
Re: [time-nuts] HP 1938 revisited
J. L. Trantham wrote: Thanks for the info. My plan is to develop a stable GPS disciplined reference suitable for use as a reference for Microwave work in the 10 GHz range that can be used in portable locations with relatively quick start up. Perhaps the 1938 would be better in the shop where it could be left on for weeks/months at a time and an LPRO 101 or a 10811 could be used for the portable application. Reasonably high drift rates could be accommodated if the GPS signal is reliable and the time constants (disciplining rate, drift rate, etc.) are appropriate. Thanks, Joe The advantage of the E1938A over the 10811 is that it has very low tempco, which comes into play in the holdover mode (no GPS). If you always have GPS, you might as well just use a 10811. The 5071A cesium clock uses a 10811 that is disciplined all the time. There would be no advantage in replacing it with an E1938A. The loop in the 5071A is fairly slow, but would be considered fast by GPS standards. If the portable location involves severe temperature fluctuations, then possibly the E1938A could make sense. Rick Karlquist N6RK E1938A designer ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Spring cleaning and antenna placement
Fellow timenuts, You know you are a time-nut when your spring cleanup dutines involves getting onto the rooftop and clean your GPS antennas. In my case my dual frequency antenna (really a survey antenna, a Novatel GPS 502 L1/L2) was due to lack of patience temporarily put up there in a not so fixed position, so that the wind shifter position and attitude as it slided down the tiles that it was effectively useless, especially as it was pointing mostly north into the big GPS hole and putting the antennas nuling power towards most of the low-attitude sats. Let's just say that I'm not too proud of that mounting... I know better, I know far better... but I am impatient to play with the news toys, so I hope people recognice the feeling. My biggest problem is that I have a large TV antenna pole sitting ontop of one of the chimneys. It is needed as the house is partly down a valey to get a nice and clean TV signal. Fairly new also, as the old antennas was broken. I have got part of the gear to mount the GPS antennas on the other chimney, but lack the proper antenna tubes. My big L1 choke rings (AT575) can be mounted without the central tube but I wish to use the combination of that and the three outer mounting screws. I need to build a mounting platform for it anyway. Besides, I think the way I pull cables and what cables I use could improve. My RG 58 is certainly not optimum, but it is cheap, easily available alongside connectors and suitable strip and crimp tools. So... hints for tubes and other mounting aspects would be nice. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Spring cleaning and antenna placement
Hi Magnus: Why not just put the GPS antenna at the top of the existing TV mast? http://www.prc68.com/I/Ant.shtml#SBant Have Fun, Brooke Clarke http://www.PRC68.com Fellow timenuts, You know you are a time-nut when your spring cleanup dutines involves getting onto the rooftop and clean your GPS antennas. In my case my dual frequency antenna (really a survey antenna, a Novatel GPS 502 L1/L2) was due to lack of patience temporarily put up there in a not so fixed position, so that the wind shifter position and attitude as it slided down the tiles that it was effectively useless, especially as it was pointing mostly north into the big GPS hole and putting the antennas nuling power towards most of the low-attitude sats. Let's just say that I'm not too proud of that mounting... I know better, I know far better... but I am impatient to play with the news toys, so I hope people recognice the feeling. My biggest problem is that I have a large TV antenna pole sitting ontop of one of the chimneys. It is needed as the house is partly down a valey to get a nice and clean TV signal. Fairly new also, as the old antennas was broken. I have got part of the gear to mount the GPS antennas on the other chimney, but lack the proper antenna tubes. My big L1 choke rings (AT575) can be mounted without the central tube but I wish to use the combination of that and the three outer mounting screws. I need to build a mounting platform for it anyway. Besides, I think the way I pull cables and what cables I use could improve. My RG 58 is certainly not optimum, but it is cheap, easily available alongside connectors and suitable strip and crimp tools. So... hints for tubes and other mounting aspects would be nice. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Spring cleaning and antenna placement
bro...@pacific.net skrev: Hi Magnus: Why not just put the GPS antenna at the top of the existing TV mast? http://www.prc68.com/I/Ant.shtml#SBant It would be a little bit of a challenge... I think I have some 4-5 meter of additional heigth, the tiling is slippery and then antenna rig is kind of heavy, so it is not one of those things you do alone. At least I don't do it alone... I'm not a well trained radio amateur and the rooftop is not very well suited for elaborate exercises... I sure would like to have things like safety-line when working up there. I don't feel that safe up there. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Spring cleaning and antenna placement
Hi Magnus: Safety is the # 1 concern. It may be that if you can minimize the multipath problems having the GPS antenna near the roof is not a problem? I'm starting to do some antenna related tests using the Polaris Guide (DAGR) GPS receiver, see: http://www.prc68.com/I/DAGR.shtml#GLS and scroll down for some zero base line tests of how well it does carrier phase distance and angle measurements (aka Gun Laying, Azimuth Determination, North Finding, Relative Survey Mode). Another test I'd like to do is measure multipath, but so far have not come up with a direct way to do it other that infer based on how poor/good some test result is. For example are the variations in the above tests due to multipath? Have Fun, Brooke http://www.PRC68.com bro...@pacific.net skrev: Hi Magnus: Why not just put the GPS antenna at the top of the existing TV mast? http://www.prc68.com/I/Ant.shtml#SBant It would be a little bit of a challenge... I think I have some 4-5 meter of additional heigth, the tiling is slippery and then antenna rig is kind of heavy, so it is not one of those things you do alone. At least I don't do it alone... I'm not a well trained radio amateur and the rooftop is not very well suited for elaborate exercises... I sure would like to have things like safety-line when working up there. I don't feel that safe up there. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Characterising frequency standards
2009/4/13 Magnus Danielson mag...@rubidium.dyndns.org: But randomly throwing out data points would introduce ZDT. It would introduce dead-time, it would not introduce zero dead-time (ZDT). Dropping every second sample of a phase/time-error series can maintain the zero dead-time property, but you loose the resolution for higher taus. First rule of posting, engage brain before typing :-) Yes, of course you are right Magnus and that was what I really meant. 73, Steve -- Steve Rooke - ZL3TUV G8KVD JAKDTTNW Omnium finis imminet ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Spring cleaning and antenna placement
Joseph M Gwinn skrev: time-nuts-boun...@febo.com wrote on 04/13/2009 11:32:48 AM: bro...@pacific.net skrev: Hi Magnus: Why not just put the GPS antenna at the top of the existing TV mast? http://www.prc68.com/I/Ant.shtml#SBant It would be a little bit of a challenge... I think I have some 4-5 meter of additional height, the tiling is slippery and then antenna rig is kind of heavy, so it is not one of those things you do alone. At least I don't do it alone... I'm not a well trained radio amateur and the rooftop is not very well suited for elaborate exercises... I sure would like to have things like safety-line when working up there. I don't feel that safe up there. I don't know what it costs in Sweden, but in the US one can hire roofers or antenna installers to do such things, and they have the equipment. How did the TV mast get on the roof? A professional TV mast rigger did the job. I had a look at it again... and I'd say it is more like 3 meters up from the chimney, but it is still 4 m up from the roof at the chimney so it is a bit hard to get to. Anyway, the repositioned antenna now makes the borrowed Ashtech Z-12 lock up at L1 and L2 for sufficient amount of sats, but the S/N is lowish, which I think I will blame on my RG-58, not too professionally done coax work... could TDR it thought. Hmm... why didn't I think of that? Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Characterising frequency standards
Steve Rooke skrev: 2009/4/13 Magnus Danielson mag...@rubidium.dyndns.org: But randomly throwing out data points would introduce ZDT. It would introduce dead-time, it would not introduce zero dead-time (ZDT). Dropping every second sample of a phase/time-error series can maintain the zero dead-time property, but you loose the resolution for higher taus. First rule of posting, engage brain before typing :-) Yes, of course you are right Magnus and that was what I really meant. I thought that you just made the mistake... :) Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Spring cleaning and antenna placement
Magnus, If you replace your RG-58 with RG-6 TV coax, your loss issues will disappear. The difference in impedance (75 ohms instead of 50) is not enough to cause problems. Adapters to the pre-fitted F connectors are readily available. And it's not expensive. Regards, Peter Magnus Danielson wrote: Joseph M Gwinn skrev: [1]time-nuts-boun...@febo.com wrote on 04/13/2009 11:32:48 AM: [2]bro...@pacific.net skrev: Hi Magnus: Why not just put the GPS antenna at the top of the existing TV mast? [3]http://www.prc68.com/I/Ant.shtml#SBant It would be a little bit of a challenge... I think I have some 4-5 meter of additional height, the tiling is slippery and then antenna rig is kind of heavy, so it is not one of those things you do alone. At least I don't do it alone... I'm not a well trained radio amateur and the rooftop is not very well suited for elaborate exercises... I sure would like to have things like safety-line when working up there. I don't feel that safe up there. I don't know what it costs in Sweden, but in the US one can hire roofers or antenna installers to do such things, and they have the equipment. How did the TV mast get on the roof? A professional TV mast rigger did the job. I had a look at it again... and I'd say it is more like 3 meters up from the chimney, but it is still 4 m up from the roof at the chimney so it is a bit hard to get to. Anyway, the repositioned antenna now makes the borrowed Ashtech Z-12 lock up at L1 and L2 for sufficient amount of sats, but the S/N is lowish, which I think I will blame on my RG-58, not too professionally done coax work... could TDR it thought. Hmm... why didn't I think of that? Cheers, Magnus ___ time-nuts mailing list -- [4]time-n...@febo.com To unsubscribe, go to [5]https://www.febo.com/cgi-bin/mailman/listinfo/time-nut s and follow the instructions there. References 1. mailto:time-nuts-boun...@febo.com 2. mailto:bro...@pacific.net 3. http://www.prc68.com/I/Ant.shtml#SBant 4. mailto:time-nuts@febo.com 5. https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Power Supplies for Rubidiums OCXOs - etc...
I recently purchased a lot of 24 volt 5 amp switching power supplies. Well, they are marked 5 amps, but, I was able to get 7 out of them without any problem, with the exception that my resistive load was getting too hot. These are very nice supplies, look like an oversized laptop power supply. I have a bunch of Rubidium sources that require 24 volts and these obviously would be great. Packaged, with AC cord, they weigh a little less than 3 pounds. If you are interested in obtaining one or more, let me know. I would like $35 each, plus shipping. I do not think you will be disappointed. There are a lot of possible applications, besides Rubidiums and OCXOs. Regards - Mike Mike B. Feher, N4FS 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Spring cleaning and antenna placement
time-nuts-boun...@febo.com wrote on 04/13/2009 11:32:48 AM: bro...@pacific.net skrev: Hi Magnus: Why not just put the GPS antenna at the top of the existing TV mast? http://www.prc68.com/I/Ant.shtml#SBant It would be a little bit of a challenge... I think I have some 4-5 meter of additional height, the tiling is slippery and then antenna rig is kind of heavy, so it is not one of those things you do alone. At least I don't do it alone... I'm not a well trained radio amateur and the rooftop is not very well suited for elaborate exercises... I sure would like to have things like safety-line when working up there. I don't feel that safe up there. I don't know what it costs in Sweden, but in the US one can hire roofers or antenna installers to do such things, and they have the equipment. How did the TV mast get on the roof? Joe Gwinn ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Characterising frequency standards
The whole purpose of taking a data set from a known ZDT counter and then throwing out random samples is to simulate the kind of data that a normal counter would produce. You could compare the results and get an idea of how using a normal counter for calculating adevs would compare to using a ZDT counter. I would start by generating random numbers from 0-3 and throwing out that many samples. With most normal counters you cannot guarantee that you would get a sample every other interval. It all depends upon how the counter works, what its timebase is, how it triggers and retriggers, how it is being read out, what the input signal is, etc. I would suspect that most counters would give a reading every two or three intervals. I have seen some counters give two consecutive back-to-back readings then a long dead time. But randomly throwing out data points would introduce ZDT. The whole point I was making was that the data set is well defined the missing data occurs every other sample therefore tau0 = 2 x (sample period of each sample). _ Windows Live™: Keep your life in sync. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_042009 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Characterising frequency standards
Mark, Mark Sims skrev: The whole purpose of taking a data set from a known ZDT counter and then throwing out random samples is to simulate the kind of data that a normal counter would produce. You could compare the results and get an idea of how using a normal counter for calculating adevs would compare to using a ZDT counter. I would start by generating random numbers from 0-3 and throwing out that many samples. With most normal counters you cannot guarantee that you would get a sample every other interval. It all depends upon how the counter works, what its timebase is, how it triggers and retriggers, how it is being read out, what the input signal is, etc. I would suspect that most counters would give a reading every two or three intervals. I have seen some counters give two consecutive back-to-back readings then a long dead time. Most counters I know of would make one frequency measure, then miss the directly following just to trigger directly ontop the next, those for a PPS pulse it would measure the period between the first and second pulse, then dwell until the third pps pulse and measure until the fourth pulse, but then happily repeat this pattern. But measuring frequency/period like this is not very useful for post-processing in any Allan Deviation measure. The lack of back-to-back measures prohibits you from achieving the data you need. We rather use time-interval measures. Let's consider the same counter, we arm it with a PPS pulse from either of the sources, but then measure the time interval between two 1 kHz variants of the signal, or use the PPS as start of the TI and the stop channel sees the 1 kHz signal, I'll use the later as a reference, but the cases are equalent. The same counter can now dwell between the measurements, but most counters can withstand 1 measurement per second without too much trouble. The 1 kHz signal allow for a maximum of 1 ms delay from arming/start trigger to stop trigger. This still allows for plenty of time for the counter post-processing to occur and re-arming. As the clocks drift, dynamically would stop-channels choice of 1 kHz flanks shift, but it would be a fairly simple task to post-process that into a continous stream of PPS marks. Using these time-interval measures of tau0 being 1 s, we can now make any set of back-to-back frequency measures as we please, as long as they are integer multiples of tau0 by dropping n-1 samples inbetween and recall that the sample-series has converted to a tau0 of n seconds. We can also use the series directly for the Allan Deviation estimator of choice in either time or frequency form. Thus, the lack of zero dead time does not necesserilly prohibits the use, but care in setting up the signals and I/O can curcumvent the problem. Many counters is being used one way or another for continuous measures even if they are not exclusive ZDT counters, but it takes care. Having one or two of TVBs PIC dividers at hand should certainly be handy for doing tricks like this. Time-resolution of the counters as well as trigger noise may be issues to look at. When do one need true ZDT counters then? Well, if you want to make measurements for higher frequency modulations, you need that power, but most of the time they are just very handy tools. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
