Re: [time-nuts] Making the most of SRS Rb source
On Wed, Jan 18, 2006 at 09:17:31AM -0700, David Forbes wrote: > Our need for accurate time/frequency is: > 1) Local oscillator frequency +/-1kHz at several hundred GigaHertz; > 2) Time correct to +/-10 milliseconds to keep the telescope pointed correctly. > > I suppose it would make a lot more sense to run the GPS 1PPS into the > Rb source so that it will discipline itself, and the phase error be > monitored to make sure it stays at zero. > > Am I barking up the right tree here? For some purposes, where short-term stability is needed, it's better to let it free run and just record the difference and tweak / reset it ocasionally at a time when you know people aren't using it. For our radar and VLBI observations that's the case, and to some extent for pulsar timing as well. Depending on the timescale and resolution requirements of your observations, either way could be better. But 1-us phase jumps sound pretty bad. Is that within spec for the Rb? That would wipe out my observations, and we used Rb until we got the H-maser for VLBI. -Mike ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
In message <[EMAIL PROTECTED]>, David Forbes writes: >At 6:43 AM -0800 1/18/06, James Maynard wrote: >I suppose it would make a lot more sense to run the GPS 1PPS into the >Rb source so that it will discipline itself, and the phase error be >monitored to make sure it stays at zero. > >Am I barking up the right tree here? Sorry if this is a bit longwinded: You would want to be quite carefull in your choice of timeconstant on the PRS10, if you set it too short you will ruin your short term stability and if you set it too long you will get worse performance than you have today. I've been playing a lot with the PRS10 and one of the things I found is that a raw PPS signal from a GPS is badly suited as input for the PRS10 because of the short term jitter (+/- N nanoseconds for some N > 20 typically, dependent on your GPS receiver. The PRS10 can enable a a 256 sample running average on the PPS input, but that is not nearly long enough to filter out the sampling artifacts in the GPS/PPS (The ones Tom calls "Hanging Bridges"). In an experiment I slaved an OCXO oscillator to the PPS from the GPS, and divided that down to 1Hz as input to the PRS10. That allowed me to use about four times longer timeconstant in the PRS10 and disable the "256 filter" in the PRS10 and the stability was better overall. An even better result was obtained by moving the PLL from the PRS10 onto a computer where the "negative-sawtooth" information from the GPS could be taken into account. I fed the GPS/PPS into the PRS10, used the TT command to read the measured phase as input to the PLL and the SF command to set the frequency. The best results where had using my HP5370B for the timestamping. My conclusion was that even though the timestamping circuitry in the PRS10 is good, it is not good enough to do the Rb part justice and leaving the negative sawtooth correction out of the PLL is ruinous for the Rb parts good performance. So if I were you, I would use the SR620 to measure between the GPS and the PRS10 and implement the PLL in a small computer which talks to both, or alternatively, use the PRS10 for the timestamping, but have the external computer do the PLL. Ideally, it should be possible to have a microcontroller read the GPS data stream, extract the negative sawtooth and send TO commands to the PRS10, but at least in my firmware version that is not possible. SRS told me that they have allowed that in more recent firmware. Poul-Henning PS: If you read the PRS10 docs, it's pretty simple to replicate their PLL parameters. Just remember that they had to do it on a small microcontroller and you'll see how simple it is. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
Poul, Thanks for the informative reply. I guess the short answer is that we should use a very clean long-term GPS trend record to correct the PRS10 rather than the 1PPS from the GPS unit, which has horrible short-term stability. I believe that Tom VB mentioned once that a daily update to the PRS10 EFC should be sufficient. I can see his point. It would be bad to add all that short-term noise to the rubidium's PLL. I will discuss with the other engineers here the possibility of using the phase info from the counter to bump the EFC control on the Rb source. It seems like a fairly easy thing to do. I imagine a simple proportional frequency control loop (human or computerized) would be sufficient for our needs since the integral error would be much smaller than our milliseond-level time accuracy requirement. I'll also look into setting up the SR620 counter to average the phase over perhaps 1000 samples to do the first-level noise reduction. Then we'll get 86 useful numbers per day instead of 86000. Finally, we will be getting a maser again in a couple months and we will probably want to stabilize it this way (with a 30 day adjustment period, perhaps) after the VLBI run, unless the VLBI folks have a better idea. I'll have to ask them. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
HI Poul: That's very interesting. How about using the PRS10 in time stamping mode where you feed it the 1 PPS from the GPS and use a uC to read back the time stamp and the GPS saw tooth to get the actual delta time? It would be easy to record these every hour and after a day or so make a tweak to the PRS10 frequency. This way the only additional equipment would the the uC connecting to the PRS10 serial port. 73, Brooke Clarke -- w/Java http://www.PRC68.com w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml http://www.precisionclock.com Poul-Henning Kamp wrote: >In message <[EMAIL PROTECTED]>, David Forbes writes: > > >>At 6:43 AM -0800 1/18/06, James Maynard wrote: >> >> > > > >>I suppose it would make a lot more sense to run the GPS 1PPS into the >>Rb source so that it will discipline itself, and the phase error be >>monitored to make sure it stays at zero. >> >>Am I barking up the right tree here? >> >> > >Sorry if this is a bit longwinded: > >You would want to be quite carefull in your choice of timeconstant >on the PRS10, if you set it too short you will ruin your short term >stability and if you set it too long you will get worse performance >than you have today. > >I've been playing a lot with the PRS10 and one of the things I found >is that a raw PPS signal from a GPS is badly suited as input for >the PRS10 because of the short term jitter (+/- N nanoseconds for >some N > 20 typically, dependent on your GPS receiver. > >The PRS10 can enable a a 256 sample running average on the PPS >input, but that is not nearly long enough to filter out the sampling >artifacts in the GPS/PPS (The ones Tom calls "Hanging Bridges"). > >In an experiment I slaved an OCXO oscillator to the PPS >from the GPS, and divided that down to 1Hz as input to the PRS10. > >That allowed me to use about four times longer timeconstant in the >PRS10 and disable the "256 filter" in the PRS10 and the stability >was better overall. > >An even better result was obtained by moving the PLL from the PRS10 >onto a computer where the "negative-sawtooth" information from the >GPS could be taken into account. I fed the GPS/PPS into the PRS10, >used the TT command to read the measured phase as input to the PLL >and the SF command to set the frequency. > >The best results where had using my HP5370B for the timestamping. > >My conclusion was that even though the timestamping circuitry in >the PRS10 is good, it is not good enough to do the Rb part justice >and leaving the negative sawtooth correction out of the PLL is >ruinous for the Rb parts good performance. > >So if I were you, I would use the SR620 to measure between the >GPS and the PRS10 and implement the PLL in a small computer >which talks to both, or alternatively, use the PRS10 for the >timestamping, but have the external computer do the PLL. > >Ideally, it should be possible to have a microcontroller read >the GPS data stream, extract the negative sawtooth and send >TO commands to the PRS10, but at least in my firmware version >that is not possible. SRS told me that they have allowed that >in more recent firmware. > >Poul-Henning > >PS: If you read the PRS10 docs, it's pretty simple to replicate >their PLL parameters. Just remember that they had to do it >on a small microcontroller and you'll see how simple it is. > > > ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
Hi, You should definately ask the VLBI people how to deal with the maser. You definately don't want to touch the frequency reference during an observation. They really do need the 10^-15 short-term stability. They can solve for pretty substantial frequency offsets (and linear drifts) after the fact, but phase noise will kill the experiment. If you want a GPS-disciplined Rb, you'd probably be better off buying one than fiddling with yours if it's really running the telescope. But I suspect you can do better yourself with practice. Is this the 12m? When we got our maser, we did about what you suggest: futz with it for a month and get it stabilized, then leave it alone as much as you can. We reset the time if it gets off by 250 ns, and the frequency occasionally. Mostly, log what you do, or the pulsar timing people will have a fit. You may not get much pulsar timing at 100 GHz, but the pulsar folk have their hands in all pies. Sorry if this information is redundant, but and experiment that's blown becaus of a bad frequency reference is a bummer. Cheers, -Mike -- Mike Nolan +1 787 878 2612x334 Fax: +1 787 878 1861 [EMAIL PROTECTED] Arecibo Observatory, HC 3 Box 53995, Arecibo, Puerto Rico 00612 USA ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
This is an interesting discussion and aimed directly at my homebrew efforts. > How about using the PRS10 in time stamping mode where you feed it > the 1 PPS from the GPS and use a uC to read back the time stamp > and the GPS saw tooth to get the actual delta time? It would be > easy to record these every hour and after a day or so make a > tweak to the PRS10 frequency. This way the only additional > equipment would the the uC connecting to the PRS10 serial port. That's an interesting idea. I like it. My goal is to put together a good GPSDO with isolated distribution. I'm currently have a PRS10, an M12+ timing receiver, a couple TAPR TADD-1's tied together on my bench. I aim to get it put into a single box with an appropriate power supply and with some uC to drive a display and perhaps USB. I was going to use the uC simply to intialize and monitor the PRS10 and M12+, but after Poul's comments about the PRS10, I think I had better re-think this plan. Currently the first bits of the initialization and monitoring code are together and running on a spare Linux box. (Is this the sort of thing that others would like to collaborate on? Some help would be great.) BTW, I asked Stanford Research last week about settings for the PRS10 with 1PPS driven from a GPS timing receiver. The recommendation was stock settings with a long integration constant, i.e, PT14. I am currently running this setting, but I haven't measured it yet ... -- Christopher ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
In message <[EMAIL PROTECTED]>, Brooke Clarke writes: >HI Poul: > >That's very interesting. How about using the PRS10 in time stamping >mode where you feed it the 1 PPS from the GPS and use a uC to read back >the time stamp and the GPS saw tooth to get the actual delta time? It >would be easy to record these every hour and after a day or so make a >tweak to the PRS10 frequency. This way the only additional equipment >would the the uC connecting to the PRS10 serial port. That's more or less what I did, except I used a box from www.soekris.com instead of a uC to do the job. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
From: Mike Nolan <[EMAIL PROTECTED]> Subject: Re: [time-nuts] Making the most of SRS Rb source Date: Wed, 18 Jan 2006 21:58:00 -0400 Message-ID: <[EMAIL PROTECTED]> > Hi, Hi Mike, Now, considering that I would like to probe a pulsar what would be the typical gear needed? At what frequency do I probe? Just out of curiosity, I don't intend to overtrash my VISA card for that purpose, so you can be calm about that aspect. PS. I should not have bought the GPS clock from that german fellow Heisenberg and put the antenna on my house, because either I know where it is but not when or I don't know when it is but where. :P Cheers, Magnus > > You should definately ask the VLBI people how to deal with the maser. > You definately don't want to touch the frequency reference during an > observation. They really do need the 10^-15 short-term stability. They > can solve for pretty substantial frequency offsets (and linear drifts) > after the fact, but phase noise will kill the experiment. > > If you want a GPS-disciplined Rb, you'd probably be better off buying > one than fiddling with yours if it's really running the telescope. > But I suspect you can do better yourself with practice. Is this the 12m? > > When we got our maser, we did about what you suggest: futz with it > for a month and get it stabilized, then leave it alone as much as you > can. We reset the time if it gets off by 250 ns, and the frequency > occasionally. > Mostly, log what you do, or the pulsar timing people will have a > fit. You may not get much pulsar timing at 100 GHz, but the pulsar folk > have their hands in all pies. > > Sorry if this information is redundant, but and experiment that's blown > becaus of a bad frequency reference is a bummer. > > Cheers, > > -Mike > > -- > Mike Nolan +1 787 878 2612x334 Fax: +1 787 878 1861 [EMAIL PROTECTED] > Arecibo Observatory, HC 3 Box 53995, Arecibo, Puerto Rico 00612 USA > > ___ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
At 2:05 AM +0100 1/20/06, Magnus Danielson wrote: >From: Mike Nolan <[EMAIL PROTECTED]> >Subject: Re: [time-nuts] Making the most of SRS Rb source >Date: Wed, 18 Jan 2006 21:58:00 -0400 >Message-ID: <[EMAIL PROTECTED]> > >> Hi, > >Hi Mike, > >Now, considering that I would like to probe a pulsar what would be the typical >gear needed? At what frequency do I probe? Just out of curiosity, I don't >intend to overtrash my VISA card for that purpose, so you can be calm about >that aspect. > >PS. I should not have bought the GPS clock from that german fellow Heisenberg >and put the antenna on my house, because either I know where it is >but not when >or I don't know when it is but where. :P > >Cheers, >Magnus Magnus, There are several good books available to get you started on pulsars. They should be available in the astronomy section of your local science library. There are even books out there for amateur radio astronomers that show how to use an old satellite TV dish for this purpose. I think just about any frequency you look at will reveal them. You'll need a tunable downconverter to get a decent IF signal a few MHz wide. An old satellite tuner set to an astronomy-friendly frequency should work, although it will need a motorized equatorial mount or computerized alt-azimuth drive system to take long-term data. A bunch of amplification is also needed, but this is fairly cheap. Lots of IF filtering to keep out bad signals is a must. The detector is as simple as a diode AM detector feeding an integrator feeding an ADC card and a computer data logger. The pulsar detection software is available for free on the Internet. The sample rate is dependent on the pulse rate you're looking for - the idea is to take lots of samples at a regular rate, then have software 'fold' them (stack them up at a particular periodic rate) to detect a bump in the data which is the pulsar. The result is a wiggly line on a graph, which is what most radio astronomy results look like. You might also need to know what time it is. I think you can handle that part. -- --David Forbes, Tucson, AZ http://www.cathodecorner.com/ ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
David Forbes wrote: > > The result is a wiggly line on a graph, which is what most radio > astronomy results look like. > So in the movie Contact they were right when saying that very few, if any, radio astronomers actually "listen" to what is received... 73 Alberto I2PHD ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
At 08:56 AM 1/20/2006, you wrote: >David Forbes wrote: > > > > The result is a wiggly line on a graph, which is what most radio > > astronomy results look like. > > > >So in the movie Contact they were right when saying that very few, >if any, radio astronomers actually "listen" to what is received... True, having done a very little amateur radio astronomy, there really is nothing to hear! John >73 Alberto I2PHD > > >___ >time-nuts mailing list >time-nuts@febo.com >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] Making the most of SRS Rb source
At 2:56 PM +0100 1/20/06, Alberto di Bene wrote: >David Forbes wrote: >> >> The result is a wiggly line on a graph, which is what most radio >> astronomy results look like. >> > >So in the movie Contact they were right when saying that very few, >if any, radio astronomers actually "listen" to what is received... > >73 Alberto I2PHD True, in the same way that no one looks into the eyepiece on a big optical telescope. There's always some piece of electronic mumbo-jumbo getting between the astronomer and the sky. The Hubble people are lucky in that they get to publish big color photographs of the sky as it was 10 billion years ago. Most astronomers just look at numbers or wiggly lines. I remember my gather participated in a groundbreaking infrared Fourier spectrum of the sun in 1972, which produced a wiggly line that wrapped all the way around his laboratory when printed on chart paper. If you're curious as to what the computers of the day looked like... http://www.nixiebunny.com/datareduction.jpg There's a Nova in there, a counter that uses Dekatron tubes, and a variety of ancient surplus test equipment. Dad's boss was a real surplus hound. I can't explain the presence of the Tek 5000 series scope - it had to have been brand-new! -- --David Forbes, Tucson, AZ http://www.cathodecorner.com/ ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
