Re: [time-nuts] EFC info on Trimble 34310-T OXCO
I found the parameters for the X72's 1PPS input. The range of Tau is 5 - 10,000 seconds, or about 2 hrs, 45 min. at the longest setting. It defaults to 400 seconds. However, mine doesn't have the firmware to enable the 1PPS input, so it really doesn't matter. Out of curiosity, does anyone know of any Rb units that can be programmed to a 24 hr (or longer) Tau, either by EFC or 1PPS input? Dave M Bob Camp wrote: Hi It is not what is done in the Efratom Rb’s. Their pps input is set up to get things on frequency / on time quickly. The assumption is that you plug it into a pps to get it “right” and then take off on your mission. That takes them into the short (for a Rb) time constant region. Bob On Aug 24, 2014, at 5:56 PM, Brooke Clarke bro...@pacific.net wrote: Hi Bob: I think that's what's done in the SRS PRS10 http://www.prc68.com/I/PRS10.shtml Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html http://www.prc68.com/I/DietNutrition.html Bob Camp wrote: Hi If you lock an Rb to GPS, you need / want / should do it with a *very* long time constant. Numbers in the one day to several days range are commonly seen. If you lock it up with a tighter (shorter time constant) loop, it will just wander around as it follows the GPS input. That’s what would happen if you hook your Rb to your Trimble and turn on the disciplining on the Rb. It will significantly degrade the stability of the Rb. If you have a temperature stable environment (or create one) you can get some very good results with an (good) Rb locked to a (good) GPS via a proper long time constant setup. It’s not easy, but it can be done. Bob ___ 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] EFC info on Trimble 34310-T OXCO
Charles Steinmetz wrote: Dave wrote: does anyone know of any Rb units that can be programmed to a 24 hr (or longer) Tau, either by EFC or 1PPS input? The SRS PRS10 has a very versatile PPS input that can discipline its frequency with a truly comprehensive array of adjustments (to the point of potential bafflement). The PRS10 and a good PPS source can make an excellent GPSDO. (Note that the PRS10 is, by itself, one of the best compact Rb oscillators available.) The PRS10 manual has a good explanation of the PPS operation at pp.14-17 and 33-36: http://www.thinksrs.com/downloads/PDFs/Manuals/PRS10m.pdf Best regards, Charles Thanks for that, Charles. I've seen a number of articles about the PRS-10, but never really read them closely. I'll download the manual and see if I can understand what it's all about. I might have to put a PRS10 on my wish list. Thanks again, Dave M ___ 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] EFC info on Trimble 34310-T OXCO
Dave wrote: does anyone know of any Rb units that can be programmed to a 24 hr (or longer) Tau, either by EFC or 1PPS input? The SRS PRS10 has a very versatile PPS input that can discipline its frequency with a truly comprehensive array of adjustments (to the point of potential bafflement). The PRS10 and a good PPS source can make an excellent GPSDO. (Note that the PRS10 is, by itself, one of the best compact Rb oscillators available.) The PRS10 manual has a good explanation of the PPS operation at pp.14-17 and 33-36: http://www.thinksrs.com/downloads/PDFs/Manuals/PRS10m.pdf Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Hi The only Rb’s I know of that go out to the 24 to 72 hour tau’s are full blown GPSDO Rb’s. Bob On Aug 25, 2014, at 1:53 PM, Dave M dgmin...@mediacombb.net wrote: I found the parameters for the X72's 1PPS input. The range of Tau is 5 - 10,000 seconds, or about 2 hrs, 45 min. at the longest setting. It defaults to 400 seconds. However, mine doesn't have the firmware to enable the 1PPS input, so it really doesn't matter. Out of curiosity, does anyone know of any Rb units that can be programmed to a 24 hr (or longer) Tau, either by EFC or 1PPS input? Dave M Bob Camp wrote: Hi It is not what is done in the Efratom Rb’s. Their pps input is set up to get things on frequency / on time quickly. The assumption is that you plug it into a pps to get it “right” and then take off on your mission. That takes them into the short (for a Rb) time constant region. Bob On Aug 24, 2014, at 5:56 PM, Brooke Clarke bro...@pacific.net wrote: Hi Bob: I think that's what's done in the SRS PRS10 http://www.prc68.com/I/PRS10.shtml Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html http://www.prc68.com/I/DietNutrition.html Bob Camp wrote: Hi If you lock an Rb to GPS, you need / want / should do it with a *very* long time constant. Numbers in the one day to several days range are commonly seen. If you lock it up with a tighter (shorter time constant) loop, it will just wander around as it follows the GPS input. That’s what would happen if you hook your Rb to your Trimble and turn on the disciplining on the Rb. It will significantly degrade the stability of the Rb. If you have a temperature stable environment (or create one) you can get some very good results with an (good) Rb locked to a (good) GPS via a proper long time constant setup. It’s not easy, but it can be done. Bob ___ 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] EFC info on Trimble 34310-T OXCO
Charles I agree with every thing you wrote and I am implementing many of your recommendations. Forty years ago I bought a 15 foot Alu channel to make small frequency counter housings, always small, and at the time I did have access to a machine shop so I made end plates. Still have five foot pieces now I cut then off in 1 lb pieces and use them for tbolt, FE 405 B, FE 5650 and even a HP 10811 taken out of the can. As I said before am waiting for the small spheres and will see what happens. Working on a GPSDO for the FE 5680A and the FE 405 B I did find out the hard way what moving air will do. When AC season started my 405 tests showed the AC cycling it has a digital tuning resolution of 5.7 E-15.. The nicely assembled packaged unit ended up in an other RS chassis with bubble pack on each end reduced AC influence but you can still see it. If you like to see some data contact me off list file is to large to post. Picture of my Alu channel is attached. Bert Kehren In a message dated 8/23/2014 10:20:19 P.M. Eastern Daylight Time, csteinm...@yandex.com writes: Ed wrote: I agree with your statement regarding the determination of the optimum time constant, but, as Bob Camp mentioned, temperature change has a significant impact on setting the value. My 'lab' is a non-airconditioned bedroom. My Tbolt doesn't have any active temperature control. If I set the time constant to the point that Lady Heather thinks is optimum, I see large swings in PPS offset when I open the window and the temperature changes by a few degrees C. If I leave the time constant at the default of 100 seconds, the swimgs are drastically reduced. Active temperature control is on my 'round tuit' list. Bert wrote: As to Ed's and Bob's comments our projects are not able to compete with commercial products and I do not think that should be our goals. Having spend extensive time on temperature control, I limit my self to 10 C and use fans on all Rb's and passive on OCXO's. Concern about vibration induced noise on the OCXO made me remove the fan on the tbolt. Added a lot of mass and now ordered some foam balls from China to fill the enclosure as some one recommended. Well, yeah, it goes without saying (or at least I thought it would) that one must keep the rate of change of temperature of the OCXO low enough that its oven can keep the crystal temperature within design bounds at all times. I just assume that any time nut would do this, since it is extremely simple and costs next to nothing (look in the archives for my previous posts about metal boxes, metal enclosures, and thermal capacitance in connection with OCXOs). Active temperature control is NOT necessary. Which is not to say it's a bad idea, it's just not necessary to stabilize any OCXO worth owning by a time nut. (I'm not sure the MV-89 qualifies, even if you are lucky enough to get a good one. There has been some discussion on this list about the temperature control loop being quasi-stable and tending to oscillate or even latch under some conditions.) I also see no reason why amateur efforts cannot surpass the performance of commercial products, particularly if we assume that the environmental conditions are limited to those encountered in living space, not a radio shelter exposed to the elements at a remote tower. That is why I've been critical of designs that aim only to do the best that can be done for $5, or the best that can be done with a small ARM and 3 transistors. Given good design, there is no reason why an inexpensive DIY GPSDO shouldn't handily outperform a Thunderbolt (using the same OCXO), with two conditions: (i) environmental conditions are limited to those encountered in living space, and (ii) performance during holdover is neglected. The reasons why most DIY designs do not work as well as commercial designs, even if they use OCXOs of equal quality, is that their designers evidently cannot design ADPLLs of sufficient performance to do justice to the OCXO. (This includes implementing whatever means of phase comparison and sampling are chosen, the DSP loop filter, sawtooth correction, and the NCO or DAC/EFC design.) Doing all of this right isn't particularly expensive, it just takes a designer who has the skills and is willing to devote the effort. As a mentor once told me, Good thinking isn't any more expensive than bad thinking. Some of the performance gain would be in reducing the rate of temperature change seen by the OCXO, either passively as I have advocated and described before, or actively. The other main improvement would be setting the PLL crossover out where it belongs, which becomes possible when the rate of change of temperature is controlled. Avoiding a few common mistakes would provide some additional performance gains. While the foam peanuts, which I mentioned in a previous post, are helpful in
Re: [time-nuts] EFC info on Trimble 34310-T OXCO
Hi Dave, On 8/23/2014 3:51 PM, Dave M wrote: Thanks for that suggestion, Ed. After a bit of reading in the X72 Reference Guide, it appears that the X72 does have a 1PPS input. That would be considerably easier than trying to interface the Rb into the GPSDO. Still trying to understand what the manual is telling me. Next thing is to determine if my unit has that option enabled (firmware option). That will be a chore for after the holiday... really busy next week. What would that (1PPS disciplining) do for me... in terms of maintaining the Rb frequency accurately set? Would it be as accurate as having the Rb disciplined via the EFC input? It's kind of overkill, but by connecting the 1 PPS from the NTBW50AA to the X72, the X72 will be disciplined to the 1 PPS so the frequency will be accurate. The question is how well will it be disciplined, i.e. what will the Allen Deviation graph look like. I have a few X72 and SA-22c (X72's cousin), but none of them have that option. I don't know of any published data on it. Maybe you can tell us how well it performs. In general, I just don't see the point of disciplining a Rb standard to GPS. I don't understand what will be gained by doing it. I have a Z3801A and a Tbolt plus a free-running FRK as a house standard. I occasionally compare the FRK to the Z3801A but the drift is so low (~1e-12 per month over 9 months) that I see no reason to link them. One exception that I recently discussed on another forum was a guy who lives in a ground floor, north-facing condo. He might need to have a disciplined Rb standard due to poor GPS visibility. Ed Sorry for forgetting to change the Subject line on my last post. I see the futility of trying to integrate a Rb oscillator into a GPS receiver. As it turns out, my X72 doesn't have the 1PPS input option enabled, so that's a moot point. end result: I now have a couple of GPSDOs and a Rb that I can use separately, as needed. I would like to have a frequency comparator that can handle 10 MHz inputs natively without having to divide them down to 5 MHz. I have a Fluke/Montronics model 103A Frequency Comparator, but its max frequency input is 5MHz. I have a TADD-2 divider board, but I want to build a two-channel divider board to dedicate to the Fluke comparator. Which logic family is most suitable for such use; ALS, AC, etc.? Low jitter would be the critical parameter? I only need to divide by and 10, and maybe 100. My original dream, and the impetus for this thread, was to have a frequency standard having the excellent short-term stability of the Rb, but have it disciplined to the GPS to maintain its long-term accuracy without having to correct the Rb manually. My conclusion; it's now quite obvious that the old-school manual method is probably easiest and best. I'll set the frequency on the Rb, watch its performance for a few months, and use it as the main frequency source for my bench. Thanks for all the advice. Dave M ___ 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] EFC info on Trimble 34310-T OXCO
Charles I use double bobble pack inside the Alu channel and I always start out by monitoring the OCXO and make sure it is at least 10 C below the spec range. The tbolt is center located and I use a combination of rubber mounts but suspended that they sell for hard drives and squares of double bubble pack. Vibration and thermal. Bert Kehren In a message dated 8/24/2014 8:26:02 P.M. Eastern Daylight Time, csteinm...@yandex.com writes: Bert wrote: As I said before am waiting for the small spheres and will see what happens. Monitor carefully, as I suspect the spheres will pack too tightly leaving too little airspace. You could easily burn down an OCXO if this proves to be the case and the oven control loop goes unstable. I'd put a thermal sensor on the OCXO itself for testing. Even if it doesn't burn down, you could find that the oven performance is degraded by (i) instability or quasi-instability of the oven controller, or (ii) too much thermal resistance (remember, you want to add as little thermal resistance as possible). On the other hand, mounting the OCXO as centrally as possible inside the outer enclosure on thermally non-conducting standoffs (teflon or nylon), with at least an inch of air on all six sides, has proven to work extremely well. When I described using packing peanuts (or similar) to break up the airflow, it was in the context of having already mounted the OCXO as centrally as possible inside the outer enclosure on thermally non-conducting standoffs with at least an inch of air on all six sides. And as I said, I have not found the additional step necessary once you have done this. It may even be counterproductive. Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Bert wrote: As I said before am waiting for the small spheres and will see what happens. Monitor carefully, as I suspect the spheres will pack too tightly leaving too little airspace. You could easily burn down an OCXO if this proves to be the case and the oven control loop goes unstable. I'd put a thermal sensor on the OCXO itself for testing. Even if it doesn't burn down, you could find that the oven performance is degraded by (i) instability or quasi-instability of the oven controller, or (ii) too much thermal resistance (remember, you want to add as little thermal resistance as possible). On the other hand, mounting the OCXO as centrally as possible inside the outer enclosure on thermally non-conducting standoffs (teflon or nylon), with at least an inch of air on all six sides, has proven to work extremely well. When I described using packing peanuts (or similar) to break up the airflow, it was in the context of having already mounted the OCXO as centrally as possible inside the outer enclosure on thermally non-conducting standoffs with at least an inch of air on all six sides. And as I said, I have not found the additional step necessary once you have done this. It may even be counterproductive. Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Hi Keep in mind that the OCXO is likely (if it’s a modern part) optimized for TC at it’s normal thermal gain. The gain and set point are adjusted for a flat curve. If you bump either the gain or the set point you rotate the curve. Bob On Aug 24, 2014, at 8:36 PM, ewkeh...@aol.com wrote: Charles I use double bobble pack inside the Alu channel and I always start out by monitoring the OCXO and make sure it is at least 10 C below the spec range. The tbolt is center located and I use a combination of rubber mounts but suspended that they sell for hard drives and squares of double bubble pack. Vibration and thermal. Bert Kehren In a message dated 8/24/2014 8:26:02 P.M. Eastern Daylight Time, csteinm...@yandex.com writes: Bert wrote: As I said before am waiting for the small spheres and will see what happens. Monitor carefully, as I suspect the spheres will pack too tightly leaving too little airspace. You could easily burn down an OCXO if this proves to be the case and the oven control loop goes unstable. I'd put a thermal sensor on the OCXO itself for testing. Even if it doesn't burn down, you could find that the oven performance is degraded by (i) instability or quasi-instability of the oven controller, or (ii) too much thermal resistance (remember, you want to add as little thermal resistance as possible). On the other hand, mounting the OCXO as centrally as possible inside the outer enclosure on thermally non-conducting standoffs (teflon or nylon), with at least an inch of air on all six sides, has proven to work extremely well. When I described using packing peanuts (or similar) to break up the airflow, it was in the context of having already mounted the OCXO as centrally as possible inside the outer enclosure on thermally non-conducting standoffs with at least an inch of air on all six sides. And as I said, I have not found the additional step necessary once you have done this. It may even be counterproductive. Best regards, Charles ___ 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. ___ 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] EFC info on Trimble 34310-T OXCO
Hi Keep in mind that watching the OCXO in a fixed ambient may not tell the whole story. Setting a PLL to 10,000 seconds on an OCXO and then ramping the temperature 40 or 80C is likely to create issues….. It’s very easy to fall into the “I only have a room that swings 0.2C” loop when looking at OCXO’s or systems. The main design goal on an OCXO is performance when the temperature moves 50 to 120 C (depending on the design). Even GPSDO systems have temperature profile specifications. A common assumption is “we loose the GPS and the air-conditioning at the same time”. Another one is “this goes in a hut out in the sun”. Bob On Aug 22, 2014, at 8:39 PM, Charles Steinmetz csteinm...@yandex.com wrote: Bob wrote: The GPSTM is not as tweak friendly (no filter changes allowed) as some of the other GPSDO's. And that is a major problem. The correct filter settings for a Rb local oscillator are very different from the settings for an OCXO, which in turn are different from the correct settings for a TCXO. As a general matter, almost all of the DIY GPSDO designs I have seen use PLL loop filter settings that are not optimal. Many are not even close (several orders of magnitude, or more, from optimal). Generally speaking, the PLL loop filter cutoff should be set approximately where the GPS xDEV curve intersects the local oscillator xDEV curve. That puts the better device (GPS or local oscillator) in charge of the composite xDEV at all tau -- the local oscillator at short and medium tau, and the GPS at long tau. Optimal crossover tau will generally be in the range of seconds for a TCXO, hundreds of seconds for an OCXO, and hours to tens of hours for a Rb. Sometimes, there are good reasons to depart from this general rule. In particular, if a speedy recovery from holdover is required, then one might choose a PLL filter cutoff tau that is lower than optimal. The default crossover tau for the Trimble Thunderbolt is chosen quite low, presumably for this reason. See, for example, http://www.leapsecond.com/pages/gpsdo/, where the GPS contributes significantly to the ADEV way down at tau = 1 second, where the local oscillator is clearly much better than GPS and continues to be for more than two decades. The Miller DIY GPSDO on that page is crossed over about 3 decades lower than optimal. (The Miller GPSDO uses a Shera DIY controller; I presume the Shera has the same crossover tau.). Compare this to the HP z3801A and Jackson Labs Fury on the same page. The HP crosses over about 2 decades higher than the Thunderbolt and Miller GPSDOs, but that is still premature by about two decades given the very high quality of the OCXO in that particular unit. The Fury crossover is set well, but the overall ADEV is let down by the low stability of the OCXO in that particular unit. (Note that the crossover in commercially produced GPSDOs must accommodate the range in production ADEVs of the local oscillators used, and are likely set a bit lower than optimal for most of the actual OCXOs on this account.) If the filter parameters are adjustable -- as they are in the case of the Thunderbolt -- then a time nut can tune his or her individual sample to get the best possible performance that particular oscillator can deliver. As I have mentioned before, rather than just setting the time constant low to speed up holdover recovery, a better solution is to implement a switchable PLL loop filter. A GPSDO designed this way uses a suitably long time constant for normal locked operation to minimize xDEV at all frequencies, and a faster time constant for turn-on warmup and holdover recovery. It is rumored that the z3801 is designed this way. Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Hi Charles, I agree with your statement regarding the determination of the optimum time constant, but, as Bob Camp mentioned, temperature change has a significant impact on setting the value. My 'lab' is a non-airconditioned bedroom. My Tbolt doesn't have any active temperature control. If I set the time constant to the point that Lady Heather thinks is optimum, I see large swings in PPS offset when I open the window and the temperature changes by a few degrees C. If I leave the time constant at the default of 100 seconds, the swimgs are drastically reduced. Active temperature control is on my 'round tuit' list. I don't think you're correct about the Miller GPSDO containing a Shera controller. The Miller design is famous for having no processor - just simple analog hardware. It even says that on the page you referenced. That's one reason why his settings aren't optimum. It's impractical to get long time constants with the simple analog circuitry that he uses. Despite that, his design still has a 'time nuts' level of performance. Ed On 8/22/2014 6:39 PM, Charles Steinmetz wrote: Bob wrote: The GPSTM is not as tweak friendly (no filter changes allowed) as some of the other GPSDO's. And that is a major problem. The correct filter settings for a Rb local oscillator are very different from the settings for an OCXO, which in turn are different from the correct settings for a TCXO. As a general matter, almost all of the DIY GPSDO designs I have seen use PLL loop filter settings that are not optimal. Many are not even close (several orders of magnitude, or more, from optimal). Generally speaking, the PLL loop filter cutoff should be set approximately where the GPS xDEV curve intersects the local oscillator xDEV curve. That puts the better device (GPS or local oscillator) in charge of the composite xDEV at all tau -- the local oscillator at short and medium tau, and the GPS at long tau. Optimal crossover tau will generally be in the range of seconds for a TCXO, hundreds of seconds for an OCXO, and hours to tens of hours for a Rb. Sometimes, there are good reasons to depart from this general rule. In particular, if a speedy recovery from holdover is required, then one might choose a PLL filter cutoff tau that is lower than optimal. The default crossover tau for the Trimble Thunderbolt is chosen quite low, presumably for this reason. See, for example, http://www.leapsecond.com/pages/gpsdo/, where the GPS contributes significantly to the ADEV way down at tau = 1 second, where the local oscillator is clearly much better than GPS and continues to be for more than two decades. The Miller DIY GPSDO on that page is crossed over about 3 decades lower than optimal. (The Miller GPSDO uses a Shera DIY controller; I presume the Shera has the same crossover tau.). Compare this to the HP z3801A and Jackson Labs Fury on the same page. The HP crosses over about 2 decades higher than the Thunderbolt and Miller GPSDOs, but that is still premature by about two decades given the very high quality of the OCXO in that particular unit. The Fury crossover is set well, but the overall ADEV is let down by the low stability of the OCXO in that particular unit. (Note that the crossover in commercially produced GPSDOs must accommodate the range in production ADEVs of the local oscillators used, and are likely set a bit lower than optimal for most of the actual OCXOs on this account.) If the filter parameters are adjustable -- as they are in the case of the Thunderbolt -- then a time nut can tune his or her individual sample to get the best possible performance that particular oscillator can deliver. As I have mentioned before, rather than just setting the time constant low to speed up holdover recovery, a better solution is to implement a switchable PLL loop filter. A GPSDO designed this way uses a suitably long time constant for normal locked operation to minimize xDEV at all frequencies, and a faster time constant for turn-on warmup and holdover recovery. It is rumored that the z3801 is designed this way. Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Ed wrote: I don't think you're correct about the Miller GPSDO containing a Shera controller. Is there more than one Miller GPSDO? I was referring to this one, by James Miller G3RUH, which uses a Shera controller and 10811 OCXO: http://www.jrmiller.demon.co.uk/projects/freqstd/frqstd.htm Answering my own question: yes, there appear to be at least two Miller GPSDOs, both by the same Miller. Here is another (presumably the one Ed was referring to): http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Now that I see it again, I think I knew about Miller's Shera version, but I purged it from my brain in horror and disgust. He bought a Z3801A, threw away the controller and added a Shera board. The Shera board is good, but . Ed On 8/23/2014 1:40 PM, Charles Steinmetz wrote: Ed wrote: I don't think you're correct about the Miller GPSDO containing a Shera controller. Is there more than one Miller GPSDO? I was referring to this one, by James Miller G3RUH, which uses a Shera controller and 10811 OCXO: http://www.jrmiller.demon.co.uk/projects/freqstd/frqstd.htm Answering my own question: yes, there appear to be at least two Miller GPSDOs, both by the same Miller. Here is another (presumably the one Ed was referring to): http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm Best regards, Charles ___ 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] EFC info on Trimble 34310-T OXCO
Ed wrote: I agree with your statement regarding the determination of the optimum time constant, but, as Bob Camp mentioned, temperature change has a significant impact on setting the value. My 'lab' is a non-airconditioned bedroom. My Tbolt doesn't have any active temperature control. If I set the time constant to the point that Lady Heather thinks is optimum, I see large swings in PPS offset when I open the window and the temperature changes by a few degrees C. If I leave the time constant at the default of 100 seconds, the swimgs are drastically reduced. Active temperature control is on my 'round tuit' list. Bert wrote: As to Ed's and Bob's comments our projects are not able to compete with commercial products and I do not think that should be our goals. Having spend extensive time on temperature control, I limit my self to 10 C and use fans on all Rb's and passive on OCXO's. Concern about vibration induced noise on the OCXO made me remove the fan on the tbolt. Added a lot of mass and now ordered some foam balls from China to fill the enclosure as some one recommended. Well, yeah, it goes without saying (or at least I thought it would) that one must keep the rate of change of temperature of the OCXO low enough that its oven can keep the crystal temperature within design bounds at all times. I just assume that any time nut would do this, since it is extremely simple and costs next to nothing (look in the archives for my previous posts about metal boxes, metal enclosures, and thermal capacitance in connection with OCXOs). Active temperature control is NOT necessary. Which is not to say it's a bad idea, it's just not necessary to stabilize any OCXO worth owning by a time nut. (I'm not sure the MV-89 qualifies, even if you are lucky enough to get a good one. There has been some discussion on this list about the temperature control loop being quasi-stable and tending to oscillate or even latch under some conditions.) I also see no reason why amateur efforts cannot surpass the performance of commercial products, particularly if we assume that the environmental conditions are limited to those encountered in living space, not a radio shelter exposed to the elements at a remote tower. That is why I've been critical of designs that aim only to do the best that can be done for $5, or the best that can be done with a small ARM and 3 transistors. Given good design, there is no reason why an inexpensive DIY GPSDO shouldn't handily outperform a Thunderbolt (using the same OCXO), with two conditions: (i) environmental conditions are limited to those encountered in living space, and (ii) performance during holdover is neglected. The reasons why most DIY designs do not work as well as commercial designs, even if they use OCXOs of equal quality, is that their designers evidently cannot design ADPLLs of sufficient performance to do justice to the OCXO. (This includes implementing whatever means of phase comparison and sampling are chosen, the DSP loop filter, sawtooth correction, and the NCO or DAC/EFC design.) Doing all of this right isn't particularly expensive, it just takes a designer who has the skills and is willing to devote the effort. As a mentor once told me, Good thinking isn't any more expensive than bad thinking. Some of the performance gain would be in reducing the rate of temperature change seen by the OCXO, either passively as I have advocated and described before, or actively. The other main improvement would be setting the PLL crossover out where it belongs, which becomes possible when the rate of change of temperature is controlled. Avoiding a few common mistakes would provide some additional performance gains. While the foam peanuts, which I mentioned in a previous post, are helpful in some circumstances, I have never seen the need for them in the case of an OCXO inside a cast aluminum box. In that post, I mentioned my gut feeling that spheres (balls) likely pack too tightly to allow sufficient air circulation. I think irregularly-shaped pieces of foam (like packing peanuts), which leave much more air space between them, are required. The intent is NOT to impede air flow, but to randomize it. One point that I think gets lost in many of these discussions: The quality of individual OCXOs, even of the same model, varies rather widely, and you often won't know how good a particular OCXO is until you have run it continuously for at least 90 days (preferably 180 or more). The job of any GPS discipline is to gently keep the OCXO on frequency, without lowering its xDEV performance at tau where the OCXO is better than GPS. The most effective thing you can do to construct a very stable GPSDO is to start with a very stable OCXO. Often, this means buying a bunch of OCXOs (even if you have to do it one at a time for budgetary reasons), selecting the best one(s), and moving the rest along. This can take a
[time-nuts] EFC info on Trimble 34310-T OXCO
Does anyone have any info on the OXCO in the Nortel/Trimble NTBW50AA-17 GPSTM receiver? The OXCO is labeled as Trimble 34310-T. I see some Trimble 34310-T oscillators on Ebay with pinouts labeled, but no other info. Specifically, I'd like to know the EFC characteristics for it. I'm thinking of the possibility of pulling the OXCO out of the GPSTM and subbing in a 10 MHz Rubidium, and using the GPSTM to discipline the Rubidium. My Rubidium is a Symmetricom X72, recently purchased. It seems to be working well. Does anyone know the differences between the three OXCOs used in the GPSTM receivers (T, T2 and Oak)? Thanks for some insight, Dave M ___ 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] EFC info on Trimble 34310-T OXCO
I've been using one for over a year. They take 12V to power, and they have a VRef output around +6.25V, which implies an EFC range of 0-6V. Unless you get one that's aged out, an EFC range of 0-5V should be fine. The VRef has a bit of 10MHz on it on mine. All in all, it seems to be a good OCXO, though I don't have the equipment to do a real careful test on it. However, I wonder if you'd really get anything from subbing in an Rb, other than the fun of doing it? Don't Rb standards usually just have a DDS output? If so, wouldn't that be a step backwards? Bob - AE6RV From: Dave M dgmin...@mediacombb.net To: FEBO Time Nuts time-nuts@febo.com Sent: Friday, August 22, 2014 1:39 PM Subject: [time-nuts] EFC info on Trimble 34310-T OXCO Does anyone have any info on the OXCO in the Nortel/Trimble NTBW50AA-17 GPSTM receiver? The OXCO is labeled as Trimble 34310-T. I see some Trimble 34310-T oscillators on Ebay with pinouts labeled, but no other info. Specifically, I'd like to know the EFC characteristics for it. I'm thinking of the possibility of pulling the OXCO out of the GPSTM and subbing in a 10 MHz Rubidium, and using the GPSTM to discipline the Rubidium. My Rubidium is a Symmetricom X72, recently purchased. It seems to be working well. Does anyone know the differences between the three OXCOs used in the GPSTM receivers (T, T2 and Oak)? Thanks for some insight, Dave M ___ 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] EFC info on Trimble 34310-T OXCO
Hi All of the OCXO’s in the GPSDO’s pull 100’s (or more) times the pull of a Rb. The firmware that they have on that board for the OCXO is going to need some serious tweaks to get it to work properly with an Rb. The GPSTM is not as tweak friendly (no filter changes allowed) as some of the other GPSDO’s. Bob On Aug 22, 2014, at 2:39 PM, Dave M dgmin...@mediacombb.net wrote: Does anyone have any info on the OXCO in the Nortel/Trimble NTBW50AA-17 GPSTM receiver? The OXCO is labeled as Trimble 34310-T. I see some Trimble 34310-T oscillators on Ebay with pinouts labeled, but no other info. Specifically, I'd like to know the EFC characteristics for it. I'm thinking of the possibility of pulling the OXCO out of the GPSTM and subbing in a 10 MHz Rubidium, and using the GPSTM to discipline the Rubidium. My Rubidium is a Symmetricom X72, recently purchased. It seems to be working well. Does anyone know the differences between the three OXCOs used in the GPSTM receivers (T, T2 and Oak)? Thanks for some insight, Dave M ___ 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] EFC info on Trimble 34310-T OXCO
Have you checked your X72 to see if it has the 1 PPS discipline option? That would be a lot easier (and probably better) than your proposed transplant. Ed On 8/22/2014 12:39 PM, Dave M wrote: Does anyone have any info on the OXCO in the Nortel/Trimble NTBW50AA-17 GPSTM receiver? The OXCO is labeled as Trimble 34310-T. I see some Trimble 34310-T oscillators on Ebay with pinouts labeled, but no other info. Specifically, I'd like to know the EFC characteristics for it. I'm thinking of the possibility of pulling the OXCO out of the GPSTM and subbing in a 10 MHz Rubidium, and using the GPSTM to discipline the Rubidium. My Rubidium is a Symmetricom X72, recently purchased. It seems to be working well. Does anyone know the differences between the three OXCOs used in the GPSTM receivers (T, T2 and Oak)? Thanks for some insight, Dave M ___ 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] EFC info on Trimble 34310-T OXCO
Bob wrote: The GPSTM is not as tweak friendly (no filter changes allowed) as some of the other GPSDO's. And that is a major problem. The correct filter settings for a Rb local oscillator are very different from the settings for an OCXO, which in turn are different from the correct settings for a TCXO. As a general matter, almost all of the DIY GPSDO designs I have seen use PLL loop filter settings that are not optimal. Many are not even close (several orders of magnitude, or more, from optimal). Generally speaking, the PLL loop filter cutoff should be set approximately where the GPS xDEV curve intersects the local oscillator xDEV curve. That puts the better device (GPS or local oscillator) in charge of the composite xDEV at all tau -- the local oscillator at short and medium tau, and the GPS at long tau. Optimal crossover tau will generally be in the range of seconds for a TCXO, hundreds of seconds for an OCXO, and hours to tens of hours for a Rb. Sometimes, there are good reasons to depart from this general rule. In particular, if a speedy recovery from holdover is required, then one might choose a PLL filter cutoff tau that is lower than optimal. The default crossover tau for the Trimble Thunderbolt is chosen quite low, presumably for this reason. See, for example, http://www.leapsecond.com/pages/gpsdo/, where the GPS contributes significantly to the ADEV way down at tau = 1 second, where the local oscillator is clearly much better than GPS and continues to be for more than two decades. The Miller DIY GPSDO on that page is crossed over about 3 decades lower than optimal. (The Miller GPSDO uses a Shera DIY controller; I presume the Shera has the same crossover tau.). Compare this to the HP z3801A and Jackson Labs Fury on the same page. The HP crosses over about 2 decades higher than the Thunderbolt and Miller GPSDOs, but that is still premature by about two decades given the very high quality of the OCXO in that particular unit. The Fury crossover is set well, but the overall ADEV is let down by the low stability of the OCXO in that particular unit. (Note that the crossover in commercially produced GPSDOs must accommodate the range in production ADEVs of the local oscillators used, and are likely set a bit lower than optimal for most of the actual OCXOs on this account.) If the filter parameters are adjustable -- as they are in the case of the Thunderbolt -- then a time nut can tune his or her individual sample to get the best possible performance that particular oscillator can deliver. As I have mentioned before, rather than just setting the time constant low to speed up holdover recovery, a better solution is to implement a switchable PLL loop filter. A GPSDO designed this way uses a suitably long time constant for normal locked operation to minimize xDEV at all frequencies, and a faster time constant for turn-on warmup and holdover recovery. It is rumored that the z3801 is designed this way. Best regards, Charles ___ 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.
