Re: [time-nuts] GPSDO control loops and correcting quantization error
In message 1225454799-1347767280-cardhu_decombobulator_blackberry.rim.net-1901 834519-@b27.c1.bise6.blackberry, li...@lazygranch.com writes: The PWM DAC should have perfect differential linearity, which I believe is all that matters in this application. (That and no missing codes.) Not so when you try to combine two DACs to make one higher resolution DAC. The main problem with two staggered DAC's is actually that all OCXO's drift and eventually you will have to step the major DAC which will give a glitch in the lower bits, almost no matter how much you calibrate beforehand. The PRS10 uses a staggered DAC internally and the steps on the major DAC are measurable in the output signal. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | 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 To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] GPSDO control loops and correcting quantization error
gandal...@aol.com said: When some other form of external control is used, such as a DAC output for example, it's not uncommon to find the voltage reference output left disconnected and the control circuit fed from an alternative supply. On the other hand, many DACs need an external reference. A reference coming out of an OCXO is probably going to have a good temperature coefficient. :) -- These are my opinions. I hate spam. ___ 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] GPSDO control loops and correcting quantization error
Hal Murray wrote: d...@montana.com said: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. The trick for this application is that you don't need full accuracy over the full range of the DAC. All you need is roughly linear and stable around the operating point. The PLL will take care of any offset. Any gain error is just a minor change to the overall gain. This thread started with 16 bit PWM DAC. I think that matches the requirements. I would expect a problem area would be filtering the PWM output. Anything you don't filter out will turn into close in spikes. It might be fun to try to measure them. 64K/72 MHz is about 1 ms. 32bits at 72 MHz is 60 seconds. Has anybody compared DDS style DACs with PWM? The idea is to spread the pulses out to make the filtering easier. Instead of 10, you would get to work with 1010101010 Using a synchronous filter for the PWM DAC eases the additional filtering required considerably. 24 bit resolution is readily achieved by combining the outputs of a pair of PWM sources sharing a single synchronous filter. 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.
Re: [time-nuts] GPSDO control loops and correcting quantization error
On 09/15/2012 12:08 AM, Hal Murray wrote: d...@montana.com said: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. The trick for this application is that you don't need full accuracy over the full range of the DAC. All you need is roughly linear and stable around the operating point. The PLL will take care of any offset. Any gain error is just a minor change to the overall gain. This thread started with 16 bit PWM DAC. I think that matches the requirements. I would expect a problem area would be filtering the PWM output. Anything you don't filter out will turn into close in spikes. It might be fun to try to measure them. 64K/72 MHz is about 1 ms. 32bits at 72 MHz is 60 seconds. Has anybody compared DDS style DACs with PWM? The idea is to spread the pulses out to make the filtering easier. Instead of 10, you would get to work with 1010101010 PWM has the fantastic power of putting most energy into the lowest frequency, which makes analog filtering extra hard, so you need to move the bandwidth down or use higher degrees filter for a good filter slope. The filter bandwidth puts an upper limit to the PLL bandwidth. I've done a inverse PWM spectrum modulation, which isn't all that hard, and it has significant improvements over PWM. Another approach is to use the sigma-delta approach which smooths the frequency spikes out to noise. 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] GPSDO control loops and correcting quantization error
Second the comments on implementing a 16 bit DAC. You need separate analogue/digital grounds, superb voltage references, and lots of attempts to get a good design that actually uses the L.S. bit (rather than losing it in the noise). What you can do is use a second DAC to offset the 16 bit DAC. The offset DAC need only be 8 bit, as long as it is stable. I used this to autozero the output of a photomultiplier amplifier, and I needed about 20 bits to get the correct resolution. However, it can be tricky to adjust the offset DAC without jumps in the output. Incidentally superb experimental design, circuit boards taped to an odd piece of cardboard, with jumpers leads to tie everything together :). I use a dab of hot melt glue to do similar, and it can be used to secure wiring as well. On 15 September 2012 07:01, Don Latham d...@montana.com wrote: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. Don Chris Albertson On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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. -- Neither the voice of authority nor the weight of reason and argument are as significant as experiment, for thence comes quiet to the mind. De Erroribus Medicorum, R. Bacon, 13th century. If you don't know what it is, don't poke it. Ghost in the Shell Dr. Don Latham AJ7LL Six Mile Systems LLP 17850 Six Mile Road POB 134 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com ___ 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. -- Tom Harris celephi...@gmail.com ___ 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] GPSDO control loops and correcting quantization error
The PWM DAC should have perfect differential linearity, which I believe is all that matters in this application. (That and no missing codes.) Not so when you try to combine two DACs to make one higher resolution DAC. -Original Message- From: Tom Harris celephi...@gmail.com Sender: time-nuts-boun...@febo.com Date: Sun, 16 Sep 2012 12:00:55 To: Discussion of precise time and frequency measurementtime-nuts@febo.com Reply-To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] GPSDO control loops and correcting quantization error Second the comments on implementing a 16 bit DAC. You need separate analogue/digital grounds, superb voltage references, and lots of attempts to get a good design that actually uses the L.S. bit (rather than losing it in the noise). What you can do is use a second DAC to offset the 16 bit DAC. The offset DAC need only be 8 bit, as long as it is stable. I used this to autozero the output of a photomultiplier amplifier, and I needed about 20 bits to get the correct resolution. However, it can be tricky to adjust the offset DAC without jumps in the output. Incidentally superb experimental design, circuit boards taped to an odd piece of cardboard, with jumpers leads to tie everything together :). I use a dab of hot melt glue to do similar, and it can be used to secure wiring as well. On 15 September 2012 07:01, Don Latham d...@montana.com wrote: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. Don Chris Albertson On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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. -- Neither the voice of authority nor the weight of reason and argument are as significant as experiment, for thence comes quiet to the mind. De Erroribus Medicorum, R. Bacon, 13th century. If you don't know what it is, don't poke it. Ghost in the Shell Dr. Don Latham AJ7LL Six Mile Systems LLP 17850 Six Mile Road POB 134 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com ___ 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. -- Tom Harris celephi...@gmail.com ___ 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] GPSDO control loops and correcting quantization error
Greetings nuts, I've been working on a simple GPSDO as a starting point for further experimentation. I'm using a STM32 microcontroller running at 72MHz as the heart, with the input capture peripheral comparing the phase of the pulses-per-second and a 16 bit PWM DAC to drive the VFC. It's all working quite well from a functional standpoint although I'm sure the performance will be quite terrible by time-nuts standards, unfortunately I don't yet have the equipment to characterize precisely how terrible it is, but that will come later. So now that the basic functionality is there I've got a few questions about improving it. First off a technical question. I'm using a Trimble Resolution SMT as the pulse-per-second source. It sends a supplemental timing packet that contains an estimate of the quantization error in its pulse output. But the manual isn't clear on whether that applies to the next pulse or to the previous one. I've seen people correct the delay by using a programmable delay line which seems like it would only be possible if the measurement was for the next pulse. But on the other hand there is a pulse was not generated alarm that definitely applies to the previous (non)-pulse which suggests that maybe other fields refer exclusively to the previous pulse. I can handle either way since the pulses are timestamped asynchronously and can be post-processed at any time but from some preliminary data collection it's not clear which way it's meant to go. Does anyone know for sure whether the quantization error is for the next or preceding pulse? Secondly, a more general design question. Right now the feedback is done through a relatively fast PI controller. For example, here's a chart showing convergence at various integration coefficients: http://partiallystapled.com/~gxti/circuits/2012/09/13-pid-ki.png The Ki coefficient units are somewhat arbitrary due to the fixed-point math, but the X axis is seconds and the Y axis is number of counts at 72MHz (13.89ns). Right now I'm using Ki=1 because it converges quickly enough and also don't oscillate, but these parameters are only particularly interesting on startup. Something much, much slower seems more suitable for continuous operation. But I'm thinking that the best solution might be to start out with fast convergence like this, then switch to slower parameters (for PI controller and smoothing) once some desired level of stability is reached. Any thoughts on this change of parameters, or PI tuning in general, or perhaps an entirely different control topology? Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. There is some room for expansion if I want to replace the DAC or add a third oscillator input for holdover. In fact, this board seems to have more connectors than ICs: http://partiallystapled.com/~gxti/trash/2012/09/08-serafine.jpg Cheers, -- m. tharp ___ 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] GPSDO control loops and correcting quantization error
Hi M. welcome to the world of GPSDO optimization, one thing you will find is that there never is a time when there is no chance to improve something :) On the 1PPS sawtooth correction, the usual convention is for the following 1PPS. The easiest thing to do rather than trying to guess the GPS unit's behavior is to try it out on both pulses, and also try adding and subtracting the number from your raw phase measurement, then you get four streams of values, and it will be instantly obvious which one is the one that reduces the noise most. The other three will increase the noise over your raw, uncorrected measurements. On the DAC resolution, it depends on the OCXO control range, and the ADEV performance you want to achieve. For example if your OCXO has +/-2Hz control range, then a 16 bit DAC will only give you an LSB resolution of about 61 microhertz, or 6.1E-012 (4Hz divided by 2^16). This may or may not be acceptable to you. If your OCXO has a more typical +/-20Hz control range, then this would go up to 6.1E-011 per LSB, which will definitely affect your ADEV. Usually, GPSDO's use at least 20 bit control range due to this quantization effect. But in the end it may also be limited by your time-interval-counter resolution, because every tick in your counter will equal to so many steps in your DAC (depending on what gains you use for your loop prediction). Also, make sure to put filtering for errand pulses into your code, every GPS WILL generate an errand pulse from time to time in my experience, and these can be off by 10's of microseconds.. if you don't filter these properly, they will lead to jumps in your frequency. Hope that helps, Said In a message dated 9/14/2012 11:21:57 Pacific Daylight Time, g...@partiallystapled.com writes: First off a technical question. I'm using a Trimble Resolution SMT as the pulse-per-second source. It sends a supplemental timing packet that contains an estimate of the quantization error in its pulse output. But the manual isn't clear on whether that applies to the next pulse or to the previous one. I've seen people correct the delay by using a programmable delay line which seems like it would only be possible if the measurement was for the next pulse. But on the other hand there is a pulse was not generated alarm that definitely applies to the previous (non)-pulse which suggests that maybe other fields refer exclusively to the previous pulse. I can handle either way since the pulses are timestamped asynchronously and can be post-processed at any time but from some preliminary data collection it's not clear which way it's meant to go. Does anyone know for sure whether the quantization error is for the next or preceding pulse? ___ 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] GPSDO control loops and correcting quantization error
On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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] GPSDO control loops and correcting quantization error
Also you need a super ultra fantastic voltage reference for a 32bit DAC. Anyway, yes, in my GPSDO the controller has 3 levels: at startup is fast, then slow and then very slow. The levels trigger when the precision estimate is 10E-9 and 10E-11. If you have a resolution of 10nS then take 10 averages and your resolution will be 1nS and so on. When I switch level, the number of averages is increased too but this leads to a slower DAC update rate. This is the problem: now I'm trying to figure out if the corrective action can be predicted (Kalman filtering) and applied at the same speed. On Fri, Sep 14, 2012 at 8:52 PM, Chris Albertson albertson.ch...@gmail.comwrote: On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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] GPSDO control loops and correcting quantization error
Azelio Boriani wrote: Also you need a super ultra fantastic voltage reference for a 32bit DAC. Not really, the reference only needs to have low noise and good short term stability. Long term drift in the reference voltage will be corrected by the feedback loop. Anyway, yes, in my GPSDO the controller has 3 levels: at startup is fast, then slow and then very slow. The levels trigger when the precision estimate is 10E-9 and 10E-11. If you have a resolution of 10nS then take 10 averages and your resolution will be 1nS and so on. However the noise associated with the timing resolution doesn't average down so quickly. If such noise is random than at best it is reduced by SQRT(10) by averaging 10 measurements. There is no real substitute for lower noise, higher resolution measurements. When I switch level, the number of averages is increased too but this leads to a slower DAC update rate. This is the problem: now I'm trying to figure out if the corrective action can be predicted (Kalman filtering) and applied at the same speed. Bruce On Fri, Sep 14, 2012 at 8:52 PM, Chris Albertson albertson.ch...@gmail.comwrote: On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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] GPSDO control loops and correcting quantization error
Yes, you are right: but actually I have a 2.5nS simple time interval counter in the FPGA and the only way to go beyond is the average. The sophisticated way would be to implement a tapped delay line or vernier delay line time-to-digital converter in a bigger FPGA than the XC3S50. And, yes, I have recently started my first GPS disciplined Rb with the same hardware. I have eliminated the fast and slow steps from the processing, using only the slowest one. On Fri, Sep 14, 2012 at 9:50 PM, Bruce Griffiths bruce.griffi...@xtra.co.nz wrote: Azelio Boriani wrote: Also you need a super ultra fantastic voltage reference for a 32bit DAC. Not really, the reference only needs to have low noise and good short term stability. Long term drift in the reference voltage will be corrected by the feedback loop. Anyway, yes, in my GPSDO the controller has 3 levels: at startup is fast, then slow and then very slow. The levels trigger when the precision estimate is 10E-9 and 10E-11. If you have a resolution of 10nS then take 10 averages and your resolution will be 1nS and so on. However the noise associated with the timing resolution doesn't average down so quickly. If such noise is random than at best it is reduced by SQRT(10) by averaging 10 measurements. There is no real substitute for lower noise, higher resolution measurements. When I switch level, the number of averages is increased too but this leads to a slower DAC update rate. This is the problem: now I'm trying to figure out if the corrective action can be predicted (Kalman filtering) and applied at the same speed. Bruce On Fri, Sep 14, 2012 at 8:52 PM, Chris Albertson albertson.ch...@gmail.comwrote: On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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. ___ 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] GPSDO control loops and correcting quantization error
Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. Don Chris Albertson On Fri, Sep 14, 2012 at 11:21 AM, Michael Tharp g...@partiallystapled.com wrote: Finally, do people think a 16 bit DAC is adequate or should I consider building a 32-bit one? I looked at a few designs when putting this together but decided to keep it simple until things were up and running. Having a 32-bit DAC would give you enough range so that you could drop in any OCXO you might have. But if you can have trimmer resisters to selected for your specif OCXO then 16-bits should be enough. If it were me, I'd want the DAC steps to be smaller than what the phase detector can measure. Said another way a 32-bit DAC might eliminate the need for scale and offset trimmer resistors. Chris Albertson Redondo Beach, California ___ 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. -- Neither the voice of authority nor the weight of reason and argument are as significant as experiment, for thence comes quiet to the mind. De Erroribus Medicorum, R. Bacon, 13th century. If you don't know what it is, don't poke it. Ghost in the Shell Dr. Don Latham AJ7LL Six Mile Systems LLP 17850 Six Mile Road POB 134 Huson, MT, 59846 VOX 406-626-4304 www.lightningforensics.com www.sixmilesystems.com ___ 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] GPSDO control loops and correcting quantization error
-- Forwarded message -- From: Don Latham d...@montana.com Date: Fri, Sep 14, 2012 at 2:01 PM Subject: Re: [time-nuts] GPSDO control loops and correcting quantization error To: Discussion of precise time and frequency measurement time-nuts@febo.com Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. I think the step size would be close to the same with the 32 bit DAC but the reason you use it is so you can control just about any OCXO, Rb or other things you drop in. In other words I'd use the extra bits to extend the voltage range. But while in use, I doubt you'd ever change the highest 16 or 18 bits Also if you are building a general purpose controller for OCXOs and Rb, remember that some Rb oscillators use RS-232 control to set the frequqecy. It might be good to build in an RS232 port. The firmware in the uP can always be changed but hard to add the DB9 connector later. One otherthing I was doing when I was working on a design like this was to discipline both an XO and an Rb from the same GPS. Almost like building two controllers but you save some because you can use one uP and one GPS interface. Now that you have a disciplined Rb it can be used for hold over in case the GPS goes away. I thought it would be unlikely for GPS to actually fail but allowing for hold over would make the entire unit portable. Chris Albertson Redondo Beach, California ___ 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] GPSDO control loops and correcting quantization error
On 09/14/2012 05:31 PM, Chris Albertson wrote: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. Probably true, luckily as others have mentioned the long-term stability of the DAC and its voltage source are less important. I think the step size would be close to the same with the 32 bit DAC but the reason you use it is so you can control just about any OCXO, Rb or other things you drop in. In other words I'd use the extra bits to extend the voltage range. But while in use, I doubt you'd ever change the highest 16 or 18 bits Also if you are building a general purpose controller for OCXOs and Rb, remember that some Rb oscillators use RS-232 control to set the frequqecy. It might be good to build in an RS232 port. The firmware in the uP can always be changed but hard to add the DB9 connector later. One otherthing I was doing when I was working on a design like this was to discipline both an XO and an Rb from the same GPS. Almost like building two controllers but you save some because you can use one uP and one GPS interface. Now that you have a disciplined Rb it can be used for hold over in case the GPS goes away. I thought it would be unlikely for GPS to actually fail but allowing for hold over would make the entire unit portable. I had this idea as well, although not for disciplining the Rb (which unfortunately mine cannot, it's a less popular Efratom model clearly pulled from telecom application and has no external control that I can see) but just as a backup timing source for holdover. I've mentioned it here before, but the gist would be to estimate its frequency while GPS was working, then if GPS fails use the Rb instead either by dividing it by the last known frequency or by adding the error to the measurement loop. That said, I would like the holdover performance with just the OCXO to be as good as possible in its own right. I'm planning to make a future version of this project available but this first revision is mainly an experimentation platform and wouldn't be of much use to someone who doesn't have the same equipment as me. Stay tuned... -- m. tharp ___ 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] GPSDO control loops and correcting quantization error
d...@montana.com said: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. The trick for this application is that you don't need full accuracy over the full range of the DAC. All you need is roughly linear and stable around the operating point. The PLL will take care of any offset. Any gain error is just a minor change to the overall gain. This thread started with 16 bit PWM DAC. I think that matches the requirements. I would expect a problem area would be filtering the PWM output. Anything you don't filter out will turn into close in spikes. It might be fun to try to measure them. 64K/72 MHz is about 1 ms. 32bits at 72 MHz is 60 seconds. Has anybody compared DDS style DACs with PWM? The idea is to spread the pulses out to make the filtering easier. Instead of 10, you would get to work with 1010101010 -- These are my opinions. I hate spam. ___ 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] GPSDO control loops and correcting quantization error
Hello all, Most voltage controlled XOs have a voltage reference output. Is it necessary for the DAC output / frequency control input, to track this voltage reference output? Thank You Don O'Connor ___ 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] GPSDO control loops and correcting quantization error
Hi Don, I don't know if I've misunderstood your question, but as I understand it the voltage reference output is a fixed voltage from an internal regulator that can then be used as the supply to an external control circuit. For example, it could be used as the feed voltage for a variable resistor that has its wiper connected to the EFC input to allow for manual frequency adjustment. When some other form of external control is used, such as a DAC output for example, it's not uncommon to find the voltage reference output left disconnected and the control circuit fed from an alternative supply. Regards Nigel GM8PZR In a message dated 14/09/2012 23:10:59 GMT Daylight Time, eg...@wowway.com writes: Hello all, Most voltage controlled XOs have a voltage reference output. Is it necessary for the DAC output / frequency control input, to track this voltage reference output? Thank You Don O'Connor ___ 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] GPSDO control loops and correcting quantization error
If it is a reference then tracking is not the correct term: it shouldn't move. If it is a reference then can be used for the DAC. On Sat, Sep 15, 2012 at 12:10 AM, Don Oconnor eg...@wowway.com wrote: Hello all, Most voltage controlled XOs have a voltage reference output. Is it necessary for the DAC output / frequency control input, to track this voltage reference output? Thank You Don O'Connor ___ 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] GPSDO control loops and correcting quantization error
Nigel, Yes, you understood and answered the question. Thank you Don O I don't know if I've misunderstood your question, but as I understand it the voltage reference output is a fixed voltage from an internal regulator that can then be used as the supply to an external control circuit. For example, it could be used as the feed voltage for a variable resistor that has its wiper connected to the EFC input to allow for manual frequency adjustment. When some other form of external control is used, such as a DAC output for example, it's not uncommon to find the voltage reference output left disconnected and the control circuit fed from an alternative supply. Regards Nigel GM8PZR In a message dated 14/09/2012 23:10:59 GMT Daylight Time, eg...@wowway.com writes: Hello all, Most voltage controlled XOs have a voltage reference output. Is it necessary for the DAC output / frequency control input, to track this voltage reference output? ___ 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] GPSDO control loops and correcting quantization error
On Fri, 14 Sep 2012 15:08:53 -0700, Hal Murray hmur...@megapathdsl.net wrote: d...@montana.com said: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. The trick for this application is that you don't need full accuracy over the full range of the DAC. All you need is roughly linear and stable around the operating point. The PLL will take care of any offset. Any gain error is just a minor change to the overall gain. One thing you sure want is a DAC that is monotonic. Differential nonlinearity larger than the least significant bit can cause some rather peculiar servo loop problems. ___ 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] GPSDO control loops and correcting quantization error
My 2.5nS TIC? Very simple: a 400MHz counter start-stop gated with the two signal to compare. I have published here the VHDL code for it few months ago. Really nothing new but simple and useful for a 35-40nS GPSDO PPS output from an OCXO. The Rb PPS wondering is actually under evaluation against the Z3815A. On Sat, Sep 15, 2012 at 12:08 AM, Hal Murray hmur...@megapathdsl.netwrote: d...@montana.com said: Michael: Actually implementing a 16 bit DAC to its 1-bit minimum resolution will be headache enough. You will gain a real education in good grounding practice, shielding, power supply stability and noise, and other Murphy intrusion. A 32 bit DAC IMHO, is impossible, and that's the name of that tune. The trick for this application is that you don't need full accuracy over the full range of the DAC. All you need is roughly linear and stable around the operating point. The PLL will take care of any offset. Any gain error is just a minor change to the overall gain. This thread started with 16 bit PWM DAC. I think that matches the requirements. I would expect a problem area would be filtering the PWM output. Anything you don't filter out will turn into close in spikes. It might be fun to try to measure them. 64K/72 MHz is about 1 ms. 32bits at 72 MHz is 60 seconds. Has anybody compared DDS style DACs with PWM? The idea is to spread the pulses out to make the filtering easier. Instead of 10, you would get to work with 1010101010 -- These are my opinions. I hate spam. ___ 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] GPSDO control loops and correcting quantization error
Hi Very few VCXO's have reference outputs. Some OCXO's have reference outputs. The gotcha there is the oven current. You can easily get multiple mV sort of changes in the OCXO ground voltage as the oven current cycles over a fairly narrow range. That significantly impacts the usefulness of the reference, since they share a common ground. Bob On Sep 14, 2012, at 6:10 PM, Don Oconnor eg...@wowway.com wrote: Hello all, Most voltage controlled XOs have a voltage reference output. Is it necessary for the DAC output / frequency control input, to track this voltage reference output? Thank You Don O'Connor ___ 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.
