[time-nuts] Re: Catching range of GPSDO
Thanks for all the nice feedback. The 16 bit DAC being used is actually constructed as the sum two 12 bit DAC's where the low DAC has about a factor 100 less gain. Exactly to ensure all bits are usable. The multi step catching being testing now is: 1: Calibrate high dac gain 2: Calibrate low dac gain 3: Tune dac till frequency error < 5e-8 (takes 2 or 3 seconds) 4: Jam sync 5: Tune dac till phase change per second < 5e-8 (takes 1 to 10 seconds) 6: Jam sync 7: Tune dac to keep average(n=20) phase error < 1e-9, if above go back to stage 5. Now I have to solve why the simulation (incl quantization) behaves differently from the actual implementation. Erik. On 4-3-2022 1:37, Tom Van Baak wrote: Hi Erik, 1) When your device powers up you wait for GPS acquisition. The wait could be seconds, or maybe minutes. It depends on many factors, some of which you control -- such as the choice of make / model receiver, capacitor / battery backup, saved state in eeprom, a good RTC, etc. Some of which you cannot control -- such as what kind of antenna the user has, how long and what kind of cable is used, where in the world they are located, what kind of reception indoors, what level of interference is present, how long since they last powered up your device, etc. During this waiting time you don't know your TCXO frequency or phase. This is normal. 2) When the first 1PPS arrives, you can tell if your TCXO phase is wrong to within, say, 25 ns. Part of that error is your GPS receiver, part of that error is TCXO noise, part of that error is TIC resolution. This is normal. Set the TCXO/counter phase (aka "jam sync"). Now you've gone from not knowing the time or frequency to having your TCXO in phase to within a few clock cycles. This gets you 99.9% of the way there (in phase). On the very first 1PPS. Note that since your device lacks 1PPS output there is no need to physically sync TCXO phase or zero a h/w counter. It can all be done with a virtual counter inside the MCU. 3) Now wait for the next 1PPS. How much did the phase change? If your TCXO is off by 1e-6 you will see 1 us phase change. If it's 1e-7 then you will see 100 ns. Your system is able to detect this. Adjust the DAC immediately. Now not only is your 1PPS close to correct but your frequency is also. This gets you 99.9% in frequency, already by the 2nd 1PPS. If your TCXO is within 1e-8 you may have to wait a few seconds before you can reliably detect phase and frequency offsets on the order of 10 ns and 10 ns/s, respectively. The key is not to wait some fixed N seconds and make a measurement. Instead check the phase and frequency offset every second and let the frequency error drive the process, not some fixed gate time. 4) So within a few seconds you can achieve 1e-8 levels of accuracy. Similarly, it may take 10 s or 20 s to set the TCXO to 1e-9 levels. At this point you can pre-load your parameters, filters, and enable your PLL / FLL algorithm for longer-term use. The dynamics at power up can be completely different than steady-state so it's not clear to me that the same locking process should be used for both. For instrument-grade GPSDO this doesn't matter that much because they tend to be powered up once and then left running for days or years. But for a portable, battery, handheld unit frequent power cycling is expected and the user would like accurate results within seconds, not wait for some PhD level PLL / FLL / Kalman filter to settle to textbook perfection. Note also all of this can be simulated. You have lots of raw data. Collect both warm- and cold-start data too. Then simulate power up at random points within your data set. See how fast you can get to 1e-9. 5) You DAC sounds insufficient to me. 1e-11 * 32768 = 0.3 ppm. What are the drift specs of your TCXO? Do you want all your products in the trash after the first year because the DAC runs out of tuning range? Even the famous hp 10811 has a drift spec of 5e-10/day and 0.1 ppm/year. One solution is to provide a screwdriver hole so that the user can make a manual EFC adjustment when the DAC gets out of range. This is not something you'd do on a professional instrument-grade GPSDO, but it might be totally acceptable, even clever, on a low cost handheld unit. /tvb On 3/3/2022 12:36 PM, Erik Kaashoek wrote: The GPSDO I'm building started with frequency locking but now I'm adding phase locking so the time stamping counter can be on GPS time. A first version works with a PI controller setting the vc-tcxo Vtune DAC based on the phase difference of the 10 MHz with the PPS phase. Due to tolerances the tcxo frequency range is big and is set by a 16 bit DAC where 1 bit is about 2e-11 frequency change. Once the DAC value is close to the correct frequency the loop catches nicely but if the setting is far off catching takes a long time. A possible solution is to use the frequency error to set the DAC close to the optimal freq
[time-nuts] Re: Catching range of GPSDO
Erik, On 2022-03-03 21:36, Erik Kaashoek wrote: The GPSDO I'm building started with frequency locking but now I'm adding phase locking so the time stamping counter can be on GPS time. A first version works with a PI controller setting the vc-tcxo Vtune DAC based on the phase difference of the 10 MHz with the PPS phase. Due to tolerances the tcxo frequency range is big and is set by a 16 bit DAC where 1 bit is about 2e-11 frequency change. Once the DAC value is close to the correct frequency the loop catches nicely but if the setting is far off catching takes a long time. A possible solution is to use the frequency error to set the DAC close to the optimal frequency for catching. Speed of catching is important as the design is intended to only be switched on when needed. Does anyone have pointers to info on how to do quick catching in such a control loop? The track-in time scales proportionally with the square of the frequency error and inversely cube of the loop bandwidth. The formula becomes garbled in ascii-math and I am too tired to convey it in readable form. The lesson is: Make bandwidth high for quick lock-in so frequency acquisition is achieved, then switch over to a different bandwidth of the loop from that point. The second lesson is: Pre-setting the frequency from previous learning can quick the process up. Notice that the PI-loop will lock if the frequency error is within capture range, with is in effective that the oscillator can be steered to align. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: Catching range of GPSDO
and how you handle these control transitions between a slow loop, and the next fast loop, and next fast etc etc , whether in software, or in hardware , example selecting which divider output to use, is non trivial. In hardware, I wll normally freeze the value, and wait a full cycle until the hardware change of divider that interrupts the counter has rippled through the system. In software, there is a littl emore determinicity as it can be known which and what edge and counter ststus etc can be jam loaded , and also progressively slower loop constants can run simultaneously. Also, step changes to (software) loop constants can be realized (difficult in hardware) On 4/03/2022 11:16 am, Bob kb8tq wrote: Hi On Mar 3, 2022, at 3:36 PM, Erik Kaashoek wrote: The GPSDO I'm building started with frequency locking but now I'm adding phase locking so the time stamping counter can be on GPS time. A first version works with a PI controller setting the vc-tcxo Vtune DAC based on the phase difference of the 10 MHz with the PPS phase. Due to tolerances the tcxo frequency range is big and is set by a 16 bit DAC where 1 bit is about 2e-11 frequency change. Take a look at just how good the LSB ( or maybe even LSB’s … ) on your DAC really are. Some are not all that great and others are quite impressive. Cost is indeed part of that. Summing a pair of lower resolution parts often turns out to be pretty cost competitive. With 2^16 steps a LSB of 2x10^-11 (and perfect linearity) you would get a 1.3 ppm tune range. If it’s symmetric you would get +/- 0.65 ppm. That probably is not “enough” to compensate for much aging on a low cost TCXO….. Once the DAC value is close to the correct frequency the loop catches nicely but if the setting is far off catching takes a long time. A possible solution is to use the frequency error to set the DAC close to the optimal frequency for catching. Speed of catching is important as the design is intended to only be switched on when needed. Does anyone have pointers to info on how to do quick catching in such a control loop? The “normal answer” is to use a sequence of control loops. You start off with a fast / wide band loop and then once it gets “good enough” you go to a somewhat narrower loop. Your state monitoring tells you when it’s got to an adequately settled point and you step again. It is not uncommon to have anywhere from a few to a dozen or so of these steps. Bob Erik ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. -- Glen English RF Communications and Electronics Engineer CORTEX RF Pacific Media Technologies Pty Ltd trading as Cortex RF ABN 40 075 532 008 PO Box 5231 Lyneham ACT 2602, Australia. au mobile : +61 (0)418 975077 ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: Catching range of GPSDO
Hi Erik, 1) When your device powers up you wait for GPS acquisition. The wait could be seconds, or maybe minutes. It depends on many factors, some of which you control -- such as the choice of make / model receiver, capacitor / battery backup, saved state in eeprom, a good RTC, etc. Some of which you cannot control -- such as what kind of antenna the user has, how long and what kind of cable is used, where in the world they are located, what kind of reception indoors, what level of interference is present, how long since they last powered up your device, etc. During this waiting time you don't know your TCXO frequency or phase. This is normal. 2) When the first 1PPS arrives, you can tell if your TCXO phase is wrong to within, say, 25 ns. Part of that error is your GPS receiver, part of that error is TCXO noise, part of that error is TIC resolution. This is normal. Set the TCXO/counter phase (aka "jam sync"). Now you've gone from not knowing the time or frequency to having your TCXO in phase to within a few clock cycles. This gets you 99.9% of the way there (in phase). On the very first 1PPS. Note that since your device lacks 1PPS output there is no need to physically sync TCXO phase or zero a h/w counter. It can all be done with a virtual counter inside the MCU. 3) Now wait for the next 1PPS. How much did the phase change? If your TCXO is off by 1e-6 you will see 1 us phase change. If it's 1e-7 then you will see 100 ns. Your system is able to detect this. Adjust the DAC immediately. Now not only is your 1PPS close to correct but your frequency is also. This gets you 99.9% in frequency, already by the 2nd 1PPS. If your TCXO is within 1e-8 you may have to wait a few seconds before you can reliably detect phase and frequency offsets on the order of 10 ns and 10 ns/s, respectively. The key is not to wait some fixed N seconds and make a measurement. Instead check the phase and frequency offset every second and let the frequency error drive the process, not some fixed gate time. 4) So within a few seconds you can achieve 1e-8 levels of accuracy. Similarly, it may take 10 s or 20 s to set the TCXO to 1e-9 levels. At this point you can pre-load your parameters, filters, and enable your PLL / FLL algorithm for longer-term use. The dynamics at power up can be completely different than steady-state so it's not clear to me that the same locking process should be used for both. For instrument-grade GPSDO this doesn't matter that much because they tend to be powered up once and then left running for days or years. But for a portable, battery, handheld unit frequent power cycling is expected and the user would like accurate results within seconds, not wait for some PhD level PLL / FLL / Kalman filter to settle to textbook perfection. Note also all of this can be simulated. You have lots of raw data. Collect both warm- and cold-start data too. Then simulate power up at random points within your data set. See how fast you can get to 1e-9. 5) You DAC sounds insufficient to me. 1e-11 * 32768 = 0.3 ppm. What are the drift specs of your TCXO? Do you want all your products in the trash after the first year because the DAC runs out of tuning range? Even the famous hp 10811 has a drift spec of 5e-10/day and 0.1 ppm/year. One solution is to provide a screwdriver hole so that the user can make a manual EFC adjustment when the DAC gets out of range. This is not something you'd do on a professional instrument-grade GPSDO, but it might be totally acceptable, even clever, on a low cost handheld unit. /tvb On 3/3/2022 12:36 PM, Erik Kaashoek wrote: The GPSDO I'm building started with frequency locking but now I'm adding phase locking so the time stamping counter can be on GPS time. A first version works with a PI controller setting the vc-tcxo Vtune DAC based on the phase difference of the 10 MHz with the PPS phase. Due to tolerances the tcxo frequency range is big and is set by a 16 bit DAC where 1 bit is about 2e-11 frequency change. Once the DAC value is close to the correct frequency the loop catches nicely but if the setting is far off catching takes a long time. A possible solution is to use the frequency error to set the DAC close to the optimal frequency for catching. Speed of catching is important as the design is intended to only be switched on when needed. Does anyone have pointers to info on how to do quick catching in such a control loop? Erik ___ time-nuts mailing list --time-nuts@lists.febo.com -- To unsubscribe send an email totime-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
[time-nuts] Re: Catching range of GPSDO
Hi > On Mar 3, 2022, at 3:36 PM, Erik Kaashoek wrote: > > The GPSDO I'm building started with frequency locking but now I'm adding > phase locking so the time stamping counter can be on GPS time. > A first version works with a PI controller setting the vc-tcxo Vtune DAC > based on the phase difference of the 10 MHz with the PPS phase. Due to > tolerances the tcxo frequency range is big and is set by a 16 bit DAC where > 1 bit is about 2e-11 frequency change. Take a look at just how good the LSB ( or maybe even LSB’s … ) on your DAC really are. Some are not all that great and others are quite impressive. Cost is indeed part of that. Summing a pair of lower resolution parts often turns out to be pretty cost competitive. With 2^16 steps a LSB of 2x10^-11 (and perfect linearity) you would get a 1.3 ppm tune range. If it’s symmetric you would get +/- 0.65 ppm. That probably is not “enough” to compensate for much aging on a low cost TCXO….. > Once the DAC value is close to the correct frequency the loop catches > nicely but if the setting is far off catching takes a long time. > A possible solution is to use the frequency error to set the DAC close to > the optimal frequency for catching. > Speed of catching is important as the design is intended to only be > switched on when needed. > Does anyone have pointers to info on how to do quick catching in such a > control loop? The “normal answer” is to use a sequence of control loops. You start off with a fast / wide band loop and then once it gets “good enough” you go to a somewhat narrower loop. Your state monitoring tells you when it’s got to an adequately settled point and you step again. It is not uncommon to have anywhere from a few to a dozen or so of these steps. Bob > Erik > ___ > time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an > email to time-nuts-le...@lists.febo.com > To unsubscribe, go to and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com -- To unsubscribe send an email to time-nuts-le...@lists.febo.com To unsubscribe, go to and follow the instructions there.
