Thanks for your very well described study of SDRs processing GPS signals and resulting timing accuracy.
Best regards Glen > On Dec 11, 2021, at 3:28 AM, jmfriedt <jean-michel.fri...@femto-st.fr> wrote: > > If it can be of any help, we discussed generating a GPS-aligned 1-PPS > in http://jmfriedt.free.fr/ifcs2021.pdf and the oral presentation at > http://jmfriedt.free.fr/ifcs2021_presentation_jmfriedt.mp4 prepared > for the International Frequency Control Symposium (IFCS). > > What we realized when working on this topic is that the only place > where time is known in an SDR processing sequence is at the ADC sample > acquisition step, since all subsequent processing and data transfer > afterwards are asynchronous, including the multitasking non-real time > operating system GNU Radio is running on. Hence, the control on the > 1-PPS alignement can only be achieved at the FPGA level clocking the > ADC: the host clock is irrelevant since we do not know how much time > was needed to transfer and process data. Luckily, GNU Radio provides a > timestamp on each sample, assuming no data was lost since streaming > started, so the control signal can be fed back to the SDR clock if the > delay between reference and generated 1-PPS is measured using the same > time reference of the ADC clock. > > Best, JM > >>> On 2021-12-10 17:25, isaac mario tupac davila wrote: >>>> Hello everyone! >>>> >>>> My name is Isaac. I have a curious situation here... I've >>>> generated a pulsed signal per second. I've saved one second >>>> period in a .h5 file, so that I'm sure I'm having a fixed signal >>>> per second, and then repeat it in my GRC flowgraph. . >>>> >>>> To use this signal as Tx I'm trying to synchronize a USRP N200 and >>>> a trimble in GRC. After run my flowgraph, this is what I see: >>>> >>>> imagen.png >>>> >>>> imagen.png >>>> >>>> The yellow signals are the PPS of my trimble and my purple signals >>>> are my tx signal per second. They are periodic in time but when >>>> you see it deeper, the difference between the timble PPS and my Tx >>>> signal is approx 97 ms. I think this difference should be close to >>>> cero, as my USRP and trimble are synchronized. ¿What is happening >>>> here? ¿Is this a normal behavior? I'm using unknown PPS to >>>> configure my UHD:USRP sink in GRC.... >>>> >>>> I'll appreciate any help to clarify this behavior >>>> >>>> Regards >>>> Isaac T. >>>> >>> Your flow-graph made very little sense to me. If you want to >>> synchronize your TX, you have to take explicit measures to ASK the >>> USRP to schedule your transmits at specific times. >>> The 1PPS signal only synchronizes an internal time-stamp clock in >>> the unit. It has NO WAY of knowing what the *meaning* of your >>> samples are, so it can't possibly >>> synchronize some arbitrary event in your continuous sample stream >>> to 1PPS without you explicitly asking for when your bursts need to >>> be sent. >>> >>> https://kb.ettus.com/Synchronizing_USRP_Events_Using_Timed_Commands_in_UHD >>> >> >> >> > > > > -- > JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 > Besancon, France