[time-nuts] ``direct'' RS-232 vs. RS-232 via USB vs. PPS decoding cards

Sat Feb 18 08:36:51 EST 2017

Hi

> On Feb 18, 2017, at 4:53 AM, David J Taylor <david-taylor at blueyonder.co.uk> wrote:
> 
> Hi,
> 
> I was wondering whether there is some data/information available on the
> claimed +/- 100 ns jitter?

I guess the previous was not complete enough.

I routinely measure PPS jitter on GPS modules down well below 10 ns on a 53131. 
That’s after sawtooth correction is applied. Without sawtooth correction, the +/- 24 
or +/- 12 or +/- 1.25 ns of the sawtooth adds into that.

The reference used is an HP 5071 with a high performance tube option. 

Bob

> 
> Regarding the PPS -> USB (using the CTS line of a FTDI FT232R), I
> plotted, using some lines of Python, the time offset as attached. Just
> to get an overview how it is 'worst case', i.e., user program, python,
> etc. The 1PPS signal comes from a GPS rx.
> Looks like a standard deviation of around 150 us.
> y-axis:  measured pps offset from full second (computer time) in us,
> x-axis pps pulse number.
> 
> On the long term it looks interesting (while measuring I played with the
> NTP server on this computer)
> Until ca. second 10000: ntpd synchronization via internet
> Until ca. second 17000: made an additional LAN NTP server (GPS) available
> Until the end: replaced ntpd with chrony (still using internet and local
> servers)
> 
> Interesting points:
> -It looks surprisingly bad with using the normal ntpd (especially, there
> is not really an improvement having an local GPS based server
> available, did I do something wrong? Only the offset changes by ca. 3
> ms.)
> -It looks surprisingly good with chrony. But there are continuously
> outliers of up to 4500 us, is this a result of the chrony control loop?
> The time is wandering around with ntpd, but has less jitter.
> 
> Conclusion:
> Despite the 150 us stddev, the using PPS over USB gives some interesting
> inside of what the local ntp server is actually doing. It looks to me
> like it would be an improvement to use it when using ntpd, but not when
> using chrony.
> 
> Best regards,
>  Thomas
>  DK6KD
>  SA6CID
> 
> PS:
> Raw data is here, if you want to zoom in: (1.7 MiB, one row per PPS
> offset in us)
> http://petig.eu/pps-usb.txt
> =================================================
> 
> Thomas,
> 
> I've done some tests with PPS over USB with Windows some time back, which showed that PPS?USB could be better than LAN-sync alone, but that also included a reduction of the poll interval from possibly 64 seconds for LAN sync to 16 seconds for PPS sync, which may have influenced the results.
> 
> <pet-peeve>It would be helpful to have some units on the axes - 10000 what? I'm guessing microseconds....</pet-peeve>
> 
> For comparison, here is a Raspberry Pi running NTP, with the reported offset plotted against time.
> 
> http://www.satsignal.eu/mrtg/raspi14_ntp_3.html
> 
> This Raspberry Pi (running a seismic detector) is using an Ethernet connection via Power-line Ethernet (yes, I know, QRM etc. etc.), and a couple of switches to a very good stratum-1 server.  I would estimate from your graph that the jitter in offset is about 1 millisecond peak-to-peak, but it seems that I get less than that - perhaps 100 microseconds peak-to-peak with occasional excursions outside that.  This is with the latest reference version of NTP.
> 
>    remote           refid      st t when poll reach   delay   offset jitter
> ==============================================================================
> *192.168.0.20    .GPS.            1 u   17   32  377   12.351    0.000 0.428
> +192.168.0.11    .uPPS.           1 u    2   32  377   12.432   -0.106 0.824
> -192.168.0.3     .kPPS.           1 u   13   32  377   21.366   -4.524 0.804
> +192.168.0.83    .kPPS.           1 u   27   32  377   21.614   -4.511 1.206
> uk.pool.ntp.org .POOL.          16 p    -   64    0    0.000    0.000 0.001
> -193.150.34.2    133.150.251.233  3 u   38   64  137   32.343    2.738 1.477
> -80.87.128.17    94.125.129.7     3 u   30   64  375   53.337   -1.225 1.516
> -192.146.137.13  82.148.230.254   2 u   56   64  377   46.089    2.220 2.535
> -129.250.35.250  249.224.99.213   2 u  169   64  214   42.499   -3.015 12.507
> -213.130.44.252  145.238.203.14   2 u  487   64  200   37.210   -0.725 13.232
> 
> 73,
> David GM8ARV
> -- 
> SatSignal Software - Quality software written to your requirements
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
> Twitter: @gm8arv 
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