[time-nuts] An Other Ublox 8

Thu Jan 28 07:05:42 UTC 2021

Hello,

I have been playing lately with the M8F, but did not had a detailed look 
to the 30.72MHz frequency as John did. I have used it along with a 
Zynq-based board (a very unexpensive surplus EBAZ4205, that can be get 
for ~10 EUR) to made an ntp server, replacing the kernel clocksource 
from the ARM timer with an external timer implemented at the FPGA 
running from the 30.72MHz output (multiplied inside the FPGA to 250MHz), 
and using that counter to provide the PPS timestamps, in order to 
eliminate any software jitter due to interrupt latencies.

I have been using chrony instead of ntp. The PPS timestamps are almost 
ever ending in .000000000:

# cat /sys/class/pps/pps0/assert
1611817180.000000000#94736

The result of chornyc sources is:

# chronyc sources
210 Number of sources = 8
MS Name/IP address         Stratum Poll Reach LastRx Last sample
===============================================================================
#* GPS                           0   0   377     1     +0ns[ +0ns] +/-  
901ns
^- 10.0.2.24                     1   3   377     6   +124us[ +124us] +/- 
1335us

10.0.2.24 is a RPi-based ntpd. Both the adjusted offset and the measured 
offset are ever 0ns, and the estimated error stucks at 901ns, that I 
think that is something like a chrony floor (I have seen it drop to zero 
along time with other chorny version)

After a short time from boot, the results of the tracking command are:

# chronyc tracking
Reference ID    : 47505300 (GPS)
Stratum         : 1
Ref time (UTC)  : Thu Jan 28 06:53:36 2021
System time     : 0.000000000 seconds fast of NTP time
Last offset     : +0.000000000 seconds
RMS offset      : 0.000000000 seconds
Frequency       : 0.000 ppm slow
Residual freq   : +0.000 ppm
Skew            : 0.000 ppm
Root delay      : 0.000000001 seconds
Root dispersion : 0.000002380 seconds
Update interval : 1.0 seconds
Leap status     : Normal

All offsets 0 :)

So although it does not seem optimal for a GPSDO, it looks nice for 
applications like this (only caveat is that the M8F price is around 8x 
the FPGA board price... :) )

Regards,

Javier, EA1CRB

On 27/1/21 16:09, John Ackermann N8UR wrote:
> Hi Bert --
>
> That's very interesting.  Can you explain a bit more about how you are 
> generating the 24 MHz and the role of the Si5351?
>
> I took a close look at the M8F a year or so ago, looking at PPS and 
> the 30.72 MHz output it makes directly available:
>
> 1.  PPS jitter is very, very good compared to other single-frequency 
> receivers -- it's about the same as the dual frequency ZED-F9.  
> However, there is no sawtooth correction available to deal with the 
> jitter that remains.  When you look at the M8F PPS vs. M8T with 
> sawtooth correction, the results are about the same.
>
> 2.  The internal 30.72 oscillator seems to be adjusted with a sort of 
> PWM control -- it does a bang-bang of about +/- 3e-10 around the true 
> frequency, getting the average frequency correct by spending more or 
> less time above or below nominal. (See attached.)
>
> 3.  It's an interesting question whether you could get better results 
> disciplining an external oscillator, but I think that would depend on 
> the DAC sensitivity and whether the PLL time constant was 
> appropriately set (see point 4).
>
> 4.  One considerable disadvantage to using the M8F as a GPSDO is that 
> there doesn't seem to be any way to adjust the loop time constant (or 
> even to know what it is).  The only adjustable parameter I've been 
> able to find is the EFC sensitivity.
>
>
>
> On 1/27/21 7:27 AM, ew via time-nuts wrote:
>> Over the last couple of years I have looked at the LEA M8F, it is a 
>> GPSDO with 1 pps.  not needing Saw Tooth correction. How ever its 
>> frequency output is not time-nut friendly. 10 MHz is only 5 E-9. An 
>> ebay post showed an 8F board with intriguing 24 MHz data. Juerg did 
>> some tests with an SI 5351 at 24 MHz. Spec is 25 to 27 MHz but his 
>> tests at 24 MHz shows 10 MHz at 1 E-12.  So I bought some, one on the 
>> way to Switzerland. Also made some of our heavies aware of our work. 
>> The seller initially was not going to make more boards but changed 
>> his mind. I suspect the parts a pulls and he has to make boards to 
>> make sure the F8's work. My goal is to use it for aging tests always 
>> use 24 to 48 hour average testing. See my attached results exceed my 
>> expectations. 2.39 E-13 mean! Will update once Juerg has his unit and 
>> uses a SI 5131 to generate 10 MHz.  I can use the 24 MHz it is a true 
>> representation of the GPS signal. Can also use other GNS signals. 
>> Leave that up to others.                                           
>>  Bert Kehren
>>
>>
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