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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