[time-nuts] An Other Ublox 8

[John Ackermann N8UR]

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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