Re: K3S XREF GPSDO recommendation needed

Posted by Clay Autery-2 on Jun 06, 2018; 7:17pm
URL: http://elecraft.85.s1.nabble.com/K3S-XREF-GPSDO-recommendation-needed-tp7641686p7641753.html

Thank you for the response, Ed.  Why discipline the OXCO, or even the DOXCO?

Because you can.

Because the ability is there to progressively steer the correction
instead of stomp on it twice a year.

Lots of reasons, not the least of which is not having to open a chassis
and/or drill a strategically placed hole in a chassis in order to allow
you to manually make the correctly.

And there are those of us who use the frequency reference for things
other than just our radios. <smile>

BUT I understand why you do it the way you do, now.  Which was the
intent of the original question.  Thanks!

And I do appreciate the information that the K3 is not phase locked and
why the choice was made.
Does anyone have any idea how often that "check and nudge" procedure is
done or better the logic behind how it is determined when to make a
correction?  I assume it is implemented in firmware.

Have a great day! <smile>

______________________
Clay Autery, KY5G
[phone]

On 06/06/18 02:45, Edward R Cole wrote:

> Wes,
>
> You got it pretty close.  I was kind of stumped by Clay's question. 
> Disciplining I take to mean controlling from an external source - huh?
> why?
>
> If you want GPS frequency accuracy then buy a GPSDO for a couple
> hundred dollars, but as far as affecting K3 frequency accuracy it will
> not be significantly more accurate than using an OCXO (Oven Controlled
> Xtal Oscillator).  The Morion spec for short term stability is +/- 5
> E-12.  Long term stability is+/- 5 E-5.  The OCXO drifts about 1-Hz
> per 6-months from my actual measurements. So, yes it needs to be
> disciplined occasionally (twice a year) manually by adjusting the
> freq. adjust pot comparing its output to an accurate 10-MHz source (I
> use my LPRO Rubidium as my reference).   The short term stability of
> the Rb is +/- 5 E-11 but that is "good enough".
>
> BTW the K3 frequency is established by the TCXO and the external
> reference merely performs a periodic check. Any freq error causes a
> correction instruction to the TCXO to bring it back onto frequency. 
> The TCXO-3 is rated at 0.5 ppm which is +/-5 E-7. Adding the EXREF
> limits drift to 0.1ppm or E-7.
>
> At 28-MHz that implies a 28 E+6/E-7 or 2.8 Hz.  The TCXO drift is
> limited to +/- 2.8 Hz.  The OCXO is 10,000 times more stable and thus
> more accurate.
>
> The reason you do not get the OCXO level of stability is that the
> EXREF system does not phase-lock the TCXO.  It periodically corrects
> the error but the TCXO drifts normally between freg corrections which
> happen often enough to reduce the amount of drift from 5.6 Hz to 2.8
> Hz at 28-MHz.
>
> The OCXO returns the TCXO to between 28,000,000.0005 Hz to
> 27,999,999,9995 Hz but then the TCXO drifts 2.8 Hz by the next
> frequency check.  I think 0.0005 Hz is close enough.  So maybe the
> GPSDO is +/- 5 E-13.  That would correct the K3 to 0.000005 Hz, but it
> would still drift 2.8Hz by the next frequency check.
>
> My actual frequency measurements show 0.5 to 1.5 Hz inaccuracy at
> 28-MHz and 1.5 to 2.5 Hz inaccuracy at 50-MHz so my TCXO-3 is doing
> better than 0.5ppm.
>
> Now if you want to know why Elecraft chose periodic freq correction
> over true phase-locking, the answer is low LO phase noise.  Much
> harder to get low phase noise in a PLL than in a TCXO.  And now you
> know the rest of the story.
>
> 73, Ed - KL7UW
>
> From: Wes Stewart <[hidden email]>
> To: [hidden email]
> Subject: Re: [Elecraft] K3S XREF GPSDO recommendation needed
>
> I'm not answering for Ed, and I don't have the Ref Lock doodad for my K3.
>
> That said, a decent OCXO, like a Morion, will have, after warm up and
> a little
> aging, stability more than adequate to discipline a K3.? The K3 isn't
> phase
> locked to an external reference anyway, it's just "nudged" on frequency
> periodically.
>
> If GPS lock is lost on the Bodnar device it reverts to a TCXO that is
> likely no
> better than the one in the K3, assuming the high stability option.?
> (Note: I
> haven't pursued all of the relevant specs, this is just gut feel.)
>
> I have the Bodnar unit because my sweetie asked me what I wanted for
> Christmas a
> few years ago and I had just read about it.? I have no compelling
> need, but I
> did use it with a modified SDR-IQ to try my hand at the last ARRL
> FMT.? I also
> have a Morion, off eBay, that I thought I should have.? I have yet to
> apply
> power to it :-)? I suppose I was remembering the days when I would
> have killed
> to have something that accurate.
>
> Likewise after reading timenuts I thought I should get a TAPR GPS time
> standard
> kit.? I have absolutely no need for that either but Lady Heather looks
> nice on a
> spare computer screen ticking away at one second increments with
> nanosecond
> accuracy :-).
>
> Wes? N7WS
>
>
>  ?On 6/5/2018 12:55 PM, Clay Autery wrote:
>
> > Just curious...? Why would you choose NOT to discipline the OXCO?
>
>
> 73, Ed - KL7UW
>   http://www.kl7uw.com
> Dubus-NA Business mail:
>   [hidden email]
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]