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
(318) 518-1389
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 <[email protected]>
To: [email protected]
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:
[email protected]
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