[Elecraft] K3S XREF GPSDO recommendation needed

[Edward R Cole]

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 <wes_n7ws at triconet.org>
To: elecraft at mailman.qth.net
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:
   dubususa at gmail.com