[Elecraft] K3EXREF and Trimble Thunderbolts

[Edward R. Cole]

Julian, and all:

As far as stability and accuracy, I believe the GPS are referenced to 
a better atomic standard than the Rb, but for ham needs probably not 
a significant difference (unless you do microwave, like >10-GHz).

I have the LPRO Rubidium (5xE-11), an OCXO (5xE-12), a couple Jupiter 
GPS which I have yet to set up.

I chose to run my OCXO 24/7 as my station frequency standard because 
its phase noise is best (-150 dB/Hz at 1-KHz).  The Rubidium is much 
better for long-term accuracy (the OCXO drifts over months time by a 
Hz or two).  But if you are needing a low phase reference the OCXO is 
considerably better than the Rb (-119 dB/Hz at 1-KHz).  Phase noise 
is important for good receiver sensitivity.  If the Rx uses a PLL 
phase-locked to an external reference, then that is linked to the 
phase-noise of the reference.

My 1296/28 transverter has a PLL so it is locked to my OCXO with 
error < 1 Hz.  The PLL is rated at -80 dBc/Hz of phase noise so the 
-150 dB/Hz phase noise rating of the OCXO is overkill.  My mw counter 
has an ext. ref. so I can use the Rb when making critical frequency 
measurements (in practise, I cannot see a difference using the Rb vs. 
the internal TCXO in the counter at 1296-MHz).  I found that my OCXO 
had drifted down about 1 Hz in three months since installing the OCXO 
(it runs continuously on a battery).  It implies 2-3 Hz annual 
drift.  If I ran the Rb there would be no drift beyond +/- 
5xE-11.  But the Rb has a finite life and the ones for sale are used 
so unknown how much life is left.  The OCXO was $39.99 on e-bay.

On the other hand the K3EXREF is not a PLL so the reference does not 
need to have super low phase noise.  It does need to be stable and 
accurate.  As Wayne explained it to me they chose not to PLL the TCXO 
as then the phase-noise spec of the K3 would be subject to whatever a 
ham used for a reference.  By using a method of periodic Frequency 
correction, the phase noise is only affected during the frequency 
adjustment that occurs about every 4 seconds.  The rest of the time 
the K3 enjoys the excellent phase noise performance of the TCXO which 
all the synthesizers in the K3 are locked to.

So discussion of best phase-noise reference sources is not really 
germane to the K3EXREF (Wayne and others correct me if I am 
misunderstanding this).  A OCXO, Rb, or GPS-DO will all work well as 
external ref. for the K3.  You want short term stability and 
long-term accuracy in the reference.

For the record my OCXO keeps the K3 within about 1.5 Hz at 28-MHz:
http://www.kl7uw.com/K3EXREF.htm

When you monitor the TCXO (CONFIG: REF CAL) you will see the 
frequency shift over time.  That is the action of the K3EXREF 
shifting the TCXO to match the external reference.  I found that my 
TCXO-3 after a year's use was 36-Hz high at 28-MHz when measured on 
my counter (with ext. ref off).  The ext. ref moved the TCXO to 
49.380.074 after about an hour.  But with the ext. ref. the K3 was on 
frequency from the moment of power on.  With just the TXCO there is a 
warm up period when the K3 will drift (about 30-minutes).  This is 
the time that the K3 internal temperature comes up to working 
level.  The amount of drift is much less using a TCXO.  And almost 
non-existent if using a ext. ref. like a OCXO.
(sorry, probably way more than you wished to know)

73, Ed - KL7UW
------------------------------

Message: 20
Date: Tue, 5 Apr 2011 08:09:26 -0700 (PDT)
From: "Julian, G4ILO" <julian.g4ilo at gmail.com>
Subject: Re: [Elecraft] K3EXREF and Trimble Thunderbolts
To: elecraft at mailman.qth.net
Message-ID: <1302016166946-6242619.post at n2.nabble.com>
Content-Type: text/plain; charset=us-ascii

What are the advantages / disadvantages of this type of frequency standard
over the Efratom LPRO-101 which is a rubidium standard?

Julian, G4ILO




73, Ed - KL7UW, WD2XSH/45
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