Posted by
Edward R Cole on
Apr 05, 2011; 7:43pm
URL: http://elecraft.85.s1.nabble.com/K3EXREF-and-Trimble-Thunderbolts-tp6242158p6243611.html
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.htmWhen 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" <
[hidden email]>
Subject: Re: [Elecraft] K3EXREF and Trimble Thunderbolts
To:
[hidden email]
Message-ID: <
[hidden email]>
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
======================================
BP40IQ 500 KHz - 10-GHz www.kl7uw.com
EME: 144-1.4kw, 432-100w, 1296-testing*, 3400-winter?
DUBUS Magazine USA Rep
[hidden email]
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