[Elecraft] 10 mHZ external reference VS the K3S's optional KTCXO3-1

[Edward R Cole]

Doug,

Many do not understand what how K3EXREF improves accuracy and 
stability.  Its commonly thought that its a phase locking system 
(which it is not doing).

Basic frequency accuracy and stability is established by the TCXO in 
the K3 which provides frequency reference to the VFO's.  Per Elecraft 
K3 manual - page:
TCXO-1 is 5ppm and TCXO-3 is 1 ppm

Using 28-MHz as an example:
5ppm = 28 E+6 x 5 E-6 = 28 x 5 = 140 Hz
1ppm = 28 E+6 x 1 E-6 = 28 Hz
note these are +140 to -140 Hz and +28 to -28 Hz

My measurement show that EXREF improves this to 0.1ppm
0.1ppm = 28 E+6 x 1 E-7 = 2.8 Hz*

*ONLY IF you have the TCXO-3 installed.  Also the TCXO is in warm-up 
for the first ten minutes after the K3 is powered on.  EXREF corrects 
all the warm-up drift, as well, so you can start using the radio 
immediately without frequency accuracy concerns.

HOW:  The EXREF ckt provides a more accurate 10-MHz reference for the 
TCXO to be compared with about every 4-seconds.  If the TCXO is off 
frequency during the 4-second interval then sw writes an offset to 
the TCXO to bring into agreement with the reference.  Drift is no 
different than running the TCXO alone.  The improvement is that drift 
is only allowed a few seconds between corrections which provides the 
improved accuracy.  Stability is still that of the TCXO that is installed.

<caveat> This is only my interpretation of the EXREF process - and 
does not represent Elecraft.

73, Ed
Whether you need this accuracy is for you to decide.  I like it since 
I do microwave ham radio where my transverters are phase locked to < 
1Hz.  It also ensures minimum drift using JT65 on VHF+.

Date: Mon, 2 Nov 2015 14:34:50 -0600
From: Doug Hensley <w5jv at hotmail.com>
To: "elecraft at mailman.qth.net" <elecraft at mailman.qth.net>
Subject: [Elecraft] 10 mHZ external reference VS the K3S's optional
         KTCXO3-1
Message-ID: <COL129-W84C7DCF545DE8AA3B4B74A912C0 at phx.gbl>
Content-Type: text/plain; charset="iso-8859-1"

I've been following the thread on using a 10 mHZ external reference 
and the prospect seems both affordable and at least initially fun BUT 
I'd like to ask is it really a practical improvement over Elecraft's 
available KTCXO3 internal 1ppm reference?  Or is this just a way to 
achieve more accuracy at less cost than the KTCXO3 ?  Isn't the K3S 
better served with its own oscillator?  What am I missing here?
Thanks,
Doug W5JV / AFA6DL


73, Ed - KL7UW
http://www.kl7uw.com
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