[Elecraft] 10 MHz external reference

[Edward R Cole]

If you just need a 10-MHz reference then you do not need a GPSDO.  A 
simple surplus OCXO (oven controlled xtal oscillator) off e-bay will 
suffice.  I bought a one for about $45 initially (Morion).  Since 
then I bought two sine-wave OCXO for $25.  You do need to supply very 
clean 12v dc for most of these and a stable bias 0-5v for tuning the 
OCXO to 10.000,000 MHz.

Short term stability is 5 E-12 which will do fine with the 
K3EXREF.  The TCXO is the limiting factor for frequency in the K3 so 
really no need for GPS accuracy.  If you are feeding a PLL system 
then the GPSDO will be better.

I use a LM7805 to 100K 15 turn pot to supply the tuning bias.  Adding 
a medium value electrolytic cap before the OCXO is extra insurance to 
eliminate any ac ripple or noise from modulating the OCXO.  One could 
also float charge a battery to feed the OCXO which also damps out any 
ac.  Why I mention this is I was using a 1.5A RS wall wart charger 
initially until I was told I had 120-Hz spurs on my signal.  There 
was about 0.3vac ripple from the so-called 12v wall wart.

I could have built or bought a clean low power 12v PS but I just 
hooked the OCXO to my 50A Astron PS.  No spurs, now.

To set up the OCXO use a good frequency counter or net the oscillator 
to WWV 10-MHz (if you can hear it).  I have a Rubidium source that I 
use as my standard and measure with a microwave frequency 
counter.  But that is not needed for basic use on HF/6m with the 
K3EXREF.  Note that OCXO will drift off frequency about 1-Hz in six 
months so regular checking a couple times per year is needed.  GPSDO 
does not need this.

73, Ed - KL7UW

Date: Wed, 09 Mar 2016 12:37:04 -0500
From: "Gary Smith" <Gary at ka1j.com>
To: Elecraft at mailman.qth.net
Subject: [Elecraft] 10 MHz external reference
Message-ID: <56E05F40.12472.7B2FFCE at Gary.ka1j.com>
Content-Type: text/plain; charset=US-ASCII

I ordered the K3EXREF and thought it was the oscillator itself which
would provide the external signal source to the K3s. I thought this
was all I was going to need but I see now that is not the case.
Looking on the Elecraft order page, they don't sell a reference
source.

What external 10-MHz reference signal source might I find get that is
not terribly expensive that will do the proper job and allow me to
get this working?

Thanks & 73,

Gary
KA1J

73, Ed - KL7UW
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