Elecraft has under beta-testing the new K3EXREF option board that locks the K3's master oscillator (TCXO)
   to an external 10-MHz source which automatically controls the REF CAL function.  Manual control of 
   REF CAL is disabled upon application of an external 10-MHz reference source (+4 dBm to +16 dBm).

   Elecraft will supply a manual for installing the option board and cable upon release for general sale.
   What follows are some details of the installation during beta-testing at my station:  (sorry for the poor photos)

   

















 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 


 
















   Installation is fairly easy following the manual and can be accomplished in under 1-1/2 hours with hand tools 
   (no soldering).  Operation requires uploading new firmware to the K3.

   Upon connecting my 10-MHz reference I see an asterisk appear at CONFIG: REF*CAL and the TCXO freq. adjusted to
   49.380,006, 49.380,007, 49.380,008, ... 49.380,064/074 about thirty minutes after k3 power on.

   TEST RESULTS
   
   Measurements taken on 3/21/2011:

   REF OSC: Morion MV-62 OCXO, Freq = 10 MHz +/- 5 E-12, 10 dBm
   FREQ STD: Effracom LPRO Rubidium, Freq = 10 MHz +/- 5 E-11, 7.8 dBm
   FREQ COUNTER: EIP-538, 10 Hz - 26 GHz, ext ref: LPRO Rb

   OCXO Fo = 10.000,000 MHz (measured by EIP-538*)
   K3 (VFO-A set to 3.920,000 MHz)  Fo = could not make measurement (inadequate signal level to trigger counter)
   K3 (VFO-A set to 14.205,000 MHz) Fo = 14.204,999/14.204,998  (-1.5)
   K3 (VFO-A set to 28.100,000 MHz) Fo = 28.099,999/28.099,998  (-1.5)
   K3 (VFO-A set to 50.100,000 MHz) Fo = 50.100.002/50.100,003  (+2.5)

   Measurements taken on 3/31/2011: 
   OCXO required no adjustment to Rb.

   K3 (VFO-A set to 28.100,000 MHz) Fo = 28.100,000/28.099,999  (-0.5)
   K3 (VFO-A set to 50.100,000 MHz) Fo = 50.100.001/50.100,002  (+1.5)
   k3+144 Transverter** 
      (VFO-A set to 28.100,000 MHz) Fo = 144.100,005/144.100,006  (+6.0)
   K3+1296 Transverter*** 
      (VFO-A set to 28.100,000 MHz) Fo= 1296.099,998/1296.099,999  (-1.0)

   Transverter errors compensated for K3 error on 28.100 MHz.
 
   Notes:
     Fo = F1/F2 means frequency measurement toggles between these two values; (x.x) indicates freq. error in Hz.
     *  EIP-538 using ext. Rb reference after 30-min warm-up period; OCXO required slight freq. adjustment (3/21/2011) 
        of approx. 1-Hz; EIP-538 using its internal TCXO is less than 1-Hz off from the Rb.  
     ** 144/28 DEMI Transverter has xtal-osc. not locked to a reference.
     *** 1296/28 DEMI Transverter LO phase-locked to 10-MHz ext. reference.

  
   Disconnecting the 10-MHz reference and cycling K3 power restored normal TCXO operation (49.380,000):
   K3 (VFO-A set to 28.100,000 MHz) Fo = 28.100,036
 
 

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