[Elecraft] KREF3 External Reference Option: Detailed proposal

[Eugene Balinski]

10 MHz is an industry standard.  This is goof.  However,
you might want to consider a 1PPS GPS-trained input as
well.   

73,
Gene K1NR  

On Mon, 14 Jun 2010 16:15:39 -0700
 Wayne Burdick <n6kr at elecraft.com> wrote:
> There appears to be more demand for this than we thought,
> so we're  
> looking into it again.
> 
> Note that our planned external reference implementation
> would not  
> require an exact 49.380000-MHz reference oscillator
> signal. It also  
> doesn't require a pin-1 control voltage input on the TCXO
> module  
> itself, as suggested earlier. It should work with any
> existing K3  
> TCXO, either +/- 5 or +/- 1 PPM.
> 
> If you're interested, here are the details:
> 
> A new REF-lock module would be designed that plugs into
> the small 8- 
> pin connector near the top of the KREF3 PCB. The user's
> 10-MHz  
> reference signal would be routed from the rear panel to
> the TMP  
> connector on the KRE3 (this connector faces forward, into
> the DSP/ 
> front panel area).
> 
> A microcontroller on the REF-lock module would be clocked
> by the 10- 
> MHz reference. If the signal were present, the
> microcontroller would  
> continuously measure (and average) the frequency of the
> 49.38 MHz  
> oscillator, obtaining a value to the nearest 1 Hz.
> Hysteresis would be  
> applied to prevent "hunting," so the actual accuracy spec
> would  
> probably be +/- 2 Hz. This translates to about +/- 1 Hz
> in operating  
> frequency on 20 m.
> 
> Approximately once per second, the module would report
> the actual  
> reference frequency to the K3's main MCU via a slow
> digital data line  
> (labeled "100HZ" on the RF and KREF3 schematics). The
> main MCU would  
> use this to calibrate the REF CAL value normally entered
> by the user.  
> There would be no noticeable impact on performance.
> 
> If the 10 MHz external signal were not present, the
> user's normal REF  
> CAL value would be used.
> 
> Let me know if you have thoughts on this implementation.
> 
> 73,
> Wayne
> N6KR
> 
>
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