[Elecraft] KREF3 External Reference Option: Detailed proposal

[Wayne Burdick]

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