[Elecraft] KREF3 External Reference Option: Detailed proposal

[David Pratt]

This is coming, Nige; see Wayne's msg on the reflector on 15 June at 
00:15:39 ....

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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.

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73 de David G4DMP

In a recent message, Nige <nige1 at hotmail.com> wrote ...
>
>Wow that was a close one. I was planning to buy a loaded K3 + P3 as a 
>back-up radio for my Flex 5K which does have an external reference 
>input. The cart was loaded to go but now I know the K3 doesn't have 
>this capability this is very disappointing and I'll have to reconsider 
>what to do with my money.
>
>This really is a shame.
>
>Nige.
>G7CNF
-- 
David G4DMP
Leeds, England, UK
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