[Elecraft] K1 RIT and VFO counter hysteresis

[Wayne Burdick]

There has been some discussion lately about a change in the K1's displayed VFO
frequency on exit from RIT mode. This is due to our use of "hysteresis"
(lead/lag updates) in the K1's VFO frequency counter firmware. Applying
hysteresis prevents flickering of the 100 Hz digit on the LCD. But as a
side-effect, a shift of as much as 200 Hz is possible when changing bands or
turning RIT on/off. 

For those of you who are still awake :) here is further and still more arcane
detail: Suppose, due to the hystereis algorithm, you had a "lead" of one count
(100 Hz) in one direction at the time RIT is turned on. By the time it is turned
off, the counter will have sampled again many times, and it could now exhibit a
"lag" of 100 Hz in the other direction. The net (200 Hz) represents the
worst-case shift that you would ever see, assuming that VFO drift during the
interval is less than 100 Hz.

It would be possible to change the firmware to catch this condition explicitly
and guarantee that you'd never see the 200 Hz apparent frequency shift. But the
truth is that the display is still accurate to within +/- 100 Hz even in this
case, which meets the intended spec for the K1 VFO. And of course the VFO itself
will exhibit no shift at all--it's just the display that changes.

So I think we'll reserve the firmware space for something more interesting!

73,
Wayne
N6KR

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