[Elecraft] Thoughts about K2 freq shift & FINE RIT...

[David Woolley]

Steve Kallal wrote:
> 
> I understand there is a FINE RIT feature that can correct this frequency
> shift. I have the RTTY mode filters set up as:

I would say the RIT steps were part of the problem, rather than part of 
cure, but basically you will need frequency measuring equipment with 1Hz 
or better resolution.

> Next I'd like to ask about FINE RIT. A reading of the manual and this
> reflector tells me there are a number of variables affecting FINE RIT. It
> has a finer frequency correction on the lower bands. And the shift amount

If I understand how it works, this should read bands with lower local 
oscillator frequencies, rather than lower bands.  I'm not sure that the 
two are the same.

> could change even within the same band. Also the FINE RIT correction needed
> may change from filter to filter.

It will cycle over about every 5kHz (need to check the manual to find 
the actual value), with an overall drift over a wider frequency range. 
What happens is that a stepped voltage is applied to the PLL frequency 
reference to interpolate between the 5kHz (?) steps of the PLL.  The 
actual step in the reference frequency will vary because the varactor 
control voltage to frequency curve is non-linear, so different steps 
will occur at different points in the 5kHz (?) tweaking range.

Each step will be multiplied by the ratio between the local oscillator 
frequency and the reference frequency, so higher local oscillator 
frequencies will result in larger steps.  On the other hand, a smaller 
control voltage range will be needed, so the varactor characteristics 
will be more linear and one may be able to select a part of the range 
that supports relatively small steps, whereas, for low local oscillator 
frequencies one would need a larger range, so might end up with some 
quite small steps at one end of the range.

There will also be a variability because the the actual BFO frequencies 
are not exact round frequencies, but depend on the result of applying 
different control voltages to the varactors.  That means there will be 
an additional error due to the resolution of the frequency counter, and, 
I think to any change in the 4MHz frequency since the CAL PLL was run. 
I think that is 10kHz.

> 
> How can I determine the correct FINE RIT value needed for each band / filter
> width combination?

You need the output from CAL PLL, unfortunately that's not available 
externally.  You also need to measure the BFO frequency more accurately 
than the RIT step size.  That probably means you need to use an external 
frequency counter, with 1Hz, or better, resolution

> popular digital frequency. I don't know if the frequency shift will vary
> much on 40 meters when 30 kHz apart. The other bands are 80, 30, and 20
> meters right now.

The variation across the band will be the result of RIT step resolution, 
so not correctable by RIT, although the number of RIT steps needed to 
correct the BFO frequency measurement error will vary, but will tend to 
cycle over whatever is the intrinsic PLL frequency step.
-- 
David Woolley
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